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1
Chechelnitsky, Michael Y. (Michael Yurievich) 1972. "Adaptive error estimation in linearized ocean general circulation models." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/58516.
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Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1999.Includes bibliographical references (p. 206-211).
Data assimilation methods, such as the Kalman filter, are routinely used in oceanography. The statistics of the model and measurement errors need to be specified a priori. In this study we address the problem of estimating model and measurement error statistics from observations. We start by testing the Myers and Tapley (1976, MT) method of adaptive error estimation with low-dimensional models. We then apply the MT method in the North Pacific (5°-60° N, 132°-252° E) to TOPEX/POSEIDON sea level anomaly data, acoustic tomography data from the ATOC project, and the MIT General Circulation Model (GCM). A reduced state linear model that describes large scale internal (baroclinic) error dynamics is used. The MT method, closely related to the maximum likelihood methods of Belanger (1974) and Dee (1995), is shown to be sensitive to the initial guess for the error statistics and the type of observations. It does not provide information about the uncertainty of the estimates nor does it provide information about which structures of the error statistics can be estimated and which cannot. A new off-line approach is developed, the covariance matching approach (CMA), where covariance matrices of model-data residuals are "matched" to their theoretical expectations using familiar least squares methods. This method uses observations directly instead of the innovations sequence and is shown to be related to the MT method and the method of Fu et al. (1993). The CMA is both a powerful diagnostic tool for addressing theoretical questions and an efficient estimator for real data assimilation studies. It can be extended to estimate other statistics of the errors, trends, annual cycles, etc. Twin experiments using the same linearized MIT GCM suggest that altimetric data are ill-suited to the estimation of internal GCM errors, but that such estimates can in theory be obtained using acoustic data. After removal of trends and annual cycles, the low frequency /wavenumber (periods> 2 months, wavelengths> 16°) TOPEX/POSEIDON sea level anomaly is of the order 6 cm2. The GCM explains about 40% of that variance. By covariance matching, it is estimated that 60% of the GCM-TOPEX/POSEIDON residual variance is consistent with the reduced state linear model. The CMA is then applied to TOPEX/POSEIDON sea level anomaly data and a linearization of a global GFDL GCM. The linearization, done in Fukumori et al.(1999), uses two vertical mode, the barotropic and the first baroclinic modes. We show that the CMA method can be used with a global model and a global data set, and that the estimates of the error statistics are robust. We show that the fraction of the GCMTOPEX/ POSEIDON residual variance explained by the model error is larger than that derived in Fukumori et al.(1999) with the method of Fu et al.(1993). Most of the model error is explained by the barotropic mode. However, we find that impact of the change in the error statistics on the data assimilation estimates is very small. This is explained by the large representation error, i.e. the dominance of the mesoscale eddies in the TIP signal, which are not part of the 20 by 10 GCM. Therefore, the impact of the observations on the assimilation is very small even after the adjustment of the error statistics. This work demonstrates that simultaneous estimation of the model and measurement error statistics for data assimilation with global ocean data sets and linearized GCMs is possible. However, the error covariance estimation problem is in general highly underdetermined, much more so than the state estimation problem. In other words there exist a very large number of statistical models that can be made consistent with the available data. Therefore, methods for obtaining quantitative error estimates, powerful though they may be, cannot replace physical insight. Used in the right context, as a tool for guiding the choice of a small number of model error parameters, covariance matching can be a useful addition to the repertory of tools available to oceanographers.
by Michael Y. Chechelnitsky.
Ph.D.
2
Vimont, Daniel J. "The seasonal footprinting mechanism in the CSIRO coupled general circulation models and in observations /." Thesis, Connect to this title online; UW restricted, 2002. http://hdl.handle.net/1773/10074.
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Beare, Matthew Ivor. "The development of a general purpose parallel ocean circulation model." Thesis, University of East Anglia, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.266748.
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Font, i. Ferré Jordi. "La circulació general a la mar Catalana." Barcelona : Centre de Publicacions, Intercanvi Cientific i Extensio Universitaria, Universitat de Barcelona, 1986. http://catalog.hathitrust.org/api/volumes/oclc/32908084.html.
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Weaver, Anthony T. "On assimilating sea surface temperature data into an ocean general circulation model." Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/29204.
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The feasibility of sea surface temperature (SST) data improving the performance of an ocean general circulation model (OGCM) is investigated through a series of idealized numerical experiments. The GFDL Bryan-Cox-Semtner primitive equation model is set-up as an eddy resolving, unforced, flat bottomed channel of uniform depth. 'Observed'
SST data taken from a reference ocean established in a control run are continuously
assimilated into an 'imperfect' model using a simple 'nudging' scheme based on a surface relaxation condition of the form Q = C(SST — T₁) where Q is the heat flux and T₁ is the temperature at the top level of the model. The rate of assimilation is controlled by adjusting the constant inverse relaxation time parameter C.
Numerical experiments indicate that the greatest improvement in the model fields is achieved in the extreme case of infinite assimilation (i.e., C = ᅇ) in which the 'observed' SST is directly inserted into the model. This improvement is quantified by monitoring the reduction in the root mean square (RMS) errors relative to the simulated
reference ocean. Assimilation with longer relaxation time-scales (i.e., smaller C's) proves quite ineffective in reducing the RMS errors. The improvement in the direct insertion numerical experiment stems from the model's ability to transfer assimilated SST into subsurface information through strong advective processes. The assimilation of cool surface data induces convective overturning which transfers the 'cool' information
downward rapidly but adversely affects the vertical thermal structure by an unrealistic deepening of the mixed layer. By contrast, warm surface data do not penetrate
downward readily. Thus, the systematically biased downward flux of coolness gradually produces unrealistically cool subsurface waters.Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate
6
Brown, Catherine Alicia. "Oscillatory behavior in an ocean general circulation model of the North Atlantic." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape9/PQDD_0004/MQ46006.pdf.
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Dubois, Clotilde. "The role of diapycnal mixing in coupled atmosphere-ocean general circulation models." Thesis, University of Southampton, 2006. https://eprints.soton.ac.uk/63133/.
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The value of ocean diapycnal diffusivity (v) sets the rate at which dense bottom water can be mixed up through the stratified water column and thus plays an important role in the meridional overturning circulation (MOC). Previous idealised experiments and simplified theory suggest that the strength of the MOC and the ocean heat transport scale with the v. This study investigates the dependence of the MOC and other parameters on v using atmosphere-ocean general circulation models (AOGCM). Firstly, the dependence of the MOC strength on v is studied using a low resolution AOGCM with realistic geometry, FORTE, with spatially constant v values ranging from 0.1 cm2/s to an unrealistic high value of 5 cm2/s. At the cyclostationary state, global MOC strength is found to scale with v (in agreement with previous studies) according to a power law of 0.5. No power law is found for the MOC in the individual basins. The increase in MOC strength in the Atlantic and Pacific Oceans is associated with an increase in the ocean heat transport. The atmosphere responds to the change in the ocean state by a decrease of its energy transport and surface winds. Only a partial compensation is found between the ocean and atmosphere energy transport. The strength of v is found to have a strong impact on coupled phenomena, such as a cessation of El Niño at high v. Secondly, similar experiments are conducted with a state-of-the-art AOGCM, ECHAM5/ MPIOM. In this model, v is derived from a constant background diapycnal diffusion (b), wind induced mixing, the Richardson number and the convective adjustment. A set of 3 coupled experiments is conducted, with b = 0.1, 0.25 and 1 cm2/s. The scaling law from simple theory and the previous experiments with FORTE is not observed with this coupled model. At the cyclostationary state, the MOC strength weakens by 16% as b increases from 0.1 to 1 cm2/s. This behavior is not found when the experiments are repeated with an ocean-only model. The reduction in MOC in the coupled model is linked to a strong reduction in the convective mixing at high latitudes. The convective mixing is reduced by a continuous strong freshening in the Arctic region due to an increase in surface air temperature and melting of the sea-ice in the coupled experiments, which is not observed in the ocean-only experiments. The responses of the two coupled models show many similarities as b increases. Both models show convection in the Pacific for high values of b. The main difference is the response of the MOC in the Atlantic is linked to the different locations of the deep convection and their relative changes in the models. I conclude that the diapycnal mixing and the ocean-atmosphere interactions both control the strength of the MOC, and their influences cannot be considered separately.
8
Eichelberger, Scott James. "The effects of meridional heating gradients on the atmospheric general circulation and its variability /." Thesis, Connect to this title online; UW restricted, 2005. http://hdl.handle.net/1773/10029.
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Weddle, Charles A. OCEAN CURRENTS PACIFIC OCEAN OCEAN MODELS WIND STRESS STRESSES MATHEMATICAL MODELS WIND SENSITIVITY DEPTH CIRCULATION TROPICAL CYCLONES LONGITUDE INSTABILITY CYCLONES AIR WATER INTERACTIONS MIXED LAYER(MARINE) IMAGES METEOROLOGICAL SATELLITES THESES EQUATORIAL REGIONS TEMPERATURE. "The effect of westerly wind bursts on a tropical ocean general circulation model /." Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1993. http://handle.dtic.mil/100.2/ADA276423.
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Thesis (M.S. in Meteorology and M.S. in Physical Oceanography) Naval Postgraduate School, December 1993.Thesis advisor(s): Murphree, James Thomas ; Garwood, Roland W. "December 1993." Bibliography: p. 115-118. Also available online.
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Weddle, Charles A., and CURRENTS PACIFIC OCEAN OCEAN MODELS WIND STRESS STRESSES MATHEMATICAL MODELS WIND SENSITIVITY DEPTH CIRCULATION TROPICAL CYCLONES LONGITUDE INSTABILITY CYCLONES AIR WATER INTERACTIONS MIXED LAYER(MARINE) IMAGES ME OCEAN. "The effect of westerly wind bursts on a tropical ocean general circulation model." Thesis, Monterey, California. Naval Postgraduate School, 1993. http://hdl.handle.net/10945/26516.
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A primitive equation general circulation model with imbedded mixed layer physics has been used to investigate the response of the equatorial Pacific ocean to daily varying winds and westerly wind bursts. The major issue addressed by this study is the impact of daily varying winds, including westerly wind bursts, in the modeling of the tropical Pacific ocean and El Nino. In the developmental phase, the sensitivity of the model to the integration time step and the domain size were investigated. The results of this work were used to determine the optimal time step and model domain size for the main experimental model runs. In the experimental phase, the model was spun-up using time averaged wind stresses. The model ocean was then exposed to two years of realistic daily varying wind stresses covering the period of 1991 and 1992. The model developed an El Nino like response that corresponded in several respects with observed features of the 1991-92 El Nino. The model also developed tropical instability waves in the eastern Pacific similar to those observed in situ and in satellite SST images. The model's responses to the tropical cyclones that occurred during 1991-92 were also consistent in several ways with observations
11
Senan, Retish. "Intraseasonal Variability Of The Equatorial Indian Ocean Circulation." Thesis, Indian Institute Of Science, 2004. http://hdl.handle.net/2005/297.
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Climatological winds over the equatorial Indian Ocean (EqlO) are westerly most of the year. Twice a year, in April-May ("spring") and October-December ("fall"), strong, sustained westerly winds generate eastward equatorial jets in the ocean. There are several unresolved issues related to the equatorial jets. They accelerate rapidly to speeds over lms"1 when westerly wind stress increases to about 0.7 dyne cm"2 in spring and fall, but decelerate while the wind stress continues to be westerly; each jet is followed by westward flow in the upper ocean lasting a month or longer.
In addition to the semi-annual cycle, the equatorial winds and currents have strong in-traseasonal fluctuations. Observations show strong 30-60 day variability of zonal flow, and suggest that there might be variability with periods shorter than 20 days in the central EqlO. Observations from moored current meter arrays along 80.5°E south of Sri Lanka showed a distinct 15 day oscillation of equatorial meridional velocity (v) and off-equatorial zonal velocity (u). Recent observations from current meter moorings at the equator in the eastern EqlO show continuous 10-20 day, or biweekly, oscillations of v. The main motivation for the present study is to understand the dynamics of intraseasonal variability in the Indian Ocean that has been documented in the observational literature.
What physical processes are responsible for the peculiar behavior of the equatorial jets? What are the relative roles of wind stress and large scale ocean dynamics? Does intraseasonal variability of wind stress force intraseasonal jets? What is the structure and origin of the biweekly variability? The intraseasonal and longer timescale variability of the equatorial Indian Ocean circulation is studied using an ocean general circulation model (OGCM) and recent in
Abstract ii
situ observations. The OGCM simulations are validated against other available observations. In this thesis, we document the space-time structure of the variability of equatorial Indian Ocean circulation, and attempt to find answers to some of the questions raised above.
The main results are based on OGCM simulations forced by high frequency reanalysis and satellite scatterometer (QuikSCAT) winds. Several model experiments with idealized winds are used to interpret the results of the simulations. In addition to the OGCM simulations, the origin of observed intraseasonal anomalies of sea surface temperature (SST) in the eastern EqlO and Bay of Bengal, and related air-sea interaction, are investigated using validated satellite data.
The main findings of the thesis can be summarized as:
• High frequency accurate winds are required for accurate simulation of equatorial Indian
Ocean currents, which have strong variability on intraseasonal to interannual time scales.
• The variability in the equatorial waveguide is mainly driven by variability of the winds;
there is some intraseasonal variability near the western boundary and in the equatorial
waveguide due to dynamic instability of seasonal "mean" flows.
• The fall equatorial jet is generally stronger and longer lived than the spring jet; the fall
jet is modulated on intraseasonal time scales. Westerly wind bursts can drive strong
intraseasonal equatorial jets in the eastern EqlO during the summer monsoon.
• Eastward equatorial jets create a westward zonal pressure gradient force by raising sea
level, and deepening the thermocline, in the east relative to the west. The zonal pressure
force relaxes via Rossby wave radiation from the eastern boundary.
• The zonal pressure force exerts strong control on the evolution of zonal flow; the decel
eration of the eastward jets, and the subsequent westward flow in the upper ocean in the
presence of westerly wind stress, is due to the zonal pressure force.
• Neither westward currents in the upper ocean nor subsurface eastward flow (the ob
served spring and summer "undercurrent") requires easterly winds; they can be gener
ated by equatorial adjustment due to Kelvin (Rossby) waves generated at the western
(eastern) boundary.
• The biweekly variability in the EqlO is associated with forced mixed Rossby-gravity
(MRG) waves generated by intraseasonal variability of winds. The biweekly MRG wave in has westward and upward phase propagation, zonal wavelength of 3000-4500 km and phase speed of 4 m s"1; it is associated with deep off equatorial upwelling/downwelling.
Intraseasonal SST anomalies are forced mainly by net heat flux anomalies in the central and eastern EqlO; the large northward propagating SST anomalies in summer in the Bay of Bengal are due to net heat flux anomalies associated with the monsoon active-break cycle. Coherent variability in the atmosphere and ocean suggests air-sea interaction.
12
Kuo, Yu-Heng. "Errors caused by incompatible wind and buoyancy forcing in the ocean general circulation models." Thesis, Monterey, California. Naval Postgraduate School, 1992. http://hdl.handle.net/10945/23658.
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Approved for public release; distribution is unlimitedThe Geophysical Fluid Dynamics Laboratory Modular Ocean Model (GFDL MOM) is used to investigate the model difference between compatible and incompatible surface wind and buoyancy forcing. The atmosphere is a physical system in which surface wind and temperature fields are related, however in most ocean numerical models, the wind stress and buoyancy forcing are usually specified separately, i.e., no constraint between the surface wind stress and surface air temperature is considered. In reality, only one of these two fields can be prescribed in the atmosphere-driven ocean models. When the surface wind field is prescribed, the surface air temperature should be derived, and vice versa. If the two related fields are treated as totally independent in forcing the ocean models the results will be distorted. Since the model solutions depend upon the atmospheric forcing, it is important that we study the compatibility between the wind and buoyancy forcings and the effect which incompatibility might have on the ocean numerical models. This study shows that the surface wind and buoyancy forcing widely used in ocean numerical models are incompatible. Such an incompatibility results in 21 % error in the total northward transport of heat, 16% error in the total northward transport of salt, 25% error in v velocity, and 16% error in w velocity.
13
Warrior, Hari V. "Parameterization of the light models in various general ocean circulation models for shallow waters." [Tampa, Fla.] : University of South Florida, 2004. http://purl.fcla.edu/fcla/etd/SFE0000394.
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Warrior, Hari. "Parameterization of the Light Models in Various General Ocean Circulation Models for shallow waters." Scholar Commons, 2004. https://scholarcommons.usf.edu/etd/1292.
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Solar energy is incident on the earth's surface in both short-wave and long-wave parts of the spectrum. The short-wave part of the spectrum is of special interest to oceanographers since the vertical distribution of temperature in the top layer of the ocean is mostly determined by the vertical attenuation of short-wave radiation. There are numerous studies regarding the temperature evolution as a function of time (see Chapter 2 for details). The diurnal and seasonal variation of the heat content (and hence temperature) of the ocean is explored in this thesis. The basis for such heat budget simulation lies in the fact that the heat budget is the primary driver of ocean currents (maybe secondary to wind effects) and these circulation features affect the biological and chemical effects of that region.
The vertical attenuation of light (classified to be in the 300-700 nm range) in the top layer of the ocean has been parameterized by several authors. Simpson and Dickey (1981) in their paper have listed the various attenuation schemes in use till then. This includes a single-exponential form, a bimodal exponential form, and a spectral decomposition into nine spectral bands, each with their specific exponential functions with depth.
The effects of vertical light attenuation have been investigated by integrating the light models into a 1D and a 3D turbulence closure model. The main part of the thesis is the inclusion of a bottom effect in the shallow waters. Bottom serves two purposes, it reflects some light based on its albedo and it radiates the rest of the light as heat. 1-D simulation including bottom effects clearly indicates the effect of light on the temperature profile and also the corresponding effect on salinity profiles.
An extension of the study includes a 3D simulation of the heat budget and the associated circulation and hydrodynamics. Intense heating due to the bottom leads to the formation of hyper-saline waters that percolate down to depths of 50 m in the summer. Such plumes have been simulated by using a 3D numerical ocean model and it is consistent with observations from the Bahamas banks.
15
Lavender, Kara L. "The general circulation and open-ocean deep convection in the Labrador Sea : a study using subsurface floats /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2001. http://wwwlib.umi.com/cr/ucsd/fullcit?p3035893.
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Gough, William A. (William Arthur). "Lateral and isopycnal mixing of passive and active tracers in an ocean general circulation model." Thesis, McGill University, 1991. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=70297.
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The parameterization of isopycnal mixing is examined in an ocean general circulation model. The results are compared to those obtained with lateral mixing.In the equilibrium experiments, the isopycnal case has more gyre kinetic energy, a less intense thermohaline circulation, and less interior downwelling than the lateral case. Convection is replaced by enhanced vertical diffusion in the isopycnal case.
In the time dependent passive tracer experiments, the isopycnal case has smaller depth penetration of a surface released tracer. This is likely due to induced recirculation rather than numerical limitations.
The active tracer experiments examine the long term asymmetric behaviour of warm and cold surface anomalies introduced in an abrupt and gradual fashion for the lateral and isopycnal models. The thermal anomalies produce asymmetric transient responses. The abrupt and gradual changes produce the same equilibrium but different transient responses. The isopycnal case responds more rapidly and energetically than the lateral case.
17
Kim, Joong Tae. "Enhancing the resolution of sea ice in long-term global ocean general circulation model (gcm) integrations." Texas A&M University, 2003. http://hdl.handle.net/1969.1/5746.
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Open water in sea ice, such as leads and polynyas, plays a crucial
role in determining the formation of deep- and bottom-water, as well
as their long-term global properties and circulation. Ocean general
circulation models (GCMs) designed for studies of the long-term
thermohaline circulation have typically coarse resolution, making it
inevitable to parameterize subgrid-scale features such as leads and
convective plumes. In this study, a hierarchy of higher-resolution
sea-ice models is developed to reduce uncertainties due to coarse
resolution, while keeping the ocean component at coarse resolution to
maintain the efficiency of the GCM to study the long-term deep-ocean
properties and circulation. The higher-resolved sea-ice component is restricted to the Southern Ocean. Compared with the coarse sea-ice
model, the intermediate, higher-resolution version yields more
detailed coastal polynyas, a realistically sharp ice edge, and an
overall enhanced lead fraction. The latter gives enhanced rates of
Antarctic Bottom Water formation through enhanced near-boundary
convection. Sensitivity experiments revealed coastal katabatic winds
accounted for in the higher resolution version, are the main reason
for producing such an effect. For a more realistic coastline,
satellite passive-microwave data for fine-grid land/ice-shelf â seaice/
ocean boundary were used.
With a further enhancement of the resolution of the Southern OceanâÂÂs
sea-ice component, a grid spacing of 22 km is reached. This is about
the size of the pixel resolution of satellite-passive microwave data
from which ice concentration is retrieved. This product is used in
this study to validate the sea-ice component of the global ocean GCM.
The overall performance of the high-resolution sea-ice component is
encouraging, particularly the representation of the crucial coastal polynyas. Enhancing the resolution of the convection parameterization
reduces spurious coarse-grid polynyas. Constraining the upper-ocean
temperature and modifying the plume velocity removes unrealistic
small-scale convection within the ice pack. The observed highfrequency
variability along the ice edge is to some extent captured
by exposing the ice pack to upper-ocean currents that mimic tidal
variability. While these measures improve several characteristics of
the Southern Ocean sea-ice pack, they deteriorate the global deepocean
properties and circulation, calling for further refinements and
tuning to arrive at presently observed conditions.
18
Kiss, Andrew Elek, and Andrew Kiss@anu edu au. "Dynamics of laboratory models of the wind-driven ocean circulation." The Australian National University. Research School of Earth Sciences, 2001. http://thesis.anu.edu.au./public/adt-ANU20011018.115707.
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This thesis presents a numerical exploration of the dynamics governing
rotating flow driven by a surface stress in the " sliced cylinder " model
of Pedlosky & Greenspan (1967) and Beardsley (1969), and its close
relative, the " sliced cone " model introduced by Griffiths & Veronis
(1997). The sliced cylinder model simulates the barotropic wind-driven
circulation in a circular basin with vertical sidewalls, using a depth
gradient to mimic the effects of a gradient in Coriolis parameter. In the
sliced cone the vertical sidewalls are replaced by an azimuthally uniform
slope around the perimeter of the basin to simulate a continental slope.
Since these models can be implemented in the laboratory, their dynamics
can be explored by a complementary interplay of analysis and numerical
and laboratory experiments. ¶
In this thesis a derivation is presented of a generalised
quasigeostrophic formulation which is valid for linear and moderately
nonlinear barotropic flows over large-amplitude topography on an f-plane,
yet retains the simplicity and conservation properties of the standard
quasigeostrophic vorticity equation (which is valid only for small depth
variations). This formulation is implemented in a numerical model based
on a code developed by Page (1982) and Becker & Page (1990). ¶
The accuracy of the formulation and its implementation are confirmed by
detailed comparisons with the laboratory sliced cylinder and sliced cone
results of Griffiths (Griffiths & Kiss, 1999) and Griffiths & Veronis
(1997), respectively. The numerical model is then used to provide insight
into the dynamics responsible for the observed laboratory flows. In the
linear limit the numerical model reveals shortcomings in the sliced cone
analysis by Griffiths & Veronis (1998) in the region where the slope and
interior join, and shows that the potential vorticity is dissipated in an
extended region at the bottom of the slope rather than a localised region
at the east as suggested by Griffiths & Veronis (1997, 1998). Welander's
thermal analogy (Welander, 1968) is used to explain the linear
circulation pattern, and demonstrates that the broadly distributed
potential vorticity dissipation is due to the closure of geostrophic
contours in this geometry. ¶
The numerical results also provide insight into features of the flow at
finite Rossby number. It is demonstrated that separation of the western
boundary current in the sliced cylinder is closely associated with a
" crisis " due to excessive potential vorticity dissipation in the viscous
sublayer, rather than insufficient dissipation in the outer western
boundary current as suggested by Holland & Lin (1975) and Pedlosky
(1987). The stability boundaries in both models are refined using the
numerical results, clarifying in particular the way in which the western
boundary current instability in the sliced cone disappears at large
Rossby and/or Ekman number. A flow regime is also revealed in the sliced
cylinder in which the boundary current separates without reversed flow,
consistent with the potential vorticity " crisis " mechanism. In addition
the location of the stability boundary is determined as a function of the
aspect ratio of the sliced cylinder, which demonstrates that the flow is
stabilised in narrow basins such as those used by Beardsley (1969, 1972,
1973) and Becker & Page (1990) relative to the much wider basin used by
Griffiths & Kiss (1999). ¶
Laboratory studies of the sliced cone by Griffiths & Veronis (1997)
showed that the flow became unstable only under anticyclonic forcing. It
is shown in this thesis that the contrast between flow under cyclonic and
anticyclonic forcing is due to the combined effects of the relative
vorticity and topography in determining the shape of the potential
vorticity contours. The vorticity at the bottom of the sidewall smooths
out the potential vorticity contours under cyclonic forcing, but distorts
them into highly contorted shapes under anticyclonic forcing. In
addition, the flow is dominated by inertial boundary layers under
cyclonic forcing and by standing Rossby waves under anticyclonic forcing
due to the differing flow direction relative to the direction of Rossby
wave phase propagation. The changes to the potential vorticity structure
under strong cyclonic forcing reduce the potential vorticity changes
experienced by fluid columns, and the flow approaches a steady free
inertial circulation. In contrast, the complexity of the flow structure
under anticyclonic forcing results in strong potential vorticity changes
and also leads to barotropic instability under strong forcing. ¶
The numerical results indicate that the instabilities in both models
arise through supercritical Hopf bifurcations. The two types of
instability observed by Griffiths & Veronis (1997) in the sliced cone are
shown to be related to the western boundary current instability and
" interior instability " identified by Meacham & Berloff (1997). The
western boundary current instability is trapped at the western side of
the interior because its northward phase speed exceeds that of the
fastest interior Rossby wave with the same meridional wavenumber, as
discussed by Ierley & Young (1991). ¶
Numerical experiments with different lateral boundary conditions are also
undertaken. These show that the flow in the sliced cylinder is
dramatically altered when the free-slip boundary condition is used
instead of the no-slip condition, as expected from the work of Blandford
(1971). There is no separated jet, because the flow cannot experience a
potential vorticity " crisis " with this boundary condition, so the western
boundary current overshoots and enters the interior from the east. In
contrast, the flow in the sliced cone is identical whether no-slip,
free-slip or super-slip boundary conditions are applied to the horizontal
flow at the top of the sloping sidewall, except in the immediate vicinity
of this region. This insensitivity results from the extremely strong
topographic steering near the edge of the basin due to the vanishing
depth, which demands a balance between wind forcing and Ekman pumping on
the upper slope, regardless of the lateral boundary condition. The
sensitivity to the lateral boundary condition is related to the
importance of lateral friction in the global vorticity balance. The
integrated vorticity must vanish under the no-slip condition, so in the
sliced cylinder the overall vorticity budget is dominated by lateral
viscosity and Ekman friction is negligible. Under the free-slip condition
the Ekman friction assumes a dominant role in the dissipation, leading to
a dramatic change in the flow structure. In contrast, the much larger
depth variation in the sliced cone leads to a global vorticity balance in
which Ekman friction is always dominant, regardless of the boundary
condition.
19
Dansereau, Véronique. "Ice shelf-ocean interactions in a general circulation model : melt-rate modulation due to mean flow and tidal currents." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/78549.
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Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 121-123).
Interactions between the ocean circulation in sub-ice shelf cavities and the overlying ice shelf have received considerable attention in the context of observed changes in flow speeds of marine ice sheets around Antarctica. Modeling these interactions requires parameterizing the turbulent boundary layer processes to infer melt rates from the oceanic state at the ice-ocean interface. Here we explore two such parameterizations in the context of the MIT ocean general circulation model coupled to the z-coordinates ice shelf cavity model of Losch (2008). We investigate both idealized ice shelf cavity geometries as well as a realistic cavity under Pine Island Ice Shelf (PIIS), West Antarctica. Our starting point is a three-equation melt rate parameterization implemented by Losch (2008), which is based on the work of Hellmer and Olbers (1989). In this form, the transfer coefficients for calculating heat and freshwater fluxes are independent of frictional turbulence induced by the proximity of the moving ocean to the fixed ice interface. More recently, Holland and Jenkins (1999) have proposed a parameterization in which the transfer coefficients do depend on the ocean-induced turbulence and are directly coupled to the speed of currents in the ocean mixed layer underneath the ice shelf through a quadratic drag formulation and a bulk drag coefficient. The melt rate parameterization in the MITgcm is augmented to account for this velocity dependence. First, the effect of the augmented formulation is investigated in terms of its impact on melt rates as well as on its feedback on the wider sub-ice shelf circulation. We find that, over a wide range of drag coefficients, velocity-dependent melt rates are more strongly constrained by the distribution of mixed layer currents than by the temperature gradient between the shelf base and underlying ocean, as opposed to velocity-independent melt rates. This leads to large differences in melt rate patterns under PIIS when including versus not including the velocity dependence. In a second time, the modulating effects of tidal currents on melting at the base of PIIS are examined. We find that the temporal variability of velocity-dependent melt rates under tidal forcing is greater than that of velocity-independent melt rates. Our experiments suggest that because tidal currents under PIIS are weak and buoyancy fluxes are strong, tidal mixing is negligible and tidal rectification is restricted to very steep bathymetric features, such as the ice shelf front. Nonetheless, strong tidally-rectified currents at the ice shelf front significantly increase ablation rates there when the formulation of the transfer coefficients includes the velocity dependence. The enhanced melting then feedbacks positively on the rectified currents, which are susceptible to insulate the cavity interior from changes in open ocean conditions.
by Véronique Dansereau.
S.M.
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Stemmler, Irene. "The role of the ocean in global cycling of persistent organic contaminants refinement and application of a global multicompartment chemistry transport model." Berlin Heidelberg Springer, 2009. http://d-nb.info/996662642/04.
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Pimenta, de Miranda Anne. "Application d'un modèle numérique de circulation générale océanique permettant la génération de turbulence de méso-échelle à l'étude de l'Atlantique sud." Université Joseph Fourier (Grenoble), 1996. http://www.theses.fr/1996GRE10203.
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L'objectif de notre travail est l'application a haute resolution (1/3) d'un modele numerique de circulation generale oceanique a l'etude de l'atlantique sud afin de mieux resoudre les courants de bord ouest et la dynamique tourbillonnaire. Les principales difficultes numeriques rencontrees sont la consequence des choix originaux que sont l'utilisation d'une coordonnee verticale epousant la topographie, et la presence de trois frontieres ouvertes. Le lissage de la topographie, base sur l'experience acquise au cours de simulations basse resolution, s'est avere par endroits trop important et a parfois limite le realisme des resultats. Les frontieres ouvertes se sont revelees efficaces, et ont fait apparaitre un manque de donnees d'observation, notamment sur le bord ouest de la frontiere nord. La simulation a obtenu de nombreux resultats interessants reproduisant fidelement les grandes lignes de la circulation generale, comme par exemple la region de confluence et la retroflexion du courant des aiguilles. On observe la presence de courants de bord ouest (courants du bresil, des malouines, des aiguilles) tres intenses, le positionnement des fronts dans le courant circumpolaire fortement lie a la topographie. Les resultats du modele proposent un schema de circulation des eaux intermediaires avec une divergence sur le bord ouest a 25s et une recirculation dans le gyre subtropical confirmes par des analyses recentes de trajectoires de flotteurs. Malgre la relativement haute resolution utilisee le niveau de variabilite du modele est un peu faible, sans doute a cause d'un manque de resolution verticale, egalement responsable d'une representation incomplete des eaux de fond. La simulation fournit de plus des informations tres interessantes sur le cycle saisonnier, comme le montre une etude originale de la subduction des eaux modales
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Parsons, Arthur Rost. "On the Barents Sea Polar Front in summer and interpretations of the associated regional oceanography using an Arctic Ocean general circulation model." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1995. http://handle.dtic.mil/100.2/ADA307933.
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Dissertation (Ph.D. in Physical Oceanography) Naval Postgraduate School, September 1995.Dissertation supervisor(s): Robert H. Bourke, Albert J. Semtner. "September 1995." Bibliography: p. 171-178. Also available online.
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Knochel, Hervé. "Développement et validation d'un modèle numérique de circulation océanique à coordonnées [sigma] pour l'étude climatique de l'Atlantique nord." Université Joseph Fourier (Grenoble), 1998. http://www.theses.fr/1998GRE10256.
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Dans cette these, un modele numerique de circulation generale oceanique est mis en uvre dans le but d'effectuer une etude climatique de l'atlantique nord. Le modele presente la particularite d'utiliser une coordonnee verticale epousant le relief sous-marin (coordonnee ). Il inclut en outre un modele unidimensionnel integral de couche melangee de type krauss-turner pour parametriser la penetration des forcages atmospheriques de surface dans l'ocean. L'etude d'une simulation climatologique type montre la capacite du modele a reproduire les traits dominants de la circulation generale avec en particulier une representation realiste de la trajectoire du gulf stream et de son decollement a cape hatteras. Des etudes de sensibilite du modele sont egalement effectuees, mettant en evidence certains aspects caracteristiques, comme l'importance de la resolution verticale dans la representation de la circulation profonde ou l'amelioration de la penetration des forcages thermohalins du fait de l'utilisation du modele de couche melangee. Ces etudes de simulations climatologiques fournissent un etat climatique moyen de l'ocean, a partir duquel la variabilite interannuelle du modele est etudiee dans le cadre d'une simulation incluant des flux atmospheriques mensuels, issus de la reanalyse ncep/ncar entre 1958 et 1997. L'etude se concentre sur l'eau de la mer du labrador (lsw), montrant une variabilite interannuelle coherente de cette masse d'eau et validant ainsi de maniere tres positive les donnees de forcage du ncep/ncar. La simulation met de plus en evidence une inadequation du parametrage du forcage salin de surface. Malgre cela, le modele reproduit de maniere realiste les periodes observees de forte et faible convection en mer du labrador et fait apparaitre une propagation d'anomalies chaudes et froides dans la circulation cyclonique du gyre subpolaire.
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Rieck, Jan Klaus [Verfasser]. "The Nature and Variability of Eddy Kinetic Energy in an Ocean General Circulation Model With a Focus on the South Pacific Subtropical Gyre and the Labrador Sea / Jan Klaus Rieck." Kiel : Universitätsbibliothek Kiel, 2019. http://d-nb.info/1182313647/34.
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Acosta, Navarro Juan Camilo. "Anthropogenic influence on climate through changes in aerosol emissions from air pollution and land use change." Doctoral thesis, Stockholms universitet, Institutionen för miljövetenskap och analytisk kemi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-137077.
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Particulate matter suspended in air (i.e. aerosol particles) exerts a substantial influence on the climate of our planet and is responsible for causing severe public health problems in many regions across the globe. Human activities have altered the natural and anthropogenic emissions of aerosol particles through direct emissions or indirectly by modifying natural sources. The climate effects of the latter have been largely overlooked. Humans have dramatically altered the land surface of the planet causing changes in natural aerosol emissions from vegetated areas. Regulation on anthropogenic and natural aerosol emissions have the potential to affect the climate on regional to global scales. Furthermore, the regional climate effects of aerosol particles could potentially be very different than the ones caused by other climate forcers (e.g. well mixed greenhouse gases). The main objective of this work was to investigate the climatic effects of land use and air pollution via aerosol changes. Using numerical model simulations it was found that land use changes in the past millennium have likely caused a positive radiative forcing via aerosol climate interactions. The forcing is an order of magnitude smaller and has an opposite sign than the radiative forcing caused by direct aerosol emissions changes from other human activities. The results also indicate that future reductions of fossil fuel aerosols via air quality regulations may lead to an additional warming of the planet by mid-21st century and could also cause an important Arctic amplification of the warming. In addition, the mean position of the intertropical convergence zone and the Asian monsoon appear to be sensitive to aerosol emission reductions from air quality regulations. For these reasons, climate mitigation policies should take into consideration aerosol air pollution, which has not received sufficient attention in the past.
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Ngodock, Hans Emmanuel. "Assimilation de données et analyse de sensibilité : une application à la circulation océanique." Phd thesis, Université Joseph Fourier (Grenoble), 1996. http://tel.archives-ouvertes.fr/tel-00005006.
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Le travail mené dans cette thèse porte sur l'étude "à posteriori" de l'assimilation variationnelle de données. Il s'agit d'une démarche de faisabilité pour la mise au point des outils permettant de faire une analyse diagnostique (qualitative et quantitative) du processus d'assimilation variationnelle, notamment en ce qui concerne l'influence du bruit des observations sur le processus d'assimilation ainsi que sa propagation sur les champs reconstitués (nous sommes alors amenés à faire une étude de sensibilité), et l'influence de la configuration spatio-temporelle des observations sur le processus d'assimilation. L'application usuelle des équations adjointes pour l'analyse de sensibilité est revisée, car dans le contexte de l'assimilation variationnelle, nous avons montré par un exemple simple qu'il faut s'y prendre différemment. Nous proposons alors une méthode pour mener correctement cette analyse de sensibilité. Cette méthode est basée sur l'utilisation des équations adjointes au second ordre, obtenues en prenant l'adjoint du système d'optimalité. La sensibilité en est déduite par inversion du Hessien de la fonction coût via la minimisation d'une fonctionnelle quadratique. L'application est faite sur un modèle de circulation générale océanique de type quasi-géostrophique, et nous faisons aussi l'étude de l'existence et l'unicité de la solution de l'équation adjointe au second ordre du modèle considéré, pour justifier l'utilisation du Hessien et l'applicabilité de notre méthode. Nous étudions aussi l'influence de la configuration spatio-temporelle des observations sur le processus d'assimilation au travers du Hessien (à l'optimum) dont les éléments propres varient lorsqu'on fait varier la configuration. Enfin, nous étudions la prédicibilité du système d'optimalité.
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Baquero, Bernal Astrid. "Interannual climate variability in the tropical Indian Ocean : a study with a hierarchy of coupled general circulation models = Interannuale Klimavariabilität im tropischen Indischen Ozean : eine Untersuchung mit einer Hierarchie von globalen gekoppelten Ozean-Atmosphären-Modellen /." Hamburg : Max-Planck-Institut für Meteorologie, 2004. http://mpimet.mpg.de/de/web/download.php?src=max_erdsystem&file=pdfupload&id=15&filename=BzE_8.pdf.
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Bessières, Laurent. "Impact des marées sur la circulation générale océanique dans une perspective climatique." Phd thesis, Université Paul Sabatier - Toulouse III, 2007. http://tel.archives-ouvertes.fr/tel-00172154.
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La marée océanique a longtemps été considérée comme un phénomène haute fréquence dont la zone d'influence se limitait aux plateaux continentaux. Ainsi, la marée n'apparaissait pas susceptible d'interagir sur la circulation océanique grande échelle et in fine sur la dynamique basse fréquence du climat. Ce n'est qu'à la fin des années 1990, lorsque la mesure altimétrique et les solutions hydrodynamiques globales de marées ont atteint une précision centimétrique, qu'une connexion entre les marées et le climat est devenue envisageable. Dans cette perspective, l'objectif de cette étude est d'explorer quels sont les effets de la marée sur la circulation océanique grande échelle. Ces effets ont lieu à travers deux processus physiques essentiels liés à la marée : (i) sa dynamique fortement non-linéaire et (ii) la dissipation de son énergie en plein océan, sous la forme de mélange vertical ou de chaleur.Pour examiner chacun de ces aspects, la méthode retenue consiste à paramétriser les effets de la marée dans un modèle tridimensionnel de circulation générale océanique (OGCM) dédié au climat : NEMO. Pour ce faire nous utilisons les sorties 'off line' d'un modèle hydrodynamique bidimensionnel dédié à la marée : MOG2D-G. Dans un premier temps nous déterminons et nous décrivons pour la première fois une carte de la circulation résiduelle de marée (CRM) mondiale générée par la dynamique non-linéaire de la marée. Cette CRM obtenue par l'intermédiaire de MOG2D-G est alors introduite sous la forme d'un forçage extérieur dans l'OGCM NEMO. Dans un second temps, nous examinons la dissipation de l'énergie des marées. Tout d'abord nous quantifions la fraction de l'énergie de marée qui est dissipée en chaleur, ceci afin de déterminer si, à l'instar du flux géothermal, elle est susceptible de jouer un rôle important sur la circulation abyssale. Après avoir écarté cette possibilité, nous considérons la fraction d'énergie de marée qui se dissipe localement en mélange vertical via les ondes internes : le "tidal mixing" (TM). Le TM résulte d'un transfert d'énergie du mode barotope vers les modes baroclines. Ce transfert est diagnostiqué grâce au modèle MOG2D-G et intégré dans NEMO par l'intermédiaire d'une paramétrisation du mélange turbulent vertical.
Nous concluons : (i) que l'effet des marées sur la circulation océanique grande échelle et in fine sur le climat ne peut être significatif qu'à travers le TM, (ii) que l'introduction du TM local dans les OGCM est essentielle pour représenter correctement le transport des masses d'eaux abyssales et (iii) qu'il est désormais crucial de considérer le TM engendré loin du site de génération des ondes internes
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Mémery, Laurent. "Traceurs transitoires et circulation generale : etude du tritium dans l'atlantique nord." Paris 6, 1989. http://www.theses.fr/1989PA066346.
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Cette etude presente une methodologie qui permet d'emettre un jugement sur la valeur de l'information contenue dans des distributions de traceurs transitoires, en vue de contraindre la circulation generale de l'ocean. La circulation de l'atlantique nord, deduite d'un modele inverse, est testee par rapport aux donnees historiques de tritium. Dans un premier temps, une integration directe dans le temps est effectuee: les resultats sont apparemment en contradiction avec les donnees. Etant donne les grandes incertitudes associees aux flux d'entree, il est necessaire d'abord d'essayer de repondre a cette incompatibilite en modifiant les conditions aux limites du tritium. Cette procedure de regularisation est etroitement liee a la theorie du controle: le controle est defini ici par les conditions aux limites du champ de traceur. Dans l'exemple etudie, on montre qu'en fait des corrections des flux d'entree acceptables permettent d'expliquer les ecarts entre les donnees et les resultats: le modele de circulation n'est pas en contradiction avec les donnees historiques de tritium. D'une maniere generale, cette etude a montre l'importance jouee par les frontieres laterales ouvertes, a travers lesquelles les flux sont mal definis: l'ocean mondial dans son ensemble doit donc etre pris en compte
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Cosme, Emmanuel. "Cycle du soufre des moyennes et hautes latitudes Sud dans un modèle de circulation générale atmosphérique." Phd thesis, Université Joseph Fourier (Grenoble), 2002. http://tel.archives-ouvertes.fr/tel-00705168.
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L'objectif de ce travail est de contribuer à la compréhension du cycle atmosphérique du soufre en Antarctique par l'utilisation d'un Modèle de Circulation Générale Atmosphérique (MCGA). Les versions "Antarctique" et "soufre" du MCGA LMD-ZT ("Laboratoire de Météorologie Dynamique- Zoom Traceurs") ont été fusionnées pour l'étude à haute résolution Antarctique du cycle du soufre dans les moyennes et hautes latitudes Sud. Une méthode de forçage "latéral" de la circulation atmosphérique antarctique par des analyses du Centre Européen de Prévision Météorologique à Moyen Terme (CEPMMT) a été spécifiquement développée et appliquée. Le modèle a d'abord été évalué. Il représente correctement le cycle saisonnier des espéces soufrées aux sites d'observation en Antarctique. Plusieurs défauts ont cependant été identifiés , discutés, et certains ont été étudiés par des expériences numériques spécifiques. Ceci nous a permis, dans la suite de ce travail, de décrire de manière critique le cycle du soufre en Antarctique à partir des résultats du modèle. Le modèle a été utilisé pour trois applications. La première a été une mise en oeuvre directe du modèle, tel qu'il a été évalué, dans le but d'estimer les distributions spatiales, le cycle saisonnier dans les régions centrales, et le bilan annuel des espèces soufrées en Antarctique. La deuxième se présente sous forme d'études de sensibilité à la formulation des émissions de sulfure de diméthyle (DMS) océanique, que la phase d'évaluation a révélé comme déterminante pour la modélisation du cycle du soufre. Pour la troisième, l'adjoint du modèle de transport (qui, en première approximation, permet de remonter le temps), complété par l'adjoint du module chimique spécifiquement développé pour ce travail, a été mis en oeuvre pour une recherche quantitative de l'origine géographlque et de l'âge des espèces soufrées en Antarctique.
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Knudsen, Richard Ray. "A Study on the Integration of Multivariate MetOcean, Ocean Circulation, and Trajectory Modeling Data with Static Geographic Information Systems for Better Marine Resources Management and Protection During Coastal Oil Spill Response – A Case Study and Gap Analysis on Northeastern Gulf of Mexico Tidal Inlets." Scholar Commons, 2015. http://scholarcommons.usf.edu/etd/5974.
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The Oil Pollution Act of 1990 requires the development of Regional and Area Contingency Plans. For more than 20 years, the State of Florida, under both the Department of Environmental Protection and the Florida Fish and Wildlife Commission, has worked closely with the U.S. Coast Guard and the National Oceanic and Atmospheric Administration to develop these plans for coastal and marine oil spill response. Current plans, developed with local, state and federal stakeholder input, use geographic information systems (GIS) data such as location and extent of sensitive ecological, wildlife, and human-use features (termed Environmental Sensitivity Index data), pre-defined protection priorities, and spatially explicit protection strategies to support decision-making by responders (termed Geographic Response Plans). However, they are long overdue for improvements that incorporate modern oceanographic modeling techniques and integrated data from coastal ocean observing systems. Better understanding of circulation in nearshore and estuarine waters, at a scale consistent with other spatial data, is especially lacking in Area Contingency Plans. This paper identifies the gaps in readily available information on the circulation-driven causes and effects missing in current oil spill contingency planning and describes a sample methodology whereby multiple coastal and ocean spatial science disciplines are used to answer questions that no single, non-integrated discipline can answer by itself. A path forward for further integration and development of more comprehensive plans to better support coastal protection in Florida is proposed. The advances made here are applicable to other coastal regions of the world.
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Beismann, Jens-Olaf. "Etude des variations de la circulation thermohaline dans l'océan atlantique par simulation de la circulation et de la distribution des chlorofluorométhanes." Université Joseph Fourier (Grenoble), 1999. http://www.theses.fr/1999GRE10248.
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Dans cette these, un modele numerique de circulation generale oceanique est mis en uvre dans le but d'effectuer une etude de la variabilite interannuelle et decennale de la circulation thermohaline de l'ocean atlantique. Le modele permet la communication avec les oceans voisins a l'aide des frontieres ouvertes au passage de drake et a 30\e entre l'afrique et l'antarctique. La simulation des traceurs cfc presente un outil diagnostique supplementaire pour suivre les masses d'eau. Une etude de la circulation simulee en forcage climatologique montre la capacite du modele de reproduire de maniere realiste les caracteristiques de la circulation generale. Les transports meridiens integres, variables importantes pour le role de l'ocean dans le systeme du climat, sont en accord avec des observations et d'autres etudes numeriques. Des etudes de sensibilite montrent que le choix des conditions aux frontieres ouvertes est tres important pour une simulation realiste, et qu'une modification tres locale dans la representation de la topographie de fond peut provoquer des changements inportants de la circulation generale simulee. A partir de l'etat fourni par les simulations en forcage climatologique, le modele est force par les flux atmosphere-ocean issus de la reanalyse ncep/ncar qui couvrent la periode de 1958 a 1997 pour etudier la reaction de l'ocean a un forcage variant de facon interannuelle. L'analyse des resultats se concentre sur des phenomenes dans l'atlantique nord qui sont compares aux observations de la meme periode. Les simulations mettent en evidence une reaction de la circulation oceanique aux tendances a long terme du forcage atmospherique, caracterise par l'oscillation nord-atlantique. Cette reaction comprend un renforcement de la circulation meridienne d'overturning et du transport meridien de chaleur. Les variations interannuelles sont refletes par des variations de la temperature et de la salinite des masses d'eau formees dans des regions subpolaires, variations qui sont transmises en profondeur et vers le gyre subtropical par la propagation de ces masses d'eau dans la branche profonde de la cellule d'overturning. Les simulations montrent egalement une forte influence des variations du transport de l'eau de mer du labrador vers le sud sur les variations de la circulation d'overturning qui est plus marquee pendant les periodes de faible overturning.
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Faleiro, Frederico Augusto Martins Valtuille. "Modelagem de mudanças climáticas: do nicho fundamental à conservação da biodiversidade." Universidade Federal de Goiás, 2016. http://repositorio.bc.ufg.br/tede/handle/tede/5603.
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
The climate changes are one of the major threats to the biodiversity and it is expected to increase its impact along the 21st century. The climate change affect all levels of the biodiversity from individuals to biomes, reducing the ecosystem services. Despite of this, the prediction of climate change impacts on biodiversity is still a challenge. Overcoming these issues depends on improvements in different aspects of science that support predictions of climate change impact on biodiversity. The common practice to predict the climate change impact consists in formulate ecological niche models based in the current climate and project the changes based in the future climate predicted by the climate models. However, there are some recognized limitations both in the formulation of the ecological niche model and in the use of predictions from the climate models that need to be analyzed. Here, in the first chapter we review the science behind the climate models in order to reduce the knowledge gap between the scientific community that formulate the climate models and the community that use the predictions of these models. We showed that there is not consensus about evaluate the climate models, obtain regional models with higher spatial resolution and define consensual models. However, we gave some guidelines for use the predictions of the climate models. In the second chapter, we tested if the predictions of correlative ecological niche models fitted with presence-absence match the predictions of models fitted with abundance data on the metrics of climate change impact on orchid bees in the Atlantic Forest. We found that the presence-absence models were a partial proxy of change in abundance when the output of the models was continuous, but the same was not true when the predictions were converted to binary. The orchid bees in general will decrease the abundance in the future, but will retain a good amount of suitable sites in the future and the distance to gained climatic suitable areas can be very close, despite of great variation. The change in the species richness and turnover will be mainly in the western and some regions of southern of the Atlantic Forest. In the third chapter, we discussed the drawbacks in using the estimations of realized niche instead the fundamental niche, such as overpredicting the effect of climate change on species’ extinction risk. We proposed a framework based on phylogenetic comparative and missing data methods to predict the dimensions of the fundamental niche of species with missing data. Moreover, we explore sources of uncertainty in predictions of fundamental niche and highlight future directions to overcome current limitations of phylogenetic comparative and missing data methods to improve predictions. We conclude that it is possible to make better use of the current knowledge about species’ fundamental niche with phylogenetic information and auxiliary traits to predict the fundamental niche of poorly-studied species. In the fourth chapter, we used the framework of the chapter three to test the performance of two recent phylogenetic modeling methods to predict the thermal niche of mammals. We showed that PhyloPars had better performance than Phylogenetic Eigenvector Maps in predict the thermal niche. Moreover, the error and bias had similar phylogenetic pattern for both margins of the thermal niche while they had differences in the geographic pattern. The variance in the performance was explained by taxonomic differences and not by methodological aspects. Finally, our models better predicted the upper margin than the lower margin of the thermal niche. This is a good news for predicting the effect of climate change on species without physiological data. We hope our finds can be used to improve the predictions of climate change effect on the biodiversity in future studies and support the political decisions on minimizing the effects of climate change on biodiversity.
As mudanças climáticas são uma das principais ameaças à biodiversidade e é esperado que aumente seu impacto ao longo do século XXI. As mudanças climáticas afetam todos os níveis de biodiversidade, de indivíduos à biomas, reduzindo os serviços ecossistêmicos. Apesar disso, as predições dos impactos das mudanças climáticas na biodiversidade é ainda um desafio. A superação dessas questões depende de melhorias em diferentes aspectos da ciência que dá suporte para predizer o impacto das mudanças climáticas na biodiversidade. A prática comum para predizer o impacto das mudanças climáticas consiste em formular modelos de nicho ecológico baseado no clima atual e projetar as mudanças baseadas no clima futuro predito pelos modelos climáticos. No entanto, existem algumas limitações reconhecidas na formulação do modelo de nicho ecológico e no uso das predições dos modelos climáticos que precisam ser analisadas. Aqui, no primeiro capítulo nós revisamos a ciência por detrás dos modelos climáticos com o intuito de reduzir a lacuna de conhecimentos entre a comunidade científica que formula os modelos climáticos e a comunidade que usa as predições dos modelos. Nós mostramos que não existe consenso sobre avaliar os modelos climáticos, obter modelos regionais com maior resolução espacial e definir modelos consensuais. No entanto, nós damos algumas orientações para usar as predições dos modelos climáticos. No segundo capítulo, nós testamos se as predições dos modelos correlativos de nicho ecológicos ajustados com presença-ausência são congruentes com aqueles ajustados com dados de abundância nas medidas de impacto das mudanças climáticas em abelhas de orquídeas da Mata Atlântica. Nós encontramos que os modelos com presença-ausência foram substitutos parciais das mudanças na abundância quando o resultado dos modelos foi contínuo (adequabilidade), mas o mesmo não ocorreu quando as predições foram convertidas para binárias. As espécies de abelhas, de modo geral, irão diminuir em abundância no futuro, mas reterão uma boa quantidade de locais adequados no futuro e a distância para áreas climáticas adequadas ganhadas podem estar bem próximo, apesar da grande variação. A mudança na riqueza e na substituição de espécies ocorrerá principalmente no Oeste e algumas regiões no sul da Mata Atlântica. No terceiro capítulo, nós discutimos as desvantagens no uso de estimativas do nicho realizado ao invés do nicho fundamental, como superestimar o efeito das mudanças climáticas no risco de extinção das espécies. Nós propomos um esquema geral baseado em métodos filogenéticos comparativos e métodos de dados faltantes para predizer as dimensões do nicho fundamental das espécies com dados faltantes. Além disso, nós exploramos as fontes de incerteza nas predições do nicho fundamental e destacamos direções futuras para superar as limitações atuais dos métodos comparativos filogenéticas e métodos de dados faltantes para melhorar as predições. Nós concluímos que é possível fazer melhor uso do conhecimento atual sobre o nicho fundamental das espécies com informação filogenética e caracteres auxiliares para predizer o nicho fundamental de espécies pouco estudadas. No quarto capítulo, nós usamos o esquema geral do capítulo três para testar a performance de dois novos métodos de modelagem filogenética para predizer o nicho térmico dos mamíferos. Nós mostramos que o “PhyloPars” teve uma melhor performance que o “Phylogenetic Eigenvector Maps” em predizer o nicho térmico. Além disso, o erro e o viés tiveram um padrão filogenético similar para ambas as margens do nicho térmico, enquanto eles apresentaram diferentes padrões espaciais. A variância na performance foi explicada pelas diferenças taxonômicas e não pelas diferenças em aspectos metodológicos. Finalmente, nossos modelos melhor predizem a margem superior do que a margem inferior do nicho térmico. Essa é uma boa notícia para predizer o efeito das mudanças climáticas em espécies sem dados fisiológicos. Nós esperamos que nossos resultados possam ser usados para melhorar as predições do efeito das mudanças climáticas na biodiversidade em estudos futuros e dar suporte para decisões políticas para minimização dos efeitos das mudanças climáticas na biodiversidade.
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Neveu, Emilie. "Applications des méthodes multigrilles à l'assimilation de données en géophysique." Phd thesis, Université de Grenoble, 2011. http://tel.archives-ouvertes.fr/tel-00693486.
Full textAbstract:
Depuis ces trente dernières années, les systèmes d'observation de la Terre et les modèles numériques se sont perfectionnés et complexifiés pour nous fournir toujours plus de données, réelles et numériques. Ces données, de nature très diverse, forment maintenant un ensemble conséquent d'informations précises mais hétérogènes sur les structures et la dynamique des fluides géophysiques. Dans les années 1980, des méthodes d'optimisation, capables de combiner les informations entre elles, ont permis d'estimer les paramètres des modèles numériques et d'obtenir une meilleure prévision des courants marins et atmosphériques. Ces méthodes puissantes, appelées assimilation variationnelle de données, peinent à tirer profit de la toujours plus grande complexité des informations de par le manque de puissance de calcul disponible. L'approche, que nous développons, s'intéresse à l'utilisation des méthodes multigrilles, jusque là réservées à la résolution de systèmes d'équations différentielles, pour résoudre l'assimilation haute résolution de données. Les méthodes multigrilles sont des méthodes de résolution itératives, améliorées par des corrections calculées sur des grilles de plus basses résolutions. Nous commençons par étudier dans le cas d'un modèle linéaire la robustesse de l'approche multigrille et en particulier l'effet de la correction par grille grossière. Nous dérivons ensuite les algorithmes multigrilles dans le cadre non linéaire. Les deux types d'algorithmes étudiés reposent d'une part sur la méthode de Gauss Newton multigrille et d'autre part sur une méthode sans linéarisation globale : le Full Approximation Scheme (FAS). Ceux-ci sont appliqués au problème de l'assimilation variationnelle de données dans le cadre d'une équation de Burgers 1D puis d'un modèle Shallow-water 2D. Leur comportement est analysé et comparé aux méthodes plus traditionnelles de type incrémentale ou multi-incrémentale.
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Vinayachandran, P. N. "The Bay Of Bengal Circulation In An Ocean General Circulation Model." Thesis, 1995. http://etd.iisc.ernet.in/handle/2005/1753.
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Aneesh, C. S. "Data Assimilation Experiments Using An Indian Ocean General Circulation Model." Thesis, 2006. http://hdl.handle.net/2005/358.
Full textAbstract:
Today, ocean modeling is fast developing as a versatile tool for the study of earth’s climate, local marine ecosystems and coastal engineering applications. Though the field of ocean modeling began in the early 1950s along with the development of climate models and primitive computers,
even today, the state-of-the-art ocean models have their own limitations. Many issues still remain such as the uncertainity in the parameterisation of essential processes that occur on spatial and
temporal scales smaller than that can be resolved in model calculations, atmospheric forcing of the ocean and the boundary and initial conditions.
The advent of data assimilation into ocean modeling has heralded a new era in the field of ocean modeling and oceanic sciences. “Data assimilation” is a methodology in which observations
are used to improve the forecasting skill of operational meteorological models.
The study in the present thesis mainly focuses on obtaining a four dimensional realization (the spatial description coupled with the time evolution) of the oceanic flow that is simultaneously consistent with the observational evidence and with the dynamical equations of motion and to
provide initial conditions for predictions of oceanic circulation and tracer distribution.
A good implementation of data assimilation can be achieved with the availability of large number of good quality observations of the oceanic fields as both synoptic and in-situ data. With the technology in satellite oceanography and insitu measurements advancing by leaps over the past two decades, good synoptic and insitu observations of oceanic fields have been achieved. The current and expected explosion in remotely sensed and insitu measured oceanographic data is ushering a new age of ocean modeling and data assimilation. The thesis presents results of analysis
of the impact of data assimilation in an ocean general circulation model of the North Indian Ocean.
In this thesis we have studied the impact of assimilation of temperature and salinity profiles from Argo floats and Sea Surface height anomalies from satellite altimeters in a Sigma-coordinate Indian Ocean model. An ocean data assimilation system based on the Regional Ocean Modeling System (ROMS) for the Indian Ocean is used. This model is implemented, validated and applied
in a climatological simulation experiment to study the circulation in the Indian Ocean. The validated model is then used for the implementation of the data assimilation system for the Indian Ocean region. This dissertation presents the qualitative and quantitative comparisons of the model
simulations with and without subsurface temperature and salinity profiles and sea surface height anamoly data assimilation for the Indian Ocean region. This is the first ever reported data assimilation studies of the Argo subsurface temperature and salinity profile data with ROMS in the Indian
Ocean region.
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Chien, Mu-Hua, and 簡睦樺. "Investigating Energy Conversion in Kuroshio-Oyashio Extension by Ocean General Circulation Model." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/42382913591823468720.
Full textAbstract:
碩士國立臺灣大學
大氣科學研究所
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The Kuroshio-Oyashio Extension (KOE) involves significant barotropic and baroclinic energy conversion due to complex frontal instability. The energy conversion changes the sea surface momentum and heat flux, and further affects the local air-sea interaction. The observed sea surface height and sea surface temperature anomalies exhibit considerable variances from interannual to decadal time scales in the KOE region. To investigate the link between energy conversion and Kuroshio Extension, we simulated the KOE variability using an eddy-permitting global ocean circulation model, based on the recently-developed TaIwan Multi-scale Community Ocean Model (TIMCOM) with parallel implement. The model fully resolved the mesoscale features in the global framework, including the quasi-steady solution and the Kuroshio-Oyashio interaction . We further applied the breeding method to investigate the KOE flow instability. The approach added the rescaled bred vector into the control experiment to explore the development of fast growing mode in the KOE region. We applied the Empirical orthogonal function to find the principle component of KOE instability around east coastal of Japan, and the energy conversion in KOE region is result from the interaction of mean flow and the eddy as well as the available potential energy released form vertical density perturbation.
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Kurian, Jaison. "An Ocean General Circulation Model Study Of The Arabian Sea Mini Warm Pool." Thesis, 2007. http://hdl.handle.net/2005/564.
Full textAbstract:
The most important component of the climate system over the Indian Ocean region is the southwest monsoon, which dictates the life and economy of billions of people in the tropics. Being a phenomena that involves interaction between atmosphere, ocean and land, the southwest monsoon is strongly influenced by upper ocean, primarily through warm sea surface temperature (SST). This is particularly true about the southeastern Arabian Sea (SEAS) and the onset of southwest monsoon over the peninsular India. A localized patch of warm water, known as the Arabian Sea mini warm pool (ASMWP), forms in the SEAS during February–March. It remain as the warmest spot in the northern Indian Ocean till early April. A large region, surrounding the SEAS, attains SST exceeding 30°C during April–May, with often the ASMWP as its core. The ASMWP is believed to have a critical impact on the air-sea interaction during the onset phase of southwest monsoon and on the formation of the onset vortex, during late May or early June. This thesis addresses the formation mechanisms of ASMWP, using a high-resolution Ocean General Circulation Model (OGCM) of the Indian Ocean.
In addition to the formation of ASMWP, the SEAS is characterized by several features in its hydrography and circulation, which have been invoked in the past to explain the preferential warming of this oceanic region. During November–January, the prevailing surface currents transport low-salinity water from the Bay of Bengal into the SEAS and leads to strong haline stratification in the upper layer and formation of barrier layer (layer between mixed layer and isothermal layer). The vertical distribution of temperature in the SEAS exhibit inversions (higher subsurface temperature than that at surface) during December–February. A high in sea level and anticyclonic eddies develop in the SEAS during December and they propagate westward. These eddies modify the hydrography through downwelling and play an important role in the redistribution of advected low-salinity water within the SEAS. The seasonally reversing coastal and equatorial currents present in and around SEAS also have a major contribution in setting up the hydrography, through the advection and redistribution of cooler low-salinity water. These features make the SEAS a unique oceanographic region.
The first hypothesis on the formation of ASMWP, which has been suggested by diagnostic studies, is based on the barrier layer mechanism. The barrier layer, caused by the influx of low-salinity water at surface, is argued to maintain a shallow mixed layer which can warm more efficiently. In addition, presence of barrier layer can prevent mixed layer cooling, by cutting off the interaction of mixed layer with cooler thermocline water below. However, a coupled model study have shown that there is no significant impact on the ASMWP formation from barrier layer, but only a weak warming effect during it mature phase during April. The second hypothesis, which is based on an OGCM study, has suggested that the temperature inversions present within the barrier layer can heat the mixed layer through turbulent entrainment and in turn lead to the formation of ASMWP during February–March. Both hypotheses rule out the possibility of air-sea heat fluxes being the primary reason in its formation.
The strong salinity stratification in the SEAS during December–March is central to the hypotheses about formation of the ASMWP. Observational studies have only limited success in assessing the contribution from barrier layer and temperature inversions, as the ASMWP always form in their presence. OGCMs offer a better alternative. However, modelling processes in the northern Indian Ocean, especially that in the SEAS, is a challenging problem. Previous Indian Ocean models have had serious difficulties in simulating the low-salinity water in the Bay of Bengal and its intrusion into the SEAS. The northward advection of low-salinity water in the SEAS, along the west coast of India, is used to be absent in model simulations. Moreover, the coarse resolution inhibited those models from simulating faster surface currents and vigorous eddies as seen in the observations.
In this thesis, we use an OGCM of the Indian Ocean, based on the recent version of Modular Ocean Model (MOM4p0), to study the ASMWP. The model has high resolutions in the horizontal (1/4o x 1/4o) and vertical (40 levels, with 5 m spacing in upper 60 m), and has been forced with daily values momentum, heat and freshwater fluxes. The turbulent (latent and sensible) and long wave heat fluxes have been calculated as a function of model SST. The freshwater forcing consists of precipitation, evaporation and river runoff, and there are no surface restoring or flux adjustments. The river runoff has been distributed over several grid points about the river mouth instead of discharging into a singe grid point, which has resulted in remarkable improvements in salinity simulation.
The model simulates the Indian Ocean temperature, salinity and circulation remarkably well. The pattern of model temperature distribution and evolution matches very well with that in the observations. Significant improvements have been made in the salinity simulation, including the Bay of Bengal freshwater plume and intrusion of low-salinity water from the bay into the SEAS. The salinity distribution within the SEAS is also well represented in the model. The use of appropriate horizontal friction parameters has resulted in the simulation of realistic currents. The observed features in the SEAS, including the life cycle of the ASMWP, low-salinity water, barrier layer, temperature inversions, eddies and currents are well represented in the model.
Present study has unraveled the processes involved in the life cycle of barrier layer and temperature inversions in the SEAS. Presence of low-salinity water is necessary for their formation. Barrier layer develops in the SEAS during November, after the intrusion of low-salinity water from the Bay of Bengal. The barrier layer is thickest during January–February, and it dissipates during March–April. The variations and peak of barrier layer thickness is controlled by variations in isothermal layer depth, which in turn is dominated by the downwelling effects of anticyclonic eddies. The intense solar heating during March–April leads to the formation of shallow isothermal layer and results in the dissipation of barrier layer. Temperature inversions starts developing in the SEAS during December, reaches its peak during January–February and dissipates in the following months. Advection of cooler low-salinity water over warmer salty water and penetrating shortwave radiation is found to cause temperature inversions within the SEAS, whereas winter cooling is also important to the north and south of the SEAS. There is significant variation in the magnitude, depth of occurrence and formation mechanisms of temperature inversions within the SEAS.
Analysis of model mixed layer heat budget has shown that the SEAS SST is mainly controlled by atmospheric forcing, including the life cycle of ASMWP. It has also shown that the heating from temperature inversions do not contribute to the formation of ASMWP. In an experiment in which a constant salinity of 35 psu was maintained over the entire model domain, the ASMWP evolved very similar to that in the standard run, suggesting that the salinity effects are not necessary for the formation of ASMWP. Examination of wind field show that the winds over the SEAS during November–February are low due to the blocking of northeasterly winds by Western Ghats. Several process experiments by modifying the wind and turbulent heat fluxforcing fields have shown that these low winds lead to the formation of ASMWP in the SEAS during February–March. The low winds reduce latent heat loss, resulting in net heat gain by the ocean. This helps the SEAS to keep warmer SST while the surrounding region experience intense cooling under the strong dry northeasterly winds. As the winds are weak over the SEAS, the mixed layer is not able to feel the stratification beneath and the mixed layer depth is determined by solar heating, with or without salinity effects. In addition, the weak winds are not able to entrain the temperature inversions present in the barrier layer. The winds are weak during March–April too, and the air-sea heat fluxes dictate the SST evolution during this period. Therefore, during November–April, the SEAS acts as a low wind heat-dominated regime, where the evolution of sea surface temperature is solely determined by atmospheric forcing. We show that, in such regions, the evolution of surface layer temperature is not dependent on the characteristics of subsurface ocean, including the presence of barrier layer and temperature inversions.
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Najjar, Raymond Gabriel. "Simulations of the phosphorus and oxygen cycles in the world ocean using a general circulation model." 1990. http://catalog.hathitrust.org/api/volumes/oclc/29943465.html.
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"MULTIVARIATE MULTISITE STATISTICAL DOWNSCALING OF ATMOSPHERE-OCEAN GENERAL CIRCULATION MODEL OUTPUTS OVER THE CANADIAN PRAIRIE PROVINCES." Thesis, 2015. http://hdl.handle.net/10388/ETD-2015-12-2354.
Full textAbstract:
Atmosphere-Ocean General Circulation Models (AOGCMs) are the primary tool for modelling global climate change in the future. However, their coarse spatial resolution does not permit direct application for local scale impact studies. Therefore, either dynamical or statistical downscaling techniques are used for translating AOGCM outputs to local scale climatic variables.
The main goal of this study was to improve our understanding of the historical and future climate change at local-scale in the Canadian Prairie Provinces (CPPs) of Alberta, Saskatchewan and Manitoba, comprising 47 diverse watersheds. Given the vast nature of the study area and paucity of recorded data, a novel approach for identifying homogeneous regions for regionalization of precipitation characteristics for the CPPs was proposed. This approach incorporated information about predictors ― large-scale atmospheric covariates from the National Center for Environmental Prediction (NCEP) Reanalysis-I, teleconnection indices and geographical site attributes that impact spatial patterns of precipitation in order to delineate homogeneous precipitation regions using a combination of multivariate approaches. This resulted in the delimitation of five homogeneous climatic regions which were validated independently for homogeneity using statistics computed from observations recorded at 120 stations across the CPPs.
For multisite multivariate statistical downscaling, an approach based on the Generalized Linear Model (GLM) framework was developed to downscale daily observations of precipitation and minimum and maximum temperatures from 120 sites located across the CPPs. First, the aforementioned predictors and observed daily precipitation and temperature records were used to calibrate GLMs for the 1971–2000 period. Then the calibrated GLMs were used to generate daily sequences of precipitation and temperatures for the 1962–2005 historical (conditioned on NCEP predictors), and future period (2006–2100) using outputs from six CMIP5 (Coupled Model Intercomparison Project Phase-5) AOGCMs corresponding to Representative Concentration Pathway (RCP): RCP2.6, RCP4.5, and RCP8.5 scenarios. The results indicated that the fitted GLMs were able to capture spatiotemporal characteristics of observed climatic fields. According to the downscaled future climate, mean precipitation is projected to increase in summer and decrease in winter while minimum temperature is expected to warm faster than the maximum temperature. Climate extremes are projected to intensify with increased radiative forcing.
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"Adjusting to policy expectations in climate change modeling : an interdiciplinary study of flux adjustments in coupled atmosphere-ocean general circulation models." MIT Joint Program on the Science and Policy of Global Change, 1999. http://hdl.handle.net/1721.1/3599.
Full textAbstract:
This paper surveys and interprets the attitudes of scientists to the use of flux adjustments in climate projections with coupled Atmosphere Ocean General Circulation Models. The survey is based largely on the responses of 19 climate modellers to several questions and a discussion document circulated in 1995. We interpret the responses in terms of the following factors: the implicit assumptions which scientists hold about how the environmental policy process deals with scientific uncertainty over human-related global warming; the different scientific styles that exist in climate research; and the influence of organisations, institutions, and policy upon research agendas. We find evidence that scientists' perceptions of the policy process do play a role in shaping their scientific practices. In particular, many of our respondents expressed a preference for keeping discussion of the issue of flux adjustments within the climate modeling community, apparently fearing that climate contrarians would exploit the issue in the public domain. While this may be true, we point to the risk that such an approach may backfire. We also identify assumptions and cultural commitments lying at a deeper level which play at least as important a role as perceptions of the policy process in shaping scientific practices. This leads us to identify two groups of scientists, 'pragmatists' and 'purists,' who have different implicit standards for model adequacy, and correspondingly are or are not willing to use flux adjustments.Includes bibliographical references.
Abstract in HTML and technical report in HTML and PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/)
Funded by the UK's Economic and Social Research Council as part of the "Science, Culture and the Enviroment" program at Lancaster University, UK.
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Jiang, Xingjian. "Role of oceanic heat transport processes in CO₂-induced warming : analysis of simulations by the OSU coupled atmosphere-ocean general circulation model." Thesis, 1986. http://hdl.handle.net/1957/29322.
Full textAbstract:
The OSU global coupled atmosphere/ocean general circulation
model (A/O GCM) has been used to simulate the present (lxCO₂)
climate and to investigate a CO₂-induced (2xCO₂) climate change.
Previous analysis of the lxCO₂ simulation showed distinct errors in
the simulated sea surface temperature (SST) and sea ice which were
attributed primarily to the atmospheric GM (AGCM). Analysis of
the 2xCO₂ simulation showed that the CO₂-induced warming penetrated
into the ocean; this caused a delay in the equilibration of the
climate system with an estimated e-folding time of 50-75 years.
The present study has two objectives. The principal objective is
to answer the question: By what pathways and through which physical
processes does the simulated ocean general circulation produce
the penetration of the CO₂-induced warming into the ocean? The
secondary objective is to evaluate the performance of the oceanic
GCM (OGCM) in the lxCO₂ simulation.
The comparison of the simulated lxCO₂ internal oceanic fields
with the corresponding observations shows that although they are
basically similar, there are distinct errors. Further analysis
shows that these errors were generated by the OGCM during its spin-up
integration prior to its coupling with the AGCM. This study
thus shows that it is not sufficient to compare the simulated SST
with the observed SST to evaluate the performance of the OGCM. It
is also necessary to compare the simulated internal oceanic quantities
with the corresponding observed quantities.
The global mean analysis of the CO₂-induced climate changes
shows that the ocean gains heat at a rate of 3 W/m² due to the CO₂
doubling. This heat penetrates downward into the ocean predominantly
through the reduction in the convective overturning. The
zonal mean analysis shows that the surface warming increased from
the tropics toward the mid-latitudes of both hemispheres and penetrated
gradually to the deeper ocean. The oceanic warming penetrated
to a greater depth in the subtropics and mid-latitudes than
in the equatorial region. A zonal mean heat budget analysis shows
that the CO₂-induced warming of the ocean occurs predominantly
through the downward transport of heat, with the meridional heat
flux being only of secondary importance. In the tropics the penetration
of the CO₂-induced heating is minimized by the upwelling of
cold water. In the subtropics the heating is transported downward
more readily by the downwelling existing there. In the high latitudes
the suppressed convection plays the dominant role in the
downward penetration of the CO₂-induced heating.Graduation date: 1987
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Kumar, Suvarchal. "Impact Of Dynamical Core And Diurnal Atmosphere Occean Coupling On Simulation Of Tropical Rainfall In CAM 3.1, AGCM." Thesis, 2009. http://hdl.handle.net/2005/974.
Full textAbstract:
In first part of the study we discuss impact of dynamical core in simulation of tropical rainfall. Over years many new dynamical cores have been developed for atmospheric models to increase efficiency and reduce numerical errors. CAM3.1 gives an opportunity to study the impact of the dynamical core on simulations with its three dynamical cores namely Eulerian spectral(EUL) , Semilagrangian dynamics(SLD) and Finite volume(FV) coupled to a single parametrization package. A past study has compared dynamical cores of CAM3 in terms on tracer transport and has showed advantages using FV in terms of tracer transport. In this study we compare the dynamical cores in climate simulations and at their optimal configuration, which is the intended use of the model. The model is forced with AMIP type SST and rainfall over seasonal, interannual scales is compared. The significant differences in simulation of seasonal mean exist over tropics and over monsoon regions with observations and among dynamical cores. The differences among EUL and SLD, which use spectral transform methods are lesser compared that of with FV clearly indicating role of numerics in differences. There exist major errors in simulation of seasonal cycle in all dynamical cores and errors in simulation of seasonal means over many regions are associated with errors in simulation of seasonal cycle such as over south china sea. Seasonal cycle in FV is weaker compared to SLD and EUL. The dynamical cores exhibit different interannual variability of rainfall over Indian monsoon region, the period of maximum power corresponding to a dynamical core differs substantially with another. From this study there seems no superiority associated with FV dynamical core over all climate scales as seen in tracer transport.
The next part of the study deals with impact of diurnal ocean atmosphere coupling in an AGCM,CAM3.1. Due to relatively low magnitude of diurnal cycle of SST and lack of SST observations over diurnal scales current atmospheric models are forced with SSTs of periods grater than a day. CAM 3.1 standalone model is forced with monthly SSTs but the interpolation is linear to every time step between any two months and this linear interpolation implies a linear diurnal and intraseasonal variation of SST which is not true in nature. To test the sensitivity of CAM3.1 to coupling of SST on diurnal scales, we prescribed over tropics(20S20N) a diurnal cycle of SST over daily mean interpolated SST of different magnitudes and phase comparable to observations. This idea of using a diurnal cycle of SST retaining seasonal mean SST in an atmospheric model is novel and provides an interesting frame work to test sensitivity of model to interpolations used in coupling of boundary conditions. Our analysis shows a high impact of using diurnal cycle of SST on simulation of mean rainfall over tropics. The impact in a case where diurnal cycle of SST is fixed and retained to daily mean SST implies that changes associated with a coupled model are to some extent due to change in representation of diurnal cycle of SST. A decrease of excess rainfall over western coast of Bay of Bengal and an increase of rainfall over northern bay of Bengal in such case is similar to the improvement due to coupling atmospheric model to a slab ocean model. This also implies that problems with current AMIP models in simulation of seasonal mean Indian monsoon rainfall could be due to erroneous representation of diurnal cycle of SST in models over this region where the diurnal cycle of SST is high in observations. The high spatial variability of the impact in various cases over tropics implies that a similar spatial variation of diurnal cycle could be important for accurate simulation of rainfall over tropics. Preliminary analysis shows that impact on rainfall was due to changes in moisture convergence. We also hypothesized that diurnal cycle of SST could trigger convection over regions such as northern Bay of Bengal and rainfall convergence feedback sustains it. The impact was also found on simulation of internal interannual variability of rainfall
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Venugopal, Thushara. "Sensitivity of Sea Surface Temperature Intraseasonal Oscillation to Diurnal Atmospheric Forcings in an OGCM." Thesis, 2013. http://etd.iisc.ernet.in/2005/3347.
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Abstract
The diurnal cycle is a dominant mode of sea surface temperature (SST) variability in trop-ical oceans, that influences air-sea interaction and climate processes. Diurnal variability of SST generally ranges from ~0.1 to 2.0◦C and is controlled by atmospheric fluxes of heat and momentum. In the present study, the response of intraseasonal variability (ISV) of SST in the Bay of Bengal (BoB) to diurnal atmospheric forcings, during the summer monsoon of 2007, has been examined using an Ocean General Circulation Model (OGCM). The model is based on the Modular Ocean Model Version 4 (MOM4p0), having a horizontal resolution of 0.25◦ and 40 vertical levels, with a fine resolution of 5 m in the upper 60 m. Numerical experiments were conducted by forcing the model with daily and hourly atmospheric forcings to examine the SST-ISV modulation with the diurnal cycle. Additional experiments were performed to determine the relative role of diurnal cycle in solar radiation and winds on SST and mixed layer depth (MLD). Since salinity, which is decisive in SST variability, varies meridionally in the BoB, two locations were selected for analyses: one in the northern bay at 89◦E, 19◦N where salinity is lower and the other in the southern bay at 90◦E, 8◦N where salinity is higher, as well as observations are available from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoy for comparision with model simulation.
Diurnal atmospheric forcings modify SST-ISV in both southern and northern bay. SST-ISV in the southern bay, is dominantly controlled by the diurnal cycle of insolation, while in the northern bay, diurnal cycle of insolation and winds have comparable contribution. Diurnal cycle enhanced the amplitude of 3 selected intraseasonal events in the southern bay and 3 out of the 6 events in the northern bay, during the study period. In the southern bay, simulated SST variability with hourly forcing was closer to the observations from RAMA, implying that incorporating the diurnal cycle in model forcing rectifies SST-ISV. Moreover, SST obtained with diurnal forcing consists of additional fluctuations at higher frequencies within and in between intraseasonal events; such fluctuations are absent with daily forcing. The diurnal variability of SST is significant during the warming phase of intraseasonal events and reduces during the cooling phase. Diurnal amplitude of SST decreases with depth; depth dependence also being larger during the warming phase.
SST-ISV modulation with diurnal forcing results from the diurnal cycle of upper ocean heat fluxes and vertical mixing. Diurnal warming and cooling result in a net gain or loss of heat in the mixed layer after a day’s cycle. When the retention (loss) of heat in the mixed layer increases with diurnal forcing during the warming (cooling) phase of intraseasonal events, the daily mean SST rise (fall) becomes higher, amplifying the intraseasonal warming (cooling). In the southern bay, SST-ISV amplification is mainly controlled by the diurnal variability of MLD, which modifies the heat fluxes. Increased intraseasonal warming with diurnal forcing results from the increase in radiative heating, due to the shoaling of the daytime mixed layer. Amplified intraseasonal cooling is dominantly con-trolled by the strengthening of sub-surface processes, due to the nocturnal deepening of mixed layer and increased temperature gradients below the mixed layer.
In the northern bay, SST-ISV modulation with diurnal forcing is not as large as that in the southern bay. The mean increase in SST-ISV amplitudes with diurnal forcing is ~0.16◦C in the southern bay, while it is only ~0.03◦C in the northern bay. Reduced response of SST-ISV to diurnal forcings in the northern bay is related to the weaker diurnal variability of MLD. Salinity stratification limits diurnal variability of mixed layer in the northern bay, unlike in the southern bay. The seasonal (June - September) mean diurnal amplitude of MLD is ~15 m in the southern bay, while it is reduced to ~1.5 m in the northern bay. Diurnal variability of MLD, spanning only a few meters is not sufficient to create large modifications in mixed layer heat fluxes and SST-ISV in the northern bay. The vertical resolution of the model limits the shallowing of mixed layer to 7.5 m, thus restricting the diurnal variability of simulated MLD.
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Hameed, Saji N. "Simulations Of Tropical Surface Winds : Seasonal Cycle And Interannual Variability." Thesis, 1997. http://etd.iisc.ernet.in/handle/2005/1783.
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