Simulation of monsoon intraseasonal variability in NCEP CFSv2 and its role on systematic bias

We have evaluated the simulation of Indian summer monsoon and its intraseasonal oscillations in the National Centers for Environmental Prediction climate forecast system model version 2 (CFSv2). The dry bias over the Indian landmass in the mean monsoon rainfall is one of the major concerns. In spite of this dry bias, CFSv2 shows a reasonable northward propagation of convection at intraseasonal (30–60 day) time scale. In order to document and understand this dry bias over the Indian landmass in CFSv2 simulations, a two pronged investigation is carried out on the two major facets of Indian summer monsoon: one, the air–sea interactions and two, the large scale vertical heating structure in the model. Our analysis shows a possible bias in the co-evolution of convection and sea surface temperature in CFSv2 over the equatorial Indian Ocean. It is also found that the simulated large scale vertical heat source (Q1) and moisture sink (Q2) over the Indian region are biased relative to observational estimates. Finally, this study provides a possible explanation for the dry precipitation bias over the Indian landmass in the simulated mean monsoon on the basis of the biases associated with the simulated ocean–atmospheric processes and the vertical heating structure. This study also throws some light on the puzzle of CFSv2 exhibiting a reasonable northward propagation at the intraseasonal time scale (30–60 day) despite a drier monsoon over the Indian land mass.     

Intraseasonal Oscillation in the Tropical Indian Ocean

1. Introduction The intraseasonal oscillation (ISO or Madden- Julian Oscillation, MJO) in the tropical atmosphere has been studied extensively, including its existence, structure, evolution and propagation (Madden and Ju- lian, 1971; Murakami, et al., 198…     

Possible role of warm SST bias in the simulation of boreal summer monsoon in SINTEX-F2 coupled model

Reasonably realistic climatology of atmospheric and oceanic parameters over the Asian monsoon region is a pre-requisite for models used for monsoon studies. The biases in representing these features lead to problems in representing the strength and variability of Indian summer monsoon (ISM). This study attempts to unravel the ability of a state-of-the-art coupled model, SINTEX-F2, in simulating these characteristics of ISM. The coupled model reproduces the precipitation and circulation climatology reasonably well. However, the mean ISM is weaker than observed, as evident from various monsoon indices. A wavenumber–frequency spectrum analysis reveals that the model intraseasonal oscillations are also weaker-than-observed. One possible reason for the weaker-than-observed ISM arises from the warm bias, over the tropical oceans, especially over the equatorial western Indian Ocean, inherent in the model. This warm bias is not only confined to the surface layers, but also extends through most of the troposphere. As a result of this warm bias, the coupled model has too weak meridional tropospheric temperature gradient to drive a realistic monsoon circulation. This in turn leads to a weakening of the moisture gradient as well as the vertical shear of easterlies required for sustained northward propagation of rain band, resulting in weak monsoon circulation. It is also noted that the recently documented interaction between the interannual and intraseasonal variabilities of ISM through very long breaks (VLBs) is poor in the model. This seems to be related to the inability of the model in simulating the eastward propagating Madden–Julian oscillation during VLBs.     

The Moisture Structure of ISO in Western North Pacific Revealed by AIRS

Using the humidity profiles from the Atmospheric Infrared Sounder (AIRS) dataset, rainfall from the Tropical Rainfall Measuring Mission (TRMM) Global Precipitation Index (GPI), and surface winds from QuickSCAT (QSCAT) as well as SST from the Advanced Microwave Scanning Radiometer for NASA's Earth Observing System (AMSR-E), we analyzed the structure of summer intraseasonal oscillation (ISO) over the western North Pacific region in 2003--2004. We find that the signal of 20--90-day oscillations in the western North Pacific originates from the equatorial Indian Ocean, and propagates eastward to Philippine Sea and then moves northwestward to South China. The AIRS humidity data reveal that the boundary-layer moisture leads the mid-troposphere moisture during the ISO propagation. The positive SST anomaly may play an important role to moistening the boundary-layer, which preconditions the ISO propagation.Therefore, the intraseasonal SST anomaly could positively feed back to the atmosphere through moistening the boundary-layer,destabilizing the troposphere, and contributing to the northwestward propagation of the ISO in western North Pacific. On the other hand, the salient feature that the boundary-layer moisture anomaly leads mid-troposphere moisture does not exist in ECMWF/TOGA analysis.     

Simulation of Indian summer monsoon intraseasonal oscillations in a superparameterized coupled climate model: need to improve the embedded cloud resolving model

The characteristic features of Indian summer monsoon (ISM) and monsoon intraseasonal oscillations (MISO) are analyzed in the 25 year simulation by the superparameterized Community Climate System Model (SP-CCSM). The observations indicate the low frequency oscillation with a period of 30–60 day to have the highest power with a dominant northward propagation, while the faster mode of MISO with a period of 10–20 day shows a stationary pattern with no northward propagation. SP-CCSM simulates two dominant quasi-periodic oscillations with periods 15–30 day and 40–70 day indicating a systematic low frequency bias in simulating the observed modes. Further, contrary to the observation, the SP-CCSM 15–30 day mode has a significant northward propagation; while the 40–70 day mode does not show prominent northward propagation. The inability of the SP-CCSM to reproduce the observed modes correctly is shown to be linked with inability of the cloud resolving model (CRM) to reproduce the characteristic heating associated with the barotropic and baroclinic vertical structures of the high-frequency and the low-frequency modes. It appears that the superparameterization in the General Circulation Model (GCM) certainly improves seasonal mean model bias significantly. There is a need to improve the CRM through which the barotropic and baroclinic modes are simulated with proper space and time distribution.     

Daily modes of South Asian summer monsoon variability in the NCEP climate forecast system

The leading modes of daily variability of the Indian summer monsoon in the climate forecast system (CFS), a coupled general circulation model, of the National Centers for Environmental Predictions (NCEP) are examined. The space?Ctime structures of the daily modes are obtained by applying multi-channel singular spectrum analysis (MSSA) on the daily anomalies of rainfall. Relations of the daily modes to intraseasonal and interannual variability of the monsoon are investigated. The CFS has three intraseasonal oscillations with periods around 106, 57 and 30?days with a combined variance of 7%. The 106-day mode has spatial structure and propagation features similar to the northeastward propagating 45-day mode in the observations except for its longer period. The 57-day mode, despite being in the same time scale as of the observations has poor eastward propagation. The 30-day mode is northwestward propagating and is similar to its observational counterpart. The 106-day mode is specific to the model and should not be mistaken for a new scale of variability in observations. The dominant interannual signal is related to El Ni?o-Southern Oscillation (ENSO), and, unlike in the observations, has maximum variance in the eastern equatorial Indian Ocean. Although the Indian Ocean Dipole (IOD) mode was not obtained as a separate mode in the rainfall, the ENSO signal has good correlations with the dipole variability, which, therefore, indicates the dominance of ENSO in the model. The interannual variability is largely determined by the ENSO signal over the regions where it has maximum variance. The interannual variability of the intraseasonal oscillations is smaller in comparison.     

Dynamical linkage of tropical and subtropical weather systems to the intraseasonal oscillations of the Indian summer monsoon rainfall. Part I: observations

The Indian summer monsoon rainfall (ISMR) over the Western Ghats (WG) and the Bay of Bengal (BoB) is marked by the intraseasonal oscillations (ISOs) with preferred 10–20-day and 30–50-day bands. On the basis of pentad Climate Prediction Center Merged Analysis Precipitation and daily sea level pressure and winds at 850?hPa derived from European Center for Medium-range Weather Forecast reanalysis, we present the structure and evolution of the ISOs linked to the ISMR variations over the WG and the BoB and the associated anomalies of the atmospheric circulation using the approaches of wavelet analysis, bandpass filtering and composite analysis. This study reveals that the activities of both the intertropical convergence zone (ITCZ) and the western Pacific subtropical high (WPSH) contribute strongly to the structure and propagation of the ISOs on intraseasonal time scales. Northward development and propagation of the ITCZ plays a critical role in the northward-propagating ISOs, but not in the westward-propagating BoB 10–20-day ISOs. The latter ISOs may be linked, instead, to the activity of synoptic-scale weather systems to the east over the western tropical Pacific. The enhanced ITCZ in the tropical Indian Ocean plays a strong role in the sudden strengthening of the WPSH during the transition from the break to active phase of the 30–50-day ISOs. We find that the strong WPSH in the Asian summer monsoon season, with generally northward advance and eastward withdrawal, promotes the formation of a northwest to southeast tilted anomalous rainfall belt over the East Asian tropical summer monsoon region and the western tropical Pacific in the 30–50-day low-frequency band. Positive (Negative) elongated rainfall anomalies with an unbroken northwest-southeast tilt, strong easterly (westerly) anomalies in the tropical Pacific, and northward advance and eastward movement of strong (weak) WPSH are favorable for maintaining the eastward propagation of the 30–50-day ISOs in the Pacific. Daily high-resolution sea surface temperature obtained from the National Oceanic and Atmospheric Administration is used to explain the propagation features of the 10–20-day ISOs in the Indian Ocean.     

Intraseasonal variability of latent-heat flux in the South China Sea

Intraseasonal variability (ISV) of latent-heat flux in the South China Sea (SCS) is examined using 9 years of weekly data from January 1998 to December 2006. Using harmonic and composite analysis, some fundamental features of the latent-heat flux ISVs are revealed. Intraseasonal latent-heat flux has two spectral peaks around 28–35 and 49–56 days, comparable with the timescales of the atmospheric ISV in the region. Active monsoon is clearly correlated with positive and negative phases of the ISV of latent-heat flux in the SCS. The characteristics of the intraseasonal latent-heat flux variations in summer are remarkably different from those in winter. The amplitudes of significant intraseasonal oscillations are about 35 and 80 W?m^?2 during summer and winter monsoons, respectively. In summer, the intraseasonal latent-heat flux perturbations are characterized by slow eastward (about 1° latitude/day) and slower northward (about 0.75° longitude/day) propagations, probably in a response to eastward and northward propagating Madden-Julian oscillations (MJOs) from the equatorial Indian Ocean. In contrast, the perturbations appear to remain in the northern SCS region like a quasi-stationary wave in winter. In summer, the intraseasonal latent-heat flux fluctuations are highly correlated with wind speed. In winter, however, they are primarily associated with winds and near-surface air humidity. In addition, the intraseasonal SST variation is estimated to significantly reduce the amplitude of the intraseasonal latent-heat flux by 20% during winter.     

Daily atmospheric variability in the South American monsoon system

The space–time structure of the daily atmospheric variability in the South American monsoon system has been studied using multichannel singular spectrum analysis of daily outgoing longwave radiation. The three leading eigenmodes are found to have low-frequency variability while four other modes form higher frequency oscillations. The first mode has the same time variability as that of El Nino-Southern Oscillation (ENSO) and exhibits strong correlation with the Pacific sea surface temperature (SST). The second mode varies on a decadal time scale with significant correlation with the Atlantic SST suggesting an association with the Atlantic multidecadal oscillation (AMO). The third mode also has decadal variability but shows an association with the SST of the Pacific decadal oscillation (PDO). The fourth and fifth modes describe an oscillation that has a period of about 165 days and is associated with the North Atlantic oscillation (NAO). The sixth and seventh modes describe an intraseasonal oscillation with a period of 52 days which shows strong relation with the Madden-Julian oscillation. There exists an important difference in the variability of convection between Amazon River Basin (ARB) and central-east South America (CESA). Both regions have similar variations due to ENSO though with higher magnitude in ARB. The AMO-related mode has almost identical variations in the two regions, whereas the PDO-related mode has opposite variations. The interseasonal NAO-related mode also has variations of opposite sign with comparable magnitudes in the two regions. The intraseasonal variability over the CESA is robust while it is very weak over the ARB region. The relative contributions from the low-frequency modes mainly determine the interannual variability of the seasonal mean monsoon although the interseasonal oscillation may contribute in a subtle way during certain years. The intraseasonal variability does not seem to influence the interannual variability in either region.     

热带东风急流的气候学和中期振荡

本文用13年夏半年(5—10月)月平均风场和两年逐日风场资料研究了200hPa南亚热带东风急流的气候学特征和中期振荡过程。研究表明,南亚夏季热带东风急流显著的非季节性变动和年际差异与低纬对流层高层大尺度环流变化和南亚夏季风活动密切相关,相对于多年平均而言,存在5类异常的东风急流。 各种分析表明,热带东风带存在三种主要的中期振荡。准50天周期振荡与夏季南亚对流层上部大尺度散度场的变化相关联,表现为十分显著的向南的位相传播。准50天和25天振荡均存在显著的年际变化。准50天周期振荡系统性不强的年份,准25天周期振荡是低纬行星波的主要振荡,在东风急流区除表现为系统性向西传播外也表现为向南的位相传播。准双周振荡在东风带一般向西传播。      

Evaluation of mean and intraseasonal variability of Indian summer monsoon simulation in ECHAM5: identification of possible source of bias

The performance of ECHAM5 atmospheric general circulation model (AGCM) is evaluated to simulate the seasonal mean and intraseasonal variability of Indian summer monsoon (ISM). The model is simulated at two different vertical resolutions, with 19 and 31 levels (L19 and L31, respectively), using observed monthly mean sea surface temperature and compared with the observation. The analyses examine the biases present in the internal dynamics of the model in simulating the mean monsoon and the evolution of the boreal summer intraseasonal oscillation (BSISO) and attempts to unveil the reason behind them. The model reasonably simulates the seasonal mean-state of the atmosphere during ISM. However, some notable discrepancies are found in the simulated summer mean moisture and rainfall distribution. Both the vertical resolutions, overestimate the seasonal mean precipitation over the oceanic regions, but underestimate the precipitation over the Indian landmass. The performance of the model improves with the increment of the vertical resolution. The AGCM reasonably simulates some salient features of BSISO, but fails to show the eastward propagation of the convection across the Maritime Continent in L19 simulation. The propagation across the Maritime Continent and tilted rainband structure improve as one moves from L19 to L31. The model unlikely shows prominent westward propagation that originates over the tropical western Pacific region. L31 also produces some of the observed characteristics of the northward propagating BSISOs. However, the northward propagating convection becomes stationary in phase 5–7. The simulation of shallow diabatic heating structure and the heavy rainfall activity over the Bay of Bengal indicate the abundance of the premature convection-generated precipitation events in the model. It is found that the moist physics is responsible for the poor simulation of the northward propagating convection anomalies.     

季节内印度洋-西太平洋对流涛动对次季节-季节尺度大气可预报性的影响

利用非线性局部Lyapunov指数和条件非线性局部Lyapunov指数定量估计了季节内印度洋-西太平洋对流涛动（IPCO）和实时多变量Madden-Julian指数（RMM指数）可预报期限,量化了季节内IPCO对S2S尺度大气可预报性的贡献,深入研究了季节内IPCO演变下S2S尺度可预报期限空间分布的变化规律。结果表明：（1）与RMM指数相比,季节内IPCO指数可预报性更强,可预报期限达到31天左右,比RMM指数高出2周以上;（2）印度洋-西太平洋区域S2S尺度大气可预报性最强,可预报期限达到30天以上,其中季节内IPCO是该地区的主要可预报性来源之一,其贡献达到6天,占总可预报期限的25%以上;（3）随着季节内IPCO的演变,印度洋-西太平洋地区S2S尺度大气可预报性有空间结构变化,表现为可预报期限异常的传播和振荡。S2S尺度大气可预报期限正负异常沿季节内IPCO传播路径,一支以赤道中西印度洋为起点北传至印度半岛,一支向东传播,经过海洋性大陆到赤道西太平洋后向北传播,到达日本南部。同时,可预报性异常的传播在在东印度洋和西太平洋表现出反向变化的特征,形成东西两极振荡,当季节内IPCO向正位相发展时,东印度洋具有更强的可预报性,西太平洋具有更弱的可预报性,反之亦然。季节内IPCO的发展（衰退）可使东印度洋（西太平洋）S2S尺度大气可预报性更强,表明模式预报技巧对此具有更大的提升空间。     

Boreal summer quasi-monthly oscillation in the global tropics

The boreal summer intraseasonal oscillation (ISO) in the global tropics is documented here using a 7-year suite (1998–2004) of satellite measurements. A composite scenario was made of 28 selected events with reference to the oscillation in the eastern equatorial Indian Ocean (EIO), where the oscillation is most regular and its intensity is indicative of the strength of the subsequent northward propagation. The average oscillation period is about 32 days, and this quasi-monthly oscillation (QMO) is primarily confined to the tropical Indian and Pacific Oceans. Topics that were investigated are the partition of convective versus stratiform clouds, the vertical structure of precipitation rates, and the evolution of cloud types during the initial organization and the development of intraseasonal convective anomalies in the central Indian Ocean. During the initiation of the convective anomalies, the stratiform and convective rains have comparable rates; the prevailing cloud type experiences a trimodal evolution from shallow to deep convection, and finally to anvil and extended stratiform clouds. A major northwest/southeast-slanted rainband forms as the equatorial rainfall anomalies reach Sumatra, and the rainband subsequently propagates northeastward into the west Pacific Ocean. The enhanced precipitation in the west Pacific then rapidly traverses the Pacific along the Intertropical Convergence Zone, meanwhile migrating northward to the Philippine Sea. A seesaw teleconnection in rainfall anomalies is found between the southern Bay of Bengal (5–15°N, 80–100°E) and the eastern Pacific (5–15°N, 85–105°W). Local sea-surface temperature (SST)-rainfall anomalies display a negative simultaneous correlation in the off-equatorial regions but a zero correlation (quadrature phase relationship) near the equator. We propose that atmosphere–ocean interaction and the vertical monsoon easterly shear are important contributors to the northeastward propagation component of the intraseasonal rainband. The observed evidence presented here provides critical information for validating the numerical models, and it supports the self-induction mechanism theory for maintenance of the boreal summer ISO.     

Dynamical linkage of tropical and subtropical weather systems to the intraseasonal oscillations of the Indian summer monsoon rainfall. Part II: Simulations in the ENSEMBLES project

We assess the ability of individual models (single-model ensembles) and the multi-model ensemble (MME) in the European Union-funded ENSEMBLES project to simulate the intraseasonal oscillations (ISOs; specifically in 10–20-day and 30–50-day frequency bands) of the Indian summer monsoon rainfall (ISMR) over the Western Ghats (WG) and the Bay of Bengal (BoB), respectively. This assessment is made on the basis of the dynamical linkages identified from the analysis of observations in a companion study to this work. In general, all models show reasonable skill in simulating the active and break cycles of the 30–50-day ISOs over the Indian summer monsoon region. This skill is closely associated with the proper reproduction of both the northward propagation of the intertropical convergence zone (ITCZ) and the variations of monsoon circulation in this band. However, the models do not manage to correctly simulate the eastward propagation of the 30–50-day ISOs in the western/central tropical Pacific and the eastward extension of the ITCZ in a northwest to southeast tilt. This limitation is closely associated with a limited capacity of models to accurately reproduce the magnitudes of intraseasonal anomalies of both the ITCZ in the Asian tropical summer monsoon regions and trade winds in the tropical Pacific. Poor reproduction of the activity of the western Pacific subtropical high on intraseasonal time scales also amplify this limitation. Conversely, the models make good reproduction of the WG 10–20-day ISOs. This success is closely related to good performance of the models in the representation of the northward propagation of the ITCZ, which is partially promoted by local air–sea interactions in the Indian Ocean in this higher-frequency band. Although the feature of westward propagation is generally represented in the simulated BoB 10–20-day ISOs, the air–sea interactions in the Indian Ocean are spuriously active in the models. This leads to active WG rainfall, which is not present in the observed BoB 10–20-day ISOs. Further analysis indicates that the intraseasonal variability of the ISMR is generally underrepresented in the simulations. Skill of the MME in seasonal ISMR forecasting is strongly dependent on individual model performance. Therefore, in order to improve the model skill with respect to seasonal ISMR forecasting, we suggest it is necessary to better represent the robust dynamical links between the ISOs and the relevant circulation variations, as well as the proportion of intraseasonal variability in the individual models.     

Intra-seasonal variability in Oceansat-2 scatterometer sea-surface winds over the Indian summer monsoon region

In September 2009, the Indian Space Research Organisation launched a Ku-band microwave scatterometer (OSCAT) onboard the polar orbiting satellite ‘Oceansat-2’. In this article, the capabilities of the newly available OSCAT sea-surface winds are demonstrated by studying the monsoon intra-seasonal variabilities during the 2010 summer monsoon season. A preliminary validation of OSCAT surface winds with European Centre for Medium Range Weather Forecasting (ECMWF) analysis surface winds carried out during June to August 2010 suggests that the quality of the OSCAT winds are able to meet the mission specifications. The observed mean monthly features of the Indian summer monsoon in July and August 2010 from OSCAT match well with those of ECMWF reanalysis winds. The OSCAT winds capture the known characteristics of the Indian summer monsoon, such as the northward propagation of a low level jet, and its preferred locations during active and break monsoon conditions, reasonably well. The Morlet wavelet transform is used for time series analysis. The OSCAT measured sea-surface winds were found to possess two dominant modes of variability during the 2010 monsoon season: one with a periodicity between 32 and 64?days, and another with a periodicity between 8 and 16?days. Rainfall activity over the Indian summer monsoon region is closely associated with the phases of the two above-mentioned dominant intra-seasonal variabilities. This study demonstrates that the OSCAT winds can be used very well and with confidence for meteorological studies.     

The upper-level circulation anomaly over Central Asia and its relationship to the Asian monsoon and mid-latitude wave train in early summer

A large intraseasonal variation in geopotential height over the Central Asia region, where the Asian subtropical jet is located, occurs between May and June, and the most dominant variation has a wave-like distribution. This variation in geopotential height influences precipitation across South and Southeast Asia. In this paper, we use composite analysis to determine the causes of this intraseasonal variation over Central Asia. The wave train propagates from the northern Atlantic Ocean to Central Asia over a period of a week, and generates an anomaly in geopotential height over the region. The tropical disturbance, which is similar to the Madden–Julian oscillation, appears a few days before the maximum of the anticyclonic anomaly over Central Asia, and is accompanied by active convection over the Indian Ocean and suppressed convection over Central America. Results of numerical experiments using a linear baroclinic model show that the active convection over the northern Indian Ocean causes the anticyclonic anomaly over Central Asia. The wave train that extends from the northern Atlantic Ocean to Central Asia is generated by negative thermal forcing over Central America, and the phase distribution of this wave train is similar to that observed in the composite analysis. Central Asia is the region where the effects of the tropics and middle latitudes overlap, and it is an important connection point between the Asian monsoon and middle latitudes.     

亚洲两个季风区大气季节内振荡的比较分析

本文利用ECMWF逐日再分析资料(1961～2000年),主要从结构和水平传播包括经向传播和纬向传播方面对南海和南亚两个季风区的30～60天振荡进行了比较分析.研究结果发现,两个季风区的大气季节内振荡都存在明显的年际变化,但并没有出现同相或反相的变化关系.两个季风区ISO在结构上既有相似的地方也有不同点,在垂直结构上都...     

西北太平洋夏季风的变化对台风生成的影响

研究了西北太平洋夏季风特征及其季风槽结构对台风生成的影响。当西北太平洋季风槽增强并向东扩展使季风加强时,西北太平洋的风速垂直切变、高低空辐散风、湿度和海温等都对台风的生成产生有利的影响,台风数明显比季风槽弱时多。而且对台风生成的位置也有很大的影响,即季风槽强时,台风的生成位置偏东,季风槽弱时台风的位置偏西。这表明西北太平洋夏季风主要是通过季风槽活动影响台风的生成。而夏季风的强弱对台风也有影响,在西北太平洋夏季风的活跃阶段,西北太平洋夏季风强时,台风生成的比较多,夏季风中断时台风生成的比较少。西北太平洋夏季风通过季风的季节内振荡对西北太平洋台风也有显著的影响。季节内振荡对台风生成的影响主要以30—60 d振荡为主。在这种低频振荡对流活动的湿位相时期台风生成个数明显多,干位相时期台风生成的少。而且低频振荡的西风位相也有利于台风生成,在东风位相时生成的台风少。另外,还研究了多台风期西北太平洋夏季的特征(群发性),发现在这些时期,存在强的季风槽,弱的垂直切变与充足的水汽供应。这表明西北太平洋台风时空的群发性与夏季风活动的异常密切相关。     

Performance of FGOALS-s2 in simulating intraseasonal oscillation over the south Asian monsoon region

The capability of the current version of the air-sea coupled climate model, the Flexible Global Ocean-Atmosphere-Land System model, Spectral Version 2 (FGOALS-s2), in simulating the boreal summer intraseasonal oscillation (ISO) over the south Asian monsoon (SAM) region is diagnosed, in terms of dominant period, propagation direction, and vertical structure. Results show that the coupled model can reasonably simulate the main features of observed ISO propagation compared to the chosen AGCM. These features include the eastward movement of intraseasonal 850-hPa zonal wind over the Arabian Sea and Bay of Bengal, the vertical structure in active phases, and the realistic phase relationship between ISO and underlying SST. However, the eastward propagation cannot be reproduced in the uncoupled model. This suggests that air-sea interaction is important in generating intraseasonal variability over the SAM region. Nevertheless, some deficiencies remain in the coupled model, which may relate to physical processes depicted by the cumulus parameterization and PBL schemes within its atmospheric component.