Summer monsoon circulation and precipitation over the tropical Indian Ocean during ENSO in the NCEP climate forecast system

This study investigates the El Niño Southern Oscillation (ENSO) teleconnections to tropical Indian Ocean (TIO) and their relationship with the Indian summer monsoon in the coupled general circulation model climate forecast system (CFS). The model shows good skill in simulating the impact of El Niño over the Indian Oceanic rim during its decay phase (the summer following peak phase of El Niño). Summer surface circulation patterns during the developing phase of El Niño are more influenced by local Sea Surface Temperature (SST) anomalies in the model unlike in observations. Eastern TIO cooling similar to that of Indian Ocean Dipole (IOD) is a dominant model feature in summer. This anomalous SST pattern therefore is attributed to the tendency of the model to simulate more frequent IOD events. On the other hand, in the model baroclinic response to the diabatic heating anomalies induced by the El Niño related warm SSTs is weak, resulting in reduced zonal extension of the Rossby wave response. This is mostly due to weak eastern Pacific summer time SST anomalies in the model during the developing phase of El Niño as compared to observations. Both eastern TIO cooling and weak SST warming in El Niño region combined together undermine the ENSO teleconnections to the TIO and south Asia regions. The model is able to capture the spatial patterns of SST, circulation and precipitation well during the decay phase of El Niño over the Indo-western Pacific including the typical spring asymmetric mode and summer basin-wide warming in TIO. The model simulated El Niño decay one or two seasons later, resulting long persistent warm SST and circulation anomalies mainly over the southwest TIO. In response to the late decay of El Niño, Ekman pumping shows two maxima over the southern TIO. In conjunction with this unrealistic Ekman pumping, westward propagating Rossby waves display two peaks, which play key role in the long-persistence of the TIO warming in the model (for more than a season after summer). This study strongly supports the need of simulating the correct onset and decay phases of El Niño/La Niña for capturing the realistic ENSO teleconnections. These results have strong implications for the forecasting of Indian summer monsoon as this model is currently being adopted as an operational model in India.     

Different impacts of various El Niño events on the Indian Ocean Dipole

Our early work (Wang and Wang in J Clim 26:1322–1338, 2013) separates El Niño Modoki events into El Niño Modoki I and II because they show different impacts on rainfall in southern China and typhoon landfall activity. The warm SST anomalies originate in the equatorial central Pacific and subtropical northeastern Pacific for El Niño Modoki I and II, respectively. El Niño Modoki I features a symmetric SST anomaly distribution about the equator with the maximum warming in the equatorial central Pacific, whereas El Niño Modoki II shows an asymmetric distribution with the warm SST anomalies extending from the northeastern Pacific to the equatorial central Pacific. The present paper investigates the influence of the various groups of El Niño events on the Indian Ocean Dipole (IOD). Similar to canonical El Niño, El Niño Modoki I is associated with a weakening of the Walker circulation in the Indo-Pacific region which decreases precipitation in the eastern tropical Indian Ocean and maritime continent and thus results in the surface easterly wind anomalies off Java-Sumatra. Under the Bjerknes feedback, the easterly wind anomalies induce cold SST anomalies off Java- Sumatra, and thus a positive IOD tends to occur in the Indian Ocean during canonical El Niño and El Niño Modoki I. However, El Niño Modoki II has an opposite impact on the Walker circulation, resulting in more precipitation and surface westerly wind anomalies off Java-Sumatra. Thus, El Niño Modoki II is favorable for the onset and development of a negative IOD on the frame of the Bjerknes feedback.     

与不同El Nino相伴的IOD事件的季节演变特征对比

基于1951—2012年逐月海洋和大气多种要素的再分析资料,分析了与两类El Nino相伴的IOD(Indian Ocean Dipole,印度洋偶极子)事件盛期的海洋和大气异常特征,并进一步对比了与不同类型El Nino相伴的IOD事件的季节演变及对应的海气耦合过程。结果表明:两类IOD事件盛期时,暖海温强度和位置有显著差异。发生在东部型El Nino期间的IOD事件(简称EP-IOD)盛期,正(负)SSTA中心出现在热带西北(赤道东南)印度洋,强度相当,对应的热带印度洋—海洋大陆异常Walker环流强度较强、范围较大;与中部型CP El Nino相伴的IOD事件(简称CP-IOD)的正SSTA相对较弱,且偏于南印度洋,异常Walker环流较弱、较窄。在季节演变中,两类IOD事件期间的局地海气过程差异显著,伴随着西印度洋西南季风减弱和东印度洋异常东风加强,EP-IOD事件的发展以西正东负的偶极型异常海温的出现及加强为主要特征;而CP-IOD事件的发生发展则与西北印度洋异常冷海温的生消及南印度洋暖水的堆积相伴,表现为"-+-"三极型SSTA的出现并转为西正东负偶极型的过程,夏季时出现在东印度洋的异常东风以及赤道中印度洋低层负涡度异常水平环流对其发展具有重要作用。     

La Niña diversity and Northwest Indian Ocean Rim teleconnections

The differences in tropical Pacific sea surface temperature (SST) expressions of El Niño-Southern Oscillation (ENSO) events of the same phase have been linked with different global atmospheric circulation patterns. This study examines the dynamical forcing of precipitation during October–December (OND) and March–May (MAM) over East Africa and during December–March (DJFM) over Central-Southwest Asia for 1950–2010 associated with four tropical Pacific SST patterns characteristic of La Niña events, the cold phase of ENSO. The self-organizing map method along with a statistical distinguishability test was used to isolate La Niña events, and seasonal precipitation forcing was investigated in terms of the tropical overturning circulation and thermodynamic and moisture budgets. Recent La Niña events with strong opposing SST anomalies between the central and western Pacific Ocean (phases 3 and 4), force the strongest global circulation modifications and drought over the Northwest Indian Ocean Rim. Over East Africa during MAM and OND, subsidence is forced by an enhanced tropical overturning circulation and precipitation reductions are exacerbated by increases in moisture flux divergence. Over Central-Southwest Asia during DJFM, the thermodynamic forcing of subsidence is primarily responsible for precipitation reductions, with moisture flux divergence acting as a secondary mechanism to reduce precipitation. Eastern Pacific La Niña events in the absence of west Pacific SST anomalies (phases 1 and 2), are associated with weaker global teleconnections, particularly over the Indian Ocean Rim. The weak regional teleconnections result in statistically insignificant precipitation modifications over East Africa and Central-Southwest Asia.     

热带印度洋和太平洋增暖对东亚夏季风趋势的相反影响

利用多成员集合试验结果,比较分析了热带印度洋和太平洋增暖各自对东亚夏季风趋势变化的影响。试验所用模式是GFDLAM2大气环流模式,增暖是通过在气候平均海洋表面温度（SST）基础上,叠加随时间线性增加的、相当于实际50a左右达到的SST异常来实现的。结果表明：热带印度洋和太平洋共同增暖有使东亚夏季风减弱的趋势。相比较而言,单独印度洋增暖有使东亚夏季风增强、华北降水增多的趋势,而单独太平洋增暖有使东亚夏季风减弱的趋势,即印度洋增暖与太平洋增暖对东亚夏季风存在相反的、竞争性影响。进一步分析指出,热带太平洋特别是热带中东太平洋的增温可能对20世纪70年代末期开始的夏季风年代际减弱有更重要的贡献;在未来热带印度洋和太平洋持续增暖、但增暖强度纬向差异减小的新情况下,东亚夏季风减弱的趋势可能还将持续。     

Links between Indo-Pacific climate variability and drought in the Monsoon Asia Drought Atlas

Drought patterns across monsoon and temperate Asia over the period 1877–2005 are linked to Indo-Pacific climate variability associated with the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Using the Monsoon Asia Drought Atlas (MADA) composed of a high-resolution network of hydroclimatically sensitive tree-ring records with a focus on the June–August months, spatial drought patterns during El Niño and IOD events are assessed as to their agreement with an instrumental drought index and consistency in the drought response amongst ENSO/IOD events. Spatial characteristics in drought patterns are related to regional climate anomalies over the Indo-Pacific basin, using reanalysis products, including changes in the Asian monsoon systems, zonal Walker circulation, moisture fluxes, and precipitation. A weakening of the monsoon circulation over the Indian subcontinent and Southeast Asia during El Niño events, along with anomalous subsidence over monsoon Asia and reduced moisture flux, is reflected in anomalous drought conditions over India, Southeast Asia and Indonesia. When an IOD event co-occurs with an El Niño, severe drought conditions identified in the MADA for Southeast Asia, Indonesia, eastern China and central Asia are associated with a weakened South Asian monsoon, reduced moisture flux over China, and anomalous divergent flow and subsidence over Indonesia. Insights into the relative influences of Pacific and Indian Ocean variability for Asian monsoon climate on interannual to decadal and longer timescales, as recorded in the MADA, provide a useful tool for assessing long-term changes in the characteristics of Asian monsoon droughts in the context of Indo-Pacific climate variability.     

Spring asymmetric mode in the tropical Indian Ocean: role of El Niño and IOD

The spring asymmetric mode over the Tropical Indian Ocean (TIO) is characterized by contrasting patterns of rainfall and surface wind anomalies north and south of Equator. The asymmetric pattern in rainfall has evolved as a leading mode of variability in the TIO and is strongly correlated with El Niño-Southern Oscillation (ENSO) and positive Indian Ocean Dipole (IOD). The evolution of the asymmetric pattern in rainfall and surface wind during pure El Niño/IOD and co-occurrence years are examined in the twentieth century reanalysis for the period of 1871–2008 and atmospheric general circulation model (AGCM) simulations. The study revealed that spring asymmetric mode is well developed when El Niño co-occurred with IOD (positive) and is driven by the associated meridional gradients in sea surface temperature (SST) and sea level pressure (SLP). The pure El Niño composites are characterized by homogeneous (spatially) SST anomalies (positive) and weaker SLP gradients and convection, leading to weak asymmetric mode. The asymmetric mode is absent in the pure IOD (positive) composites due to the persistence of east west SST gradient for a longer duration than the co-occurrence years. The meridional gradient in SST anomalies over the TIO associated with the ENSO-IOD forcing is therefore crucial in developing/strengthening the spring asymmetric mode. The northwest Pacific anticyclonic circulation further strengthen the asymmetric mode in surface winds by inducing northeasterlies in the north Indian Ocean during pure El Niño and co-occurrence years. The simulations based on AGCM, forced by observed SSTs during the period of 1871–2000 supported the findings. The analysis of available station and ship track data further strengthens our results.     

Modification of the southern African rainfall variability/ENSO relationship since the late 1960s

Analysis of 149 raingauge series (1946–1988) shows a weak positive correlation between late summer rainfalls (January–March) in tropical southern Africa and the Southern Oscillation Index (SOI). The correlation coefficients have been unstable since World War II. They were close to zero before 1970 and significant thereafter. Before 1970, southern African late summer rainfalls were more specifically correlated with regional patterns of sea surface temperature (SST), mainly over the southwestern Indian Ocean. After 1970, teleconnections with near global SST anomaly patterns, i.e. over the central Pacific and Indian oceans, dominate the regional connections. The increase in the sensitivity of the southern African rainfall to the global SO-related circulation anomalies is simultaneous with the correlation between SOI and more extensive SST anomalies, particularly over the southern Indian Ocean. This feature is part of longer term (decadal), global SST variability, as inferred from statistical analyses. Numerical experiments, using the Météo-France general circulation model ARPEGE-Climat, are performed to test the impact of the observed SST warming in the southern Indian and extratropical oceans during El Niño Southern Oscillation (ENSO) events on southern African rainfall. Simulated results show that ENSO events, which occurred in the relatively cold background of the pre-1970 period in the southern oceans, had a little effect on southern Africa climatic conditions and atmospheric circulation. By contrast, more recent ENSO events, with warmer SST over the southern oceans, lead to a climatic bipolar pattern between continental southern African and the western Indian Ocean, which is characterized by reduced (enhanced) deep convection and rainfall over the subcontinent (the western Indian Ocean). A weaker subtropical high-pressure belt in the southwestern Indian Ocean is also simulated, along with a reduced penetration of the moist southern Indian Ocean trade winds over the southern African plateau. These results are consistent with the strong droughts observed over all southern Africa during ENSO events since 1970.     

LINKAGE BETWEEN INDIAN OCEAN DIPOLE AND TWO TYPES OF El NI?O AND ITS POSSIBLE MECHANISMS

After compositing three representative ENSO indices,El Nio events have been divided into an eastern pattern(EP) and a central pattern(CP).By using EOF,correlation and composite analysis,the relationship and possible mechanisms between Indian Ocean Dipole(IOD) and two types of El Nio were investigated.IOD events,originating from Indo-Pacific scale air-sea interaction,are composed of two modes,which are associated with EP and CP El Ni o respectively.The IOD mode related to EP El Nio events(named as IOD1) is strongest at the depth of 50 to 150 m along the equatorial Indian Ocean.Besides,it shows a quasi-symmetric distribution,stronger in the south of the Equator.The IOD mode associated with CP El Nio(named as IOD2) has strongest signal in tropical southern Indian Ocean surface.In terms of mechanisms,before EP El Nio peaks,anomalous Walker circulation produces strong anomalous easterlies in equatorial Indian Ocean,resulting in upwelling in the east,decreasing sea temperature there;a couple of anomalous anticyclones(stronger in the south) form off the Equator where warm water accumulates,and thus the IOD1 occurs.When CP El Nio develops,anomalous Walker circulation is weaker and shifts its center to the west,therefore anomalous easterlies in equatorial Indian Ocean is less strong.Besides,the anticyclone south of Sumatra strengthens,and the southerlies east of it bring cold water from higher latitudes and northerlies west of it bring warm water from lower latitudes to the 15° to 25°S zone.Meanwhile,there exists strong divergence in the east and convergence in the west part of tropical southern Indian Ocean,making sea temperature fall and rise separately.Therefore,IOD2 lies farther south.     

Theories on formation of an anomalous anticyclone in western North Pacific during El Niño: A review

The western North Pacific anomalous anticyclone (WNPAC) is an important atmospheric circulation system that conveys El Niño impact on East Asian climate. In this review paper, various theories on the formation and maintenance of the WNPAC, including warm pool atmosphere–ocean interaction, Indian Ocean capacitor, a combination mode that emphasizes nonlinear interaction between ENSO and annual cycle, moist enthalpy advection/Rossby wave modulation, and central Pacific SST forcing, are discussed. It is concluded that local atmosphere–ocean interaction and moist enthalpy advection/Rossby wave modulation mechanisms are essential for the initial development and maintenance of the WNPAC during El Niño mature winter and subsequent spring. The Indian Ocean capacitor mechanism does not contribute to the earlier development but helps maintain the WNPAC in El Niño decaying summer. The cold SST anomaly in the western North Pacific, although damped in the summer, also plays a role. An interbasin atmosphere–ocean interaction across the Indo-Pacific warm pool emerges as a new mechanism in summer. In addition, the central Pacific cold SST anomaly may induce the WNPAC during rapid El Niño decaying/La Niña developing or La Niña persisting summer. The near-annual periods predicted by the combination mode theory are hardly detected from observations and thus do not contribute to the formation of the WNPAC. The tropical Atlantic may have a capacitor effect similar to the tropical Indian Ocean.     

Seasonality in the relationship between El Nino and Indian Ocean dipole

The seasonal change in the relationship between El Nino and Indian Ocean dipole (IOD) is examined using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), and the twentieth century simulations (20c3m) from the Geophysical Fluid Dynamics Laboratory Coupled Model, version 2.1. It is found that, both in ERA-40 and the model simulations, the correlation between El Nino (Nino3 index) and the eastern part of the IOD (90?C110°E; 10°S-equator) is predominantly positive from January to June, and then changes to negative from July to December. Correlation maps of atmospheric and oceanic variables with respect to the Nino3 index are constructed for each season in order to examine the spatial structure of their seasonal response to El Nino. The occurrence of El Nino conditions during January to March induces low-level anti-cyclonic circulation anomalies over the southeastern Indian Ocean, which counteracts the climatological cyclonic circulation in that region. As a result, evaporation decreases and the southeastern Indian Ocean warms up as the El Nino proceeds, and weaken the development of a positive phase of an IOD. This warming of the southeastern Indian Ocean associated with the El Nino does not exist past June because the climatological winds there develop into the monsoon-type flow, enhancing the anomalous circulation over the region. Furthermore, the development of El Nino from July to September induces upwelling in the southeastern Indian Ocean, thereby contributing to further cooling of the region during the summer season. This results in the enhancement of a positive phase of an IOD. Once the climatological circulation shifts from the boreal summer to winter mode, the negative correlation between El Nino and SST of the southeastern Indian Ocean changes back to a positive one.     

Influence of ENSO and of the Indian Ocean Dipole on the Indian summer monsoon variability

Indian summer monsoon (ISM) variability is forced from external factors (like the El Niño Southern Oscillation, ENSO) but it contains also an internal component that tends to reduce its potential for predictability. Large-scale and local monsoon indices based on precipitation and atmospheric circulation parameters are used as a measure of ISM variability. In a 9-members ensemble of AMIP-type experiments (with same boundary SST forcing and different initial conditions) their potential predictability is comparable using both local and large-scale monsoon indices. In the sample analyzed, about half of more predictable monsoon years coincide with El Niño and/or positive Indian Ocean Dipole (IOD) events. Summer monsoon characteristics during ENSO and IOD years are analyzed through composites computed over a three years period (i.e. one year before and one year after the event peak) to investigate the mutual relationship between the events lagged in time. The connection between ISM and IOD is mostly confined in the summer and autumn, while that with ENSO is stronger and extends more in time. In the coupled model results the IOD influence on the monsoon is large, even because in the model IOD events are intense and easily reproduced due to a strong air-sea feedback in the eastern side of the basin. Monsoon seasons preceding or following an El Niño or a La Niña event are not exactly symmetric, even in terms of their biennial character. In most of the cases, both in reanalysis and model, El Niño and positive IOD events tend to co-occur with larger anomalies either in the Indo-Pacific ocean sector or over India, while La Niña and negative IOD do not. From the observed record, the ENSO-IOD correlation is positive strong and significant since mid-60s and it may correspond with either strong or weak ENSO-monsoon relationship and with strong or weak IOD-monsoon relationship. A main difference between those periods is the relationship between Indian monsoon rainfall and SST in other ocean basins rather than the Indo-Pacific sector alone.     

Future change of the Indian Ocean basin-wide and dipole modes in the CMIP5

The Indian Ocean sea surface temperature (SST) variability has been represented with the two dominant variability modes: the Indian Ocean basin-wide (IOBW) and dipole (IOD) modes. Here we investigate future changes of the two modes together with mean state and El Niño and Southern Oscillation (ENSO) relationship under the anthropogenic global warming using 20 coupled models that participated in the phase five of Coupled Model Intercomparison Project by comparing the historical run from 1950 to 2005 and the RCP 4.5 run from 2050 to 2099. The five best models are selected based on the evaluation of the 20 models’ performances in simulating the two modes and Indian Ocean basic state for the latest 56 years. They are capable of capturing the IOBW and IOD modes in their spatial distribution, seasonal cycle, major periodicity, and relationship with ENSO to some extent. The five best models project the significant changes in the Indian Ocean mean state and variability including the two dominant modes in the latter part of twenty-first century under the anthropogenic warming scenario. First, the annual mean climatological SST displays an IOD-like pattern change over the Indian Ocean with enhanced warming in the northwestern Indian Ocean and relatively weaker warming off the Sumatra–Java coast. It is also noted that the monthly SST variance is increased over the eastern and southwestern Indian Ocean. Second, the IOBW variability on a quasi-biennial time scale will be enhanced due to the strengthening of the ENSO–IOBW mode relationship although the total variance of the IOBW mode will be significantly reduced particularly during late summer and fall. The enhanced air-sea coupling over the Indian-western Pacific climate in response to El Nino activity in the future projection makes favorable condition for a positive IOD while it tends to derive relatively cold temperature over the eastern Indian Ocean. This positive IOD-like ENSO response weakens the relationship between the eastern Indian Ocean and El Nino while strengthens the relationship with western Indian Ocean. Third, the IOD mode, intrinsic coupled mode of the Indian Ocean may not be changed appreciably under the anthropogenic global warming.     

亚洲夏季风环流结构与热带印度洋偶极型海温异常

使用T42L28大气环流模式就夏季风时期大气对印度洋海温偶极子型异常的响应进行了数值试验研究,结果表明,印度洋偶极子型海温异常可以引起感热和潜热加热异常并进而形成异常辐合辐散,导致热带印度洋及其邻近地区夏季降水异常。同时此热带扰动可激发或造成中纬度异常波列。通过改变季风区温度场分布,偶极子型海温强迫可以影响大气的正／斜压环流结构和斜压性强弱。强的纬向风垂直切变趋向于靠近海洋异常偏暖的地区。不论是正偶极子型强迫或负偶极子型强迫,西太平洋暖池和东亚地区的大气环流均出现异常并激发出中纬度的异常波列,但异常类型并未显著反相。     

A dynamic link between the basin-scale and zonal modes in the Tropical Indian Ocean

Summary The interannual variability of sea surface temperature (SST) anomalies in the tropical Indian Ocean is dominated mainly by a basin-scale mode (BM) and partly by an east–west contrast mode (zonal mode, ZM). The BM reflects the basin-scale warming or cooling and is highly correlated with El Nino with 3- to 6-month lags, while the ZM is marginally correlated with El Nino with 9-month lags.During an El Nino, large-scale anomalous subsidence over the maritime continent occurs as a result of an eastward shift in the rising branch of the Walker circulation suppresses convection over the eastern Indian Ocean, allowing more solar radiation over the eastern Indian Ocean. At the same time, the anomalous southeasterly wind over the equatorial Indian Ocean forces the thermocline over the western Indian Ocean to deepen, especially in the southern part. As a result, SST over the whole basin increases. As El Nino decays, the subsidence over the maritime continent ceases and so does the anomalous southeasterly wind. However, the thermocline perturbation does not quickly shoal back to normal because of inertia and it disperses as Rossby waves. These Rossby waves are reflected back as an equatorial Kelvin wave, causing deepening of the thermocline in the eastern Indian Ocean, and preventing SSTs from cooling in that region. Moreover, the weaker wind speed of the monsoon circulation results in less latent heat loss, and thus warms the eastern Indian Ocean. These two processes therefore help to maintain warm SSTs over the eastern Indian Ocean until fall. During the fall, the warm SST over the eastern Indian Ocean and the cold SST over the western Indian Ocean are enhanced by air–sea interaction and the ZM returns. The ZM dissipates through the seasonal reversal of the monsoon atmospheric circulation and the boundary-reflected Kelvin wave. In the same manner, a basin-scale cooling in the tropical Indian Ocean can induce the ZM warming in the west and cooling in the east.     

Polar Vortex Response to Pacific Ocean Warming and Its Additive Nonlinearity with the Indian Ocean

A previous modeling study about Pacific Ocean warming derived polar vortex response signals, by subtracting those in the Indian Ocean warming experiments from those in the Indo-Pacific. This approach questions the resemblance of such an indirectly derived response to one directly forced by Pacific Ocean warming. This is relevant to the additive nonlinearity of atmospheric responses to separated Indian and Pacific Ocean forcing. In the present study, an additional set of ensemble experiments are performed by prescribing isolated SST forcing in the tropical Pacific Ocean to address this issue. The results suggest a qualitative resemblance between responses in the derived and additional experiments. Thus, previous findings about the impact of Indian and Pacific Ocean warming are robust. This study has important implications for future climate change projections, considering the non-unanimous warming rates in tropical oceans in the 21st century. Nevertheless, a comparison of present direct-forced experiments with previous indirect-forced experiments suggests a significant additive nonlinearity between the Indian and Pacific Ocean warmings. Further diagnosis suggests that the nonlinearity may originate from the thermodynamic processes over the tropics.     

Variability of the Indian Ocean SST and its possible impact on summer western North Pacific anticyclone in the NCEP Climate Forecast System

The NCEP Climate Forecast System version 2 (CFSv2) provides important source of information about the seasonal prediction of climate over the Indo-Pacific oceans. In this study, the authors provide a comprehensive assessment of the prediction of sea surface temperature (SST) in the tropical Indian Ocean (IO). They also investigate the impact of tropical IO SST on the summer anomalous anticyclonic circulation over the western North Pacific (WNPAC), focusing on the relative contributions of local SST and remote forcing of tropical IO SST to WNPAC variations. The CFSv2 captures the two most dominant modes of summer tropical IO SST: the IO basin warming (IOBW) mode and the IO dipole (IOD) mode, as well as their relationship with El Niño-Southern Oscillation (ENSO). However, it produces a cold SST bias in IO, which may be attributed to deeper-than-observed mixed layer and smaller-than-observed total downward heat flux in the tropical IO. It also overestimates the correlations of ENSO with IOBW and IOD, but underestimates the magnitude of IOD and summer IOBW. The CFSv2 captures the climate anomalies related to IOBW but not those related to IOD. It depicts the impact of summer IOBW on WNPAC via the equatorial Kelvin wave, which contributes to the maintenance of WNPAC in July and August. The WNPAC in June is mostly forced by local cold SST, which is better predicted by the CFSv2 compared to July and August. The mechanism for WNPAC maintenance may vary with lead time in the CFSv2.     

初夏孟加拉湾东部降水异常对印度洋海温偶极子的触发作用

基于1982—2013年逐月NCEP资料及GODAS资料,采用回归分析、合成分析以及2.5层简化海洋模式数值模拟等方法,研究了热带东印度洋的大气和海洋过程对印度洋海温偶极子(IOD,Indian Ocean Dipole)东极(IODE,IOD East pole)海温异常的影响。结果表明,IODE海温异常的演变超前IOD西极(IODW,IOD West pole)海温异常的演变,并对IOD事件的生成和发展起到关键作用。初夏,来自阿拉伯海、中南半岛地区以及孟加拉湾西南部的水汽输送,导致孟加拉湾东部出现强降水。降水释放的潜热在热带东印度形成了一个跨越赤道的经向环流,有利于加强赤道东印度洋的过赤道气流,并在苏门答腊沿岸形成偏南风异常。该异常偏南风通过影响混合层垂向夹卷混合过程和纬向平流过程,导致IODE海温迅速下降。随后赤道东南印度洋异常东南风迅速增强以及赤道中印度洋东风异常的出现,增强了自东南印度洋向西印度洋的水汽输送,削弱了向孟加拉湾的水汽输送,使西南印度洋的降水增强,孟加拉湾东部的降水减弱。因此,IOD达到盛期前孟加拉湾东部的降水通过局地经向环流在苏门答腊沿岸形成偏南风异常,导致苏门答腊沿岸迅速的降温,并最终导致IOD事件的发生。     

ENSO, IOD and Indian Summer Monsoon in NCEP climate forecast system

El Ni?o-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Indian Summer Monsoon rainfall features are explored statistically and dynamically using National Centers for Environment Prediction (NCEP) Climate Forecast System (CFSv1) freerun in relation to observations. The 100?years of freerun provides a sufficiently long homogeneous data set to find out the mean state, periodicity, coherence among these climatic events and also the influence of ENSO and IOD on the Indian monsoon. Differences in the occurrence of seasonal precipitation between the observations and CFS freerun are examined as a coupled ocean–atmosphere system. CFS simulated ENSO and IOD patterns and their associated tropical Walker and regional Hadley circulation in pure ENSO (PEN), pure IOD (PIO) and coexisting ENSO-IOD (PEI) events have some similarity to the observations. PEN composites are much closer to the observation as compared to PIO and PEI composites, which suggest a better ENSO prediction and its associated teleconnections as compared to IOD and combined phenomenon. Similar to the observation, the model simulation also show that the decrease in the Indian summer monsoon rainfall during ENSO phases is associated with a descending motion of anomalous Walker circulation and the increase in the Indian summer monsoon rainfall during IOD phase is associated with the ascending branch of anomalous regional Hadley circulation. During co-existing ENSO and IOD years, however, the fate of Indian summer monsoon is dictated by the combined influence of both of them. The shift in the anomalous descending and ascending branches of the Walker and Hadley circulation may be somewhat attributed to the cold (warm) bias over eastern (western) equatorial Indian Ocean basin, respectively in the model. This study will be useful for identifying some of the limitations of the CFS model and consequently it will be helpful in improving the model to unravel the realistic coupled ocean–atmosphere interactions for the better prediction of Indian Summer Monsoon.     

Indo-Pacific remote forcing in summer rainfall variability over the South China Sea

This study investigates summer rainfall variability in the South China Sea (SCS) region and the roles of remote sea surface temperature (SST) forcing in the tropical Indian and Pacific Ocean regions. The SCS summer rainfall displays a positive and negative relationship with simultaneous SST in the equatorial central Pacific (ECP) and the North Indian Ocean (NIO), respectively. Positive ECP SST anomalies induce an anomalous low-level cyclone over the SCS-western North Pacific as a Rossby-wave type response, leading to above-normal precipitation over northern SCS. Negative NIO SST anomalies contribute to anomalous cyclonic winds over the western North Pacific by an anomalous east–west vertical circulation north of the equator, favoring more rainfall over northern SCS. These NIO SST anomalies are closely related to preceding La Niña and El Niño events through the “atmospheric bridge”. Thus, the NIO SST anomalies serve as a medium for an indirect impact of preceding ECP SST anomalies on the SCS summer rainfall variability. The ECP SST influence is identified to be dominant after 1990 and the NIO SST impact is relatively more important during 1980s. These Indo-Pacific SST effects are further investigated by conducting numerical experiments with an atmospheric general circulation model. The consistency between the numerical experiments and the observations enhances the credibility of the Indo-Pacific SST influence on the SCS summer rainfall variability.