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1
Bellon, Gilles, Adam H. Sobel i Jerome Vialard. "Ocean–Atmosphere Coupling in the Monsoon Intraseasonal Oscillation: A Simple Model Study". Journal of Climate 21, nr 20 (15.10.2008): 5254–70. http://dx.doi.org/10.1175/2008jcli2305.1.
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Abstract A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the effect of ocean–atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model consists of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled to a simple, linear model of the upper ocean. This model has one unstable eigenmode with a period in the 30–60-day range and a structure similar to the observed northward-propagating intraseasonal oscillation in the Bay of Bengal/west Pacific sector. The ocean–atmosphere coupling is shown to have little impact on either the growth rate or latitudinal structure of the atmospheric oscillation, but it reduces the oscillation’s period by a quarter. At latitudes corresponding to the north of the Indian Ocean, the sea surface temperature (SST) anomalies lead the precipitation anomalies by a quarter of a period, similarly to what has been observed in the Bay of Bengal. The mixed layer depth is in phase opposition to the SST: a monsoon break corresponds to both a warming and a shoaling of the mixed layer. This behavior results from the similarity between the patterns of the predominant processes: wind-induced surface heat flux and wind stirring. The instability of the seasonal monsoon flow is sensitive to the seasonal mixed layer depth: the oscillation is damped when the oceanic mixed layer is thin (about 10 m deep or thinner), as in previous experiments with several models aimed at addressing the boreal winter Madden–Julian oscillation. This suggests that the weak thermal inertia of land might explain the minima of intraseasonal variance observed over the Asian continent.
2
Roundy, Paul E., i Joseph R. Kravitz. "The Association of the Evolution of Intraseasonal Oscillations to ENSO Phase". Journal of Climate 22, nr 2 (15.01.2009): 381–95. http://dx.doi.org/10.1175/2008jcli2389.1.
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Abstract The Pacific Ocean intraseasonal Kelvin wave is a leading oceanic mode that links intraseasonal tropical atmospheric variations with interannual variations in the coupled ocean–atmosphere system. This study considers the premise that these waves may evolve differently with their associated weather patterns during different phases of El Niño–Southern Oscillation (ENSO). If atmospheric and oceanic intraseasonal modes interact and evolve differently during various stages of ENSO, this result may provide useful information with regard to the role of these intraseasonal processes in ENSO evolution. This work utilizes signals of the oceanic Kelvin wave as a statistical basis for a simple composite averaging technique that is applied during different phases of ENSO to objectively analyze the evolution of oceanic and the associated portions of atmospheric intraseasonal oscillations. Results confirm the above premise and suggest that coupling between Kelvin waves and atmospheric convection evolves differently during different stages of ENSO. Further, intraseasonal zonal wind anomalies across the east Pacific timed with oceanic Kelvin waves are stronger during adjustment toward El Niño than during adjustment away from El Niño. These and other patterns in the composites suggest the possibility that systematic changes in the evolution of intraseasonal variations over the course of ENSO might feed back upon this interannual mode to influence the evolution of ENSO itself.
3
Krishnamurthy, V., Cristiana Stan, David A. Randall, Ravi P. Shukla i James L. Kinter. "Simulation of the South Asian Monsoon in a Coupled Model with an Embedded Cloud-Resolving Model". Journal of Climate 27, nr 3 (24.01.2014): 1121–42. http://dx.doi.org/10.1175/jcli-d-13-00257.1.
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Abstract The simulation of the South Asian monsoon by a coupled ocean–atmosphere model with an embedded cloud-resolving model is analyzed on intraseasonal and interannual time scales. The daily modes of variability in the superparameterized Community Climate System Model, version 3 (SP-CCSM), are compared with those in observation, the superparameterized Community Atmospheric Model, version 3 (SP-CAM3), and the control simulation of CCSM (CT-CCSM) with conventional parameterization of convection. The CT-CCSM fails to simulate the observed intraseasonal oscillations but is able to generate the atmospheric El Niño–Southern Oscillation (ENSO) mode, although with regular biennial variability. The dominant modes of variability extracted from daily anomalies of outgoing longwave radiation, precipitation, and low-level horizontal wind in SP-CCSM consist of two intraseasonal oscillations and two seasonally persisting modes, in good agreement with observation. The most significant observed features of the intraseasonal oscillations correctly simulated by the SP-CCSM are the northward propagation of convection, precipitation, and circulation as well as the eastward and westward propagations. The observed spatial structure and the periods of the oscillations are also well captured by the SP-CCSM, although with lesser magnitude. The SP-CCSM is able to simulate the chaotic variability and spatial structure of the seasonally persisting atmospheric ENSO mode, while the evidence for the Indian Ocean dipole mode is inconclusive. The SP-CAM3 simulates two intraseasonal oscillations and the atmospheric ENSO mode. However, the intraseasonal oscillations in SP-CAM3 do not show northward propagation while their periods and spatial structures are not comparable to observation. The results of this study indicate the necessity of coupled models with sufficiently realistic cloud parameterizations.
4
Rokade, M. V., R. Kondala Rao, S. S. Nikte, R. N. Ghodpage, P. T. Patil, A. K. Sharma i S. Gurubaran. "Intraseasonal oscillation (ISO) in the MLT zonal wind over Kolhapur (16.8° N) and Tirunelveli (8.7° N)". Annales Geophysicae 30, nr 12 (5.12.2012): 1623–31. http://dx.doi.org/10.5194/angeo-30-1623-2012.
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Abstract. Simultaneous observations of the mean zonal winds at 88 km obtained by the medium-frequency (MF) radars at Kolhapur (16.8° N, 74.2° E) and Tirunelveli (8.7° N, 77.8° E) have been used to study the intraseasonal oscillation (ISO) in the MLT region. The influences of the intraseasonal variations in the lower tropospheric convective activity associated with the Madden-Julian oscillations on the latitudinal behavior of intraseasonal oscillations (ISO) of the zonal winds in the equatorial mesosphere and lower thermosphere (MLT) have been studied. The ISO activity in the lower tropospheric convective activity is examined by employing outgoing long wave radiation (OLR) as a proxy for deep convective activity occurring in the tropical lower atmosphere. The ISO activity in the zonal wind over TIR is more correlated with that in the convective activity compared to the ISO over KOL. The latitudinal and temporal variabilities of the ISO in MLT zonal winds are explained in terms of the intraseasonal variabilities in the convective activity.
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Hu, Qi, Zhaoning Liang i Michael W. Hoffman. "Detecting Source Regions of Wave Activities in the Tropical Atmosphere by Applying Beamforming to Interpolated Data Grids". Journal of Atmospheric and Oceanic Technology 26, nr 2 (1.02.2009): 270–80. http://dx.doi.org/10.1175/2008jtecha1121.1.
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Abstract Wave activities are primary sources of weather disturbances and cyclones in the tropical atmosphere. One such activity is the intraseasonal variations in wind, convection, and precipitation in the tropical Indian and western tropical Pacific region. These variations affect the intensity, break and reset, and rainfall in the Indian and the East Asian monsoons. Detecting the source regions of these wave activities is essential for understanding and for prediction of wave development. In this study, a fixed beamforming method is proposed to deduce source regions of some wave activities in the tropical atmosphere. This method is tested with simulations of single and distributed complex sources of waves and, then, fixed beamformers are applied to the ECMWF interpolated data grids to detect and identify source regions of the intraseasonal oscillations–waves in the tropical Indian and tropical Pacific Ocean region. Results show that the fixed beamforming technique can uniquely identify the source regions of the intraseasonal oscillations. Applications of this method have revealed various source regions of all major intraseasonal oscillation (ISO) events in the tropical Indian and western equatorial Pacific region during the 29 yr from 1974 to 2002. Knowing these source regions will make it possible to extract the relevant information and, thus, to better understand the development of the intraseasonal oscillations as well as their effects on the tropical weather and climate.
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Wang, Lu, Tim Li i Tianjun Zhou. "Intraseasonal SST Variability and Air–Sea Interaction over the Kuroshio Extension Region during Boreal Summer". Journal of Climate 25, nr 5 (marzec 2012): 1619–34. http://dx.doi.org/10.1175/jcli-d-11-00109.1.
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The structure and evolution characteristics of intraseasonal (20–100 day) variations of sea surface temperature (SST) and associated atmospheric and oceanic circulations over the Kuroshio Extension (KE) region during boreal summer are investigated, using satellite-based daily SST, observed precipitation data, and reanalysis data. The intraseasonal SST warming in the KE region is associated with an anomalous anticyclone in the overlying atmosphere, reduced precipitation, and northward and downward currents in the upper ocean. The corresponding atmospheric and oceanic fields during the SST cooling phase exhibit a mirror image with an opposite sign. A mixed layer heat budget analysis shows that the intraseasonal SST warming is primarily attributed to anomalous shortwave radiation and latent heat fluxes at the surface. The anomalous sensible heat flux and oceanic advection also have contributions, but with a much smaller magnitude. The SST warming caused by the atmospheric forcing further exerts a significant feedback to the atmosphere through triggering the atmospheric convective instability and precipitation anomalies. The so-induced heating leads to quick setup of a baroclinic response, followed by a baroclinic-to-barotropic transition. As a result, the atmospheric circulation changes from an anomalous anticyclone to an anomalous cyclone. This two-way interaction scenario suggests that the origin of the atmospheric intraseasonal oscillation over the KE region may partly arise from the local atmosphere–ocean interaction.
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Klingaman, Nicholas P., Steven J. Woolnough, Hilary Weller i Julia M. Slingo. "The Impact of Finer-Resolution Air–Sea Coupling on the Intraseasonal Oscillation of the Indian Monsoon". Journal of Climate 24, nr 10 (15.05.2011): 2451–68. http://dx.doi.org/10.1175/2010jcli3868.1.
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Abstract A newly assembled atmosphere–ocean coupled model, called HadKPP, is described and then used to determine the effects of subdaily air–sea coupling and fine near-surface ocean vertical resolution on the representation of the Northern Hemisphere summer intraseasonal oscillation. HadKPP comprises the Hadley Centre atmospheric model coupled to the K-Profile Parameterization ocean boundary layer model. Four 30-member ensembles were performed that vary in ocean vertical resolution between 1 and 10 m and in coupling frequency between 3 and 24 h. The 10-m, 24-h ensemble exhibited roughly 60% of the observed 30–50-day variability in sea surface temperatures and rainfall and very weak northward propagation. Enhancing only the vertical resolution or only the coupling frequency produced modest improvements in variability and just a standing intraseasonal oscillation. Only the 1-m, 3-h configuration generated organized, northward-propagating convection similar to observations. Subdaily surface forcing produced stronger upper-ocean temperature anomalies in quadrature with anomalous convection, which likely affected lower-atmospheric stability ahead of the convection, causing propagation. Well-resolved air–sea coupling did not improve the eastward propagation of the boreal summer intraseasonal oscillation in this model. Upper-ocean vertical mixing and diurnal variability in coupled models must be improved to accurately resolve and simulate tropical subseasonal variability. In HadKPP, the mere presence of air–sea coupling was not sufficient to generate an intraseasonal oscillation resembling observations.
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Fu, Xiouhua, Bo Yang, Qing Bao i Bin Wang. "Sea Surface Temperature Feedback Extends the Predictability of Tropical Intraseasonal Oscillation". Monthly Weather Review 136, nr 2 (1.02.2008): 577–97. http://dx.doi.org/10.1175/2007mwr2172.1.
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Abstract The possible impacts of different sea surface temperature (SST) configurations on the predictability of the boreal summer tropical intraseasonal oscillation (TISO) are assessed with a series of ensemble forecasts. The five different lower boundary conditions examined in this study are, respectively, (i) the fully interactive ocean–atmosphere coupling, (ii) “smoothed” SST, which excludes the intraseasonal signal from sea surface forcing, (iii) damped persistent SST, (iv) coupling to a slab mixed-layer ocean, and (v) daily SST from the coupled forecast. The full atmosphere–ocean coupling generates an interactive SST that results in the highest TISO predictability of about 30 days over Southeast Asia. The atmosphere-only model is capable of reaching this predictability if the ensemble mean daily SST forecast by the coupled model is used as the lower boundary condition, which suggests that, in principle, the so-called tier-one and tier-two systems have the same predictability for the boreal summer TISO. The atmosphere-only model driven by either smoothed or damped persistent SSTs, however, has the lowest predictability (∼20 days). The atmospheric model coupled to a slab mixed-layer ocean achieves a predictability of 25 days. The positive SST anomalies in the northern Indo–western Pacific Oceans trigger convective disturbances by moistening and warming up the atmospheric boundary layer. The seasonal mean easterly shear intensifies the anomalous convection by enhancing the surface convergence. An overturning meridional circulation driven by the off-equatorial anomalous convection suppresses the near-equatorial convection and enhances the northward flows, which further intensify the off-equatorial surface convergence and the TISO-related convection. Thus, the boreal summer mean easterly shear and the overturning meridional circulation in the northern Indo–western Pacific sector act as “amplifiers” for the SST feedback to the convection of the TISO.
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Rydbeck, Adam V., Eric D. Maloney, Shang-Ping Xie, Jan Hafner i Jeffrey Shaman. "Remote Forcing versus Local Feedback of East Pacific Intraseasonal Variability during Boreal Summer". Journal of Climate 26, nr 11 (31.05.2013): 3575–96. http://dx.doi.org/10.1175/jcli-d-12-00499.1.
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Abstract During boreal summer (June–October), interactions between intraseasonal variability in the Eastern Hemisphere and east Pacific warm pool are often described as a local amplification of the Madden–Julian oscillation (MJO), the dominant mode of tropical intraseasonal variability. The MJO in the Eastern Hemisphere emits eastward-propagating dry Kelvin waves that are a source of rapid communication with the east Pacific. However, the precise mechanism by and degree to which intraseasonal variability in the Eastern Hemisphere interacts with the east Pacific are not well understood. To quantify the relationship, sensitivity tests in two separate models are used: the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM) and the International Pacific Research Center Regional Atmosphere Model (IRAM). Different methods are employed to isolate the east Pacific from outside intraseasonal signals in each model. When isolated from Kelvin wave fronts associated with the MJO, the CAM produces similar east Pacific intraseasonal variability to observations. In the CAM, the communication of intraseasonal signals by Kelvin waves does not appear necessary to the initiation and maintenance of east Pacific intraseasonal variability, suggesting that such events can be independent of the MJO. However, communication by MJO-initiated Kelvin waves provides a possible phase locking mechanism between hemispheres. When the east Pacific is isolated from all remote intraseasonal signals in the IRAM, intraseasonal events there are weak and incoherent. In the IRAM communication across the Pacific appears necessary to the representation of east Pacific intraseasonal variability. However, the IRAM contains an important bias in the climatological low-level winds that may suppress east Pacific intraseasonal events.
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Wang, Xu, i Guang J. Zhang. "Evaluation of the Quasi-Biweekly Oscillation over the South China Sea in Early and Late Summer in CAM5". Journal of Climate 32, nr 1 (4.12.2018): 69–84. http://dx.doi.org/10.1175/jcli-d-18-0072.1.
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Abstract Low-frequency intraseasonal oscillations in the tropical atmosphere in general circulation models (GCMs) were studied extensively in many previous studies. However, the simulation of the quasi-biweekly oscillation (QBWO), which is an important component of the intraseasonal oscillations, in GCMs has not received much attention. This paper evaluates the QBWO features over the South China Sea in early [May–June (MJ)] and late [August–September (AS)] summer in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5.3 (CAM5), using observations and reanalysis data. Results show that the major features of the spatial distribution of the QBWO in both MJ and AS are simulated reasonably well by the model, although the amplitude of the variation is overestimated. CAM5 captures the local oscillation in MJ and the westward propagation in AS of the QBWO. Although there are important biases in geographical location and intensity in MJ, the model represents the QBWO horizontal and vertical structure qualitatively well in AS. The diagnosis of the eddy vorticity budget is conducted to better understand the QBWO activities in the model. Both horizontal advection of relative vorticity and that of planetary vorticity (Coriolis parameter) are important for the local evolution of the QBWO in MJ in observations as well as model simulation, whereas advection of planetary vorticity contributes to the westward propagation of QBWO vorticity anomalies in AS. Since the Coriolis parameter f only changes with latitude, this suggests that the correct simulation of anomalous meridional wind is a key factor in the realistic simulation of the QBWO in the model.
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Pegion, Kathy, i Ben P. Kirtman. "The Impact of Air–Sea Interactions on the Simulation of Tropical Intraseasonal Variability". Journal of Climate 21, nr 24 (15.12.2008): 6616–35. http://dx.doi.org/10.1175/2008jcli2180.1.
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Abstract The impact of coupled air–sea feedbacks on the simulation of tropical intraseasonal variability is investigated in this study using the National Centers for Environmental Prediction Climate Forecast System. The simulation of tropical intraseasonal variability in a freely coupled simulation is compared with two simulations of the atmospheric component of the model. In one experiment, the uncoupled model is forced with the daily sea surface temperature (SST) from the coupled run. In the other, the uncoupled model is forced with climatological SST from the coupled run. Results indicate that the overall intraseasonal variability of precipitation is reduced in the coupled simulation compared to the uncoupled simulation forced by daily SST. Additionally, air–sea coupling is responsible for differences in the simulation of the tropical intraseasonal oscillation between the coupled and uncoupled models, specifically in terms of organization and propagation in the western Pacific. The differences between the coupled and uncoupled simulations are due to the fact that the relationships between precipitation and SST and latent heat flux and SST are much stronger in the coupled model than in the uncoupled model. Additionally, these relationships are delayed by about 5 days in the uncoupled model compared to the coupled model. As demonstrated by the uncoupled simulation forced with climatological SST, some of the intraseasonal oscillation can be simulated by internal atmospheric dynamics. However, the intraseasonally varying SST appears to be important to the amplitude and propagation of the oscillation beyond the Maritime Continent.
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Maloney, Eric D., i Adam H. Sobel. "Surface Fluxes and Ocean Coupling in the Tropical Intraseasonal Oscillation". Journal of Climate 17, nr 22 (15.11.2004): 4368–86. http://dx.doi.org/10.1175/jcli-3212.1.
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Abstract Sensitivity of tropical intraseasonal variability to mixed layer depth is examined in the modified National Center for Atmospheric Research Community Atmosphere Model 2.0.1 (CAM), with relaxed Arakawa–Schubert convection, coupled to a slab ocean model (SOM) whose mixed layer depth is fixed and geographically uniform, but varies from one experiment to the next. Intraseasonal west Pacific precipitation variations during boreal winter are enhanced relative to a fixed-SST (infinite mixed layer depth) simulation for mixed layer depths from 5 to 50 m, with a maximum at 20 m [interestingly, near the observed value in the regions where the Madden– Julian oscillation (MJO) is active], but are strongly diminished in the 2-m depth simulation. This nonmonotonicity of intraseasonal precipitation variance with respect to mixed layer depth was predicted by Sobel and Gildor using a highly idealized model. Further experiments with the same idealized model help to interpret results derived from the modified NCAR CAM. A sensitivity study shows that the convection–surface flux feedback [wind-induced surface heat exchange (WISHE)] is important to the intraseasonal variability in the CAM. This helps to explain the behavior of the 2-m SOM simulation and the agreement with the idealized model. Although intraseasonal SST variations are stronger in the 2-m SOM simulation than in any of the other simulations, these SST variations are phased in such a way as to diminish the amplitude of equatorial latent heat flux variations. Reducing the mixed layer depth is thus nearly equivalent to eliminating WISHE, which in this model reduces intraseasonal variability. The WISHE mechanism in the model is nonlinear and occurs in a region of mean low-level westerlies. Since a very shallow mixed layer is effectively similar to wet land, it is suggested that the mechanism described here may explain the local minimum in MJO amplitude observed over the Maritime Continent region.
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Maloney, Eric D., i Steven K. Esbensen. "A Modeling Study of Summertime East Pacific Wind-Induced Ocean–Atmosphere Exchange in the Intraseasonal Oscillation". Journal of Climate 18, nr 4 (15.02.2005): 568–84. http://dx.doi.org/10.1175/jcli-3280.1.
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Abstract Intraseasonal precipitation variability over the northeast Pacific warm pool during June–October in the National Center for Atmospheric Research Community Atmosphere Model 2.0.1 with a relaxed Arakawa–Schubert convection parameterization is found to be strongly sensitive to wind-induced variations in surface latent heat flux. A control simulation with interactive surface fluxes produces northeast Pacific warm pool intraseasonal wind and precipitation variations that are of similar magnitude and structure to those associated with the observed intraseasonal oscillation (ISO). Periods of low-level westerly intraseasonal wind anomalies are associated with enhanced surface latent heat fluxes and enhanced precipitation, as in observations. Variations in surface wind speed primarily control the surface flux anomalies. A simulation in which eastern North Pacific oceanic latent heat fluxes are fixed produces intraseasonal precipitation variations that are significantly weaker than those in the control simulation and in observations. These results support the observational findings of Maloney and Esbensen, who suggested that wind-induced latent heat flux variability is a significant driver of ISO-related convective variability over the northeast Pacific warm pool during Northern Hemisphere summer. East Pacific ISO-related convection in this model, thus, appears to be forced by an analogous wind-induced surface heat exchange mechanism to that proposed by Maloney and Sobel to explain the forcing of west Pacific ISO-related convection. The surface exchange mechanism is apparently active within regions of mean westerly low-level flow. In contrast, summertime eastern North Pacific intraseasonal wind variance and spatial structure does not differ significantly between the control and fixed-evaporation simulations. A strong coupling between the east Pacific flow and precipitation over Central America may be responsible for the relatively small changes in wind variability between the simulations. Interactions among the coarsely resolved Central American orography, the large-scale flow, and the convection parameterization in the model likely contribute to this anomalous coupling.
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Lin, Hai, Gilbert Brunet i Jacques Derome. "Intraseasonal Variability in a Dry Atmospheric Model". Journal of the Atmospheric Sciences 64, nr 7 (1.07.2007): 2422–41. http://dx.doi.org/10.1175/jas3955.1.
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Abstract A long integration of a primitive equation dry atmospheric model with time-independent forcing under boreal winter conditions is analyzed. A variety of techniques such as time filtering, space–time spectral analysis, and lag regressions are used to identify tropical waves. It is evident that oscillations with intraseasonal time scales and a Kelvin wave structure exist in the model tropical atmosphere. Coherent eastward propagations in the 250-hPa velocity potential and zonal wind are found, with a speed of about 15 m s−1. The oscillation is stronger in the Eastern Hemisphere than in the Western Hemisphere. Interactions between the tropical and extratropical flows are found to be responsible for the simulated intraseasonal variability. Wave activity flux analysis reveals that a tropical influence occurs in the North Pacific region where a northeastward wave activity flux is found associated with the tropical divergent flow in the western and central Pacific. In the North Atlantic sector, on the other hand, a strong extratropical influence is observed with a southward wave activity flux into the Tropics. The extratropical low-frequency variability develops by extracting kinetic energy from the basic mean flow and through interactions with synoptic-scale transient eddies. Linear experiments show that the tropical atmospheric response to the extratropical forcing in the North Atlantic leads to an eastward-propagating wave in the tropical easterly mean flow of the Eastern Hemisphere.
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Henderson, Gina R., Bradford S. Barrett, Ashley Lois i Haadi Elsaawy. "Time-Lagged Response of the Antarctic and High-Latitude Atmosphere to Tropical MJO Convection". Monthly Weather Review 146, nr 4 (kwiecień 2018): 1219–31. http://dx.doi.org/10.1175/mwr-d-17-0224.1.
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Intraseasonal tropical variability has important implications for the mid- and high-latitude atmosphere, and in recent studies has been shown to modulate a number of weather processes in the Northern Hemisphere, such as snow depth, sea ice concentration, precipitation, atmospheric rivers, and air temperature. In such studies, the extratropical atmosphere has tended to respond to the tropical convection of the leading mode of intraseasonal variability, the Madden–Julian oscillation (MJO), with a time lag of approximately 7 days. However, the time lag between the MJO and the Antarctic atmosphere has been found to vary between less than 7 and greater than 20 days. This study builds on previous work by further examining the time-lagged response of Southern Hemisphere tropospheric circulation to tropical MJO forcing, with specific focus on the latitude belt associated with the Antarctic Oscillation, during the months of June (austral winter) and December (austral summer) using NCEP–DOE Reanalysis 2 data for the years 1979–2016. Principal findings indicate that the time lag with the strongest height anomalies depends on both the location of the MJO convection (e.g., the MJO phase) and the season, and that the lagged height anomalies in the Antarctic atmosphere are fairly consistent across different vertical levels and latitudinal bands. In addition, certain MJO phases in December displayed lagged height anomalies indicative of blocking-type atmospheric patterns, with an approximate wavenumber of 4, whereas in June most phases were associated with more progressive height anomaly centers resembling a wavenumber-3-type pattern.
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Ge, Xuyang, Wanqiu Wang, Arun Kumar i Ying Zhang. "Importance of the Vertical Resolution in Simulating SST Diurnal and Intraseasonal Variability in an Oceanic General Circulation Model". Journal of Climate 30, nr 11 (4.05.2017): 3963–78. http://dx.doi.org/10.1175/jcli-d-16-0689.1.
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Abstract In this paper, the influence of high vertical resolution near the surface in an oceanic general circulation model in simulating the observed sea surface temperature (SST) variability is investigated. In situ observations of vertical temperature profiles are first used to quantify temperature variability with depth near the ocean surface. The analysis shows that there is a sharp vertical temperature gradient within the top 10 m of the ocean. Both diurnal and intraseasonal variabilities of the ocean temperatures are largest near the surface and decrease with the ocean depth. Numerical experiments with an oceanic general circulation model are next carried out with 1- and 10-m vertical resolutions for the upper ocean to study the dependence of the simulated SST and vertical temperature structure on the vertical resolution. It is found that the simulated diurnal and intraseasonal variabilities, as well as the associated vertical temperature gradient near the surface, are strongly influenced by the oceanic vertical resolution, with the 1-m vertical resolution producing a stronger vertical temperature gradient and temporal variability than the 10-m vertical resolution. These results suggest that a realistic representation of SST variability with a high vertical resolution in the upper ocean is required for a coupled atmosphere–ocean model to correctly simulate the observed tropical intraseasonal oscillations, including the Madden–Julian oscillation and the boreal summer monsoon intraseasonal oscillation, which are strongly linked with the underlying SST.
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Batstone, Crispian P., Adrian J. Matthews i David P. Stevens. "Coupled Ocean–Atmosphere Interactions between the Madden–Julian Oscillation and Synoptic-Scale Variability over the Warm Pool". Journal of Climate 18, nr 12 (15.06.2005): 2004–20. http://dx.doi.org/10.1175/jcli3398.1.
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Abstract A principal component analysis of the combined fields of sea surface temperature (SST) and surface zonal and meridional wind reveals that the dominant mode of intraseasonal (30 to 70 day) covariability during northern winter in the tropical Eastern Hemisphere is that of the Madden–Julian oscillation (MJO). Regression calculations show that the submonthly (30-day high-pass filtered) surface wind variability is significantly modulated during the MJO. Regions of increased (decreased) submonthly surface wind variability propagate eastward, approximately in phase with the intraseasonal surface westerly (easterly) anomalies of the MJO. Because of the dependence of the surface latent heat flux on the magnitude of the total wind speed, this systematic modulation of the submonthly surface wind variability produces a significant component in the intraseasonal latent heat flux anomalies, which partially cancels the latent heat flux anomalies due to the slowly varying intraseasonal wind anomalies, particularly south of 10°S. A method is derived that demodulates the submonthly surface wind variability from the slowly varying intraseasonal wind anomalies. This method is applied to the wind forcing fields of a one-dimensional ocean model. The model response to this modified forcing produces larger intraseasonal SST anomalies than when the model is forced with the observed forcing over large areas of the southwest Pacific Ocean and southeast Indian Ocean during both phases of the MJO. This result has implications for accurate coupled modeling of the MJO. A similar calculation is applied to the surface shortwave flux, but intraseasonal modulation of submonthly surface shortwave flux variability does not appear to be important to the dynamics of the MJO.
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Pegion, Kathy, i Ben P. Kirtman. "The Impact of Air–Sea Interactions on the Predictability of the Tropical Intraseasonal Oscillation". Journal of Climate 21, nr 22 (15.11.2008): 5870–86. http://dx.doi.org/10.1175/2008jcli2209.1.
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Abstract This study investigates whether air–sea interactions contribute to differences in the predictability of the boreal winter tropical intraseasonal oscillation (TISO) using the NCEP operational climate model. A series of coupled and uncoupled, “perfect” model predictability experiments are performed for 10 strong model intraseasonal events. The uncoupled experiments are forced by prescribed SST containing different types of variability. These experiments are specifically designed to be directly comparable to actual forecasts. Predictability estimates are calculated using three metrics, including one that does not require the use of time filtering. The estimates are compared between these experiments to determine the impact of coupled air–sea interactions on the predictability of the tropical intraseasonal oscillation and the sensitivity of the potential predictability estimates to the different SST forcings. Results from all three metrics are surprisingly similar. They indicate that predictability estimates are longest for precipitation and outgoing longwave radiation (OLR) when the ensemble mean from the coupled model is used. Most importantly, the experiments that contain intraseasonally varying SST consistently predict the control events better than those that do not for precipitation, OLR, 200-hPa zonal wind, and 850-hPa zonal wind after the first 10 days. The uncoupled model is able to predict the TISO with similar skill to that of the coupled model, provided that an SST forecast that includes these intraseasonal variations is used to force the model. This indicates that the intraseasonally varying SSTs are a key factor for increased predictability and presumably better prediction of the TISO.
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Rydbeck, Adam V., i Tommy G. Jensen. "Oceanic Impetus for Convective Onset of the Madden–Julian Oscillation in the Western Indian Ocean". Journal of Climate 30, nr 11 (9.05.2017): 4299–316. http://dx.doi.org/10.1175/jcli-d-16-0595.1.
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Abstract A theory for intraseasonal atmosphere–ocean–atmosphere feedback is supported whereby oceanic equatorial Rossby waves are partly forced in the eastern Indian Ocean by the Madden–Julian oscillation (MJO), reemerge in the western Indian Ocean ~70 days later, and force large-scale convergence in the atmospheric boundary layer that precedes MJO deep convection. Downwelling equatorial Rossby waves permit high sea surface temperature (SST) and enhance meridional and zonal SST gradients that generate convergent circulations in the atmospheric boundary layer. The magnitude of the SST and SST gradient increases are 0.25°C and 1.5°C Mm−1 (1 megameter is equal to 1000 km), respectively. The atmospheric circulations driven by the SST gradient are estimated to be responsible for up to 45% of the intraseasonal boundary layer convergence observed in the western Indian Ocean. The SST-induced boundary layer convergence maximizes 3–4 days prior to the convective maximum and is hypothesized to serve as a trigger for MJO deep convection. Boundary layer convergence is shown to further augment deep convection by locally increasing boundary layer moisture. Warm SST anomalies facilitated by downwelling equatorial Rossby waves are also associated with increased surface latent heat fluxes that occur after MJO convective onset. Finally, generation of the most robust downwelling equatorial Rossby waves in the western Indian Ocean is shown to have a distinct seasonal distribution.
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Tang, Yue, Gang Zeng, Xiaoye Yang, Vedaste Iyakaremye i Zhongxian Li. "Intraseasonal Oscillation of Summer Extreme High Temperature in Northeast China and Associated Atmospheric Circulation Anomalies". Atmosphere 13, nr 3 (25.02.2022): 387. http://dx.doi.org/10.3390/atmos13030387.
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Previous studies have demonstrated the important effects of intraseasonal oscillations in the tropics on the occurrence of extreme-high-temperature events (EHTs), whereas the influence of intraseasonal oscillations in mid-high latitudes on EHTs has been less discussed. In this study, the intraseasonal oscillation of summer extreme high temperatures from 1981 to 2019 in northeast China and its associated atmospheric circulation were studied using conventional statistic methods. The results show that the summer extreme-high-temperature distribution in northeast China is consistent throughout the whole region, with a low-frequency oscillation period of 10–30 d. The low-frequency extreme-high-temperature events (LFEHTs) in northeast China account for 88.8% of all EHTs during the summer. The corresponding low-frequency circulation anomalies with 10–30 d oscillations exhibit a barotropic wave-train moving from west to east in the mid-high latitudes of Eurasia. A low-frequency wave-train index (LFWI) was defined to characterize the wave-train anomaly system in the mid-high latitudes of the Eurasian continent. The LFWI may be a potential precursor for forecasting LFEHTs about 7 days in advance. It could explain 15–30% of the summertime low-frequency daily maximum temperature variability in northeast China.
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Alexander, Romeo, Zhizhen Zhao, Eniko Székely i Dimitrios Giannakis. "Kernel Analog Forecasting of Tropical Intraseasonal Oscillations". Journal of the Atmospheric Sciences 74, nr 4 (1.04.2017): 1321–42. http://dx.doi.org/10.1175/jas-d-16-0147.1.
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Abstract This paper presents the results of forecasting the Madden–Julian oscillation (MJO) and boreal summer intraseasonal oscillation (BSISO) through the use of satellite-obtained global brightness temperature data with a recently developed nonparametric empirical method. This new method, referred to as kernel analog forecasting, adopts specific indices extracted using the technique of nonlinear Laplacian spectral analysis as baseline definitions of the intraseasonal oscillations of interest, which are then extended into forecasts through an iterated weighted averaging scheme that exploits the predictability inherent to those indices. The pattern correlation of the forecasts produced in this manner remains above 0.6 for 50 days for both the MJO and BSISO when 23 yr of training data are used and 37 days for the MJO when 9 yr of data are used.
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Alessandri, Andrea, Andrea Borrelli, Annalisa Cherchi, Stefano Materia, Antonio Navarra, June-Yi Lee i Bin Wang. "Prediction of Indian Summer Monsoon Onset Using Dynamical Subseasonal Forecasts: Effects of Realistic Initialization of the Atmosphere". Monthly Weather Review 143, nr 3 (27.02.2015): 778–93. http://dx.doi.org/10.1175/mwr-d-14-00187.1.
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Abstract Ensembles of retrospective 2-month dynamical forecasts initiated on 1 May are used to predict the onset of the Indian summer monsoon (ISM) for the period 1989–2005. The subseasonal predictions (SSPs) are based on a coupled general circulation model and recently they have been upgraded by the realistic initialization of the atmosphere with initial conditions taken from reanalysis. Two objective large-scale methods based on dynamical-circulation and hydrological indices are applied to detect the ISM onset. The SSPs show some skill in forecasting earlier-than-normal ISM onsets, while they have difficulty in predicting late onsets. It is shown that significant contribution to the skill in forecasting early ISM onsets comes from the newly developed initialization of the atmosphere from reanalysis. On one hand, atmospheric initialization produces a better representation of the atmospheric mean state in the initial conditions, leading to a systematically improved monsoon onset sequence. On the other hand, the initialization of the atmosphere allows some skill in forecasting the northward-propagating intraseasonal wind and precipitation anomalies over the tropical Indian Ocean. The northward-propagating intraseasonal modes trigger the monsoon in some early-onset years. The realistic phase initialization of these modes improves the forecasts of the associated earlier-than-normal monsoon onsets. The prediction of late onsets is not noticeably improved by the initialization of the atmosphere. It is suggested that late onsets of the monsoon are too far away from the start date of the forecasts to conserve enough memory of the intraseasonal oscillation (ISO) anomalies and of the improved representation of the mean state in the initial conditions.
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Hudson, Debra, Andrew G. Marshall, Yonghong Yin, Oscar Alves i Harry H. Hendon. "Improving Intraseasonal Prediction with a New Ensemble Generation Strategy". Monthly Weather Review 141, nr 12 (25.11.2013): 4429–49. http://dx.doi.org/10.1175/mwr-d-13-00059.1.
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Abstract The Australian Bureau of Meteorology has recently enhanced its capability to make coupled model forecasts of intraseasonal climate variations. The Predictive Ocean Atmosphere Model for Australia (POAMA, version 2) seasonal prediction forecast system in operations prior to March 2013, designated P2-S, was not designed for intraseasonal forecasting and has deficiencies in this regard. Most notably, the forecasts were only initialized on the 1st and 15th of each month, and the growth of the ensemble spread in the first 30 days of the forecasts was too slow to be useful on intraseasonal time scales. These deficiencies have been addressed in a system upgrade by initializing more often and through enhancements to the ensemble generation. The new ensemble generation scheme is based on a coupled-breeding approach and produces an ensemble of perturbed atmosphere and ocean states for initializing the forecasts. This scheme impacts favorably on the forecast skill of Australian rainfall and temperature compared to P2-S and its predecessor (version 1.5). In POAMA-1.5 the ensemble was produced using time-lagged atmospheric initial conditions but with unperturbed ocean initial conditions. P2-S used an ensemble of perturbed ocean initial conditions but only a single atmospheric initial condition. The improvement in forecast performance using the coupled-breeding approach is primarily reflected in improved reliability in the first month of the forecasts, but there is also higher skill in predicting important drivers of intraseasonal climate variability, namely the Madden–Julian oscillation and southern annular mode. The results illustrate the importance of having an optimal ensemble generation strategy.
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Wang, Shuguang, i Adam H. Sobel. "A Unified Moisture Mode Theory for the Madden–Julian Oscillation and the Boreal Summer Intraseasonal Oscillation". Journal of Climate 35, nr 4 (15.02.2022): 1267–91. http://dx.doi.org/10.1175/jcli-d-21-0361.1.
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Abstract The Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO) are fundamental modes of variability in the tropical atmosphere on the intraseasonal time scale. A linear model, using a moist shallow water equation set on an equatorial beta plane, is developed to provide a unified treatment of the two modes and to understand their growth and propagation over the Indian Ocean. Moisture is assumed to increase linearly with longitude and to decrease quadratically with latitude. Solutions are obtained through linear stability analysis, considering the gravest (n = 1) meridional mode with nonzero meridional velocity. Anomalies in zonal moisture advection and surface fluxes are both proportional to those in zonal wind, but of opposite sign. With observation-based estimates for both effects, the zonal advection dominates, and drives the planetary-scale instability. With a sufficiently small meridional moisture gradient, the horizontal structure exhibits oscillations with latitude and a northwest–southeast horizontal tilt in the Northern Hemisphere, qualitatively resembling the observed BSISO. As the meridional moisture gradient increases, the horizontal tilt decreases and the spatial pattern transforms toward the “swallowtail” structure associated with the MJO, with cyclonic gyres in both hemispheres straddling the equatorial precipitation maximum. These results suggest that the magnitude of the meridional moisture gradient shapes the horizontal structures, leading to the transformation from the BSISO-like tilted horizontal structure to the MJO-like neutral wave structure as the meridional moisture gradient changes with the seasons. The existence and behavior of these intraseasonal modes can be understood as a consequence of phase speed matching between the equatorial mode with zero meridional velocity (analogous to the dry Kelvin wave) and a local off-equatorial component that is characterized by considering an otherwise similar system on an f plane.
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Alaka, Ghassan J., i Eric D. Maloney. "Internal Intraseasonal Variability of the West African Monsoon in WRF". Journal of Climate 30, nr 15 (sierpień 2017): 5815–33. http://dx.doi.org/10.1175/jcli-d-16-0750.1.
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The West African monsoon (WAM) and its landmark features, which include African easterly waves (AEWs) and the African easterly jet (AEJ), exhibit significant intraseasonal variability in boreal summer. However, the degree to which this variability is modulated by external large-scale phenomena, such as the Madden–Julian oscillation (MJO), remains unclear. The Weather Research and Forecasting (WRF) Model is employed to diagnose the importance of the MJO and other external influences for the intraseasonal variability of the WAM and associated AEW energetics by removing 30–90-day signals from initial and lateral boundary conditions in sensitivity tests. The WAM produces similar intraseasonal variability in the absence of external influences, indicating that the MJO is not critical to produce WAM variability. In control and sensitivity experiments, AEW precursor signals are similar near the AEJ entrance in East Africa. For example, an eastward extension of the AEJ increases barotropic and baroclinic energy conversions in East Africa prior to a 30–90-day maximum of perturbation kinetic energy in West Africa. The WAM appears to prefer a faster oscillation when MJO forcing is removed, suggesting that the MJO may serve as a pacemaker for intraseasonal oscillations in the WAM. WRF results show that eastward propagating intraseasonal signals (e.g., Kelvin wave fronts) are responsible for this pacing, while the role of westward propagating intraseasonal signals (e.g., MJO-induced Rossby waves) appears to be limited. Mean state biases across the simulations complicate the interpretation of results.
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Rahaman, Hasibur, G. N. Bharath Raj i M. Ravichandran. "Coupled Ocean–Atmosphere Summer Intraseasonal Oscillation over the Bay of Bengal". Pure and Applied Geophysics 176, nr 12 (29.07.2019): 5415–29. http://dx.doi.org/10.1007/s00024-019-02275-4.
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Bellon, Gilles. "Monsoon intraseasonal oscillation and land–atmosphere interaction in an idealized model". Climate Dynamics 37, nr 5-6 (24.08.2010): 1081–96. http://dx.doi.org/10.1007/s00382-010-0893-0.
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Wu, Qigang. "Forcing of Tropical SST Anomalies by Wintertime AO-like Variability". Journal of Climate 23, nr 10 (15.05.2010): 2465–72. http://dx.doi.org/10.1175/2009jcli2749.1.
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Abstract A lagged maximum covariance analysis (MCA) is utilized to investigate large-scale patterns of covariability between sea surface temperature (SST) in the global tropics and 500-mb geopotential height (Z500) in the extratropics at monthly to interannual time scales distinct from the conventional El Niño–Southern Oscillation (ENSO) signal during the Northern Hemisphere (NH) winter. The first MCA mode indicates a strong impact of tropical SST anomalies associated with ENSO on the extratropical atmosphere. The second MCA mode corresponds with coupling between Arctic Oscillation (AO)-like atmospheric variations and tropical SST anomalies. An AO-like MCA mode appears to depict an atmosphere-to-ocean forcing, in which the tropical ocean responds to the higher extratropical AO-like atmospheric anomalies with an intraseasonal time lag. In winter, AO-like atmospheric variability is associated with the northern tropical Atlantic mode and the tropical Pacific ENSO Modoki mode through enhanced or weakened trade winds. The above forced SST anomalies by the AO-like variability may play a role in the subsequent evolution of the conventional ENSO phenomena.
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Rimbu, N., G. Lohmann, G. König-Langlo, C. Necula i M. Ionita. "Daily to intraseasonal oscillations at Antarctic research station Neumayer". Antarctic Science 26, nr 2 (13.08.2013): 193–204. http://dx.doi.org/10.1017/s0954102013000540.
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AbstractHigh temporal resolution (three hours) records of temperature, wind speed and sea level pressure recorded at Antarctic research station Neumayer (70°S, 8°W) during 1982–2011 are analysed to identify oscillations from daily to intraseasonal timescales. The diurnal cycle dominates the three-hourly time series of temperature during the Antarctic summer and is almost absent during winter. In contrast, the three-hourly time series of wind speed and sea level pressure show a weak diurnal cycle. The dominant pattern of the intraseasonal variability of these quantities, which captures the out-of-phase variation of temperature and wind speed with sea level pressure, shows enhanced variability at timescales of ∼ 40 days and ∼ 80 days, respectively. Correlation and composite analysis reveal that these oscillations may be related to tropical intraseasonal oscillations via large-scale eastward propagating atmospheric circulation wave-trains. The second pattern of intraseasonal variability, which captures in-phase variations of temperature, wind and sea level pressure, shows enhanced variability at timescales of ∼ 35, ∼ 60 and ∼ 120 days. These oscillations are attributed to the Southern Annular Mode/Antarctic Oscillation (SAM/AAO) which shows enhanced variability at these timescales. We argue that intraseasonal oscillations of tropical climate and SAM/AAO are related to distinct patterns of climate variables measured at Neumayer.
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Sandery, Paul A., Terence J. O’Kane, Vassili Kitsios i Pavel Sakov. "Climate Model State Estimation Using Variants of EnKF Coupled Data Assimilation". Monthly Weather Review 148, nr 6 (21.05.2020): 2411–31. http://dx.doi.org/10.1175/mwr-d-18-0443.1.
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Abstract Data assimilation (DA) experiments are performed to assess impacts of observations in climate model state estimation through the cross-domain ocean–atmosphere forecast error covariances (cross covariances). Specifically, we explore strongly and weakly coupled DA variants using the Climate Analysis Forecast Ensemble (CAFE) system. This comprises 96 ensemble members of the Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 climate model assimilating observational data from the ocean, atmosphere, and sea ice realms with the ensemble Kalman filter (EnKF). Sequences of atmospheric synoptic time-scale coupled forecasts (7 days) are carried out with model consistent initialization. Unassimilated forward-independent observations are used to quantify forecast innovation error-growth rates. The results show benefit for the slow components of the atmosphere and ocean subsurface when strongly coupling ocean observations to the atmosphere. In the present system, projecting fast atmospheric observations onto the ocean subsurface through the cross covariances benefits the oceanic and atmospheric near-surface layers; however, this leads to deterioration in the ocean subsurface. Particular variants of coupled DA are able to constrain the ocean and atmosphere. The forecasts initialized with these variants have predictability at intraseasonal time scales. Errors associated with the dominant intraseasonal mode of variability, the Madden–Julian oscillation (MJO), are decomposed into normal mode functions. Consistent with recent studies showing large MJO events are concurrent with rapid error growth associated with nonlinear interactions, we find a clear relationship between the strength of a given MJO event and the related forecast innovations. Our results demonstrate consistent system behavior in relation to capturing real-world disturbances that affect climate predictability.
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Wang, Shuguang. "A Precipitation-Based Index for Tropical Intraseasonal Oscillations". Journal of Climate 33, nr 3 (1.02.2020): 805–23. http://dx.doi.org/10.1175/jcli-d-19-0019.1.
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AbstractCharacteristic patterns of precipitation-associated tropical intraseasonal oscillations, including the Madden–Julian oscillation (MJO) and boreal summer intraseasonal oscillation (BSISO), are identified using local empirical orthogonal function (EOF) analysis of the Tropical Rainfall Measuring Mission (TRMM) precipitation data as a function of the day of the year. The explained variances of the EOF analysis show two peaks across the year: one in the middle of the boreal winter corresponding to the MJO and the other in the middle of summer corresponding to the BSISO. Comparing the fractional variance indicates that the BSISO is more coherent than the MJO during the TRMM period. Similar EOF analyses with the outgoing longwave radiation (OLR) confirm this result and indicate that the BSISO is less coherent before the TRMM era (1979–98). In contrast, the MJO exhibits much less decadal variability. A precipitation-based index for tropical intraseasonal oscillation (PII) is derived by projecting bandpass-filtered precipitation anomalies to the two leading EOFs as a function of day of the year. A real-time version that approximates the PII is further developed using precipitation anomalies without any bandpass filtering. It is further shown that this real-time PII index may be used to diagnose precipitation in the subseasonal forecasts.
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Benedict, James J., i David A. Randall. "Structure of the Madden–Julian Oscillation in the Superparameterized CAM". Journal of the Atmospheric Sciences 66, nr 11 (1.11.2009): 3277–96. http://dx.doi.org/10.1175/2009jas3030.1.
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Abstract The detailed dynamic and thermodynamic space–time structures of the Madden–Julian oscillation (MJO) as simulated by the superparameterized Community Atmosphere Model version 3.0 (SP-CAM) are analyzed. Superparameterization involves substituting conventional boundary layer, moist convection, and cloud parameterizations with a configuration of cloud-resolving models (CRMs) embedded in each general circulation model (GCM) grid cell. Unlike most GCMs that implement conventional parameterizations, the SP-CAM displays robust atmospheric variability on intraseasonal space and time (30–60 days) scales. The authors examine a 19-yr SP-CAM simulation based on the Atmospheric Model Intercomparison Project protocol, forced by prescribed sea surface temperatures. Overall, the space–time structures of MJO convective disturbances are very well represented in the SP-CAM. Compared to observations, the model produces a similar vertical progression of increased moisture, warmth, and heating from the boundary layer to the upper troposphere as deep convection matures. Additionally, important advective and convective processes in the SP-CAM compare favorably with those in observations. A deficiency of the SP-CAM is that simulated convective intensity organized on intraseasonal space–time scales is overestimated, particularly in the west Pacific. These simulated convective biases are likely due to several factors including unrealistic boundary layer interactions, a lack of weakening of the simulated disturbance over the Maritime Continent, and mean state differences.
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Bony, Sandrine, i Kerry A. Emanuel. "On the Role of Moist Processes in Tropical Intraseasonal Variability: Cloud–Radiation and Moisture–Convection Feedbacks". Journal of the Atmospheric Sciences 62, nr 8 (1.08.2005): 2770–89. http://dx.doi.org/10.1175/jas3506.1.
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Abstract Recent observations of the tropical atmosphere reveal large variations of water vapor and clouds at intraseasonal time scales. This study investigates the role of these variations in the large-scale organization of the tropical atmosphere, and in intraseasonal variability in particular. For this purpose, the influence of feedbacks between moisture (water vapor, clouds), radiation, and convection that affect the growth rate and the phase speed of unstable modes of the tropical atmosphere is investigated. Results from a simple linear model suggest that interactions between moisture and tropospheric radiative cooling, referred to as moist-radiative feedbacks, play a significant role in tropical intraseasonal variability. Their primary effect is to reduce the phase speed of large-scale tropical disturbances: by cooling the atmosphere less efficiently during the rising phase of the oscillations (when the atmosphere is moister) than during episodes of large-scale subsidence (when the atmosphere is drier), the atmospheric radiative heating reduces the effective stratification felt by propagating waves and slows down their propagation. In the presence of significant moist-radiative feedbacks, planetary disturbances are characterized by an approximately constant frequency. In addition, moist-radiative feedbacks excite small-scale disturbances advected by the mean flow. The interactions between moisture and convection exert a selective damping effect upon small-scale disturbances, thereby favoring large-scale propagating waves at the expense of small-scale advective disturbances. They also weaken the ability of radiative processes to slow down the propagation of planetary-scale disturbances. This study suggests that a deficient simulation of cloud radiative interactions or of convection-moisture interactions may explain some of the difficulties experienced by general circulation models in simulating tropical intraseasonal oscillations.
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Jiang, Xun, i Yuk L. Yung. "Global Patterns of Carbon Dioxide Variability from Satellite Observations". Annual Review of Earth and Planetary Sciences 47, nr 1 (30.05.2019): 225–45. http://dx.doi.org/10.1146/annurev-earth-053018-060447.
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Advanced satellite technology has been providing unique observations of global carbon dioxide (CO2) concentrations. These observations have revealed important CO2variability at different timescales and over regional and planetary scales. Satellite CO2retrievals have revealed that stratospheric sudden warming and the Madden-Julian Oscillation can modulate atmospheric CO2concentrations in the mid-troposphere. Atmospheric CO2also demonstrates variability at interannual timescales. In the tropical region, the El Niño–Southern Oscillation and the Tropospheric Biennial Oscillation can change atmospheric CO2concentrations. At high latitudes, mid-tropospheric CO2concentrations can be influenced by the Northern Hemispheric annular mode. In addition to modulations by the large-scale circulations, sporadic events such as wildfires, volcanic eruptions, and droughts, which change CO2surface emissions, can cause atmospheric CO2concentrations to increase significantly. The natural variability of CO2summarized in this review can help us better understand its sources and sinks and its redistribution by atmospheric motion. ▪ Global satellite CO2data offer a unique opportunity to explore CO2variability in different regions. ▪ Atmospheric CO2concentration demonstrates variations at intraseasonal, seasonal, and interannual timescales. ▪ Both large-scale circulations and variations of surface emissions can modulate CO2concentrations in the atmosphere.
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Gerber, Edwin P., i Geoffrey K. Vallis. "On the Zonal Structure of the North Atlantic Oscillation and Annular Modes". Journal of the Atmospheric Sciences 66, nr 2 (1.02.2009): 332–52. http://dx.doi.org/10.1175/2008jas2682.1.
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Abstract The zonal structure and dynamics of the dipolar patterns of intraseasonal variability in the extratropical atmosphere—namely, the North Atlantic Oscillation (NAO) and the so-called annular modes of variability—are investigated in an idealized general circulation model. Particular attention is focused on the relationships linking the zonal structure of the stationary waves, synoptic variability (i.e., the storm tracks), and the zonal structure of the patterns of intraseasonal variability. Large-scale topography and diabatic anomalies are introduced to modify and concentrate the synoptic variability, establishing a recipe for a localized storm track. Comparison of the large-scale forcing, synoptic variability, and patterns of intraseasonal variability suggests a nonlinear relationship between the large-scale forcing and the variability. It is found that localized NAO-like patterns arise from the confluence of topographic and diabatic forcing and that the patterns are more localized than one would expect based on superposition of the responses to topography and thermal forcing alone. The connection between the eddy life cycle of growth and decay and the localization of the intraseasonal variability is investigated. Both the termination of the storm track and the localization of the intraseasonal variability in the GCM depend on a difluent region of weak upper-level flow, where eddies break and dissipate rather than propagate energy forward through downstream development. The authors' interpretation suggests that the North Atlantic storm track and the NAO are two manifestations of the same phenomenon. Conclusions from the GCM study are critiqued by comparison with observations.
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Bernie, D. J., S. J. Woolnough, J. M. Slingo i E. Guilyardi. "Modeling Diurnal and Intraseasonal Variability of the Ocean Mixed Layer". Journal of Climate 18, nr 8 (15.04.2005): 1190–202. http://dx.doi.org/10.1175/jcli3319.1.
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Abstract The intraseasonal variability of SST associated with the passage of the Madden–Julian oscillation (MJO) is well documented; yet coupled model integrations generally underpredict the magnitude of this SST variability. Observations from the Improved Meteorological Instrument (IMET) mooring in the western Pacific during the intensive observing period (IOP) of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) showed a large diurnal signal in SST that is modulated by the passage of the MJO. In this study, observations from the IOP of the TOGA COARE and a one-dimensional (1D) ocean mixed layer model incorporating the K-Profile Parameterization (KPP) vertical mixing scheme have been used to investigate the rectification of the intraseasonal variability of SST by the diurnal cycle and the implied impact of the absence of a representation of this process on the modeled intraseasonal variability in coupled GCMs. Analysis of the SST observations has shown that the increase of the daily mean SST by the diurnal cycle of SST accounts for about one-third of the magnitude of intraseasonal variability of SST associated with the Madden–Julian oscillation in the western Pacific warm pool. Experiments from the 1D model forced with fluxes at a range of temporal resolutions and with differing vertical resolution of the model have shown that to capture 90% of the diurnal variability of SST, and hence 95% of the intraseasonal variability of SST, requires a 3-h or better temporal resolution of the fluxes and a vertical grid with an upper-layer thickness of the order of 1 m. In addition to the impact of the representation of the diurnal cycle on the intraseasonal variability of SST, the strength of the mixing across the thermocline was found to be enhanced by the proper representation of the nighttime deep mixing in the ocean, implying a possible impact of the diurnal cycle onto the mean climate of the tropical ocean.
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Subramanian, Aneesh C., Markus Jochum, Arthur J. Miller, Raghu Murtugudde, Richard B. Neale i Duane E. Waliser. "The Madden–Julian Oscillation in CCSM4". Journal of Climate 24, nr 24 (15.12.2011): 6261–82. http://dx.doi.org/10.1175/jcli-d-11-00031.1.
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Abstract This study assesses the ability of the Community Climate System Model, version 4 (CCSM4) to represent the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical atmosphere. The U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group’s prescribed diagnostic tests are used to evaluate the model’s mean state, variance, and wavenumber–frequency characteristics in a 20-yr simulation of the intraseasonal variability in zonal winds at 850 hPa (U850) and 200 hPa (U200), and outgoing longwave radiation (OLR). Unlike its predecessor, CCSM4 reproduces a number of aspects of MJO behavior more realistically. The CCSM4 produces coherent, broadbanded, and energetic patterns in eastward-propagating intraseasonal zonal winds and OLR in the tropical Indian and Pacific Oceans that are generally consistent with MJO characteristics. Strong peaks occur in power spectra and coherence spectra with periods between 20 and 100 days and zonal wavenumbers between 1 and 3. Model MJOs, however, tend to be more broadbanded in frequency than in observations. Broad-scale patterns, as revealed in combined EOFs of U850, U200, and OLR, are remarkably consistent with observations and indicate that large-scale convergence–convection coupling occurs in the simulated MJO. Relations between MJO in the model and its concurrence with other climate states are also explored. MJO activity (defined as the percentage of time the MJO index exceeds 1.5) is enhanced during El Niño events compared to La Niña events, both in the model and observations. MJO activity is increased during periods of anomalously strong negative meridional wind shear in the Asian monsoon region and also during strong negative Indian Ocean zonal mode states, in both the model and observations.
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Rajendran, K., i A. Kitoh. "Modulation of Tropical Intraseasonal Oscillations by Ocean–Atmosphere Coupling". Journal of Climate 19, nr 3 (1.02.2006): 366–91. http://dx.doi.org/10.1175/jcli3638.1.
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Abstract The impact of ocean–atmosphere coupling on the structure and propagation characteristics of 30–60-day tropical intraseasonal oscillations (TISOs) is investigated by analyzing long-term simulations of the Meteorological Research Institute coupled general circulation model (CGCM) and its stand-alone atmospheric general circulation model (AGCM) version forced with SSTs derived from the CGCM and comparing them with recent observation datasets [Global Precipitation Climatology Project (GPCP) precipitation, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), and Reynolds SST]. Composite events of (i) eastward propagating Madden–Julian oscillations (MJOs) during boreal winter and (ii) northward propagating intraseasonal oscillations (NPISOs) during boreal summer, constructed based on objective criteria, show that the three-dimensional structure, amplitude, and speed of propagation, and the phase relationship among surface fluxes, SST, and convection, are markedly improved in the CGCM simulation. Consistent with the frictional wave conditional instability of the second kind mechanism, successive development of low-level convergence to the east (north) of deep convection was found to be important for eastward (northward) propagation of MJO (NPISO). Complex interaction between large-scale dynamics and convection reveals the importance of atmospheric dynamics and suggests that they are intrinsic modes in the atmosphere where coupling is not essential for their existence. However, as in observations, realistic coupling in the CGCM is found to result in the evolution of TISOs as coupled modes through a coherent coupled feedback process. This acts as an amplifying mechanism for the existing propagating convective anomalies and plays an important modifying role toward a more realistic simulation of TISOs. In contrast, the simulated TISOs in its atmosphere-alone component lack many of the important features associated with their amplitude, phase, and life cycle. Thus, a realistic representation of the interaction between sea surface and the atmospheric boundary layer is crucial for a better simulation of TISOs.
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Wang, Wanqiu, Mingyue Chen i Arun Kumar. "Impacts of Ocean Surface on the Northward Propagation of the Boreal Summer Intraseasonal Oscillation in the NCEP Climate Forecast System". Journal of Climate 22, nr 24 (15.12.2009): 6561–76. http://dx.doi.org/10.1175/2009jcli3007.1.
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Abstract Impacts of the ocean surface on the representation of the northward-propagating boreal summer intraseasonal oscillation (NPBSISO) over the Indian monsoon region are analyzed using the National Centers for Environmental Prediction (NCEP) coupled atmosphere–ocean Climate Forecast System (CFS) and its atmospheric component, the NCEP Global Forecast System (GFS). Analyses are based on forecasts of five strong NPBSISO events during June–September 2005–07. The inclusion of an interactive ocean in the model is found to be necessary to maintain the observed NPBSISO. The atmosphere-only GFS is capable of maintaining the convection that propagates from the equator to 12°N with reasonable amplitude within the first 15 days, after which the anomalies become very weak, suggesting that the atmospheric internal dynamics alone are not sufficient to sustain the anomalies to propagate to higher latitudes. Forecasts of the NPBSISO in the CFS are more realistic, with the amplitude of precipitation and 850-mb zonal wind anomalies comparable to that in observations for the entire 30-day target period, but with slower northward propagation compared to that observed. Further, the phase relationship between precipitation, sea surface temperature (SST), and surface latent heat fluxes associated with the NPBSISO in the CFS is similar to that in the observations, with positive precipitation anomalies following warm SST anomalies, which are further led by positive anomalies of the surface latent heat and solar radiation fluxes into the ocean. Additional experiments with the atmosphere-only GFS are performed to examine the impacts of uncertainties in SSTs. It is found that intraseasonal SST anomalies 2–3 times as large as that of the observational bulk SST analysis of Reynolds et al. are needed for the GFS to produce realistic northward propagation of the NPBSISO with reasonable amplitude and to capture the observed phase lag between SST and precipitation. The analysis of the forecasts and the experiments suggests that a realistic representation of the observed propagation of the oscillation by the NCEP model requires not only an interactive ocean but also an intraseasonal SST variability stronger than that of the bulk SST analysis.
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Bürger, Gerd. "Intraseasonal Oscillation Indices from Complex EOFs". Journal of Climate 34, nr 1 (styczeń 2021): 107–22. http://dx.doi.org/10.1175/jcli-d-20-0427.1.
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AbstractIndices of oscillatory behavior are conveniently obtained by projecting the fields in question into a phase space of a few (mostly just two) dimensions; empirical orthogonal functions (EOFs) or other, more dynamical, modes are typically used for the projection. If sufficiently coherent and in quadrature, the projected variables simply describe a rotating vector in the phase space, which then serves as the basis for predictions. Using the boreal summer intraseasonal oscillation (BSISO) as a test case, an alternative procedure is introduced: it augments the original fields with their Hilbert transform (HT) to form a complex series and projects it onto its (single) dominant EOF. The real and imaginary parts of the corresponding complex pattern and index are compared with those of the original (real) EOF. The new index explains slightly less variance of the physical fields than the original, but it is much more coherent, partly from its use of future information by the HT. Because the latter is in the way of real-time monitoring, the index can only be used in cases with predicted physical fields, for which it promises to be superior. By developing a causal approximation of the HT, a real-time variant of the index is obtained whose coherency is comparable to the noncausal version, but with smaller explained variance of the physical fields. In test cases the new index compares well to other indices of BSISO. The potential for using both indices as an alternative is discussed.
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Lebsock, Matthew D., Christian Kummerow i Graeme L. Stephens. "An Observed Tropical Oceanic Radiative–Convective Cloud Feedback". Journal of Climate 23, nr 8 (15.04.2010): 2065–78. http://dx.doi.org/10.1175/2009jcli3091.1.
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Abstract Anomalies of precipitation, cloud, thermodynamic, and radiation variables are analyzed on the large spatial scale defined by the tropical oceans. In particular, relationships between the mean tropical oceanic precipitation anomaly and radiative anomalies are examined. It is found that tropical mean precipitation is well correlated with cloud properties and radiative fields. In particular, the tropical mean precipitation anomaly is positively correlated with the top of the atmosphere reflected shortwave anomaly and negatively correlated with the emitted longwave anomaly. The tropical mean relationships are found to primarily result from a coherent oscillation of precipitation and the area of high-level cloudiness. The correlations manifest themselves radiatively as a modest decrease in net downwelling radiation at the top of the atmosphere, and a redistribution of energy from the surface to the atmosphere through reduced solar radiation to the surface and decreased longwave emission to space. Integrated over the tropical oceanic domain, the anomalous atmospheric column radiative heating is found to be about 10% of the magnitude of the anomalous latent heating. The temporal signature of the radiative heating is observed in the column mean temperature that indicates a coherent phase-lagged oscillation between atmospheric stability and convection. These relationships are identified as a radiative–convective cloud feedback that is observed on intraseasonal time scales in the tropical atmosphere.
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Gualdi, Silvio, Antonio Navarra i Martin Fischer. "The tropical intraseasonal oscillation in a coupled ocean-atmosphere general circulation model". Geophysical Research Letters 26, nr 19 (1.10.1999): 2973–76. http://dx.doi.org/10.1029/1999gl010414.
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Inness, P. M., i D. Gregory. "Aspects of the intraseasonal oscillation simulated by the Hadley Centre Atmosphere Model". Climate Dynamics 13, nr 6 (21.07.1997): 441–58. http://dx.doi.org/10.1007/s003820050176.
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Fu, Xiouhua, i Bin Wang. "The Boreal-Summer Intraseasonal Oscillations Simulated in a Hybrid Coupled Atmosphere–Ocean Model*". Monthly Weather Review 132, nr 11 (1.11.2004): 2628–49. http://dx.doi.org/10.1175/mwr2811.1.
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Abstract The boreal-summer intraseasonal oscillation (BSISO) simulated by an atmosphere–ocean coupled model is validated with the long-term observations [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall, ECMWF analysis, and Reynolds' SST]. This validation focuses on the three-dimensional water vapor cycle associated with the BSISO and its interaction with underlying sea surface. The advantages of a coupled approach over stand-alone atmospheric approaches on the simulation of the BSISO are revealed through an intercomparison between a coupled run and two atmosphere-only runs. This coupled model produces a BSISO that mimics the one presented in the observations over the Asia– western Pacific region. The similarities with the observations include 1) the coherent spatiotemporal evolutions of rainfall, surface winds, and SST associated with the BSISO; 2) the intensity and period (or speed) of the northward-propagating BSISO; and 3) the tropospheric moistening (or drying) and overturning circulations of the BSISO. However, the simulated tropospheric moisture fluctuations in the extreme phases (both wet and dry) are larger than those in the ECMWF analysis. The simulated sea surface cooling during the wet phase is weaker than the observed cooling. Better representations of the interaction between convection and boundary layer in the GCM and including salinity effects in the ocean model are expected to further improve the simulation of the BSISO. The intercomparison between a coupled run and two atmospheric runs suggests that the air–sea coupled system is the ultimate tool needed to realistically simulate the BSISO. Though the major characteristics of the BSISO are very likely determined by the internal atmospheric dynamics, the correct interaction between the internal dynamics and underlying sea surface can only be sustained by a coupled system. The atmosphere-only approach, when forced with high-frequency (e.g., daily) SST, introduces an erroneous boundary interference on the internal dynamics associated with the BSISO. The implications for the predictability of the BSISO are discussed.
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Frederiksen, Jorgen S., i Hai Lin. "Tropical–Extratropical Interactions of Intraseasonal Oscillations". Journal of the Atmospheric Sciences 70, nr 10 (1.10.2013): 3180–97. http://dx.doi.org/10.1175/jas-d-12-0302.1.
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Abstract Tropical–extratropical interactions of intraseasonal oscillations (ISOs), based on 30 years (1979–2009) of northern winter observations and theory, are compared. The phase relationships between the tropical signal of the leading theoretical ISO for a January 1979 basic state and the development of Pacific–North America (PNA)-like and North Atlantic Oscillation (NAO) teleconnection patterns are found to compare closely with those for the observed Madden–Julian oscillation (MJO). For both observations and theory positive NAO occurs 5–15 days after MJO convection [negative outgoing longwave radiation (OLR) and positive precipitation] and negative upper-troposphere velocity potential ISO anomalies are focused over the central Indian Ocean. The fluxes of wave activity, based on the upper-troposphere streamfunction of the leading theoretical mode, indicate strong tropical–extratropical interactions and have very similar structures to those obtained by H. Lin et al. based on observations of extratropical anomalies associated with MJO convection. The second leading theoretical ISO mode for January 1979 has quite similar properties to the leading ISO mode but has a longer period of 44.5 days compared with 34.4 days and a more distinct quadrupole streamfunction structure straddling the equator. Theoretical ISO modes for other observed basic states, including January 1988 and the 30-yr average of January 1980–2009, again link the tropical ISO signal with Northern Hemisphere teleconnection patterns, particularly the NAO. The growth rates of ISO modes increase with stronger baroclinicity of the basic-state zonal winds in the main jet streams and, importantly, with increased tropical–extratropical interactions because of stronger meridional winds.
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Maloney, Eric D. "The Moist Static Energy Budget of a Composite Tropical Intraseasonal Oscillation in a Climate Model". Journal of Climate 22, nr 3 (1.02.2009): 711–29. http://dx.doi.org/10.1175/2008jcli2542.1.
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Abstract The intraseasonal moist static energy (MSE) budget is analyzed in a climate model that produces realistic eastward-propagating tropical intraseasonal wind and precipitation variability. Consistent with the recharge–discharge paradigm for tropical intraseasonal variability, a buildup of column-integrated MSE occurs within low-level easterly anomalies in advance of intraseasonal precipitation, and a discharge of MSE occurs during and after precipitation when westerly anomalies occur. The strongest MSE anomalies peak in the lower troposphere and are, primarily, regulated by specific humidity anomalies. The leading terms in the column-integrated intraseasonal MSE budget are horizontal advection and surface latent heat flux, where latent heat flux is dominated by the wind-driven component. Horizontal advection causes recharge (discharge) of MSE within regions of anomalous equatorial lower-tropospheric easterly (westerly) anomalies, with the meridional component of the moisture advection dominating the MSE budget near 850 hPa. Latent heat flux anomalies oppose the MSE tendency due to horizontal advection, making the recharge and discharge of column MSE more gradual than if horizontal advection were acting alone. This relationship has consequences for the time scale of intraseasonal variability in the model. Eddies dominate intraseasonal meridional moisture advection in the model. During periods of low-level intraseasonal easterly anomalies, eddy kinetic energy (EKE) is anomalously low due to a suppression of tropical synoptic-scale disturbances and other variability on time scales shorter than 20 days. Anomalous moistening of the equatorial lower troposphere occurs during intraseasonal easterly periods through suppression of eddy moisture advection between the equator and poleward latitudes. During intraseasonal westerly periods, EKE is enhanced, leading to anomalous drying of the equatorial lower troposphere through meridional advection. Given the importance of meridional moisture advection and wind-induced latent heat flux to the intraseasonal MSE budget, these findings suggest that to simulate realistic intraseasonal variability, climate models must have realistic basic-state distributions of lower-tropospheric zonal wind and specific humidity.
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Zhao, Haikun, Ying Lu, Xianan Jiang, Philip J. Klotzbach, Liguang Wu i Jian Cao. "A Statistical Intraseasonal Prediction Model of Extended Boreal Summer Western North Pacific Tropical Cyclone Genesis". Journal of Climate 35, nr 8 (15.04.2022): 2459–78. http://dx.doi.org/10.1175/jcli-d-21-0110.1.
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Abstract An L2 regularized logistic regression model is developed in this study to predict weekly tropical cyclone (TC) genesis over the western North Pacific (WNP) Ocean and subregions of the WNP including the South China Sea (SCS), the western WNP (WWNP), and the eastern WNP (EWNP). The potential predictors for the TC genesis model include a time-varying TC genesis climatology, the Madden–Julian oscillation (MJO), the quasi-biweekly oscillation (QBWO), and ENSO. The relative importance of the predictors in a constructed L2 regression model is justified by a forward stepwise selection procedure for each region from a 0-week to a 7-week lead. Cross-validated hindcasts are then generated for the corresponding prediction schemes out to a 7-week lead. The TC genesis climatology generally improves the regional model skill, and the importance of intraseasonal oscillations and ENSO is regionally dependent. Over the WNP, there is increased model skill over the time-varying climatology in predicting weekly TC genesis out to a 4-week lead by including the MJO and QBWO, whereas ENSO has a limited impact. On a regional scale, ENSO and then either the MJO or QBWO are the two most important predictors over the EWNP and WWNP after the TC genesis climatology. The MJO is found to be the most important predictor over the SCS. The logistic regression model is shown to have comparable reliability and forecast skill scores to the ECMWF dynamical model on intraseasonal time scales. Significance Statement Skillful forecasts of tropical cyclone activity on time scales from short-range to seasonal are now issued operationally. Although there has been great progress in the understanding of physical mechanisms driving tropical cyclone (TC) activity, intraseasonal prediction of TCs remains a significant scientific challenge. This study develops a statistically based intraseasonal model to predict weekly TC genesis over the western North Pacific Ocean basin. The intraseasonal prediction model developed here for TC genesis over the western North Pacific basin shows skill extending out to four weeks. We discuss the regional dependence of the model skill on ENSO and other subseasonal climate oscillations. This approach provides skillful intraseasonal forecasting of TCs over the western North Pacific basin.
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Martin, Elinor R., i Courtney Schumacher. "Modulation of Caribbean Precipitation by the Madden–Julian Oscillation". Journal of Climate 24, nr 3 (1.02.2011): 813–24. http://dx.doi.org/10.1175/2010jcli3773.1.
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Abstract Based on 12 years of daily satellite precipitation data and reanalysis winds, intraseasonal (30–90 days) variability in Caribbean precipitation is linked to phases of the Madden–Julian oscillation (MJO). Intraseasonal variability is largest during September–November (SON), but some modulation of precipitation by the MJO appears throughout all seasons. Precipitation anomalies up to 50% above (below) the annual mean are observed in phases 1 and 2 (5 and 6) of the MJO. The changes in Caribbean precipitation associated with the MJO are shown to be related to changes in the low-level (925 hPa) winds. When precipitation anomalies are above (below) average in phases 1 and 2 (5 and 6), wind anomalies act to decrease (increase) the strength of the prevailing easterly trade winds. The changes in the low-level winds are most apparent in the region of the Caribbean low-level jet (CLLJ), and divergence anomalies associated with the entrance and exit regions of the CLLJ precede the precipitation anomalies. The CLLJ itself is also shown to be subject to intraseasonal variability, and its magnitude varies with the phase of the MJO. Again, intraseasonal variability in the CLLJ associated with the MJO is observed in all seasons and shows a significant coherence with intraseasonal variability in the precipitation. Extreme rainfall events over islands in the Caribbean show a strong relationship with the MJO phase, with extreme events being most common in phases 1 and 2 of an MJO event. This relationship between the MJO and extreme events has important implications for the predictability of precipitation extremes in the Caribbean.
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Benedict, James J., i David A. Randall. "Impacts of Idealized Air–Sea Coupling on Madden–Julian Oscillation Structure in the Superparameterized CAM". Journal of the Atmospheric Sciences 68, nr 9 (1.09.2011): 1990–2008. http://dx.doi.org/10.1175/jas-d-11-04.1.
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Abstract Air–sea interactions and their impact on intraseasonal convective organization are investigated by comparing two 5-yr simulations from the superparameterized Community Atmosphere Model version 3.0 (SP-CAM). The first is forced using prescribed sea surface temperatures (SSTs). The second is identical except that a simplified oceanic mixed-layer model is used to predict tropical SST anomalies that are coupled to the atmosphere. This partially coupled simulation allows SSTs to respond to anomalous surface fluxes. Implementation of the idealized slab ocean model in the SP-CAM results in significant changes to intraseasonal convective variability and organization. The more realistic treatment of air–sea interactions in the coupled simulation improves many aspects of tropical convection on intraseasonal scales, from the relationships between precipitation and SSTs to the space–time structure and propagation of the Madden–Julian oscillation (MJO). This improvement is associated with a more realistic convergence structure and longitudinal gradient of SST relative to MJO deep convection. In the uncoupled SP-CAM, SST is roughly in phase with the MJO convective center and the development of the Kelvin wave response and boundary layer convergence east of the convective center is relatively weak. In the coupled SP-CAM, maxima in SST lead maxima in MJO convection by cycle. Coupling produces warmer SSTs, a stronger Kelvin wave response, enhanced low-level convergence, and increased convective heating ahead (east) of the MJO convective center. Convective development east of the MJO precipitation center is more favorable in the coupled versus the uncoupled version, resulting in more realistic organization and clearer eastward propagation of the MJO in the coupled SP-CAM.
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Zhang, Pengfei, Guoping Li, Xiouhua Fu, Yimin Liu i Laifang Li. "Clustering of Tibetan Plateau Vortices by 10–30-Day Intraseasonal Oscillation*". Monthly Weather Review 142, nr 1 (1.01.2014): 290–300. http://dx.doi.org/10.1175/mwr-d-13-00137.1.
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Abstract Tibetan Plateau (TP) vortices and the related 10–30-day intraseasonal oscillation in May–September 1998 are analyzed using the twice-daily 500-hPa synoptic weather maps, multiple reanalysis datasets, and satellite-retrieved brightness temperature. During the analysis period, distinctively active and suppressed periods of TP vortices genesis are noticed. In 1998, nine active periods of TP vortices occurred, which were largely clustered by the cyclonic circulations associated with the intraseasonal oscillation of 500-hPa relative vorticity. In addition to the well-recognized 30–60-day oscillation, the clustering of TP vorticity in the 1998 summer are more likely modulated by the 10–30-day oscillation, because all active periods of TP vortices fall into the positive phase of the 10–30-day oscillation in 1998. Even in the negative (i.e., anticyclonic) phases of the 30–60-day oscillation, the positive (i.e., cyclonic) 500-hPa 10–30-day oscillation can excite the clustering of TP vortices. This result indicates that the 10–30-day oscillation more directly modulates the activities of TP vortices by providing a favorable (unfavorable) cyclonic (anticyclonic) environment. The analysis of the 10–30-day atmospheric oscillation suggests that the westerly trough disturbances, in conjunction with convective instability due to low-level warm advection from the Indian monsoon region, are important in the clustering of TP vortex activities. In particular, the moisture flux from the southwest boundary of TP is essential to the accumulation of convective energy. Thus, a better understanding and prediction of the 10–30-day intraseasonal oscillation is needed to advance the extended-range forecasting of TP vortices and their downstream impacts on the weather and climate over East Asia.