Summer Monsoon Trough Research Articles

Western North Pacific Monsoon Vorticity Distribution as a Potential Driver of Interannual Meridional Migration of the Boreal Summer Synoptic-Scale Wave Train

Abstract On interannual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave (SSW) train relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure. A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG–TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of large-scale circulation and tropical cyclones.

Increasing tropical cyclone intensity in the western North Pacific partly driven by warming Tibetan Plateau

The increase in intense tropical cyclone (TC) activity across the western North Pacific (WNP) has often been attributed to a warming ocean. However, it is essential to recognize that the tropical WNP region already boasts high temperatures, and a marginal increase in oceanic warmth due to global warming does not exert a significant impact on the potential for TCs to intensify. Here we report that the weakened vertical wind shear is the primary driver behind the escalating trend in TC intensity within the summer monsoon trough of the tropical WNP, while local ocean surface and subsurface thermodynamic factors play a minor role. Through observational diagnoses and numerical simulations, we establish that this weakening of the vertical wind shear is very likely due to the increase in temperature of the Tibetan Plateau. With further warming of the Tibetan Plateau under the Representative Concentration Pathway 4.5 scenario, the projected TCs will likely become stronger.

An Approach to Link Climate Model Tropical Cyclogenesis Bias to Large‐Scale Wind Circulation Modes

AbstractAttributing sources of tropical cyclogenesis (TCG) bias to large‐scale circulation in global circulation models is challenging. Here, we propose the use of empirical orthogonal functions as an approach to understand model bias of TCG. Two leading modes of large‐scale wind circulations in the West Pacific can explain the TCG frequency and location in both climate reanalysis and the MetUM model. In the reanalysis, the two modes distinguish the summer monsoon trough position and the strength of the north Pacific subtropical high. However, in the model, the wind circulations are biased toward the positive phase of simulated modes thus overestimating TCG in the entire Main Development Region. This bias is further related to the north‐eastward shifted monsoon trough and a weakened subtropical high, and overly strong tropics‐subtropics connections. This approach could be deployed more widely to other basins and models to diagnose the causes of TCG bias.

Radar observed convective storm characteristics at the eastern edge of the Indian summer monsoon trough

Radar observed convective storm characteristics at the eastern edge of the Indian summer monsoon trough

Effects of Environmental Relative Vorticity and Seasonal Variation on Tropical Cyclones over the Western North Pacific

An improved understanding of the environmental factors influencing tropical cyclones (TCs) is vital to enhance the accuracy of forecasting TC intensity. More than half of TCs that were substantially affected by environmental factors were predominantly affected by low-level environmental relative vorticity (hereafter, VOR TCs). In this study, the seasonal variation and related physical features of VOR TCs from 2003–2017 during TC seasons in summer and autumn over the western North Pacific were analyzed. Autumn VOR TCs exhibited the strongest intensity among all TCs over the western North Pacific. The enhanced environmental relative vorticity during the TC intensification period was larger and more favorably distributed for VOR TC development in autumn. The vorticity diagnostic analysis showed that the convergence was the positive source of environmental relative vorticity of VOR TCs, while the contribution of convergence was larger in autumn than in summer. The increased convergence was related to seasonal variation in larger-scale systems, especially the higher environmental pressure gradient, which reflected the larger subtropical high and the compressed East Asian summer monsoon trough in autumn. In addition, the East Asian summer monsoon trough was also somewhat stronger during the intensification period of VOR TCs, especially in autumn.

Understanding the relationship of storm‐ to large‐scale environment in the monsoon trough region: results inferred from long‐term radar and reanalysis datasets

AbstractMonsoon trough (MT) convection exhibits multiscale spatial variability ranging from cellular storms to large organized convective systems. Understanding the relationships between storm‐scale convection and the large‐scale environment is crucial for accurately representing the properties and effect of convection in climate models. We investigate these relationships using nine wet seasons (June–September of 2009–2017) of S‐band radar observations over a tropical location (Kolkata) in the Indian summer monsoon trough region. A Lagrangian‐based objective cell‐tracking method, Thunderstorm Identification, Tracking, Analysis, and Nowcasting (TITAN), is applied to the volumetric reflectivity data to identify and track the convective storms. ERA5 hourly reanalysis products from the European Centre for Medium‐Range Weather Forecasts (ECMWF) are utilized for the field variables representing the large‐scale environment. We focus on the relationships of radar‐derived storm properties with large‐scale dynamic, thermodynamic, and stability variables such as convergence, mid‐level humidity, convective available potential energy (CAPE), and convective inhibition (CIN). Storm‐scale convection shows a selective nature, characterized as embedded cells in their unique environment. Furthermore, storms are classified as storm cells (simple and intense) and multicellular (linear and nonlinear) based on their geometrical properties and intensity. MT convection is predominantly composed of storm cells and characterized by an environment of moist mid‐level, low‐to‐moderate CAPE, low CIN, and strong dynamic forcing, that is, vertical ascent. In dry and weak dynamic forcing (i.e., subsiding atmosphere) with high CAPE and CIN, the highest rain intensity is associated with a few linear and nonlinear multicellular storms. In the development of such major linear and nonlinear multicellular storms over the eastern flank of the MT, the mid‐troposphere acts as a dry capping inversion. It is observed that the storm‐scale convection varies with the large‐scale dynamic forcing. During weak (strong) dynamic forcing conditions, the CAPE (mid‐level humidity) relates well with the observed storm‐scale convection.

Interannual covariability of summer monsoon precipitation in Yunnan, China, and diabatic heating anomalies over the Arabian Sea–Bay of Bengal

Covariability of summer monsoon precipitation in Yunnan and diabatic heating anomalies over the Arabian Sea–Bay of Bengal (AS–BOB) is analyzed using ERA-Interim monthly reanalysis data and monthly mean precipitation data from 124 stations across Yunnan, China. Singular value decomposition analysis indicates a seesaw pattern associated with the diabatic heating over the AS–BOB and summer monsoon precipitation in Yunnan over interannual timescales. When the diabatic heating over the AS–BOB is weaker (stronger) than the climatology during the summer monsoon season, more (less) summer monsoon precipitation generally occurs in Yunnan. A significant correlation between the diabatic heating over the AS–BOB and summer monsoon precipitation in Yunnan begins in late spring–early summer and persists throughout the summer. The AS–BOB diabatic heating in late spring–early summer may thus have a prediction potential for summer monsoon precipitation in Yunnan. The anomalous low-level winds associated with the diabatic heating anomalies over the AS–BOB influence the Indian summer monsoon trough, the vertical motion in Yunnan, and then the summer monsoon precipitation. Numerical experiments based on a linear baroclinic model confirm the key physical process revealed in the observational diagnosis. These experiments also indicate that the diabatic heating over the AS–BOB has a greater impact on summer monsoon precipitation in Yunnan than does diabatic heating around Yunnan on the atmospheric circulation over the AS–BOB. The diabatic heating anomalies over the AS–BOB contributed to the severe summer droughts in Yunnan in 2010, 2011, and 2013 by influencing the circulation.

Impact of interannually varying background circulation on summertime wave patterns and tropical cyclone tracks in the western North Pacific

This study analyzed summertime wave patterns in the western North Pacific by separating them into those during El Nino (EN) and La Nina (LN) years for 1979–2015. The background flow revealed that in the EN years, the subtropical high was weaker and the East Asian summer monsoon trough was deeper and extended farther southeastward. The composite results indicated that in the EN years, tropical cyclone (TC) tracks were more solid and stably associated with wave pattern propagation routes because the circulation anomalies were significantly stronger. However, in the LN years, when the wave patterns were weak under the shorter and less organized monsoon trough due to the enhanced subtropical high, the TC tracks and cyclonic anomalies were irregular and sporadic. Further background flow analyses indicated that the EN circulation favored the development of the monsoon trough and east–west expansion and contraction of the subtropical ridge, whereas the stronger subtropical ridge dominated most western North Pacific areas in the LN years. Therefore, the monsoon trough was weaker in the LN years and induced recurving TCs to take more scattered routes near Taiwan and China’s east coast. Therefore, Taiwan experienced more rainfall during recurving TC periods in the LN years than in the EN years. For the straight-moving TCs passing through Taiwan, rainfall was concentrated over southern Taiwan in the EN years but more evenly distributed in the LN years. Thus, background flow changes strongly affect wave patterns and TCs.

Northward Propagation, Initiation, and Termination of Boreal Summer Intraseasonal Oscillations in a Zonally Symmetric Model

AbstractA simple multilayer zonally symmetric model, using a multicloud convective parameterization and coupled to a dynamical bulk atmospheric boundary layer, is used here to simulate boreal summer intraseasonal oscillations (BSISO) in the summer monsoon trough and elucidate the underlying main physical mechanisms responsible for their initiation, propagation, and termination. Northward-moving precipitating events initiated near the equator propagate northward at roughly 1° day−1 and terminate near 20°N. Unlike earlier findings, the northward propagation of precipitation anomalies in this model is due to the propagation of positive moisture anomalies in the northward direction, resulting from an asymmetry in the meridional velocity induced by the beta effect. From a moisture-budget perspective, advection constitutes a biased intrusion of dry air into the convection center, forcing new convection events to form north of the wave disturbance, while moisture convergence supplies the precipitation sink. The BSISO events are initiated near the equator when the competing effects between first-baroclinic divergence and second-baroclinic convergence, induced by the descending branch of the Hadley cell and in situ congestus heating, respectively, become favorable to convective intensification. The termination often near 20°N and halfway stalling of these precipitating events occur when the asymmetry in the first-baroclinic meridional winds weakens and when the negative moisture gradient to the north of the convection center becomes too strong as the anomaly exits the imposed warm pool domain.

Interannual variability of the seesaw mode of the interface between the Indian and East Asian summer monsoons

The relation between the seesaw mode of the Interface between the Indian summer monsoon and East Asian summer monsoon (IIE) and the South China Sea summer monsoon trough (SCSSMT) and the Indian summer monsoon trough (ISMT) is investigated using two atmospheric reanalyses together with outgoing longwave radiation, sea surface temperature (SST), and gridded precipitation datasets. Canonical correlation analysis combined with empirical orthogonal functions, correlation, and composite analysis are employed. Results indicate that a stronger ISMT and SCSSMT resulting from colder SST over the tropical Indian Ocean and tropical east-central Pacific cause the IIE to deviate from its normal position in an anticlockwise direction, with a node at around 22°N. This leads to heavier than normal summer rainfall over the north-central Indian subcontinent and South China Sea, but weaker than normal from the low and middle reaches of the Yangtze River and South Korea to central Japan. A weaker ISMT and SCSSMT resulting from warmer SST over the tropical Indian Ocean and tropical east-central Pacific causes the IIE to deviate from its normal position in a clockwise direction, and the anomalous summer rainfall pattern is the opposite of that for the stronger troughs. Further analysis indicates that the SCSSMT plays a crucial role in the evolution of the IIE seesaw mode. The latitudinal difference between the IMST and SCSSMT may be one of the most important reasons for the formation of the IIE seesaw mode.

Synoptic climatology and sea surface temperatures teleconnections for warm season heat waves in Saudi Arabia

Little information is available about the synoptic weather patterns and physical factors contributing to the formation and intensity of heat waves (HWs) in Saudi Arabia. The research objectives were to identify the synoptic situations that are related to HW occurrence, to match different HW aspects (frequency and intensity) related to different circulation types, and to examine the possible links/associations between HW days and the sea surface temperature (SST) anomalies of closely associated water bodies (i.e., Mediterranean Sea, Black Sea, Caspian Sea, Arabian Gulf, Arabian Sea, and Red Sea).Using a time-sensitive approach, a HW was defined as a period of at least two consecutive days with a daily maximum temperature equal or higher than the 90th percentile of the monthly maximum and with a daily minimum temperature equal or higher than the 85th percentile of the monthly minimum for the decade in question. Using reanalysis data for 1985–2014, three weather/circulation types were identified using Ward's method for cluster determination. Together, weather types 1 and 3 induced 57.5% of HW days and connected with negative anomalies in sea level pressure with lower heights and warmer temperatures at the 850 hPa level. Weather type 2 induced 42.5% of HW days and was related to positive anomalies at all heights. Intensification of Indian Summer Monsoon Trough and Arabian heat lows were key atmospheric features related to weather types 1 and 3. Anomalies of SSTs of the Red Sea, Arabian Gulf, Caspian Sea, Black Sea, and Mediterranean Sea displayed positive values for both weather types 1 and 3. SSTs anomalies seem to be a more important factor for intensity of a HW day. HWs in Saudi Arabia tend to occur during regional warming due to atmospheric circulation conditions and SST teleconnections.

Interannual Variation of the Summer Rainfall Center in the South China Sea

Abstract A northwest–southeast-oriented summer monsoon trough exists between northern Indochina and northwestern Borneo. Ahead of this the South China Sea (SCS) trough is located at a convergent center west of the Philippines, which provides an environment favorable for rain-producing synoptic systems to produce rainfall over this center and form the SCS summer rainfall center. Revealed from the x–t diagram for rainfall, this rainfall center is developed by multiple-scale processes involved with the SCS trough (TR), tropical depression (TY), interaction of the SCS trough with the easterly wave/tropical depression (EI), and easterly wave (EW). It is found that 56% of this rainfall center is produced by the SCS trough, while 41% is generated by the other three synoptic systems combined. Apparently, the formation of the SCS summer monsoon rainfall center is contributed to by these four rain-producing synoptic systems from the SCS and the Philippines Sea. The Southeast Asian summer monsoon undergoes an interannual variation and exhibits an east–west-oriented cyclonic (anticyclonic) anomalous circulation centered at the western tropical Pacific east of the Luzon Strait. This circulation change is reflected by the deepening (filling) of the SCS summer monsoon trough, when the monsoon westerlies south of 15°N intensify (weaken). This interannual variation of the monsoon westerlies leads to the interannual variation of the SCS summer monsoon rainfall center to follow the Pacific–Japan oscillation of rainfall. The rainfall amount produced over this rainfall center during the weak monsoon season is about two-thirds of that produced during the strong monsoon season. The rain-production ratio between TR and TY + EI + EW is 60:38 during the strong monsoon season and 47:49 during the weak monsoon season.

Relationships between ENSO and the East Asian–western North Pacific monsoon: observations versus 18 CMIP5 models

The western North Pacific anomalous anticyclone (WNPAC) plays an essential role in linking El Nino and the East Asian–western North Pacific monsoon. The WNPAC and the corresponding WNP anomalous cyclone (WNPC) associated with La Nina are studied by examining the pre-industrial control runs of 18 Coupled Model Intercomparison Project Phase 5 models. The WNPAC is asymmetric with the WNPC during ENSO mature winters, which can be reproduced by seven out of the eighteen models. Based on the differences between the seven models and the other models, it is verified that the asymmetry results from the combined effects of the asymmetric remote forcing from the equatorial central-eastern Pacific and the asymmetric local sea surface temperature (SST) anomalies. It is further demonstrated that the asymmetric circulation anomalies over the tropical WNP contribute to the asymmetric decaying rates between El Nino and La Nina. Those models that can (cannot) simulate the asymmetry between the WNPAC and WNPC tend to reproduce ENSO with asymmetric (symmetric) decaying rates. In El Nino decaying summers, the WNPAC is still maintained due to the sequential effects of the local forcing of the underlying cold SST anomalies remaining from the previous winter and spring, and the remote forcing from the tropical Indian Ocean. Multi-model results show that the coupling of the local cold SST anomalies and the WNPAC forms a damping mode, which is only conducive to maintaining the WNPAC in the early summer, while the response of the WNPAC to the remote forcing from the tropical Indian Ocean intensifies from June to July, concurring with the establishment of the climatological WNP summer monsoon trough.

Simulations of the Present and Late-Twenty-First-Century Western North Pacific Tropical Cyclone Activity Using a Regional Model

Abstract A high-resolution regional atmospheric model is used to simulate present-day western North Pacific (WNP) tropical cyclone (TC) activity and to investigate the projected changes for the late twenty-first century. Compared to observations, the model can realistically simulate many basic features of the WNP TC activity climatology, such as the TC genesis location, track, and lifetime. A number of spatial and temporal features of observed TC interannual variability are captured, although observed variations in basinwide TC number are not. A relatively well-simulated feature is the contrast of years when the Asian summer monsoon trough extends eastward (retreats westward), more (fewer) TCs form within the southeastern quadrant of the WNP, and the corresponding TC activity is above (below) normal over most parts of the WNP east of 125°E. Future projections with the Coupled Model Intercomparison Project phase 3 (CMIP3) A1B scenario show a weak tendency for decreases in the number of WNP TCs, and for increases in the more intense TCs; these simulated changes are significant at the 80% level. The present-day simulation of intensity is limited to storms of intensity less than about 55 m s−1. There is also a weak (80% significance level) tendency for projected WNP TC activity to shift poleward under global warming. A regional-scale feature is a projected increase of the TC activity north of Taiwan, which would imply an increase in TCs making landfall in north China, the Korean Peninsula, and parts of Japan. However, given the weak statistical significance found for the simulated changes, an assessment of the robustness of such regional-scale projections will require further study.

Revisiting the Structure and Characteristics of an Early Summer Monsoon Trough over South China in 1975

Revisiting the Structure and Characteristics of an Early Summer Monsoon Trough over South China in 1975

Factors favorable to frequent extreme precipitation in the upper Yangtze River Valley

Extreme precipitation events in the upper Yangtze River Valley (YRV) have recently become an increasingly important focus in China because they often cause droughts and floods. Unfortunately, little is known about the climate processes responsible for these events. This paper investigates factors favorable to frequent extreme precipitation events in the upper YRV. Our results reveal that a weakened South China Sea summer monsoon trough, intensified Eurasian-Pacific blocking highs, an intensified South Asian High, a southward subtropical westerly jet and an intensified Western North Pacific Subtropical High (WNPSH) increase atmospheric instability and enhance the convergence of moisture over the upper YRV, which result in more extreme precipitation events. The snow depth over the eastern Tibetan Plateau (TP) in winter and sea surface temperature anomalies (SSTAs) over three key regions in summer are important external forcing factors in the atmospheric circulation anomalies. Deep snow on the Tibetan Plateau in winter can weaken the subsequent East Asian summer monsoon circulation above by increasing the soil moisture content in summer and weakening the land–sea thermal contrast over East Asia. The positive SSTA in the western North Pacific may affect southwestward extension of the WNPSH and the blocking high over northeastern Asia by arousing the East Asian-Pacific pattern. The positive SSTA in the North Atlantic can affect extreme precipitation event frequency in the upper YRV via a wave train pattern along the westerly jet between the North Atlantic and East Asia. A tripolar pattern from west to east over the Indian Ocean can strengthen moisture transport by enhancing Somali cross-equatorial flow.

A simulation study of a heavy rainfall process over the Yangtze River valley using the two-way nesting approach

In this study, the major features of a heavy rainfall event in the Yangtze River region on 3–7 June 2011 and its event-related large-scale circulation and predictability were studied. Both observational analysis and model simulation were used, the latter being based on the Weather Research and Forecasting (WRF) model forced by NCEP Global Forecast System (GFS) datasets. It was found that, during 3–5 June, the western Pacific subtropical high apparently extended to the west and was much stronger, and the Indian summer monsoon trough was slightly weaker than in normal years. The east-west oriented shear line over the middle and lower reaches of the Yangtze River was favorable for the transportation and convergence of water vapor, and the precipitation band was located slightly to the south of the shear line. During 6–7 June, the western Pacific subtropical high retreated eastward, while the trough over the Okhotsk Sea deepened. The low vortex in Northeast China intensified, bringing much more cold air to the middle and lower reaches of the Yangtze River, and the shear line over this area moved slightly southward. The convection band moved southward and became weaker, so the rainfall during 6–7 June weakened and was located slightly to the south of the previous precipitation band. Many of the observed features, including background circulation and the distribution and amount of precipitation, were reproduced reasonably by the WRF, suggesting a feasibility of this model for forecasting extreme weather events in the Yangtze River region.

The Abrupt Shift of Typhoon Activity in the Vicinity of Taiwan and Its Association with Western North Pacific–East Asian Climate Change

Abstract Bayesian analysis is applied to detect changepoints in the time series of seasonal typhoon counts in the vicinity of Taiwan. An abrupt shift in the typhoon count series occurs in 2000. On average, 3.3 typhoons per year have been noted before 2000 (1970–99), with the rate increasing to 5.7 typhoons per year since 2000 (2000–06). This abrupt change is consistent with a northward shift of the typhoon track over the western North Pacific–East Asian region and an increase of typhoon frequency over the Taiwan–East China Sea region. The northward shift of the typhoon track tends to be associated with typhoon-enhancing environmental conditions over the western North Pacific, namely, the weakening of the western North Pacific subtropical high, the strengthening of the Asian summer monsoon trough, and the enhanced positive vorticity anomalies in the lower troposphere. Based on observational analysis and model simulations, warm sea surface temperature anomalies over the equatorial western and central Pacific appear to be a major factor contributing to a northward-shifted typhoon track.

El Niño Impacts on Precipitation in the Western North Pacific–East Asian Sector

Abstract In this study, the western North Pacific–East Asian (WNP–EA) rainfall anomaly induced by the strong El Niño in 1982–83, 1991–92, and 1997–98, and its association with the mean state, are examined. Over the northern part of the WNP–EA region (north of 20°N), which is dominated by southwest–northeast tilting frontal systems, positive rainfall anomalies from the fall before the El Niño peak phase (year 0) to the first wet period after the peak phase (year 1) are affected by low- and midlevel horizontal moisture convergence anomalies induced by low-level anticyclonic circulation anomalies over the WNP region that are associated with El Niño. Over the southern part of the WNP–EA region (south of 20°N), which is dominated by tropical convection, positive precipitation anomalies in the first and second wet periods of year 0 and negative precipitation anomalies from the fall of year 0 to the second wet period of year 1 are associated with the variation of the net energy into the atmosphere, which is mainly contributed to by local evaporation anomalies. The mechanisms for inducing the precipitation anomalies over both northern and southern parts of the WNP–EA region are similar to the mechanisms of the mean precipitation in each rainy period, but the detailed processes for the southern WNP–EA precipitation anomalies are more complicated, particularly in summer. In the first wet periods of years 0 and 1 and the fall of year 0, the precipitation anomalies are induced by evaporation anomalies that are contributed to by similar effects of sea surface temperature (SST) and wind speed anomalies. In the second wet period of years 0 and 1, on the other hand, near-surface wind speed anomalies affect precipitation via the process of evaporation. These wind speed anomalies are associated with the concurrence of the low-level circulation anomalies over the WNP region and the Asian summer monsoon trough. The SST anomalies are merely a response to evaporation and downward solar radiation anomalies. The dependence of the rainfall anomalies on the mean state, that is, similar causes for the rainfall mean and anomalies in each rainy period, implies that the mean state plays a key role in simulating the interannual variation over the WNP–EA region.

Satellite Assessment of Divergent Water Vapor Transport from NCEP, ERA40, and JRA25 Reanalyses over the Asian Summer Monsoon Region

The divergent component of water vapor transports was constructed using evaporation, precipitation, and total precipitable water estimated from the Special Sensor Microwave Imager (SSM/I). The SSM/I moisture budget parameters were then compared with those from the National Centers for Environmental Prediction (NCEP), the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA40), and the Japanese 25-year Reanalysis Project (JRA25) data over the Asian monsoon region for the May to September (MJJAS) period from 1988 to 2000.The climatology of SSM/I water vapor transports clearly indicates three major water vapor sources for the Asian monsoon, i.e., the subtropical Indian Ocean and Pacific Ocean in the Southern Hemisphere, and the North Pacific Ocean. In contrast, sinks are located in the Asian summer monsoon trough, the equatorial convective zones over the Indian and western Pacific Oceans, and over the East Asian monsoon region from the northern tip of Philippine Sea to the Kuroshio extension region. These sources and sinks are linked to well-known large-scale rotational circulation features, i.e., the cross-equatorial flow associated with monsoon circulation over the Indian Ocean, the anticyclonic circulation along the western periphery of the western North Pacific High, and the cross-equatorial flow north of New Guinea. In conjunction with the fluctuation of these sources and sinks, the northward propagating climatological intraseasonal oscillations in water vapor flux convergence are evident in the South and East Asian monsoon regions in the SSM/I data.From the comparison of water budget parameters of NCEP, ERA40, and JRA25 reanalysis with SSM/I-derived features, we found that the general features of all three reanalyses are in good agreement with those from SSM/I; however, the magnitudes of water vapor transports are comparatively weaker in all three reanalyses than what SSM/I measurements suggested. In addition, much weaker water vapor transports in three reanalyses are found in the intraseasonal oscillation signals with less distinct patterns, compared to what are inferred from the SSM/I measurements.