An investigation of tropical Atlantic bias in a high-resolution coupled regional climate model

Coupled atmosphere–ocean general circulation models (AOGCMs) commonly fail to simulate the eastern equatorial Atlantic boreal summer cold tongue and produce a westerly equatorial trade wind bias. This tropical Atlantic bias problem is investigated with a high-resolution (27-km atmosphere represented by the Weather Research and Forecasting Model, 9-km ocean represented by the Regional Ocean Modeling System) coupled regional climate model. Uncoupled atmospheric simulations test climate sensitivity to cumulus, land-surface, planetary boundary layer, microphysics, and radiation parameterizations and reveal that the radiation scheme has a pronounced impact in the tropical Atlantic. The CAM radiation simulates a dry precipitation (up to ?90%) and cold land-surface temperature (up to ?8?K) bias over the Amazon related to an over-representation of low-level clouds and almost basin-wide westerly trade wind bias. The Rapid Radiative Transfer Model and Goddard radiation simulates doubled Amazon and Congo Basin precipitation rates and a weak eastern Atlantic trade wind bias. Season-long high-resolution coupled regional model experiments indicate that the initiation of the warm eastern equatorial Atlantic sea surface temperature (SST) bias is more sensitive to the local rather than basin-wide trade wind bias and to a wet Congo Basin instead of dry Amazon—which differs from AOGCM simulations. Comparisons between coupled and uncoupled simulations suggest a regional Bjerknes feedback confined to the eastern equatorial Atlantic amplifies the initial SST, wind, and deepened thermocline bias, while barrier layer feedbacks are relatively unimportant. The SST bias in some CRCM simulations resembles the typical AOGCM bias indicating that increasing resolution is unlikely a simple solution to this problem.     

What controls equatorial Atlantic winds in boreal spring?

The factors controlling equatorial Atlantic winds in boreal spring are examined using both observations and general circulation model (GCM) simulations from the coupled model intercomparison phase 5. The results show that the prevailing surface easterlies flow against the attendant pressure gradient and must therefore be maintained by other terms in the momentum budget. An important contribution comes from meridional advection of zonal momentum but the dominant contribution is the vertical transport of zonal momentum from the free troposphere to the surface. This implies that surface winds are strongly influenced by conditions in the free troposphere, chiefly pressure gradients and, to a lesser extent, meridional advection. Both factors are linked to the patterns of deep convection. Applying these findings to GCM errors indicates, that, consistent with the results of previous studies, the persistent westerly surface wind bias found in most GCMs is due mostly to precipitation errors, in particular excessive precipitation south of the equator over the ocean and deficient precipitation over equatorial South America. Free tropospheric influences also dominate the interannual variability of surface winds in boreal spring. GCM experiments with prescribed climatological sea-surface temperatures (SSTs) indicate that the free tropospheric influences are mostly associated with internal atmospheric variability. Since the surface wind anomalies in boreal spring are crucial to the development of warm SST events (Atlantic Niños), the results imply that interannual variability in the region may rely far less on coupled air–sea feedbacks than is the case in the tropical Pacific.     

Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation

Most coupled general circulation models (GCMs) perform poorly in the tropical Atlantic in terms of climatological seasonal cycle and interannual variability. The reasons for this poor performance are investigated in a suite of sensitivity experiments with the Geophysical Fluid Dynamics Laboratory (GFDL) coupled GCM. The experiments show that a significant portion of the equatorial SST biases in the model is due to weaker than observed equatorial easterlies during boreal spring. Due to these weak easterlies, the tilt of the equatorial thermocline is reduced, with shoaling in the west and deepening in the east. The erroneously deep thermocline in the east prevents cold tongue formation in the following season despite vigorous upwelling, thus inhibiting the Bjerknes feedback. It is further shown that the surface wind errors are due, in part, to deficient precipitation over equatorial South America and excessive precipitation over equatorial Africa, which already exist in the uncoupled atmospheric GCM. Additional tests indicate that the precipitation biases are highly sensitive to land surface conditions such as albedo and soil moisture. This suggests that improving the representation of land surface processes in GCMs offers a way of improving their performance in the tropical Atlantic. The weaker than observed equatorial easterlies also contribute remotely, via equatorial and coastal Kelvin waves, to the severe warm SST biases along the southwest African coast. However, the strength of the subtropical anticyclone and along-shore winds also play an important role.     

Influence of the warm pool and cold tongue El Niños on the following Caribbean rainy season rainfall

Caribbean rainfall and associated regional-scale ocean–atmosphere anomalies are analyzed during and after warm pool (WP) and cold tongue (CT) El Niño (EN) events (i.e. from the usual peak of EN events in boreal winter to next summer from 1950 to 2011). During and after a CT event, a north–south dipolar pattern with positive (negative) rainfall anomalies over the northern (southern) Caribbean during the boreal winter tends to reverse in spring, and then to vanish in summer. On the contrary, during and after a WP event, weak rainfall anomalies during the boreal winter intensify themselves from spring, with anomalous wet conditions over most of the Caribbean basin observed during summer, except over the eastern coast of Nicaragua and Costa Rica. The Caribbean rainfall anomalies associated with WP and CT events are shaped by competition between at least four different, but interrelated, mechanisms; (1) the near-equatorial large-scale subsidence anomaly over the equatorial Atlantic linked to the zonal adjustment of the Walker circulation; (2) the extra-tropical wave-like train combining positive phase of the Pacific/North American mode and negative phase of the North Atlantic Oscillation; (3) the wind-evaporation-sea surface temperature (SST) positive feedback coupling warmer-than-normal SST with weaker-than-normal low level easterlies over the tropical North Atlantic; and (4) the air-sea coupling between the speed of low level easterlies, including the Caribbean low level jet, and the SST anomaly (SSTA) gradient between the Caribbean basin and the eastern equatorial Pacific. It seems that Caribbean rainfall anomalies are shaped mostly by mechanisms (1–3) during CT events from the boreal winter to spring. These mechanisms seem less efficient during WP events when the atmospheric response seems driven mostly by mechanism (4), coupling positive west-east SSTA gradient with weaker-than-normal low level easterlies, and secondary by mechanism (3), from the boreal spring to summer.     

春季热带南大西洋海表温度异常与夏季亚澳季风区降水异常的联系

利用1979—2019年Hadley中心的海表温度资料、GPCP的降水资料以及NCEP-DOE的再分析资料等,分析了北半球春季热带南大西洋海表温度异常与北半球夏季亚澳季风区降水异常的联系。研究表明,北半球春季热带南大西洋海表温度异常与随后夏季热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区的降水异常为显著负相关（正相关）关系。北半球春季热带南大西洋的海表温度正异常可以引起热带大西洋和热带太平洋间的异常垂直环流,其中异常上升支（下沉支）位于热带大西洋（热带中太平洋）。热带中太平洋的异常下沉气流和低层辐散气流引起热带中西太平洋低层的异常东风,后者有利于热带中东太平洋海表温度出现负异常。通过Bjerknes正反馈机制,热带中东太平洋海表温度异常从北半球春季到夏季得到发展。热带中东太平洋海表温度负异常激发的Rossby波使得北半球夏季热带西太平洋低层出现一对异常反气旋。此时,850 hPa上热带西太平洋到海洋性大陆地区为显著的异常东风,有利于热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区出现异常的水汽辐散（辐合）,导致该地区降水减少（增加）。     

Sensitivity of tropical Atlantic climate to mixing in a coupled ocean–atmosphere model

The sensitivity of tropical Atlantic climate to upper ocean mixing is investigated using an ocean-only model and a coupled ocean–atmosphere model. The upper ocean thermal structure and associated atmospheric circulation prove to be strongly related to the strength of upper ocean mixing. Using the heat balance in the mixed layer it is shown that an excessively cold equatorial cold tongue can be attributed to entrainment flux at the base of the oceanic mixed layer, that is too large. Enhanced entrainment efficiency acts to deepen the mixed layer and causes strong reduction in the upper ocean divergence in the central equatorial Atlantic. As a result, the simulated sea surface temperature, thermocline structure, and upwelling velocities are close to the observed estimates. In the coupled model, the seasonal migration of the Intertropical Convergence Zone (ITCZ) reduces when the entrainment efficiency in the oceanic mixed layer is enhanced. The precipitation rates decrease in the equatorial region and increase along 10°N, resulting in a more realistic Atlantic Marine ITCZ. The reduced meridional surface temperature gradient in the eastern tropical Atlantic prohibits the development of convective precipitation in the southeastern part of the tropical Atlantic. Also, the simulation of tropical Atlantic variability as expressed in the meridional gradient mode and the eastern cold tongue mode improves when the entrainment efficiency is enhanced.     

Remotely forced variability in the tropical Atlantic Ocean

An ensemble of eight hindcasts has been conducted using an ocean-atmosphere general circulation model fully coupled only within the Atlantic basin, with prescribed observational sea surface temperature (SST) for 1950–1998 in the global ocean outside the Atlantic basin. The purpose of these experiments is to understand the influence of the external SST anomalies on the interannual variability in the tropical Atlantic Ocean. Statistical methods, including empirical orthogonal function analysis with maximized signal-to-noise ratio, have been used to extract the remotely forced Atlantic signals from the ensemble of simulations. It is found that the leading external source on the interannual time scales is the El Niño/Southern Oscillation (ENSO) in the Pacific Ocean. The ENSO signal in the tropical Atlantic shows a distinct progression from season to season. During the boreal winter of a maturing El Niño event, the model shows a major warm center in the southern subtropical Atlantic together with warm anomalies in the northern subtropical Atlantic. The southern subtropical SST anomalies is caused by a weakening of the southeast trade winds, which are partly associated with the influence of an atmospheric wave train generated in the western Pacific Ocean and propagating into the Atlantic basin in the Southern Hemisphere during boreal fall. In the boreal spring, the northern tropical Atlantic Ocean is warmed up by a weakening of the northeast trade winds, which is also associated with a wave train generated in the central tropical Pacific during the winter season of an El Niño event. Apart from the atmospheric planetary waves, these SST anomalies are also related to the sea level pressure (SLP) increase in the eastern tropical Atlantic due to the global adjustment to the maturing El Niño in the tropical Pacific. The tropical SLP anomalies are further enhanced in boreal spring, which induce anomalous easterlies on and to the south of the equator and lead to a dynamical oceanic response that causes cold SST anomalies in the eastern and equatorial Atlantic from boreal spring to summer. Most of these SST anomalies persist into the boreal fall season.

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The Equatorial Undercurrent in the central Atlantic and its relation to tropical Atlantic variability

Seasonal to interannual variations of the Equatorial Undercurrent (EUC) in the central Atlantic at 23°W are studied using shipboard observation taken during the period 1999–2011 as well as moored velocity time series covering the period May 2005–June 2011. The seasonal variations are dominated by an annual harmonic of the EUC transport and the EUC core depth (both at maximum during September), and a semiannual harmonic of the EUC core velocity (maximum during April and September). Substantial interannual variability during the period of moored observation included anomalous cold/warm equatorial Atlantic cold tongue events during 2005/2008. The easterly winds in the western equatorial Atlantic during boreal spring that represent the preconditioning of cold/warm events were strong/weak during 2005/2008 and associated with strong/weak boreal summer EUC transport. The anomalous year 2009 was instead associated with weak preconditioning and smallest EUC transport on record from January to July, but during August coldest SST anomalies in the eastern equatorial Atlantic were observed. The interannual variations of the EUC are discussed with respect to recently described variability of the tropical Atlantic Ocean.     

An overview of the performance of CMIP6 models in the tropical Atlantic: mean state,variability, and remote impacts

General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic and its linkage to the tropical Pacific. While, on average, mean state biases have improved little, relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement.

     

Capturing the Atlantic cold tongue and coastal upwelling in an intermediate-level ocean model coupled to a regional climate model

Coupled atmosphere–ocean general circulation models are known to have difficulties simulating the cold tongue in the equatorial Atlantic Ocean. Here a regional climate model coupled to an intermediate-level mixed layer ocean model with Ekman dynamics is developed and used to better understand the seasonal evolution of the equatorial Atlantic cold tongue and upwelling off western Africa. Parameterization improvements are made to an earlier version of the ocean model to account for the variations in temperature and shearing stress at the base of the mixed layer. 90-km resolution sensitivity tests demonstrate that the development of the equatorial Atlantic cold tongue in the boreal spring/summer is captured only if seasonal variations in the temperature at the base of the ocean mixed layer are included. The development of cold temperatures off the northwest African coast in the late boreal winter/spring is found to be primarily associated with the net radiation balance as shortwave warming of the mixed layer is relatively low while latent cooling is relatively high yielding a net cooling of mixed layer temperatures, consistent with other studies. The westward extension of the Atlantic cold tongue is primarily due to the horizontal advection of cool water from the South Atlantic African coast. This coastal cooling is associated with vertical diffusion and vertical entrainment, while the vertical entrainment has a secondary and more localized role over the equatorial Atlantic.     

Teleconnected influence of North Atlantic sea surface temperature on the El Ni?o onset

Influence of North Atlantic sea surface temperature (SST) anomalies on tropical Pacific SST anomalies is examined. Both summer and winter North Atlantic SST anomalies are negatively related to central-eastern tropical Pacific SST anomalies in the subsequent months varying from 5 to 13?months. In particular, when the North Atlantic is colder than normal in the summer, an El Ni?o event is likely to be initiated in the subsequent spring in the tropical Pacific. Associated with summer cold North Atlantic SST anomalies is an anomalous cyclonic circulation at low-level over the North Atlantic from subsequent October to April. Corresponded to this local response, an SST-induced heating over the North Atlantic produces a teleconnected pattern, similar to the East Atlantic/West Russia teleconnection. The pattern features two anticyclonic circulations near England and Lake Baikal, and two cyclonic circulations over the North Atlantic and near the Caspian Sea. The anticyclonic circulation near Lake Baikal enhances the continent northerlies, and strengthens the East-Asian winter monsoon. These are also associated with an off-equatorial cyclonic circulation in the western Pacific during the subsequent winter and spring, which produces equatorial westerly wind anomalies in the western Pacific. The equatorial westerly wind anomalies in the winter and spring can help initiate a Pacific El Ni?o event following a cold North Atlantic in the summer.     

Evolution of model systematic errors in the Tropical Atlantic Basin from coupled climate hindcasts

Significant systematic errors in the tropical Atlantic Ocean are common in state-of-the-art coupled ocean–atmosphere general circulation models. In this study, a set of ensemble hindcasts from the NCEP coupled forecast system (CFS) is used to examine the initial growth of the coupled model bias. These CFS hindcasts are 9-month integrations starting from perturbed real-time oceanic and atmospheric analyses for 1981–2003. The large number of integrations from a variety of initial states covering all months provides a good opportunity to examine how the model systematic errors grow. The monthly climatologies of ensemble hindcasts from various initial months are compared with both observed and analyzed oceanic and atmospheric datasets. Our analyses show that two error patterns are dominant in the hindcasts. One is the warming of the sea surface temperature (SST) in the southeastern tropical Atlantic Ocean. This error grows faster in boreal summer and fall and peaks in November–December at round 2°C in the open ocean. It is caused by an excessive model surface shortwave radiative flux in this region, especially from boreal summer to fall. The excessive radiative forcing is in turn caused by the CFS inability to reproduce the observed amount of low cloud cover in the southeastern ocean and its seasonal increase. According to a comparison between the seasonal climatologies from the CFS hindcasts and a long-term simulation of the atmospheric model forced with observed SST, the CFS low cloud and radiation errors are inherent to its atmospheric component. On the other hand, the SST error in CFS is a major cause of the model’s southward bias of the intertropical convergence zone (ITCZ) in boreal winter and spring. An analysis of the SST errors of the 6-month ensemble hindcasts by seven coupled models in the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction project shows that this SST error pattern is common in coupled climate hindcasts. The second error pattern is an excessive deepening of the model thermocline depth to the north of the equator from the western coast toward the central ocean. This error grows fastest in boreal summer. It is forced by an overly strong local anticyclonic surface wind stress curl and is in turn related to the weakened northeast trade winds in summer and fall. The thermocline error in the northwest delays the annual shoaling of the equatorial thermocline in the Gulf of Guinea remotely through the equatorial waveguide.     

Equatorial Atlantic variability and its relation to mean state biases in CMIP5

Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15°N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.     

Impacts of Indian Ocean SST biases on the Indian Monsoon: as simulated in a global coupled model

In this study, the impact of the ocean–atmosphere coupling on the atmospheric mean state over the Indian Ocean and the Indian Summer Monsoon (ISM) is examined in the framework of the SINTEX-F2 coupled model through forced and coupled control simulations and several sensitivity coupled experiments. During boreal winter and spring, most of the Indian Ocean biases are common in forced and coupled simulations, suggesting that the errors originate from the atmospheric model, especially a dry islands bias in the Maritime Continent. During boreal summer, the air-sea coupling decreases the ISM rainfall over South India and the monsoon strength to realistic amplitude, but at the expense of important degradations of the rainfall and Sea Surface Temperature (SST) mean states in the Indian Ocean. Strong SST biases of opposite sign are observed over the western (WIO) and eastern (EIO) tropical Indian Ocean. Rainfall amounts over the ocean (land) are systematically higher (lower) in the northern hemisphere and the south equatorial Indian Ocean rainfall band is missing in the control coupled simulation. During boreal fall, positive dipole-like errors emerge in the mean state of the coupled model, with warm and wet (cold and dry) biases in the WIO (EIO), suggesting again a significant impact of the SST errors. The exact contributions and the distinct roles of these SST errors in the seasonal mean atmospheric state of the coupled model have been further assessed with two sensitivity coupled experiments, in which the SST biases are replaced by observed climatology either in the WIO (warm bias) or EIO (cold bias). The correction of the WIO warm bias leads to a global decrease of rainfall in the monsoon region, which confirms that the WIO is an important source of moisture for the ISM. On the other hand, the correction of the EIO cold bias leads to a global improvement of precipitation and circulation mean state during summer and fall. Nevertheless, all these improvements due to SST corrections seem drastically limited by the atmosphere intrinsic biases, including prominently the unimodal oceanic position of the ITCZ (Inter Tropical Convergence Zone) during summer and the enhanced westward wind stress along the equator during fall.     

Impact of different convective cloud schemes on the simulation of the tropical seasonal cycle in a coupled ocean–atmosphere model

The simulation of the mean seasonal cycle of sea surface temperature (SST) remains a challenge for coupled ocean–atmosphere general circulation models (OAGCMs). Here we investigate how the numerical representation of clouds and convection affects the simulation of the seasonal variations of tropical SST. For this purpose, we compare simulations performed with two versions of the same OAGCM differing only by their convection and cloud schemes. Most of the atmospheric temperature and precipitation differences between the two simulations reflect differences found in atmosphere-alone simulations. They affect the ocean interior down to 1,000 m. Substantial differences are found between the two coupled simulations in the seasonal march of the Intertropical Convergence Zone in the eastern part of the Pacific and Atlantic basins, where the equatorial upwelling develops. The results confirm that the distribution of atmospheric convection between ocean and land during the American and African boreal summer monsoons plays a key role in maintaining a cross equatorial flow and a strong windstress along the equator, and thereby the equatorial upwelling. Feedbacks between convection, large-scale circulation, SST and clouds are highlighted from the differences between the two simulations. In one case, these feedbacks maintain the ITCZ in a quite realistic position, whereas in the other case the ITCZ is located too far south close to the equator.     

赤道太平洋冷舌的春季海表面稳定冷偏差和最小风引起的混合

The authors investigate the relationship between bias in simulated sea surface temperature (SST) in the equatorial eastern Pacific cold tongue during the boreal spring as simulated by an oceanic general circulation model (OGCM) and minimal wind mixing (MWM) at the surface. The cold bias of simulated SST is greatest during the boreal spring, at approximately 3°C. A sensitivity experiment reducing MWM by one order of magnitude greatly alleviates cold biases, especially in March-April. The decrease in bias is primarily due to weakened vertical mixing, which preserves heat in the uppermost layer and results in warmer simulated SST. The reduction in vertical mixing also leads to a weak westward current in the upper layer, which further contributes to warmer SST estimates. These findings imply that there are large uncertainties about simple model parameters such as MWM at the oceanic surface.     

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter.     

赤道不稳定波海气相互作用的数值模拟分析

赤道不稳定波(tropical instability waves)存在于热带东太平洋赤道附近, 通常于每年的春末夏初出现, 以约0.6 m/s速度向西传播, 波周期为20～40天左右, 波长约为1000～2000 km。本文利用一个全球高分辨率海气耦合模式对赤道不稳定波在赤道附近的热量输送进行分析, 表明赤道不稳定波产生指向赤道的热通量, 从而部分抵消了热带东太平洋地区由Ekman辐散和温度平流导致的强冷却效应, 维持热带地区的热量平衡。其对赤道冷舌区的增暖作用可以消除和减弱气候模式中热带东太平洋地区的系统性冷偏差, 能使冷舌的强度和分布得到合理的改善, 对气候模式的改进和发展具有潜在贡献。赤道不稳定波还可以改变赤道海洋上空低层大气层结稳定度, 导致近地层强的风场辐合辐散, 并进一步影响大气混合层的温度、 风场等气象要素。模拟分析结果还表明, 赤道不稳定波对大气强迫产生二次响应, 改变赤道上空逆温层的垂直位移和逆温强度。研究赤道不稳定波对热带海洋气候及其海气相互作用机理的理解具有重要意义。     

Changes in tropical Atlantic interannual variability from a substantial weakening of the meridional overturning circulation

In response to a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC)—from a coupled ocean–atmosphere general circulation model experiment—significant changes in the interannual variability are found over the tropical Atlantic, characterized by an increase of variance (by ~150 %) in boreal late spring-early summer and a decrease of variance (by ~60 %) in boreal autumn. This study focuses on understanding physical mechanisms responsible for these changes in interannual variability in the tropical Atlantic. It demonstrates that the increase of variability in spring is a consequence of an increase in the variance of the El Niño-Southern Oscillation, which has a large impact on the tropical Atlantic via anomalous surface heat fluxes. Winter El Niño (La Niña) affects the eastern equatorial Atlantic by decreasing (increasing) cloud cover and surface wind speed which is associated with anomalous downward (upward) short wave radiation and reduced (enhanced) upward latent heat fluxes, creating anomalous positive (negative) sea surface temperature (SST) anomalies over the region from winter to spring. On the other hand, the decrease of SST variance in autumn is due to a deeper mean thermocline which weakens the impact of the thermocline movement on SST variation. The comparison between the model results and observations is not straightforward owing to the influence of model biases and the lack of a major MOC weakening event in the instrumental record. However, it is argued that the basic physical mechanisms found in the model simulations are likely to be robust and therefore have relevance to understanding tropical Atlantic variability in the real world, perhaps with modified seasonality.     

ENSO phase-locking to the boreal winter in CMIP3 and CMIP5 models

In this study, the El Nino-Southern Oscillation (ENSO) phase-locking to the boreal winter in CMIP3 and CMIP5 models is examined. It is found that the models that are poor at simulating the winter ENSO peak tend to simulate colder seasonal-mean sea-surface temperature (SST) during the boreal summer and associated shallower thermocline depth over the eastern Pacific. These models tend to amplify zonal advection and thermocline depth feedback during boreal summer. In addition, the colder eastern Pacific SST in the model can reduce the summertime mean local convective activity, which tends to weaken the atmospheric response to the ENSO SST forcing. It is also revealed that these models have more serious climatological biases over the tropical Pacific, implying that a realistic simulation of the climatological fields may help to simulate winter ENSO peak better. The models that are poor at simulating ENSO peak in winter also show excessive anomalous SST warming over the western Pacific during boreal winter of the El Nino events, which leads to strong local convective anomalies. This prevents the southward shift of El Nino-related westerly during boreal winter season. Therefore, equatorial westerly is prevailed over the western Pacific to further development of ENSO-related SST during boreal winter. This bias in the SST anomaly is partly due to the climatological dry biases over the central Pacific, which confines ENSO-related precipitation and westerly responses over the western Pacific.