On the Atlantic–Pacific Niños connection: a multidecadal modulated mode

Atlantic and Pacific El Niño are the leading tropical oceanic variability phenomena at interannual timescales. Recent studies have demonstrated how the Atlantic Niño is able to influence on the dynamical processes triggering the development of the Pacific La Niña and vice versa. However, the stationarity of this interbasin connection is still controversial. Here we show for the first time that the Atlantic–Pacific Niños connection takes place at particular decades, coinciding with negative phases of the Atlantic Multidecadal Oscillation (AMO). During these decades, the Atlantic–Pacific connection appears as the leading coupled covariability mode between Tropical Atlantic and Pacific interannual variability. The mode is defined by a predictor field, the summer Atlantic Sea Surface Temperature (SST), and a set of predictand fields which represent a chain of atmospheric and oceanic mechanisms to generate the Pacific El Niño phenomenon: alteration of the Walker circulation, surface winds in western Pacific, oceanic Kelvin wave propagating eastward and impacting on the eastern thermocline and changes in the Pacific SST by internal Bjerknes feedback. We suggest that the multidecadal component of the Atlantic acts as a switch for El Niño prediction during certain decades, putting forward the AMO as the modulator, acting through changes in the equatorial Atlantic convection and the equatorial Pacific SST variability. These results could have a major relevance for the decadal prediction systems.     

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.     

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.     

The relative importance of ENSO and tropical Atlantic sea surface temperature anomalies for seasonal precipitation over South America: a numerical study

The role of tropical Atlantic sea surface temperature (SST) anomalies during ENSO episodes over northeast Brazil (Nordeste) is investigated using the CPTEC/COLA Atmospheric General Circulation Model (AGCM). Four sets of integrations are performed using SST in El Niño and La Niña (ENSO) episodes, changing the SST of the Atlantic Ocean. A positive dipole (SST higher than normal in the tropical North Atlantic and below normal in the tropical South Atlantic) and a negative dipole (opposite conditions), are set as the boundary conditions of SST in the Atlantic Ocean. The four experiments are performed using El Niño or La Niña SST in all oceans, except in the tropical Atlantic where the two phases of the SST dipole are applied. Five initial conditions were integrated in each case in order to obtain four ensemble results. The positive SST dipole over the tropical Atlantic Ocean and El Niño conditions over the Pacific Ocean resulted in dry conditions over the Nordeste. When the negative dipole and El Niño conditions over the Pacific Ocean were applied, the results showed precipitation above normal over the north of Nordeste. When La Niña conditions over Pacific Ocean were tested together with a negative dipole, positive precipitation anomalies occurred over the whole Nordeste. Using the positive dipole over the tropical Atlantic, the precipitation over Nordeste was below average. During La Niña episodes, the Atlantic Ocean conditions have a larger effect on the precipitation of Nordeste than the Pacific Ocean. In El Niño conditions, only the north region of Nordeste is affected by the Atlantic SST. Other tropical areas of South America show a change only in the intensity of anomalies. Central and southeast regions of South America are affected by the Atlantic conditions only during La Niña conditions, whereas during El Niño these regions are influenced only by conditions in the Pacific Ocean.     

Cross-season relation of the South China Sea precipitation variability between winter and summer

The present study reveals cross-season connections of rainfall variability in the South China Sea (SCS) region between winter and summer. Rainfall anomalies over northern South China Sea in boreal summer tend to be preceded by the same sign rainfall anomalies over southern South China Sea in boreal winter (denoted as in-phase relation) and succeeded by opposite sign rainfall anomalies over southern South China Sea in the following winter (denoted as out-of-phase relation). Analysis shows that the in-phase relation from winter to summer occurs more often in El Niño/La Niña decaying years and the out-of-phase relation from summer to winter appears more frequently in El Niño/La Niña developing years. In the summer during the El Niño/La Niña decaying years, cold/warm and warm/cold sea surface temperature (SST) anomalies develop in tropical central North Pacific and the North Indian Ocean, respectively, forming an east–west contrast pattern. The in-phase relation is associated with the influence of anomalous heating/cooling over the equatorial central Pacific during the mature phase of El Niño/La Niña events that suppresses/enhances precipitation over southern South China Sea and the impact of the above east–west SST anomaly pattern that reduces/increases precipitation over northern South China Sea during the following summer. The impact of the east–west contrast SST anomaly pattern is confirmed by numerical experiments with specified SST anomalies. In the El Niño/La Niña developing years, regional air-sea interactions induce cold/warm SST anomalies in the equatorial western North Pacific. The out-of-phase relation is associated with a Rossby wave type response to anomalous heating/cooling over the equatorial central Pacific during summer and the combined effect of warm/cold SST anomalies in the equatorial central Pacific and cold/warm SST anomalies in the western North Pacific during the mature phase of El Niño/La Niña events.     

Impacts of recent El Niño Modoki on dry/wet conditions in the Pacific rim during boreal summer

Present work uses 1979–2005 monthly observational data to study the impacts of El Niño Modoki on dry/wet conditions in the Pacific rim during boreal summer. The El Niño Modoki phenomenon is characterized by the anomalously warm central equatorial Pacific flanked by anomalously cool regions in both west and east. Such zonal SST gradients result in anomalous two-cell Walker Circulation over the tropical Pacific, with a wet region in the central Pacific. There are two mid-tropospheric wave trains passing over the extratropical and subtropical North Pacific. They contain a positive phase of a Pacific-Japan pattern in the northwestern Pacific, and a positive phase of a summertime Pacific-North American pattern in the northeastern Pacific/North America region. The western North Pacific summer monsoon is enhanced, while the East Asian summer monsoon is weakened. In the South Pacific, there is a basin-wide low in the mid-latitude with enhanced Australian high and the eastern South Pacific subtropical high. Such an atmospheric circulation pattern favors a dry rim surrounding the wet central tropical Pacific. The El Niño Modoki and its climate impacts are very different from those of El Niño. Possible geographical regions for dry/wet conditions influenced by El Niño Modoki and El Niño are compared. The two phenomena also have very different temporal features. El Niño Modoki has a large decadal background while El Niño is predominated by interannual variability. Mixing-up the two different phenomena may increase the difficulty in understanding their mechanisms, climate impacts, and uncertainty in their predictions.     

Mechanisms of tropical Pacific interannual-to-decadal variability in the ARPEGE/ORCA global coupled model

Spatial and temporal structures of interannual-to-decadal variability in the tropical Pacific Ocean are investigated using results from a global atmosphere–ocean coupled general circulation model. The model produces quite realistic mean state characteristics, despite a sea surface temperature cold bias and a thermocline that is shallower than observations in the western Pacific. The periodicity and spatial patterns of the modelled El Niño Southern Oscillations (ENSO) compare well with those observed over the last 100 years, although the quasi-biennial timescale is dominant. Lag-regression analysis between the mean zonal wind stress and the 20°C isotherm depth suggests that the recently proposed recharge-oscillator paradigm is operating in the model. Decadal thermocline variability is characterized by enhanced variance over the western tropical South Pacific (~7°S). The associated subsurface temperature variability is primarily due to adiabatic displacements of the thermocline as a whole, arising from Ekman pumping anomalies located in the central Pacific, south of the equator. Related wind anomalies appear to be caused by SST anomalies in the eastern equatorial Pacific. This quasi-decadal variability has a timescale between 8 years and 20 years. The relationship between this decadal tropical mode and the low-frequency modulation of ENSO variance is also discussed. Results question the commonly accepted hypothesis that the low-frequency modulation of ENSO is due to decadal changes of the mean state characteristics.     

Extra-tropical atmospheric response to ENSO in the CMIP5 models

The seasonal mean extra-tropical atmospheric response to El Niño/Southern Oscillation (ENSO) is assessed in the historical and pre-industrial control CMIP5 simulations. This analysis considers two types of El Niño events, characterized by positive sea surface temperature (SST) anomalies in either the central equatorial Pacific (CP) or eastern equatorial Pacific (EP), as well as EP and CP La Niña events, characterized by negative SST anomalies in the same two regions. Seasonal mean geopotential height anomalies in key regions typify the magnitude and structure of the disruption of the Walker circulation cell in the tropical Pacific, upper tropospheric ENSO teleconnections and the polar stratospheric response. In the CMIP5 ensembles, the magnitude of the Walker cell disruption is correlated with the strength of the mid-latitude responses in the upper troposphere i.e., the North Pacific and South Pacific lows strengthen during El Niño events. The simulated responses to El Niño and La Niña have opposite sign. The seasonal mean extra-tropical, upper tropospheric responses to EP and CP events are indistinguishable. The ENSO responses in the MERRA reanalysis lie within the model scatter of the historical simulations. Similar responses are simulated in the pre-industrial and historical CMIP5 simulations. Overall, there is a weak correlation between the strength of the tropical response to ENSO and the strength of the polar stratospheric response. ENSO-related polar stratospheric variability is best simulated in the “high-top” subset of models with a well-resolved stratosphere.     

气候系统模式FGOALS_gl模拟的赤道太平洋年际变率

本文分析了中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室 (LASG/IAP) 发展的气候系统模式FGOALS_gl对赤道太平洋年际变率的模拟能力。结果表明, FGOALS_gl可以较好地模拟出赤道太平洋SST异常年际变率的主要特征, 但模拟的ENSO事件振幅偏大, 且变率周期过于规则。耦合模式模拟的气候平均风应力在热带地区比ERA40再分析资料的风应力强度偏弱30%左右, 由此引起的海洋平均态的变化, 是造成模拟的ENSO振幅偏强的主要原因。FGOALS_gl模拟的ENSO峰值多出现在春季或夏季, 原因可归之于模式模拟的SST季节循环偏差。耦合模式可以合理再现ENSO演变过程, 但观测中SST异常的东传特征在模式中没有得到再现, 这与模拟的ENSO发展模态表现为单一的 “SST模态” 有关。模拟的ENSO位相转换机制与 “充电—放电” 概念模型相符合, 赤道太平洋热含量的变化是维持ENSO振荡的机制。在ENSO暖位相时期, 赤道中东太平洋与印度洋—西太平洋暖池区的海平面气压距平型表现为南方涛动型 (SO型), 200 hPa位势高度分布表现为太平洋—北美遥相关型 (PNA型)。     

Interannual and decadal variability of the subsurface thermal structure in the Pacific Ocean: 1961–90

Heat content anomalies are analyzed to understand subsurface variability on both aparticular focus on the evolving basinwide patterns and oceanic connections between the extratropics and tropics. Various analyses indicate two distinct modes, one interannual and the other decadal, that involve the tropics and the North Pacific subtropical gyre, respectively. Interannual variability is associated with El Niño in the tropics, with a prominent “see-saw” pattern alternately on and off the equator, and in the east and west, respectively. The interannual cycle features a coherent propagation of subsurface signals around the tropical Pacific, eastward along the equator but westward off the equator at 10–15?°N. Decadal signals are dominant in the subtropics and midlatitudes but also have a tropical component that appears to be independent of interannual variations. An oceanic connection can be seen between subsurface anomalies in the midlatitudes, in the subtropics and tropics on decadal time scales. Subsurface thermal anomalies associated with midlatitude decadal variability can propagate through the subtropics into the tropics, which may modulate the intensity of interannual variability in the tropics. For example, in the middle and late 1970s, a significant warm temperature anomaly appeared to penetrate into the western and central tropics at depth, warming the tropical upper ocean and depressing the thermocline. During the development of El Niño, therefore, an extratropically preconditioned subsurface state (e.g., an enhanced positive heat content anomaly) in the western and central tropical Pacific would favor a warmer sea surface temperature anomaly in the eastern equatorial Pacific, potentially increasing the intensity of ocean-atmosphere coupling. These changes in the thermocline structure and possibly in the coupling strength can further alter the very character of tropical air-sea interactions. This may help to explain decadal variability of El Niño evolution in the tropical Pacific as observed in the 1980s. Our subsurface variability analysis presents observational evidence for the detailed space-time structure of decadal oceanic links between the extratropics and the tropics.     

Impact of vertical mixing induced by small vertical scale structures above and within the equatorial thermocline on the tropical Pacific in a CGCM

Oceanic vertical mixing is known to influence the state of the equatorial ocean which affects the climate system, including the amplitude of El Niño/Southern Oscillation (ENSO). Recent measurements of ocean currents at high vertical resolution capture numerous small vertical scale structures (SVSs) within and above the equatorial thermocline that contribute significantly to vertical mixing but which are not sufficiently resolved by coarse resolution ocean models. We investigate the impact of the vertical mixing induced by the SVSs on the mean state and interannual variability in the tropical Pacific by using a coupled general circulation model. The vertical mixing induced by the SVSs is represented as an elevated vertical diffusivity from the surface down to the 20 °C isotherm depth, a proxy for the depth of the thermocline. We investigate different forms for the elevated mixing. It is found that the SVS-induced mixing strongly affect the mean state of the ocean leading to a warming of sea surface temperature (SST) and associated deepening and sharpening of the thermocline in the eastern equatorial Pacific. We find that the SST warming induced by the elevated mixing is further strengthened through the Bjerknes feedback and SST-shortwave flux feedback. We also find a reduction in the number of large amplitude ENSO events and in certain cases an increase in the skewness of ENSO.     

Interdecadal changes in the relationship between Southern China winter-spring precipitation and ENSO

Winter-spring precipitation in southern China tends to be higher (lower) than normal in El Niño (La Niña) years during 1953–1973. The relationship between the southern China winter-spring precipitation and El Niño-Southern Oscillation (ENSO) is weakened during 1974–1994. During 1953–1973, above-normal southern China rainfall corresponds to warmer sea surface temperature (SST) in the equatorial central Pacific. There are two anomalous vertical circulations with ascent over the equatorial central Pacific and ascent over southern China and a common branch of descent over the western North Pacific that is accompanied by an anomalous lower-level anticyclone. During 1974–1994, above-normal southern China rainfall corresponds to warmer SST in eastern South Indian Ocean and cooler SST in western South Indian Ocean. Two anomalous vertical circulations act to link southern China rainfall and eastern South Indian Ocean SST anomalies, with ascent over eastern South Indian Ocean and southern China and a common branch of descent over the western North Pacific. Present analysis shows that South Indian Ocean SST anomalies can contribute to southern China winter-spring precipitation variability independently. The observed change in the relationship between southern China winter-spring rainfall and ENSO is likely related to the increased SST variability in eastern South Indian Ocean and the modulation of the Pacific decadal oscillation.     

Interannual variability of the South Pacific Convergence Zone and implications for tropical cyclone genesis

The interannual variability of the South Pacific Convergence Zone (SPCZ) and its influence on tropical cyclone (TC) genesis in the South Pacific are investigated using observations and ERA40 reanalysis over the 1979?C2002 period. In austral summer, the SPCZ displays four typical structures at interannual timescales. The first three are characterized by a diagonal orientation of the SPCZ and account for 85% of the summer seasons. One is close to climatology and the other two exhibit a 3° northward or southward departure from the SPCZ climatological position. In contrast, the fourth one, that only encompasses three austral summer seasons (the extreme 1982/1983 and 1997/1998 El Ni?o events and the moderate 1991/1992 El Ni?o event), displays very peculiar behaviour where the SPCZ largely departs from its climatological position and is zonally oriented. Variability of the western/central Pacific equatorial sea surface temperature (SST) is shown to modulate moisture transport south of the equator, thereby strongly constraining the location of the SPCZ. The SPCZ location is also shown to strongly modulate the atmospheric circulation variability in the South Pacific with specific patterns for each class. However, independently of its wide year-to-year excursions, the SPCZ is always collocated with the zero relative vorticity at low levels while the maximum vorticity axis lies 6° to the south of the SPCZ position. This coherent atmospheric organisation in the SPCZ region is shown to constrain tropical cyclogenesis to occur preferentially within 10° south of the SPCZ location as this region combines all the large-scale atmospheric conditions that favour the breeding of TCs. This analysis also reveals that cyclogenesis in the central Pacific (in the vicinity of French Polynesia) only occurs when the SPCZ displays a zonal orientation while this observation was previously attributed to El Ni?o years in general. Different characteristics of El Ni?o Southern Oscillation (ENSO)-related Pacific equatorial warming are shown to impact differently on the SPCZ position, suggesting that for regional scales, such as the South Pacific, the SPCZ classification is more appropriate than a simple ENSO index to characterize TC interannual variability. These findings suggest that forecasting the strength of El Ni?o through SST variations in the eastern Pacific may not be sufficient to accurately predict cyclogenesis in the South Pacific, especially east of the dateline.     

北大西洋年际变率的海气耦合模式模拟II：热带太平洋强迫

利用一个全球海气耦合模式(BCM),结合观测资料,讨论了热带太平洋强迫对北大西洋年际气候变率的影响。研究表明,BCM能够相对合理地模拟赤道太平洋的年际变率模态及相应的海温距平型和大气遥相关型,尽管其准3年的振荡周期过于规则。来自数值模式和观测上的证据都表明,北大西洋冬季海温的主导性变率模态,即自北而南出现的“- -”的海温距平型,受到来自热带太平洋强迫的显著影响,其正位相与赤道中东太平洋冷事件相对应。换言之,赤道太平洋暖事件的发生,在太平洋-北美沿岸激发出PNA遥相关型,进而通过在北大西洋产生类似NAO负位相的气压距平型,削弱本来与NAO正位相直接联系的三核型海温距平。北大西洋三核型海温距平对热带太平洋强迫的响应,要滞后2—3个月的时间。     

ENSIP: the El Niño simulation intercomparison project

An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Niño/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.     

Global warming,low-frequency variability,and biennial oscillation: an attempt to understand the physical mechanisms driving major ENSO events

Three primary global modes of sea surface temperature (SST) variability during the period of 1871–2010 are identified through cyclostationary empirical orthogonal function analysis. The first mode exhibits a clear trend and represents global SST warming with an ‘El Niño-like’ SST pattern in the tropical Pacific. The second mode is characterized by considerable low-frequency variability in both the tropical Pacific and the North Pacific regions, indicating that there is a close connection between the two regions on interannual and decadal time scales. The third mode shows a seesaw pattern between El Niño and La Niña within a two-year period; this mode is derived by the oscillatory tendency of the tropical Pacific ocean–atmosphere coupled system. A SST reconstruction based on these three modes captures a significant portion of the SST variability in the raw data, which is primarily associated with El Niño-Southern Oscillation (ENSO) events in the tropical Pacific. Additionally, this study attempts to interpret the major ENSO events that have occurred since the 1970s in terms of the interplay originating from these three modes of variability. In particular, two key points are derived from this analysis: (1) the most extreme El Niño events occurred in 1982/1983 and 1997/1998 are attributed to the positive contributions of all three modes; and (2) the central Pacific (CP) El Niño events in the 1990s and 2000s have different physical mechanisms, that is, the CP El Niño events in the early 1990s originated mainly from the low-frequency mode, while those in the early 2000s derived mainly from the global warming mode.     

Twentieth century ENSO characteristics in the IPCC database

In this paper, we assess and compare to observations the spatial characteristics of the twentieth Century ENSO SST variability simulated by 23 models of the IPCC-AR4/CMIP3 database. The analysis is confined to the SST anomalies along the equatorial Pacific and is based on the use of a non-linear neural classification algorithm, the Self-Organizing Maps. Systematic biases include a larger than observed proportion for modelled ENSO maximum variability occurring in the Western Pacific. No clear relationship is found between this bias and the characteristics of the modelled mean state bias in the equatorial Pacific. This bias is mainly related to a misrepresentation of both El Niño and La Niña termination phases for most of the models. In contrast, the onset phase is quite well simulated. Modelled El Niño and La Niña peak phases display an asymmetric bias. Whereas the main bias of the modelled El Niño peak is to exhibit a maximum in the western Pacific, the simulated La Niña bias mainly occurs in the central Pacific. In addition, some models are able to capture the observed El Niño peak characteristics while none of them realistically simulate La Niña peaks. It also arises that the models closest to the observations score unevenly in reproducing the different phases, preventing an accurate classification of the models quality to reproduce the overall ENSO-like variability.     

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.     

Pacific decadal oscillation and variability of the Indian summer monsoon rainfall

Recent studies have furnished evidence for interdecadal variability in the tropical Pacific Ocean. The importance of this phenomenon in causing persistent anomalies over different regions of the globe has drawn considerable attention in view of its relevance in climate assessment. Here, we examine multi-source climate records in order to identify possible signatures of this longer time scale variability on the Indian summer monsoon. The findings indicate a coherent inverse relationship between the inter-decadal fluctuations of Pacific Ocean sea surface temperature (SST) and the Indian monsoon rainfall during the last century. A warm (cold) phase of the Pacific interdecadal variability is characterized by a decrease (increase) in the monsoon rainfall and a corresponding increase (decrease) in the surface air temperature over the Indian subcontinent. This interdecadal relationship can also be confirmed from the teleconnection patterns evident from long-period sea level pressure (SLP) dataset. The SLP anomalies over South and Southeast Asia and the equatorial west Pacific are dynamically consistent in showing an out-of-phase pattern with the SLP anomalies over the tropical central-eastern Pacific. The remote influence of the Pacific interdecadal variability on the monsoon is shown to be associated with prominent signals in the tropical and southern Indian Ocean indicative of coherent inter-basin variability on decadal time scales. If indeed, the atmosphere–ocean coupling associated with the Pacific interdecadal variability is independent from that of the interannual El Niño-Southern Oscillation (ENSO), then the climate response should depend on the evolutionary characteristics of both the time scales. It is seen from our analysis that the Indian monsoon is more vulnerable to drought situations, when El Niño events occur during warm phases of the Pacific interdecadal variability. Conversely, wet monsoons are more likely to prevail, when La Niña events coincide during cold phases of the Pacific interdecadal variability.     

Reduction of the thermocline feedback associated with mean SST bias in ENSO simulation

Associated with the double Inter-tropical convergence zone problem, a dipole SST bias pattern (cold in the equatorial central Pacific and warm in the southeast tropical Pacific) remains a common problem inherent in many contemporary coupled models. Based on a newly-developed coupled model, we performed a control run and two sensitivity runs, one is a coupled run with annual mean SST correction and the other is an ocean forced run. By comparison of these three runs, we demonstrated that a serious consequence of this SST bias is to severely suppress the thermocline feedback in a realistic simulation of the El Ni?o/Southern Oscillation. Firstly, the excessive cold tongue extension pushes the anomalous convection far westward from the equatorial central Pacific, prominently diminishing the convection-low level wind feedback and thus the air-sea coupling strength. Secondly, the equatorial surface wind anomaly exhibits a relatively uniform meridional structure with weak gradient, contributing to a weakened wind-thermocline feedback. Thirdly, the equatorial cold SST bias induces a weakened upper-ocean stratification and thus yields the underestimation of the thermocline-subsurface temperature feedback. Finally, the dipole SST bias underestimates the mean upwelling through (a) undermining equatorial mean easterly wind stress, and (b) enhancing convective mixing and thus reducing the upper ocean stratification, which weakens vertical shear of meridional currents and near-surface Ekman-divergence.