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1
Callies, Jörn, et Raffaele Ferrari. « Baroclinic Instability in the Presence of Convection ». Journal of Physical Oceanography 48, no 1 (janvier 2018) : 45–60. http://dx.doi.org/10.1175/jpo-d-17-0028.1.
Texte intégralRésumé :
AbstractBaroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. While the focus has so far been on the balanced dynamics of these instabilities, they occur in and depend on an environment shaped by atmospherically forced small-scale turbulence. In this study, idealized numerical simulations are presented that allow the development of both baroclinic instability and convective small-scale turbulence, with simple control over the relative strength. If the convection is only weakly forced, baroclinic instability restratifies the layer and shuts off convection, as expected. With increased forcing, however, it is found that baroclinic instabilities are remarkably resilient to the presence of convection. Even if the instability is too weak to restratify the layer and shut off convection, the instability still grows in the convecting environment and generates baroclinic eddies and fronts. This suggests that despite the vigorous atmospherically forced small-scale turbulence in winter mixed layers, baroclinic instabilities can persistently grow, generate balanced submesoscale turbulence, and modify the bulk properties of the upper ocean.
2
Straneo, Fiammetta, Mitsuhiro Kawase et Robert S. Pickart. « Effects of Wind on Convection in Strongly and Weakly Baroclinic Flows with Application to the Labrador Sea* ». Journal of Physical Oceanography 32, no 9 (1 septembre 2002) : 2603–18. http://dx.doi.org/10.1175/1520-0485-32.9.2603.
Texte intégralRésumé :
Abstract Large buoyancy loss driving deep convection is often associated with a large wind stress that is typically omitted in simulations of convection. Here it is shown that this omission is not justified when overturning occurs in a horizontally inhomogeneous ocean. In strongly baroclinic flows, convective mixing is influenced both by the background horizontal density gradient and by the across-front advection of buoyancy due to wind. The former process—known as slantwise convection—results in deeper convection, while the effect of wind depends on the relative orientation of wind with respect to the baroclinic front. For the case of the Labrador Sea, wintertime winds act to destabilize the baroclinic Labrador Current causing a buoyancy removal roughly one-third as large as the air–sea buoyancy loss. Simulations using a nonhydrostatic numerical model, initialized and forced with observed fields from the Labrador Sea, show how the combination of wind and lateral gradients can result in significant convection within the current, in contrast with previous ideas. Though the advection of buoyancy due to wind in weakly baroclinic flows is negligible compared to the surface buoyancy removal typical of convective conditions, convective plumes are substantially deformed by wind. This deformation, and the associated across-front secondary circulation, are explained in terms of the vertical advection of wind-generated vorticity from the surface boundary layer to deeper depths. This mechanism generates vertical structure within the convective layer, contradicting the historical notion that properties become vertically homogenized during convection. For the interior Labrador Sea, this mechanism may be partly responsible for the vertical variability observed during convection, which modeling studies have until now failed to reproduce.
3
Pasquero, Claudia, et Eli Tziperman. « Statistical Parameterization of Heterogeneous Oceanic Convection ». Journal of Physical Oceanography 37, no 2 (1 février 2007) : 214–29. http://dx.doi.org/10.1175/jpo3008.1.
Texte intégralRésumé :
Abstract A statistical convective adjustment scheme is proposed that attempts to account for the effects of mesoscale and submesoscale variability of temperature and salinity typically observed in the oceanic convective regions. Temperature and salinity in each model grid box are defined in terms of their mean, variance, and mutual correlations. Subgrid-scale instabilities lead to partial mixing between different layers in the water column. This allows for a smooth transition between the only two states (convection on and convection off) allowed in standard convective adjustment schemes. The advantage of the statistical parameterization is that possible instabilities associated with the sharp transition between the two states, which are known to occasionally affect the large-scale model solution, are eliminated. The procedure also predicts the generation of correlations between temperature and salinity and the presence of convectively induced upgradient fluxes that have been obtained in numerical simulations of heterogeneous convection and that cannot be represented by standard convective adjustment schemes.
4
Buch, Erik. « Physical oceanography of the Greenland Sea ». Meddelelser om Grønland. Bioscience 58 (1 janvier 2007) : 14–21. http://dx.doi.org/10.7146/mogbiosci.v58.142635.
Texte intégralRésumé :
Ocean-atmosphere interactions in the North Atlantic are responsible for heat transports that keep the Nordic region and North Western Europe 5–10°C warmer than the average of the corresponding latitude belt. This is to a large extent due to the ocean’s thermohaline circulation (THC). This circulation is driven by differences in water density, which is a function of temperature (thermo) and salinity (haline) and particularly by convection processes in the northern North Atlantic, especially the Labrador Sea and the Greenland Sea.
Therefore, the Greenland Sea has attracted much attention in the marine research community over the past decades. Scientific research in the Greenland Sea has been important to understand:
The physical processes generating deep convection
The role of sea ice in the deep convection process and biological production
Uptake of carbon dioxide from the atmosphere and further transport into the ocean interior
Variability in deep convection, especially the decrease in the Greenland Sea convection observed over the recent decades
The main incentive behind all these activities has been to understand the role of the Greenland Sea – and the rest of the Nordic Seas (Greenland, Norwegian and Iceland Seas) – in the climate system of the world.
5
Zhou, S. Q., L. Qu, Y. Z. Lu et X. L. Song. « The instability of diffusive convection and its implication for the thermohaline staircases in the deep Arctic Ocean ». Ocean Science 10, no 1 (24 février 2014) : 127–34. http://dx.doi.org/10.5194/os-10-127-2014.
Texte intégralRésumé :
Abstract. In the present study, the classical description of diffusive convection is updated to interpret the instability of diffusive interfaces and the dynamical evolution of the bottom layer in the deep Arctic Ocean. In the new consideration of convective instability, both the background salinity stratification and rotation are involved. The critical Rayleigh number of diffusive convection is found to vary from 103 to 1011 in the deep Arctic Ocean as well as in other oceans and lakes. In such a wide range of conditions, the interface-induced thermal Rayleigh number is shown to be consistent with the critical Rayleigh number of diffusive convection. In most regions, background salinity stratification is found to be the main hindrance to the occurrence of convecting layers. With the new parameterization, it is predicted that the maximum thickness of the bottom layer is 1051 m in the deep Arctic Ocean, which is close to the observed value of 929 m. The evolution time of the bottom layer is predicted to be ~ 100 yr, which is on the same order as that based on 14C isolation age estimation.
6
Zhou, S. Q., L. Qu, Y. Z. Lu et X. L. Song. « The instability of diffusive convection and its implication for the thermohaline staircases in the deep Arctic Ocean ». Ocean Science Discussions 10, no 4 (13 août 2013) : 1343–66. http://dx.doi.org/10.5194/osd-10-1343-2013.
Texte intégralRésumé :
Abstract. In the present study, the classical description of diffusive convection is updated to interpret the instability of diffusive interfaces and the dynamical evolution of the bottom layer in the deep Arctic Ocean. In the new consideration of convective instability, both the background salinity stratification and rotation are involved. The critical Rayleigh number of diffusive convection is found to vary from 103 to 1011 in the deep Arctic Ocean as well as in other oceans and lakes. In such a wide range of conditions, the interface-induced thermal Rayleigh number is indicated to be consistent with the critical Rayleigh number of diffusive convection. In most regions, background salinity stratification is found to be the main hindrance to the occurrence of convecting layers. With the new parameterization, it is predicted that the maximum thickness of the bottom layer is 1051 m, which is close to the observed value of 929 m. And the evolution time of the bottom layer is predicted to be of the same order as that based on 14C isolation age estimation.
7
Spall, Michael A. « Influences of Precipitation on Water Mass Transformation and Deep Convection ». Journal of Physical Oceanography 42, no 10 (22 mai 2012) : 1684–700. http://dx.doi.org/10.1175/jpo-d-11-0230.1.
Texte intégralRésumé :
Abstract The influences of precipitation on water mass transformation and the strength of the meridional overturning circulation in marginal seas are studied using theoretical and idealized numerical models. Nondimensional equations are developed for the temperature and salinity anomalies of deep convective water masses, making explicit their dependence on both geometric parameters such as basin area, sill depth, and latitude, as well as on the strength of atmospheric forcing. In addition to the properties of the convective water, the theory also predicts the magnitude of precipitation required to shut down deep convection and switch the circulation into the haline mode. High-resolution numerical model calculations compare well with the theory for the properties of the convective water mass, the strength of the meridional overturning circulation, and also the shutdown of deep convection. However, the numerical model also shows that, for precipitation levels that exceed this critical threshold, the circulation retains downwelling and northward heat transport, even in the absence of deep convection.
8
Su, Zhan, Andrew P. Ingersoll, Andrew L. Stewart et Andrew F. Thompson. « Ocean Convective Available Potential Energy. Part I : Concept and Calculation ». Journal of Physical Oceanography 46, no 4 (avril 2016) : 1081–96. http://dx.doi.org/10.1175/jpo-d-14-0155.1.
Texte intégralRésumé :
AbstractThermobaric convection (type II convection) and thermobaric cabbeling (type III convection) might substantially contribute to vertical mixing, vertical heat transport, and deep-water formation in the World Ocean. However, the extent of this contribution remains poorly constrained. The concept of ocean convective available potential energy (OCAPE), the thermobaric energy source for type II and type III convection, is introduced to improve the diagnosis and prediction of these convection events. OCAPE is analogous to atmospheric CAPE, which is a key energy source for atmospheric moist convection and has long been used to forecast moist convection. OCAPE is the potential energy (PE) stored in an ocean column arising from thermobaricity, defined as the difference between the PE of the ocean column and its minimum possible PE under adiabatic vertical parcel rearrangements. An ocean column may be stably stratified and still have nonzero OCAPE. The authors present an efficient strategy for computing OCAPE accurately for any given column of seawater. They further derive analytical expressions for OCAPE for approximately two-layer ocean columns that are widely observed in polar oceans. This elucidates the dependence of OCAPE on key physical parameters. Hydrographic profiles from the winter Weddell Sea are shown to contain OCAPE (0.001–0.01 J kg−1), and scaling analysis suggests that OCAPE may be substantially enhanced by wintertime surface buoyancy loss. The release of this OCAPE may substantially contribute to the kinetic energy of deep convection in polar oceans.
9
Wirth, A., et B. Barnier. « Mean Circulation and Structures of Tilted Ocean Deep Convection ». Journal of Physical Oceanography 38, no 4 (1 avril 2008) : 803–16. http://dx.doi.org/10.1175/2007jpo3756.1.
Texte intégralRésumé :
Abstract Convection in a homogeneous ocean is investigated by numerically integrating the three-dimensional Boussinesq equations in a tilted, rotating frame ( f–F plane) subject to a negative buoyancy flux (cooling) at the surface. The study focuses on determining the influence of the angle (tilt) between the axis of rotation and gravity on the convection process. To this end the following two essential parameters are varied: (i) the magnitude of the surface heat flux, and (ii) the angle (tilt) between the axis of rotation and gravity. The range of the parameters investigated is a subset of typical open-ocean deep convection events. It is demonstrated that when gravity and rotation vector are tilted with respect to each other (i) the Taylor–Proudman–Poincaré theorem leaves an imprint in the convective structures, (ii) a horizontal mean circulation is established, and (iii) the second-order moments involving horizontal velocity components are considerably increased. Tilted rotation thus leaves a substantial imprint in the dynamics of ocean convection.
10
Legg, Sonya. « A Simple Criterion to Determine the Transition from a Localized Convection to a Distributed Convection Regime* ». Journal of Physical Oceanography 34, no 12 (1 décembre 2004) : 2843–46. http://dx.doi.org/10.1175/jpo2653.1.
Texte intégralRésumé :
Abstract A recent numerical study by Noh et al. of open-ocean deep convection in the presence of a single geostrophic eddy showed that two possible regimes exist: 1) the localized convection regime in which baroclinic instability of the eddy dominates, with slantwise fluxes and restratification, and 2) the distributed convection regime in which vertical mixing dominates. Noh et al. found that localized convection dominates for relatively small buoyancy forcing, strong eddies, and strong surface ambient stratification. Here it is shown that this regime transition can be expressed in terms of a ratio of time scales: the localized convection regime appears when the time scale for lateral fluxes from eddy interior to exterior tL is short in comparison with the time scale for convective erosion of the exterior stratification tc. Scaling arguments give this ratio of time scales as tL/tc ∼ f β2R2B/(A2γ) where f is the Coriolis parameter, R is the radius of the eddy, B is the buoyancy forcing, 1/β is the depth scale of the exponentially decaying surface-intensified stratification, γ is the relative amplitude of the eddy, and Aβ is the value of the surface stratification N20. Comparison with the numerical simulations of Noh et al. shows that this parameter does indeed separate the localized and distributed convection regimes, with the transition occurring at tL/tc ≈ 0.05–0.1.
11
Chanut, Jérôme, Bernard Barnier, William Large, Laurent Debreu, Thierry Penduff, Jean Marc Molines et Pierre Mathiot. « Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification ». Journal of Physical Oceanography 38, no 8 (1 août 2008) : 1617–43. http://dx.doi.org/10.1175/2008jpo3485.1.
Texte intégralRésumé :
Abstract The cycle of open ocean deep convection in the Labrador Sea is studied in a realistic, high-resolution (4 km) regional model, embedded in a coarser (⅓°) North Atlantic setup. This configuration allows the simultaneous generation and evolution of three different eddy types that are distinguished by their source region, generation mechanism, and dynamics. Very energetic Irminger Rings (IRs) are generated by barotropic instability of the West Greenland and Irminger Currents (WGC/IC) off Cape Desolation and are characterized by a warm, salty subsurface core. They densely populate the basin north of 58°N, where their eddy kinetic energy (EKE) matches the signal observed by satellite altimetry. Significant levels of EKE are also found offshore of the West Greenland and Labrador coasts, where boundary current eddies (BCEs) are spawned by weakly energetic instabilities all along the boundary current system (BCS). Baroclinic instability of the steep isopycnal slopes that result from a deep convective overturning event produces convective eddies (CEs) of 20–30 km in diameter, as observed and produced in more idealized models, with a distinct seasonal cycle of EKE peaking in April. Sensitivity experiments show that each of these eddy types plays a distinct role in the heat budget of the central Labrador Sea, hence in the convection cycle. As observed in nature, deep convective mixing is limited to areas where adequate preconditioning can occur, that is, to a small region in the southwestern quadrant of the central basin. To the east, west, and south, BCEs flux heat from the BCS at a rate sufficient to counteract air–sea buoyancy loss. To the north, this eddy flux alone is not enough, but when combined with the effects of Irminger Rings, preconditioning is effectively inhibited here too. Following a deep convective mixing event, the homogeneous convection patch reaches as deep as 2000 m and a horizontal scale on the order of 200 km, as has been observed. Both CEs and BCEs are found to play critical roles in the lateral mixing phase, when the patch restratifies and transforms into Labrador Sea Water (LSW). BCEs extract the necessary heat from the BCS and transport it to the deep convection site, where it fluxed into convective patches by CEs during the initial phase. Later in the phase, BCE heat flux maintains and strengthens the restratification throughout the column, while solar heating establishes a near-surface seasonal stratification. In contrast, IRs appear to rarely enter the deep convection region. However, by virtue of their control on the surface area preconditioned for deep convection and the interannual variability of the associated barotropic instability, they could have an important role in the variability of LSW.
12
Frajka-Williams, Eleanor, Peter B. Rhines et Charles C. Eriksen. « Horizontal Stratification during Deep Convection in the Labrador Sea ». Journal of Physical Oceanography 44, no 1 (1 janvier 2014) : 220–28. http://dx.doi.org/10.1175/jpo-d-13-069.1.
Texte intégralRésumé :
Abstract Deep convection—the process by which surface waters are mixed down to 1000 m or deeper—forms the primary downwelling of the meridional overturning circulation in the Northern Hemisphere. High-resolution hydrographic measurements from Seagliders indicate that during deep convection—though water is well mixed vertically—there is substantial horizontal variation in density over short distances (tens of kilometers). This horizontal density variability present in winter (January–February) contains sufficient buoyancy to restratify the convecting region to observed levels 2.5 months later, as estimated from Argo floating platforms. These results highlight the importance of small-scale heterogeneities in the ocean that are typically poorly represented in climate models, potentially contributing to the difficulty climate models have in representing deep convection.
13
Schloesser, Fabian. « Large-Scale Dynamics of Circulations with Open-Ocean Convection ». Journal of Physical Oceanography 45, no 12 (décembre 2015) : 2933–51. http://dx.doi.org/10.1175/jpo-d-15-0088.1.
Texte intégralRésumé :
AbstractFormation of the densest water masses in the North Atlantic and its marginal seas involves open-ocean convection. The main goal of this study is to contribute to the general understanding of how such convective regions connect to the large-scale ocean circulation. Specifically, analytic and numerical versions of a variable density layer model are used to explore the processes underlying the circulation in an idealized ocean basin. The models are forced by a surface buoyancy flux, which generates a density maximum in the ocean interior. In response to the forcing, a region forms that is characterized by the closed Rossby wave characteristics and where the eddy–mean transport converges toward the convective site. Outside of that region, characteristics extend from the eastern boundary and a distorted β-plume circulation develops, linking the convection site with the western boundary. The overturning strength in the model can be related to several environment variables and forcings and is constrained by the surface density field, stratification, eddy mixing strength and by Rossby wave dynamics. Solutions forced by an interior ocean density minimum are also considered. Although no convection occurs, the dynamics underlying the circulation are closely related to the case with cooling.
14
Straneo, Fiammetta. « On the Connection between Dense Water Formation, Overturning, and Poleward Heat Transport in a Convective Basin* ». Journal of Physical Oceanography 36, no 9 (1 septembre 2006) : 1822–40. http://dx.doi.org/10.1175/jpo2932.1.
Texte intégralRésumé :
Abstract An isopycnal, two-layer, idealized model for a convective basin is proposed, consisting of a convecting, interior region and a surrounding boundary current (buoyancy and wind-driven). Parameterized eddy fluxes govern the exchange between the two. To balance the interior buoyancy loss, the boundary current becomes denser as it flows around the basin. Geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation. The poleward heat transport, associated with convection in the basin, can thus be viewed as a result of both an overturning and a horizontal circulation. When adapted to the Labrador Sea, the model is able to reproduce the bulk features of the mean state, the seasonal cycle, and even the shutdown of convection from 1969 to 1972. According to the model, only 40% of the poleward heat (buoyancy) transport of the Labrador Sea is associated with the overturning circulation. An exact solution is presented for the linearized equations when changes in the boundary current are small. Numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin. These results highlight how the overturning circulation is not simply related to the amount of dense water formed. A speeding up of the circulation around the basin due to wind forcing, for example, will decrease the intensity of the overturning circulation while the dense water formation remains unvaried. In general, it is shown that the fraction of poleward buoyancy (or heat) transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors.
15
Carpenter, J. R., T. Sommer et A. Wüest. « Stability of a Double-Diffusive Interface in the Diffusive Convection Regime ». Journal of Physical Oceanography 42, no 5 (1 mai 2012) : 840–54. http://dx.doi.org/10.1175/jpo-d-11-0118.1.
Texte intégralRésumé :
Abstract In this paper, the authors explore the conditions under which a double-diffusive interface may become unstable. Focus is placed on the case of a cold, freshwater layer above a warm, salty layer [i.e., the diffusive convection (DC) regime]. The “diffusive interface” between these layers will develop gravitationally unstable boundary layers due to the more rapid diffusion of heat (the destabilizing component) relative to salt. Previous studies have assumed that a purely convective-type instability of these boundary layers is what drives convection in this system and that this may be parameterized by a boundary layer Rayleigh number. The authors test this theory by conducting both a linear stability analysis and direct numerical simulations of a diffusive interface. Their linear stability analysis reveals that the transition to instability always occurs as an oscillating diffusive convection mode and at boundary layer Rayleigh numbers much smaller than previously thought. However, these findings are based on making a quasi-steady assumption for the growth of the interfaces by molecular diffusion. When diffusing interfaces are modeled (using direct numerical simulations), the authors observe that the time dependence is significant in determining the instability of the boundary layers and that the breakdown is due to a purely convective-type instability. Their findings therefore demonstrate that the relevant instability in a DC staircase is purely convective.
16
Pasetto, Stefano, Cesare Chiosi, Mark Cropper et Eva K. Grebel. « Scale-free convection theory ». Proceedings of the International Astronomical Union 11, A29B (août 2015) : 747. http://dx.doi.org/10.1017/s1743921316006700.
Texte intégralRésumé :
AbstractConvection is one of the fundamental mechanisms to transport energy, e.g., in planetology, oceanography, as well as in astrophysics where stellar structure is customarily described by the mixing-length theory, which makes use of the mixing-length scale parameter to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height of the star, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to claim that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in the literature are hampered by this basic uncertainty. In a recent paper (Pasetto et al. 2014) we presented the first fully analytical scale-free theory of convection that does not require the mixing-length parameter. Our self-consistent analytical formulation of convection determines all the properties of convection as a function of the physical behaviour of the convective elements themselves and the surrounding medium (be it a star, an ocean, or a primordial planet). The new theory of convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time dependent formalism. We obtained an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory in astrophysical environment are compared with those from the standard mixing-length paradigm in stars with exceptional results for atmosphere models of the Sun and all the stars in the Hertzsprung-Russell diagram.
17
Gnanadesikan, Anand, Cassidy M. Speller, Grace Ringlein, John San Soucie, Jordan Thomas et Marie-Aude Pradal. « Feedbacks Driving Interdecadal Variability in Southern Ocean Convection in Climate Models : A Coupled Oscillator Mechanism ». Journal of Physical Oceanography 50, no 8 (1 août 2020) : 2227–49. http://dx.doi.org/10.1175/jpo-d-20-0037.1.
Texte intégralRésumé :
AbstractNumerous climate models display large-amplitude, long-period variability associated with quasiperiodic convection in the Southern Ocean, but the mechanisms responsible for producing such oscillatory convection are poorly understood. In this paper we identify three feedbacks that help generate such oscillations within an Earth system model with a particularly regular oscillation. The first feedback involves increased (decreased) upward mixing of warm interior water to the surface, resulting in more (less) evaporation and loss of heat to the atmosphere which produces more (less) mixing. This positive feedback helps explain why temperature anomalies are not damped out by surface forcing. A second key mechanism involves convective (nonconvective) events in the Weddell Sea causing a relaxation (intensification) of westerly winds, which at some later time results in a pattern of currents that reduces (increases) the advection of freshwater out of the Weddell Sea. This allows for the surface to become lighter (denser) which in turn can dampen (trigger) convection—so that the overall feedback is a negative one with a delay—helping to produce a multidecadal oscillation time scale. The decrease (increase) in winds associated with convective (nonconvective) states also results in a decrease (increase) in the upward mixing of salt in the Eastern Weddell Sea, creating a negative (positive) salinity anomaly that propagates into the Western Weddell Sea and dampens (triggers) convection—again producing a negative feedback with a delay. A principal oscillatory pattern analysis yields a reasonable prediction for the period of oscillation. Strengths of the feedbacks are sensitive to parameterization of mesoscale eddies.
18
Grignon, L., D. A. Smeed, H. L. Bryden et K. Schroeder. « Importance of the variability of hydrographic preconditioning for deep convection in the Gulf of Lion, NW Mediterranean ». Ocean Science 6, no 2 (14 juin 2010) : 573–86. http://dx.doi.org/10.5194/os-6-573-2010.
Texte intégralRésumé :
Abstract. We study the variability of hydrographic preconditioning defined as the heat and salt contents in the Ligurian Sea before convection. The stratification is found to reach a maximum in the intermediate layer in December, whose causes and consequences for the interannual variability of convection are investigated. Further study of the interannual variability and correlation tests between the properties of the deep water formed and the winter surface fluxes support the description of convection as a process that transfers the heat and salt contents from the top and intermediate layers to the deep layer. A proxy for the rate of transfer is given by the final convective mixed layer depth, that is shown to depend equally on the surface fluxes and on the preconditioning. In particular, it is found that deep convection in winter 2004–2005 would have happened even with normal winter conditions, due to low pre-winter stratification.
19
Grignon, L., D. A. Smeed, H. L. Bryden et K. Schroeder. « Importance of the variability of hydrographic preconditioning for deep convection in the Gulf of Lion, NW Mediterranean ». Ocean Science Discussions 7, no 1 (18 janvier 2010) : 51–90. http://dx.doi.org/10.5194/osd-7-51-2010.
Texte intégralRésumé :
Abstract. We study the variability of hydrographic preconditioning defined as the heat and salt contents in the Ligurian Sea before convection. The stratification is found to reach a maximum in the intermediate layer in December, whose causes and consequences for the interannual variability of convection are investigated. Further study of the interannual variability and correlation tests between the properties of the deep water formed and the winter surface fluxes support the description of convection as a process that transfers the heat and salt contents from the top and intermediate layers to the deep layer. A proxy for the rate of transfer is given by the final convective mixed layer depth, that is shown to depend equally on the surface fluxes and on the preconditioning. In particular, it was found that deep convection in winter 2004–2005 would have happened even with normal winter conditions, due to low pre-winter stratification.
20
Verma, Vicky, Hieu T. Pham et Sutanu Sarkar. « Interaction between Upper-Ocean Submesoscale Currents and Convective Turbulence ». Journal of Physical Oceanography 52, no 3 (mars 2022) : 437–58. http://dx.doi.org/10.1175/jpo-d-21-0148.1.
Texte intégralRésumé :
Abstract The interaction between upper-ocean submesoscale fronts evolving with coherent features, such as vortex filaments and eddies, and convective turbulence generated by surface cooling of varying magnitude is investigated. Here, we decompose the flow into finescale (FS) and submesoscale (SMS) fields explicitly to investigate the energy pathways and the strong interaction between them. Most of the surface cooling flux is transferred to the FS kinetic energy through the FS buoyancy flux carried by the convective plumes. Overall, the SMS strengthens due to surface cooling. The frontogenetic tendency at the submesoscale increases, which counters the enhanced horizontal diffusion by convection-induced turbulence. Downwelling/upwelling strengthens, and the peak SMS vertical buoyancy flux increases as surface cooling is increased. Furthermore, the production of FS energy by SMS velocity gradients (the interscale transfer term, which mediates forward energy cascade) is significant, up to half of the production by convection. Examination of potential vorticity reveals that surface cooling promotes higher levels of secondary symmetric instability (SI), which coexists with the persistent baroclinic instability. The forward interscale transfer is found to increase in the regions with SI.
21
Zhang, Weiwei, et Xiao-Hai Yan. « Lateral Heat Exchange after the Labrador Sea Deep Convection in 2008 ». Journal of Physical Oceanography 44, no 12 (26 novembre 2014) : 2991–3007. http://dx.doi.org/10.1175/jpo-d-13-0198.1.
Texte intégralRésumé :
Abstract The mechanisms through which convected water restratifies in the Labrador Sea are still under debate. The Labrador Sea restratification after deep convection in the 2007/08 winter is studied with an eddy-resolving numerical model. The modeled mixed layer depth during wintertime resembles the Argo observed mixed layer very well, and the lateral heat flux during the subsequent restratification is in line with observations. The Irminger rings (IRs) are reproduced with fresher caps above the 300-m depths, and they are identified and tracked automatically. The model underestimates both the number of IRs in the convection area and the heat they carry. The underestimation is most likely caused by the errors in the direction of the west Greenland currents in the model, which causes more IRs propagating westward, and only the IRs originating south of 61.5°N are able to propagate southward, yet with speed much slower than observed speed. The model still observed three eddies propagating into the convection area during the restratification phase in 2008, and their thermal contribution ranges from 1% to 4% if the estimation is made at the time when they enter the convection area. If all newly generated eddies are considered, then the ensemble-mean contributions by the IRs become 5.3%. The more detailed and direct heat flux by IRs is difficult to derive because of the strong fluctuation of the identified eddy radius. Nevertheless, the modeled lateral heat flux is largely composed of the boundary current eddies and convective eddies, thus it is possible for the model to maintain an acceptable thermal balance.
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Fiedler, Brian H. « Mesoscale cellular convection : is it convection ? » Tellus A 37A, no 2 (mars 1985) : 163–75. http://dx.doi.org/10.1111/j.1600-0870.1985.tb00278.x.
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Colin de Verdière, A. « A Simple Model of Millennial Oscillations of the Thermohaline Circulation ». Journal of Physical Oceanography 37, no 5 (1 mai 2007) : 1142–55. http://dx.doi.org/10.1175/jpo3056.1.
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Abstract Stommel’s two-box model of thermohaline circulation is modified to include the possibility of convection. When reduced to a two-degrees-of-freedom dynamical system, the model exhibits the well-known multiple (thermal and haline) steady states as well as new convective thermal steady states. However, for some values of the control parameters (such as the freshwater flux) oscillations occur. Millennial period oscillatory regimes correspond to switches between the Stommel’s haline fixed point and the convective thermal state, both of which are unstable in a window of precipitation values. The transitions between steady and oscillatory regimes at the boundaries of the window are global bifurcations, which in some cases have an infinite period character. This character is due either to the proximity of the Stommel saddle node bifurcation or to the infinite time it takes for the convection to resume when the system is in the haline regime. The oscillations bear a close relationship to those of Welander’s flip–flop model. The physics of this class of millennial oscillations may be relevant to those observed in more complex OGCMs and may help to rationalize certain features of the millennial band oscillations that punctuate the last glacial period.
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Sandal, Cathrine, et Doron Nof. « A New Analytical Model for Heinrich Events and Climate Instability ». Journal of Physical Oceanography 38, no 2 (1 février 2008) : 451–66. http://dx.doi.org/10.1175/2007jpo3722.1.
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Abstract The authors focus on Heinrich events and the question of whether the arrest and restart of convection can explain the associated sudden changes in oceanic and atmospheric temperature. For this purpose, a new (mixed) dynamical-box model is developed in which the ocean and atmosphere communicate via both Ekman layers and convection. The conservation of heat, salt, volume flux, and a “convection condition” yields a system of algebraic equations that are solved analytically. As expected, it is found that as the freshwater flux increases, the convective ocean temperature decreases. The heat flux from the ocean to the atmosphere, the transport of the oceanic meridional overturning cell (MOC), and the corresponding atmospheric flow generated by the heat flux from the ocean all decrease. However, the outgoing air temperature increases with increasing freshwater flux. This counterintuitive increase is because a decreased latent and sensible heat flux (to a humid atmosphere) means a reduced temperature difference between the warmer ocean and the cooler atmosphere, implying a cooler ocean and warmer atmosphere. For each wind speed, there is a critical freshwater flux beyond which the convection collapses and the temperatures of both the ocean and the air plunge because equatorial water is no longer flowing northward to replace the frigid northern waters. The above points to a potentially new instability process that was probably active during glaciation periods—when ice and snow are abundant, even the smallest amount of freshwater flux will cause local warming which, in turn, will cause increased melting, resulting in an ever-increased freshwater flux until the critical flux is reached and the MOC collapses. The model suggests that switching convection on and off changed the glacial ocean temperature by 4°C and the glacial air temperature by 12.5°C, both consistent with the Greenland Ice Sheet Project (GISP II) ice core record and the Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE) alkenone record.
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Su, Zhan, Andrew P. Ingersoll, Andrew L. Stewart et Andrew F. Thompson. « Ocean Convective Available Potential Energy. Part II : Energetics of Thermobaric Convection and Thermobaric Cabbeling ». Journal of Physical Oceanography 46, no 4 (avril 2016) : 1097–115. http://dx.doi.org/10.1175/jpo-d-14-0156.1.
Texte intégralRésumé :
AbstractThe energetics of thermobaricity- and cabbeling-powered deep convection occurring in oceans with cold freshwater overlying warm salty water are investigated here. These quasi-two-layer profiles are widely observed in wintertime polar oceans. The key diagnostic is the ocean convective available potential energy (OCAPE), a concept introduced in a companion piece to this paper (Part I). For an isolated ocean column, OCAPE arises from thermobaricity and is the maximum potential energy (PE) that can be converted into kinetic energy (KE) under adiabatic vertical parcel rearrangements. This study explores the KE budget of convection using two-dimensional numerical simulations and analytical estimates. The authors find that OCAPE is a principal source for KE. However, the complete conversion of OCAPE to KE is inhibited by diabatic processes. Further, this study finds that diabatic processes produce three other distinct contributions to the KE budget: (i) a sink of KE due to the reduction of stratification by vertical mixing, which raises water column’s center of mass and thus acts to convert KE to PE; (ii) a source of KE due to cabbeling-induced shrinking of the water column’s volume when water masses with different temperatures are mixed, which lowers the water column’s center of mass and thus acts to convert PE into KE; and (iii) a reduced production of KE due to diabatic energy conversion of the KE convertible part of the PE to the KE inconvertible part of the PE. Under some simplifying assumptions, the authors also propose a theory to estimate the maximum depth of convection from an energetic perspective. This study provides a potential basis for improving the convection parameterization in ocean models.
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Li, Qing, et Baylor Fox-Kemper. « Assessing the Effects of Langmuir Turbulence on the Entrainment Buoyancy Flux in the Ocean Surface Boundary Layer ». Journal of Physical Oceanography 47, no 12 (décembre 2017) : 2863–86. http://dx.doi.org/10.1175/jpo-d-17-0085.1.
Texte intégralRésumé :
AbstractLarge-eddy simulations (LESs) with various constant wind, wave, and surface destabilizing surface buoyancy flux forcing are conducted, with a focus on assessing the impact of Langmuir turbulence on the entrainment buoyancy flux at the base of the ocean surface boundary layer. An estimate of the entrainment buoyancy flux scaling is made to best fit the LES results. The presence of Stokes drift forcing and the resulting Langmuir turbulence enhances the entrainment rate significantly under weak surface destabilizing buoyancy flux conditions, that is, weakly convective turbulence. In contrast, Langmuir turbulence effects are moderate when convective turbulence is dominant and appear to be additive rather than multiplicative to the convection-induced mixing. The parameterized unresolved velocity scale in the K-profile parameterization (KPP) is modified to adhere to the new scaling law of the entrainment buoyancy flux and account for the effects of Langmuir turbulence. This modification is targeted on common situations in a climate model where either Langmuir turbulence or convection is important and may overestimate the entrainment when both are weak. Nevertheless, the modified KPP is tested in a global climate model and generally outperforms those tested in previous studies. Improvements in the simulated mixed layer depth are found, especially in the Southern Ocean in austral summer.
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Paquin, Jean-Philippe, Youyu Lu, Simon Higginson, Frédéric Dupont et Gilles Garric. « Modelled Variations of Deep Convection in the Irminger Sea during 2003–10 ». Journal of Physical Oceanography 46, no 1 (janvier 2016) : 179–96. http://dx.doi.org/10.1175/jpo-d-15-0078.1.
Texte intégralRésumé :
AbstractResults from a high-resolution ice–ocean model are analyzed to understand the physical processes responsible for the interannual variability of ocean convection over the Irminger Sea. The modeled convection in the open Irminger Sea for the winters of 2007/08 and 2008/09 is in good agreement with observations. Deep convection is caused by strong atmospheric forcing that increases the ocean heat loss through latent and sensible heat fluxes. Greenland tip jets are found to be the only strong wind events that directly affect the deep convection area and explain up to 53% of the total turbulent heat loss during active convection years. Deep convection is modeled where there is favorable preconditioning of the water column due to isopycnal doming inside the semienclosed Irminger Gyre. The region of deep convection is also characterized by weak eddy kinetic energy. Finally, an estimation of the surface-forced water mass transformation confirms the Irminger Sea as a region of intermittent production of Labrador Sea Water, with annual averages between 0.9 and 1.9 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) of water denser than 27.7 kg m−3 for years of active convection.
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Taylor, John R., et Raffaele Ferrari. « Buoyancy and Wind-Driven Convection at Mixed Layer Density Fronts ». Journal of Physical Oceanography 40, no 6 (1 juin 2010) : 1222–42. http://dx.doi.org/10.1175/2010jpo4365.1.
Texte intégralRésumé :
Abstract In this study, the influence of a geostrophically balanced horizontal density gradient on turbulent convection in the ocean is examined using numerical simulations and a theoretical scaling analysis. Starting with uniform horizontal and vertical buoyancy gradients, convection is driven by imposing a heat loss or a destabilizing wind stress at the upper boundary, and a turbulent layer soon develops. For weak lateral fronts, turbulent convection results in a nearly homogeneous mixed layer (ML) whose depth grows in time. For strong fronts, a turbulent layer develops, but this layer is not an ML in the traditional sense because it is characterized by persistent horizontal and vertical gradients in density. The turbulent layer is, however, nearly homogeneous in potential vorticity (PV), with a value near zero. Using the PV budget, a scaling for the depth of the turbulent low PV layer and its time dependence is derived that compares well with numerical simulations. Two dynamical regimes are identified. In a convective layer near the surface, turbulence is generated by the buoyancy loss at the surface; below this layer, turbulence is generated by a symmetric instability of the lateral density gradient. This work extends classical scalings for the depth of turbulent boundary layers to account for the ubiquitous presence of lateral density gradients in the ocean. The new results indicate that a lateral density gradient, in addition to the surface forcing, can affect the stratification and the rate of growth of the surface boundary layer.
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Kumar, Pardeep. « Magneto-Rotatory Convection in Couple-Stress Fluid ». WSEAS TRANSACTIONS ON FLUID MECHANICS 18 (6 octobre 2023) : 58–65. http://dx.doi.org/10.37394/232013.2023.18.6.
Texte intégralRésumé :
Background: Thermal convection is the most convective instability when crystals are produced from a single element like silicon and the thermal instability of a fluid layer heated from below plays an important role in geophysics, oceanography, atmospheric physics, etc. The flow through porous media is of considerable interest for petroleum engineers, for geophysical fluid dynamicists and has importance in chemical technology and industry. Many of the flow problems in fluids with couple-stresses indicate some possible experiments, that could be used for determining the material constants, and the results are found to differ from those of Newtonian fluid. Keeping this in view, the present work was to study the effect of a uniform vertical magnetic field on the couple-stress fluid heated from below in the presence of a uniform vertical rotation through permeable media. Methodology: The present problem is studied using the linearized stability theory, Boussinesq approximation, normal mode analysis, and the dispersion relation is obtained. Results: The stationary convection, stability of the system, and oscillatory modes are discussed. In the case of stationary convection, the rotation postpones the onset of convection. The magnetic field and couple-stress may hasten the onset of convection in the presence of rotation while in the absence of rotation; they always postpone the onset of convection. The medium permeability hastens the onset of convection in the absence of rotation while in the presence of rotation, it may postpone the onset of convection. The rotation and magnetic field are found to introduce oscillatory modes in the system which was non-existent in their absence. A sufficient condition for the non-existence of overstability is also obtained.
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Jones, Helen, et John Marshall. « Restratification after Deep Convection ». Journal of Physical Oceanography 27, no 10 (octobre 1997) : 2276–87. http://dx.doi.org/10.1175/1520-0485(1997)027<2276:radc>2.0.co;2.
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Dewar, William K. « Convection in Small Basins ». Journal of Physical Oceanography 32, no 10 (octobre 2002) : 2766–88. http://dx.doi.org/10.1175/1520-0485(2002)032<2766:cisb>2.0.co;2.
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DELDEN, AARNOUT. « Comments on “Mesoscale cellular convection : is it convection?” ». Tellus A 37A, no 5 (octobre 1985) : 487–88. http://dx.doi.org/10.1111/j.1600-0870.1985.tb00446.x.
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Rieck, Jan K., Claus W. Böning et Klaus Getzlaff. « The Nature of Eddy Kinetic Energy in the Labrador Sea : Different Types of Mesoscale Eddies, Their Temporal Variability, and Impact on Deep Convection ». Journal of Physical Oceanography 49, no 8 (août 2019) : 2075–94. http://dx.doi.org/10.1175/jpo-d-18-0243.1.
Texte intégralRésumé :
AbstractOceanic eddies are an important component in preconditioning the central Labrador Sea (LS) for deep convection and in restratifying the convected water. This study investigates the different sources and impacts of eddy kinetic energy (EKE) and its temporal variability in the LS with the help of a 52-yr-long hindcast simulation of a 1/20° ocean model. Irminger Rings (IR) are generated in the West Greenland Current (WGC) between 60° and 62°N, mainly affect preconditioning, and limit the northward extent of the convection area. The IR exhibit a seasonal cycle and decadal variations linked to the WGC strength, varying with the circulation of the subpolar gyre. The mean and temporal variations of IR generation can be attributed to changes in deep ocean baroclinic and upper-ocean barotropic instabilities at comparable magnitudes. The main source of EKE and restratification in the central LS are convective eddies (CE). They are generated by baroclinic instabilities near the bottom of the mixed layer during and after convection. The CE have a middepth core and reflect the hydrographic properties of the convected water mass with a distinct minimum in potential vorticity. Their seasonal to decadal variability is tightly connected to the local atmospheric forcing and the associated air–sea heat fluxes. A third class of eddies in the LS are the boundary current eddies shed from the Labrador Current (LC). Since they are mostly confined to the vicinity of the LC, these eddies appear to exert only minor influence on preconditioning and restratification.
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Kimura, Satoshi, Keith W. Nicholls et Emily Venables. « Estimation of Ice Shelf Melt Rate in the Presence of a Thermohaline Staircase ». Journal of Physical Oceanography 45, no 1 (janvier 2015) : 133–48. http://dx.doi.org/10.1175/jpo-d-14-0106.1.
Texte intégralRésumé :
AbstractDiffusive convection–favorable thermohaline staircases are observed directly beneath George VI Ice Shelf, Antarctica. A thermohaline staircase is one of the most pronounced manifestations of double-diffusive convection. Cooling and freshening of the ocean by melting ice produces cool, freshwater above the warmer, saltier water, the water mass distribution favorable to a type of double-diffusive convection known as diffusive convection. While the vertical distribution of water masses can be susceptible to diffusive convection, none of the observations beneath ice shelves so far have shown signals of this process and its effect on melting ice shelves is uncertain. The melt rate of ice shelves is commonly estimated using a parameterization based on a three-equation model, which assumes a fully developed, unstratified turbulent flow over hydraulically smooth surfaces. These prerequisites are clearly not met in the presence of a thermohaline staircase. The basal melt rate is estimated by applying an existing heat flux parameterization for diffusive convection in conjunction with the measurements of oceanic conditions at one site beneath George VI Ice Shelf. These estimates yield a possible range of melt rates between 0.1 and 1.3 m yr−1, where the observed melt rate of this site is ~1.4 m yr−1. Limitations of the formulation and implications of diffusive convection beneath ice shelves are discussed.
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Oliver, K. I. C., T. Eldevik, D. P. Stevens et A. J. Watson. « A Greenland Sea Perspective on the Dynamics of Postconvective Eddies* ». Journal of Physical Oceanography 38, no 12 (1 décembre 2008) : 2755–71. http://dx.doi.org/10.1175/2008jpo3844.1.
Texte intégralRésumé :
Abstract Open ocean deep postconvection contributes to the formation of the dense waters that fill the global deep ocean. The dynamics of postconvective vortices are key to understanding the role of convection in ocean circulation. Submesoscale coherent vortices (SCVs) observed in convective regions are likely to be the anticyclonic components of hetons. Hetons are dipoles, consisting of a surface cyclone and a weakly stratified subsurface anticyclone, that can be formed by convection. Here, key postconvective processes are investigated using numerical experiments of increasing sophistication with two primary goals: 1) to understand how the ambient hydrography and topography influence the propagation of hetons and 2) to provide a theoretical context for recent observations of SCVs in the Greenland Sea. It is found that the alignment of hetons is controlled by ambient horizontal density gradients and that hetons self-propagate into lighter waters as a result. This provides a mechanism for transporting convected water out of a cyclonic gyre, but the propagation is arrested if the heton meets large-amplitude topography. Upon interaction with topography, hetons usually separate, and the surface cyclone returns toward denser water. The anticyclone usually remains close to topography and may become trapped for several hundred days. These findings may explain the observed accumulation and longevity of SCVs at the Greenland Fracture Zone, on the rim of the Greenland Sea gyre. The separation and sorting of cyclones from anticyclones have likely implications for the density and vorticity budgets of convective regions.
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Coates, M. J., G. N. Ivey et J. R. Taylor. « Unsteady, Turbulent Convection into a Rotating, Linearly Stratified Fluid : Modeling Deep Ocean Convection ». Journal of Physical Oceanography 25, no 12 (décembre 1995) : 3032–50. http://dx.doi.org/10.1175/1520-0485(1995)025<3032:utciar>2.0.co;2.
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Vauclair, S. « Thermohaline Convection in Main Sequence Stars ». Proceedings of the International Astronomical Union 4, S252 (avril 2008) : 97–101. http://dx.doi.org/10.1017/s1743921308022527.
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AbstractThermohaline convection is a well known subject in oceanography, which has long been put aside in stellar physics. In the ocean, it occurs when warm salted layers sit on top of cool and less salted ones. Then the salted water rapidly diffuses downwards even in the presence of stabilizing temperature gradients, due to double diffusion between the falling blobs and their surroundings. A similar process may occur in stars in case of inverse μ-gradients in a thermally stabilized medium. Here we describe this process and some of its stellar applications.
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Gelderloos, Renske, Caroline A. Katsman et Sybren S. Drijfhout. « Assessing the Roles of Three Eddy Types in Restratifying the Labrador Sea after Deep Convection ». Journal of Physical Oceanography 41, no 11 (1 novembre 2011) : 2102–19. http://dx.doi.org/10.1175/jpo-d-11-054.1.
Texte intégralRésumé :
Abstract Restratification after deep convection is one of the key factors in determining the temporal variability of dense water formation in the Labrador Sea. In the subsurface, it is primarily governed by lateral buoyancy fluxes during early spring. The roles of three different eddy types in this process are assessed using an idealized model of the Labrador Sea that simulates the restratification season. The first eddy type, warm-core Irminger rings, is shed from the boundary current along the west coast of Greenland. All along the coastline, the boundary current forms boundary current eddies. The third type, convective eddies, arises directly around the convection area. In the model, the latter two eddy types are together responsible for replenishing 30% of the winter heat loss within 6 months. Irminger rings add another 45% to this number. The authors’ results thus confirm that the presence of Irminger rings is essential for a realistic amount of restratification in this area. The model results are compared to observations using theoretical estimates of restratification time scales derived for the three eddy types. The time scales are also used to explain contradicting conclusions in previous studies on their respective roles.
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Brakstad, Ailin, Kjetil Våge, Lisbeth Håvik et G. W. K. Moore. « Water Mass Transformation in the Greenland Sea during the Period 1986–2016 ». Journal of Physical Oceanography 49, no 1 (janvier 2019) : 121–40. http://dx.doi.org/10.1175/jpo-d-17-0273.1.
Texte intégralRésumé :
AbstractHydrographic measurements from ships, autonomous profiling floats, and instrumented seals over the period 1986–2016 are used to examine the temporal variability in open-ocean convection in the Greenland Sea during winter. This process replenishes the deep ocean with oxygen and is central to maintaining its thermohaline properties. The deepest and densest mixed layers in the Greenland Sea were located within its cyclonic gyre and exhibited large interannual variability. Beginning in winter 1994, a transition to deeper (>500 m) mixed layers took place. This resulted in the formation of a new, less dense class of intermediate water that has since become the main product of convection in the Greenland Sea. In the preceding winters, convection was limited to <300-m depth, despite strong atmospheric forcing. Sensitivity studies, performed with a one-dimensional mixed layer model, suggest that the deeper convection was primarily the result of reduced water-column stability. While anomalously fresh conditions that increased the stability of the upper part of the water column had previously inhibited convection, the transition to deeper mixed layers was associated with increased near-surface salinities. Our analysis further suggests that the volume of the new class of intermediate water has expanded in line with generally increased depths of convection over the past 10–15 years. The mean export of this water mass from the Greenland Sea gyre from 1994 to present was estimated to be 0.9 ± 0.7 Sv (1 Sv ≡ 106 m3 s−1), although rates in excess of 1.5 Sv occurred in summers following winters with deep convection.
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Sohail, Taimoor, Bishakhdatta Gayen et Andrew McC. Hogg. « The Dynamics of Mixed Layer Deepening during Open-Ocean Convection ». Journal of Physical Oceanography 50, no 6 (juin 2020) : 1625–41. http://dx.doi.org/10.1175/jpo-d-19-0264.1.
Texte intégralRésumé :
AbstractOpen-ocean convection is a common phenomenon that regulates mixed layer depth and ocean ventilation in the high-latitude oceans. However, many climate model simulations overestimate mixed layer depth during open-ocean convection, resulting in excessive formation of dense water in some regions. The physical processes controlling transient mixed layer depth during open-ocean convection are examined using two different numerical models: a high-resolution, turbulence-resolving nonhydrostatic model and a large-scale hydrostatic ocean model. An isolated destabilizing buoyancy flux is imposed at the surface of both models and a quasi-equilibrium flow is allowed to develop. Mixed layer depth in the turbulence-resolving and large-scale models closely aligns with existing scaling theories. However, the large-scale model has an anomalously deep mixed layer prior to quasi-equilibrium. This transient mixed layer depth bias is a consequence of the lack of resolved turbulent convection in the model, which delays the onset of baroclinic instability. These findings suggest that in order to reduce mixed layer biases in ocean simulations, parameterizations of the connection between baroclinic instability and convection need to be addressed.
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Send, Uwe, et John Marshall. « Integral Effects of Deep Convection ». Journal of Physical Oceanography 25, no 5 (mai 1995) : 855–72. http://dx.doi.org/10.1175/1520-0485(1995)025<0855:ieodc>2.0.co;2.
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Jones, Helen, et John Marshall. « Convection with Rotation in a Neutral Ocean : A Study of Open-Ocean Deep Convection ». Journal of Physical Oceanography 23, no 6 (juin 1993) : 1009–39. http://dx.doi.org/10.1175/1520-0485(1993)023<1009:cwrian>2.0.co;2.
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Vankevich, R. E., E. V. Sofina, T. E. Eremina, A. V. Ryabchenko, M. S. Molchanov et A. V. Isaev. « Effects of lateral processes on the seasonal water stratification of the Gulf of Finland : 3-D NEMO-based model study ». Ocean Science Discussions 12, no 5 (12 octobre 2015) : 2395–421. http://dx.doi.org/10.5194/osd-12-2395-2015.
Texte intégralRésumé :
Abstract. This paper tries to fill the gaps in knowledge of processes affecting the seasonal water stratification in the Gulf of Finland (GOF). We used state-of-the-art modeling framework NEMO aimed for oceanographic research, operational oceanography, seasonal forecasting and climate studies to build an eddy resolving model of the GOF. To evaluate the model skill and performance two different solutions where obtained on 0.5 km eddy resolving and commonly used 2 km grids for one year simulation. We also explore the efficacy of nonhydrostatic effect (convection) parameterizations available in NEMO for coastal application. It is found that the solutions resolving sub-mesoscales have a more complex mixed layer structure in the regions of GOF directly affected by the upwelling/downwelling and intrusions from the open Baltic Sea. Presented model estimations of the upper mixed layer depth are in a good agreement with in situ CTD data. A number of model sensitivity tests to the vertical mixing parameterization confirm the model robustness.
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Pham, Hieu T., Sutanu Sarkar, William D. Smyth, James N. Moum et Sally J. Warner. « Deep-Cycle Turbulence in the Upper Pacific Equatorial Ocean : Characterization by LES and Heat Flux Parameterization ». Journal of Physical Oceanography 54, no 2 (février 2024) : 577–99. http://dx.doi.org/10.1175/jpo-d-23-0015.1.
Texte intégralRésumé :
Abstract Observations in the Pacific Equatorial Undercurrents (EUC) show that the nighttime deep-cycle turbulence (DCT) in the marginal-instability (MI) layer of the EUC exhibits seasonal variability that can modulate heat transport and sea surface temperature. Large-eddy simulations (LES), spanning a wide range of control parameters, are performed to identify the key processes that influence the turbulent heat flux at multiple time scales ranging from turbulent (minutes to hours) to daily to seasonal. The control parameters include wind stress, convective surface heat flux, shear magnitude, and thickness of the MI layer. In the LES, DCT occurs in discrete bursts during the night, exhibits high temporal variability within a burst, and modulates the mixed layer depth. At the daily time scale, turbulent heat flux generally increases with increasing wind stress, MI-layer shear, or nighttime convection. Convection is found to be important to mixing under weak wind, weak shear conditions. A parameterization for the daily averaged turbulent heat flux is developed from the LES suite to infer the variability of heat flux at the seasonal time scale. The LES-based parameterized heat flux, which takes into account the effects of all control parameters, exhibits a seasonal variability that is similar to the observed heat flux from the χ-pods.
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Cuny, Jérôme, Peter B. Rhines, Friedrich Schott et John Lazier. « Convection above the Labrador Continental Slope ». Journal of Physical Oceanography 35, no 4 (1 avril 2005) : 489–511. http://dx.doi.org/10.1175/jpo2700.1.
Texte intégralRésumé :
Abstract The Labrador Sea is one of the few regions of the World Ocean where deep convection takes place. Several moorings across the Labrador continental slope just north of Hamilton Bank show that convection does take place within the Labrador Current. Mixing above the lower Labrador slope is facilitated by the onshore along-isopycnal intrusions of low-potential-vorticity eddies that weaken the stratification, combined with baroclinic instability that sustains slanted mixing while restratifying the water column through horizontal fluxes. Above the shelf break, the Irminger seawater core is displaced onshore while the stratification weakens with the increase in isopycnal slope. The change in stratification is partially due to the onshore shift of the “classical” Labrador Current, baroclinic instability, and possibly slantwise convection.
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Zunino, Patricia, Herlé Mercier et Virginie Thierry. « Why did deep convection persist over four consecutive winters (2015–2018) southeast of Cape Farewell ? » Ocean Science 16, no 1 (20 janvier 2020) : 99–113. http://dx.doi.org/10.5194/os-16-99-2020.
Texte intégralRésumé :
Abstract. After more than a decade of shallow convection, deep convection returned to the Irminger Sea in 2008 and occurred several times since then to reach exceptional convection depths (> 1500 m) in 2015 and 2016. Additionally, deep mixed layers deeper than 1600 m were also reported southeast of Cape Farewell in 2015. In this context, we used Argo data to show that deep convection occurred southeast of Cape Farewell (SECF) in 2016 and persisted during two additional years in 2017 and 2018 with a maximum convection depth deeper than 1300 m. In this article, we investigate the respective roles of air–sea buoyancy flux and preconditioning of the water column (ocean interior buoyancy content) to explain this 4-year persistence of deep convection SECF. We analyzed the respective contributions of the heat and freshwater components. Contrary to the very negative air–sea buoyancy flux that was observed during winter 2015, the buoyancy fluxes over the SECF region during the winters of 2016, 2017 and 2018 were close to the climatological average. We estimated the preconditioning of the water column as the buoyancy that needs to be removed (B) from the end-of-summer water column to homogenize it down to a given depth. B was lower for the winters of 2016–2018 than for the 2008–2015 winter mean, especially due to a vanishing stratification from 600 down to ∼1300 m. This means that less air–sea buoyancy loss was necessary to reach a given convection depth than in the mean, and once convection reached 600 m little additional buoyancy loss was needed to homogenize the water column down to 1300 m. We show that the decrease in B was due to the combined effects of the local cooling of the intermediate water (200–800 m) and the advection of a negative S anomaly in the 1200–1400 m layer. This favorable preconditioning permitted the very deep convection observed in 2016–2018 despite the atmospheric forcing being close to the climatological average.
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Umlauf, Lars, Peter L. Holtermann, Christiane A. Gillner, Ralf D. Prien, Lucas Merckelbach et Jeffrey R. Carpenter. « Diffusive Convection under Rapidly Varying Conditions ». Journal of Physical Oceanography 48, no 8 (août 2018) : 1731–47. http://dx.doi.org/10.1175/jpo-d-18-0018.1.
Texte intégralRésumé :
AbstractIn most observations of diffusive convection in the ocean and in lakes, the characteristic diffusive staircases evolve over long time scales under quasi-stationary background conditions. In the Baltic Sea, however, diffusive staircases develop inside the flanks of intermittent intrusions that induce strong inverse temperature gradients over a vertical range of a few meters, varying on time scales of hours to days. Here, results are discussed from an extensive field campaign conducted in summer 2016 in the southern Baltic Sea, including temperature microstructure data from ocean gliders and an autonomous profiling platform. We find conditions favorable for diffusive instability in the vicinity of warm and cold intrusions with density ratios as small as Rρ = 1.3. The staircases evolving under these conditions are characterized by a small number of steps (typically 1–4) with order 0.1–1-m thickness, temperature differences exceeding 1 K across individual diffusive interfaces, and exceptionally large diffusive heat fluxes of order 10 W m−2. The standard heat flux parameterization of Kelley agrees within a factor of 2 with the directly observed interfacial heat fluxes, except for large fluxes at low Rρ, which are strongly overestimated. The glider surveys reveal a surprisingly small lateral coherency of order 100 m of the staircase patterns, and a spreading of the diffusively unstable intrusions across isopycnals.
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Byshev, V. I., V. G. Neiman, V. T. Paka et B. N. Filyushkin. « TAREEV'S PARADOXES ON THE 90th ANNIVERSARY OF DOCTOR OF PHYSICAL AND MATHEMATICAL SCIENCES BORIS TAREEV ». Journal of Oceanological Research 49, no 2 (1 septembre 2021) : 110–19. http://dx.doi.org/10.29006/1564-2291.jor-2021.49(2).8.
Texte intégralRésumé :
The article is devoted to the 90th anniversary of the prominent Russian oceanographer and former senior researcher at the P.P. Shirshov Institute of Oceanology of the Russian Academy of Sciences, Doctor of Science Boris Alexandrovich Tareev, (1931–1972). A brief information about the main fields and results of his creative scientific activity is given. The paper tells about his study of deep density convection in the ocean, as well as hydrophysics and hydrodynamics of internal gravity waves, and at last the baroclinic instability of ocean currents. The metaphorical allocation of three remarkable happenings in the life history of the hero of the day, that conventionally called Tareev's Paradoxes, enables authors of the paper to better show and appreciate the contribution made by B.A. Tareev into the ocean science and present the role of this scientist in the history of oceanography in the 20th century more substantively.
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de Boer, A. M., J. R. Toggweiler et D. M. Sigman. « Atlantic Dominance of the Meridional Overturning Circulation ». Journal of Physical Oceanography 38, no 2 (1 février 2008) : 435–50. http://dx.doi.org/10.1175/2007jpo3731.1.
Texte intégralRésumé :
Abstract North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents. Here, instead, the overturning is viewed as a global energy–driven system and the robustness of NA dominance is investigated within this framework. Using an idealized geometry ocean general circulation model coupled to an energy moisture balance model, various climatic forcings are tested for their effect on the strength and structure of the overturning circulation. Without winds or a high vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through winds or mixing (purposefully included or due to numerical diffusion) starts the deep-water formation. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e., negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP), and Southern Ocean (SO), while near the haline limit, convection is restricted to the NA. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An increase in SO winds or global winds tends to weaken the salinity gradients, also pushing the ocean to the thermal state. Paleoobservations of more distributed sinking in warmer climates in the past suggest that mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning circulation over long time scales.
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Kuzmina, Natalia, Bert Rudels, Tapani Stipa et Victor Zhurbas. « The Structure and Driving Mechanisms of the Baltic Intrusions ». Journal of Physical Oceanography 35, no 6 (1 juin 2005) : 1120–37. http://dx.doi.org/10.1175/jpo2749.1.
Texte intégralRésumé :
Abstract Data from closely spaced CTD profiling performed in the eastern Gotland Basin after the 1993 inflow event are used to study thermohaline intrusions in the Baltic Sea. Two CTD cross sections display abundant intrusive layers in the permanent halocline. Despite the overwhelming dominance of the salinity stratification, diffusive convection is shown to work in the Baltic halocline enhancing diapycnical mixing. To understand the driving mechanisms of observed intrusions, these are divided into different types depending on their structural features. Only two types of observed intrusions are suggested to be strongly influenced by diffusive convection: 1) relatively thin (3–5 m) and long (up to 8 km) intrusions inherent to high-baroclinicity regions and 2) relatively thick (∼10 m) and short (2–5 km) intrusions inherent to low-baroclinicity regions. To verify this hypothesis the linear stability models of 3D and 2D double-diffusive interleaving in approximation of a finite-width front were used. It is shown that the horizontal and vertical scales of thick and short intrusions correspond well to the 3D rotational mode for a pure thermohaline front. Since mesoscale thermohaline fronts in the Baltic halocline are shown to be essentially baroclinic, the influence of baroclinicity on the rotational mode was studied, which resulted in more adequate estimates of the growth rate of the unstable modes. The thin and long intrusions are shown to be likely driven by 2D baroclinic instability triggered by diffusive convection. The model results demonstrated that diffusion convection can be considered as a possible driver for some intrusions observed in the Baltic halocline, while most of the intrusions have a non-double-diffusive origin. Nevertheless, diffusive convection can affect all types of observed intrusions, for example, by tilting them relative to isopycnals and thereby promoting diapycnal mixing and ventilation in the Baltic halocline.