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The Bay of Bengal (BoB) plays a fundamental role in controlling the weather systems that make up the South Asian summer monsoon system. In particular, the southern BoB has cooler sea surface temperatures (SST) that influence ocean–atmosphere interaction and impact the monsoon. Compared to the southeastern BoB, the southwestern BoB is cooler, more saline, receives much less rain, and is influenced by the summer monsoon current (SMC). To examine the impact of these features on the monsoon, the BoB Boundary Layer Experiment (BoBBLE) was jointly undertaken by India and the United Kingdom during June–July 2016. Physical and biogeochemical observations were made using a conductivity–temperature–depth (CTD) profiler, five ocean gliders, an Oceanscience Underway CTD (uCTD), a vertical microstructure profiler (VMP), two acoustic Doppler current profilers (ADCPs), Argo floats, drifting buoys, meteorological sensors, and upper-air radiosonde balloons. The observations were made along a zonal section at 8°N between 85.3° and 89°E with a 10-day time series at 8°N, 89°E. This paper presents the new observed features of the southern BoB from the BoBBLE field program, supported by satellite data. Key results from the BoBBLE field campaign show the Sri Lanka dome and the SMC in different stages of their seasonal evolution and two freshening events during which salinity decreased in the upper layer, leading to the formation of thick barrier layers. BoBBLE observations were taken during a suppressed phase of the intraseasonal oscillation; they captured in detail the warming of the ocean mixed layer and the preconditioning of the atmosphere to convection. [ABSTRACT FROM AUTHOR]

This paper describes the study of the relationship between the thermal configuration of the Bay of Bengal (BOB)-Tibetan Plateau (TP) region and the precipitation anomaly in Yunnan, a province in China, in May using ERA-Interim data and precipitation data for May from 125 meteorological stations across Yunnan for 1979-2014. Results from the analysis indicate that the interannual variability of May precipitation in Yunnan is significantly modulated by the BOB-TP thermal configuration. Model runs with a linear baroclinic model suggest physical consistency. The thermal conditions over the BOB mainly impact the May precipitation anomaly in Yunnan via changes in water vapor transport from the eastern BOB northeastward to southwestern Yunnan. The second factor influencing precipitation anomalies relates to the characteristics and variability of cold air transport from the TP to northeastern Yunnan. When the BOB (the TP) is occupied by positive (negative) diabatic heating, a thermal gradient with a warmer (colder) center over the BOB and a colder (warmer) center over the TP is established, and more-than-normal (less than normal) precipitation in Yunnan will occur in May. This relationship can persist from April to the following May to some extent; therefore, the BOB-TP thermal configuration in April could be used to forecast May precipitation in Yunnan. [ABSTRACT FROM AUTHOR]

The very severe cyclonic storm (VSCS) 'Phailin (2013)' was the strongest cyclone that hit the eastern coast of the India Odisha state since the supercyclone of 1999. But the same story of casualties was not repeated as that of 1999 where approximately 10 000 fatalities were reported. In the case of Phailin, a record 1 million people were evacuated across 18 000 villages in both the Odisha and Andhra Pradesh states to coastal shelters following the improved operational forecast guidance that benefited from highly skillful and accurate numerical model guidance for the movement, intensity, rainfall, and storm surge. Thus, the property damage and death toll were minimized through the proactive involvement of three-tier disaster management agencies at central, state, and district levels. [ABSTRACT FROM AUTHOR]

This analysis examines the predictability of several key forecasting parameters using the ECMWF Variable Ensemble Prediction System (VarEPS) for tropical cyclones (TCs) in the North Indian Ocean (NIO) including tropical cyclone genesis, pregenesis and postgenesis track and intensity projections, and regional outlooks of tropical cyclone activity for the Arabian Sea and the Bay of Bengal. Based on the evaluation period from 2007 to 2010, the VarEPS TC genesis forecasts demonstrate low false-alarm rates and moderate to high probabilities of detection for lead times of 1-7 days. In addition, VarEPS pregenesis track forecasts on average perform better than VarEPS postgenesis forecasts through 120 h and feature a total track error growth of 41 n mi day−1. VarEPS provides superior postgenesis track forecasts for lead times greater than 12 h compared to other models, including the Met Office global model (UKMET), the Navy Operational Global Atmospheric Prediction System (NOGAPS), and the Global Forecasting System (GFS), and slightly lower track errors than the Joint Typhoon Warning Center. This paper concludes with a discussion of how VarEPS can provide much of this extended predictability within a probabilistic framework for the region. [ABSTRACT FROM AUTHOR]

The Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) with the capability of measuring sea surface temperature (SST) in the presence of clouds, has been providing an unprecedented view of tropical basin-scale SST variability. In this paper, an assessment of the accuracy of the SST derived from TMI over the Bay of Bengal using in situ data collected from moored buoys and research ships, is presented. The authors find that TMI captures the evolution of the SST of the bay on seasonal time scales with reasonable accuracy. The mean difference between the SST from TMI and buoys is less than 0.1°C, and the rms difference is about 0.6°C. The time scales of the intraseasonal variation of the TMI SST are realistic. However, the amplitude of the SST variation on the intraseasonal scale is overestimated by a factor of about 1.3 when compared to buoy data. It is observed that the SST derived from TMI tends to be lower during periods with deep convection or winds stronger than 10 m s-1, or both. There is better agreement during weak conditions of convection/wind. This leads to a cold bias during convectively active periods when running average SST time series are constructed from SSTs retrieved from the TMI. [ABSTRACT FROM AUTHOR]

Synergizing satellite remote sensing data with vertical profiles of atmospheric thermodynamics and regional climate model simulations, we investigate the relative importance, transport pathways, and seasonality of contribution of dust from regional (Thar Desert and adjoining arid regions) and remote (southwest Asia and northeast Africa) sources over the northeast Indian Ocean [i.e., the Bay of Bengal (BOB)]. We show that while over the northern BOB dust from the regional sources contribute more than 50% to the total dust load during the southwest monsoon period (June–September), interestingly; the remote dust sources dominate rest of the year. On the other hand, over the southern BOB, dust transported from the remote-source regions dominate throughout the year. During June, the dry elevated layer (at altitudes between 850 and 700 hPa) of dust, transported across the Indo-Gangetic Plain to the northern BOB, arises primarily from the Thar Desert. Dust from remote sources in the far west reaches the southern BOB after traversing over and around the southern Indian Peninsula. Since dust from these distinct source regions have different mineral composition (hence optical properties) and undergo distinct changes during atmospheric transport, it is important to understand source-specific dust contribution and transport pathways to address dust–climate feedback. [ABSTRACT FROM AUTHOR]

In the Bay of Bengal (BoB), surface heat fluxes play a key role in monsoon dynamics and prediction. The accurate representation of large-scale surface fluxes is dependent on the quality of gridded reanalysis products. Meteorological and surface flux variables from five reanalysis products are compared and evaluated against in situ data from the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) in the BoB. The reanalysis products: ERA-Interim (ERA-I), TropFlux, MERRA-2, JRA-55, and CFSR are assessed for their characterization of air–sea fluxes during the southwest monsoon season [June–September (JJAS)]. ERA-I captured radiative fluxes best while TropFlux captured turbulent and net heat fluxes Qnet best, and both products outperformed JRA-55, MERRA-2, and CFSR, showing highest correlations and smallest biases when compared to the in situ data. In all five products, the largest errors were in shortwave radiation QSW and latent heat flux QLH, with nonnegligible biases up to approximately 75 W m−2. The QSW and QLH are the largest drivers of the observed Qnet variability, thus highlighting the importance of the results from the buoy comparison. There are also spatially coherent differences in the mean basinwide fields of surface flux variables from the reanalysis products, indicating that the biases at the buoy position are not localized. Biases of this magnitude have severe implications on reanalysis products’ ability to capture the variability of monsoon processes. Hence, the representation of intraseasonal variability was investigated through the boreal summer intraseasonal oscillation, and we found that TropFlux and ERA-I perform best at capturing intraseasonal climate variability during the southwest monsoon season. [ABSTRACT FROM AUTHOR]

Lateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resolution data from a cruise in November 2013. The observations are from a radiator survey centered at a salinity-controlled density front, embedded in a zone of moderate mesoscale strain (0.15 times the Coriolis parameter) and forced by winds with a downfront orientation. Below a thin mixed layer, often ≤10 m, the analysis shows several dynamical signatures indicative of submesoscale processes: (i) negative Ertel potential vorticity (PV); (ii) low-PV anomalies with O(1-10) km lateral extent, where the vorticity estimated on isopycnals and the isopycnal thickness are tightly coupled, varying in lockstep to yield low PV; (iii) flow conditions susceptible to forced symmetric instability (FSI) or bearing the imprint of earlier FSI events; (iv) negative lateral gradients in the absolute momentum field (inertial instability); and (v) strong contribution from differential sheared advection at O(1) km scales to the growth rate of the depth-averaged stratification. The findings here show one-dimensional vertical processes alone cannot explain the vertical stratification and its lateral variability over O(1-10) km scales at the radiator survey. [ABSTRACT FROM AUTHOR]

The quasi-biweekly oscillation (QBW) is a dominant intraseasonal mode in summer rainfall over Bangladesh. Active phases of the QBW are often accompanied by low pressure systems (LPSs) such as vortex-type lows. This study investigated the effects of two intraseasonal modes, the QBW and the boreal summer intraseasonal oscillation (BSISO), on the genesis ofLPSs over Bangladesh during 29 summer monsoon seasons. Daily lag composites of convection and low-level atmospheric circulation were constructed for active-phase cases with LPSs (LPS case) and without LPSs (non-LPS case) based on rainfall in the QBW over Bangladesh. In the QBW mode, a westward propagation of an anticyclonic anomaly from the western Pacific to the Bay of Bengal (BoB) is common in both cases. However, the anticyclonic center in the LPS case is located slightly to the east of that in the non-LPS case, which results in stronger cyclonic vorticity over and around Bangladesh. In contrast, the BSISO mode shows an opposite phase between the two cases: a cyclonic (anticyclonic) anomaly propagating northward from the equator to the BoB in the LPS case (non-LPS case). In the LPS case, the cyclonic anomaly in the BSISO mode enhances the westerly (easterly) flow over the BoB (Bangladesh) in the active phase, resulting in the enhancement of cyclonic vorticity over the northern BoB and Bangladesh, in cooperation with the QBW mode. These results suggest that both the QBW and BSISO modes have significant influence on the environmental conditions for LPS genesis over Bangladesh. [ABSTRACT FROM AUTHOR]

Satellite observations between 2007 and 2015 are used to characterize the annual occurrence of the premonsoon drought (PMD), which causes human death and economic hardship in India, and to postulate its scientific causes. The PMDis identified as the driest and hottest weeks in central India just before the summer monsoon onset. The onset is marked by a sharp increase in precipitation and soil moisture and a decrease in air temperature. The difference between integrated moisture transported in from the Arabian Sea and out to the Bay of Bengal is largely deposited as rain over land during the summer monsoon. The PMDoccurs during the short period when moisture is drawn out to the Bay of Bengal before it can be replenished from the Arabian Sea. The time gap is caused by the earlier start of summer monsoon (southwest) winds in the Bay of Bengal than in the Arabian Sea. Sea surface temperature rise precedes the start of summer monsoon wind in both the Arabian Sea and the Bay of Bengal, and it has the potential to give advance warning of the PMD and thus allow mitigation of the adverse effects. [ABSTRACT FROM AUTHOR]

Seasonal and intraseasonal differences in mesoscale convective systems (MCSs) over South Asia are examined using A-Train satellites, a ground-based lightning network, and reanalysis fields. Premonsoon (April-May) MCSs occur primarily over Bangladesh and the eastern Bay of Bengal. During the monsoon (June-September), small MCSs occur over the Meghalaya Plateau and northeast Himalayan notch, while large and connected MCSs are most widespread over the Bay of Bengal. Monsoon MCSs produce less lightning and exhibit more extensive stratiform and anvil reflectivity structures in CloudSat observations than do premonsoon MCSs. During the monsoon, Bay of Bengal and Meghalaya Plateau MCSs vary with the 30-60-day northward-propagating intraseasonal oscillation, while northeast Himalayan notch MCSs are associated with weak large-scale anomalies but locally enhanced CAPE. During intraseasonal active periods, a zone of enhanced large and connected MCSs, precipitation, and lightning extends from the northeastern Arabian Sea southeastward over India and the Bay of Bengal, flanked by suppressed anomalies. Spatial variability is observed within this enhancement zone: lightning is most enhanced where MCSs are less enhanced, and vice versa. Reanalysis composites indicate that Bay of Bengal MCSs are associated with monsoon depressions, which are frequent during active monsoon periods, while Meghalaya Plateau MCSs are most frequent at the end of break periods, as anomalous southwesterly winds strengthen moist advection toward the terrain. Over both regions, MCSs exhibit more extensive stratiform and anvil regions and less lightning when the large-scale environment is moister, and vice versa. [ABSTRACT FROM AUTHOR]

Local sea surface temperature (SST) plays an important role in the onset of the Bay of Bengal (BoB) summer monsoon (BoBSM). Previous study indicated that the meridionally warmest SST axis (WSSTA) appears in mid-April in the central BoB, which may be a precursor for the BoBSM onset. In this study, it is found that a warm but not the meridionally warmest center, which is defined as the secondary WSSTA (SWSSTA), occurs in early April in the central BoB, leading the BoBSM onset by five pentads. Dates of the SWSSTA occurrence are significantly positively correlated with dates of the WSSTA occurrence in the central BoB and the BoBSM onset on an interannual time scale. The SWSSTA is an earlier precursor for the BoBSM onset. The formation of the oceanic precursor and its impact on the BoBSM onset are as follows. Before the BoBSM onset, resulting from more surface heat input and shallower mixed layer affected by the low-level anticyclone and subtropical high in the central BoB, local SST shows the most rapid increase. Meanwhile, the situation is adverse to the rapid increase of SST in the equatorial BoB. For this reason, the SWSSTA occurs, and the WSSTA subsequently appears in the central BoB. The WSSTA in turn enhances local convection, eliminates the low-level anticyclone, and moves the subtropical high outward away from the BoB by inducing atmospheric instability, thus developing a heating center. Convectional heating further strengthens southwesterlies in the BoB by exciting mixed planetary-gravity waves, resulting in the BoBSM onset. [ABSTRACT FROM AUTHOR]

During each summer monsoon, the Bay of Bengal is inundated by a large amount of rain and river discharge. The effects of this freshening are gradually reversed over the course of the year, with near-surface salinities typically returning to their presummer monsoon levels before the start of the next rainy season. While the forcing responsible for the summertime freshening is clear, the processes that act to restore the bay's salinity are not well understood. To examine these processes, the authors construct a basin-integrated, near-surface, seasonal salinity budget using data-assimilated output from the Hybrid Coordinate Ocean Model (HYCOM). From this salinity budget, it is deduced that vertical salt fluxes are primarily responsible for counterbalancing the near-surface freshening caused by the summertime freshwater fluxes. These vertical salt fluxes are largest during the months that immediately follow the summer monsoon, when the near-surface halocline is strongest. These results must be tempered with the knowledge that HYCOM misrepresents some key features of the bay's salinity field. In particular, the model tends to overestimate salinity along the East Indian Coastal Current during its equatorward phase. Notwithstanding these biases, these results still suggest that vertical processes have a prominent role in the bay's near-surface salinity budget. [ABSTRACT FROM AUTHOR]

Recent in situ buoy observations revealed interesting seasonal features of the diurnal sea surface temperature cycle (DSST) in the eastern tropical Indian Ocean. Composite analysis shows that areas away from the equator exhibit stronger seasonal variations of DSST, while weaker seasonal variations appear near the equator. The most interesting characteristic is the distinctive contrast of the seasonal variations of DSST between the Bay of Bengal (BOB) and the region south of the equator (particularly around 12°S). While the range of DSST is weakest in the BOB during boreal summer, it has its largest range around 12°S in austral summer. Furthermore, BOB DSST exhibits two peaks that occur during the monsoon transitions (March-April and October), whereas DSST south of the equator shows only a single peak in its annual cycle. Using a one-dimensional, oceanic, mixed layer model, the authors examined the cause of the distinctive annual cycles of DSST north and south of the equator. Two parallel experiments were conducted at buoy sites 12°N, 90°E and 12°S, 80.5°E driven by surface forcing from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) product. The results demonstrated that, in the BOB, both surface shortwave radiation and wind stress contribute to the March maximum, whereas the wind stress alone drives the October maximum. In contrast, the seasonal variation of DSST south of the equator is primarily caused by the annual cycle of the wind stress, which is extremely weak in austral summer near the intertropical convergence zone (ITCZ). How the monsoon and ITCZ modulate the distinctive annual cycles of DSST is discussed. [ABSTRACT FROM AUTHOR]

The role of the El Niño-Southern Oscillation (ENSO) on the modulation of tropical cyclone activity over the Bay of Bengal (BoB) for the 1979-2011 period is examined. It is shown that Niño-3.4 sea surface temperature (SST) anomalies are negatively correlated with the BoB tropical cyclone activity to a statistically significant percentage by a lead time of 5 months. Composites of 10-m zonal winds exhibit greater variance during La Niña events, favoring the development of low-level cyclonic vorticity. Low vertical wind shear over the central and northern BoB also aids in the development of tropical cyclones during La Niña events. Increased relative humidity is the result of enhanced moisture transport and higher precipitable water under La Niña conditions. Furthermore, storm-relative composites of relative humidity show stronger moisture pulses over the BoB during La Niña. The enhanced moisture associated with tropical cyclogenesis likely aids in the development and strengthening of the systems. ENSO forces modulations in oceanic conditions as well. The observed negative (positive) SST anomalies during La Niña (El Niño) could be seen as the result of increased (decreased) net heat flux across the sea surface. Tropical cyclone activity varies between El Niño and La Niña as a result of anomalous wind and moisture patterns during each ENSO phase. [ABSTRACT FROM AUTHOR]

Making use of the extensive shipboard and aircraft measurements of aerosol properties over the oceanic regions surrounding the Indian peninsula, under the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) field experiment during the premonsoon season (March-May), supplemented with long-term satellite data and chemical transport model simulations, investigations are made of the east-west and north-south gradients in aerosol properties and estimated radiative forcing, over the oceans around India. An eastward gradient has been noticed in most of the aerosol parameters that persisted both within the marine atmospheric boundary layer and above up to an altitude of ~6 km; the gradients being steeper at higher altitudes. It was also noticed that the north-south gradient has contrasting patterns over the Bay of Bengal and the Arabian Sea on the either side of the Indian peninsula. The aerosol-induced atmospheric heating rate increased from a low value of ≤0.1 K day−1 in the southwestern Arabian Sea to as high as ~0.5 K day−1 over the northeastern Bay of Bengal. The simulations of species-resolved spatial gradients have revealed that the observed gradients are the result of the strong modulations by anthropogenic species over the natural gradients, thereby emphasizing the role of human activities in imparting regional forcing. These large spatial gradients in aerosol forcing induced by mostly anthropogenic aerosols over the oceanic regions around the Indian peninsula can potentially affect the regional circulation patterns. [ABSTRACT FROM AUTHOR]

The annual cycle of tropical cyclone (TC) frequency over the Bay of Bengal (BoB) exhibits a notable bimodal character, different from a single peak in other basins. The causes of this peculiar feature were investigated through the diagnosis of a genesis potential index (GPI) with the use of the NCEP Reanalysis I dataset during the period 1981-2009. A methodology was developed to quantitatively assess the relative contributions of four environmental parameters. Different from a conventional view that the seasonal change of vertical shear causes the bimodal feature, it was found that the strengthened vertical shear alone from boreal spring to summer cannot overcome the relative humidity effect. It is the combined effect of vertical shear, vorticity, and SST that leads to the GPI minimum in boreal summer. It is noted that TC frequency in October-November is higher than that in April-May, which is primarily attributed to the difference of mean relative humidity between the two periods. In contrast, more supercyclones (category 4 or above) occur in April-May than in October-November. It is argued that greater ocean heat content, the first branch of northward-propagating intraseasonal oscillations (ISOs) associated with the monsoon onset over the BoB, and stronger ISO intensity in April-May are favorable environmental conditions for cyclone intensification. [ABSTRACT FROM AUTHOR]

The environmental field of tropical cyclogenesis over the Bay of Bengal is analyzed for the extended summer monsoon season (approximately May-November) using best-track and reanalysis data. Genesis potential index (GPI) is used to assess four possible environmental factors responsible for tropical cyclogenesis: lower-tropospheric absolute vorticity, vertical shear, potential intensity, and midtropospheric relative humidity. The climatological cyclogenesis is active within high GPI in the premonsoon (~May) and postmonsoon seasons (approximately October-November), which is attributed to weak vertical shear. The genesis of intense tropical cyclone is suppressed within the low GPI in the mature monsoon (approximately June-September), which is due to the strong vertical shear. In addition to the climatological seasonal transition, the authors' composite analysis based on tropical cyclogenesis identified a high GPI signal moving northward with a periodicity of approximately 30-40 days, which is associated with boreal summer intraseasonal oscillation (BSISO). In a composite analysis based on the BSISO phase, the active cyclogenesis occurs in the high GPI phase of BSISO. It is revealed that the high GPI of BSISO is attributed to high relative humidity and large absolute vorticity. Furthermore, in the mature monsoon season, when the vertical shear is climatologically strong, tropical cyclogenesis particularly favors the phase of BSISO that reduces vertical shear effectively. Thus, the combination of seasonal and intraseasonal effects is important for the tropical cyclogenesis, rather than the independent effects. [ABSTRACT FROM AUTHOR]

Cyclone Sidr, one of the most devastating tropical cyclones that resulted in several thousand deaths and substantial damages, developed in the north Indian Ocean and made landfall over the Bangladesh coast on 15 November 2007. Observation and simulation results show that Sidr was embedded in a nonuniform environment and contained an intense outer rainband to the east of its center and a significant frontal band to the northwest. A detailed study of the outer rainband is performed by numerical simulation. The eastern band was a long, quasi-straight shape in the meridional direction that remained stationary relative to the cyclone center. This band was composed of convective cells that developed southeast of the center within a synoptic-scale convergence zone and propagated along the band toward the northeast quadrant. The speed of the downwind-propagating cells was greater than that of the cyclone, which resulted in a convective cluster northeast of the center. Only the downwind portion of the band consisted of convection with stratiform rain, whereas the upwind and middle portions contained active convective cells without stratiform rain. The band was located between the synoptic-scale flows of a weakly sheared, gradient-balanced westerly and a strongly sheared, nongradient-balanced prevailing southerly caused by the complex terrain of the Bay of Bengal's southeast region. Low-level convergence along the band was dominated by cross-band flow from both sides of the band and was confined below 3 km. As the cyclone moved northward, the convergence zone resulted in the extension of band length up to ~800 km. The southerly at the eastern side of the center gradually accelerated and was directed toward the center by a strong pressure gradient force. The flow accumulated a substantial amount of water vapor from the sea in addition to the increased moisture in the lower troposphere, resulting in further intensification of the convective cells. [ABSTRACT FROM AUTHOR]

Using a dataset extended by the addition of data for 2004-08, this study reexamined the trend in the sensible heating (SH) flux at 73 meteorological stations over the Tibetan Plateau (TP) during 1980-2008 and investigated its impact on monsoon precipitation in the surrounding region. In contrast to ongoing climate warming, a weakening trend in SH is persistent over most of the plateau, despite a sharp increase in the ground-air temperature difference in 2004-08. The weakening trend in SH over the TP is primarily a response to the spatial nonuniformity of large-scale warming over the East Asian continent, which is characterized by much greater warming amplitude at mid- and high latitudes than over the tropics and subtropics. Furthermore, the suppressed air pump effect, which is driven by SH over the TP and acts as a strong forcing source, gives rise to reduced precipitation along the southern and eastern slopes of the plateau, and increased rainfall over northeastern India and the Bay of Bengal. No significantly stable correlation exists between the SH source over the TP and the overall trend or interdecadal variability in the East Asian or South Asian summer monsoon. [ABSTRACT FROM AUTHOR]

Repeat XBT transects made at near-fortnightly intervals in the Lakshadweep Sea (southeastern Arabian Sea) and ocean data assimilation products are examined to describe the year-to-year variability in the observed near-surface thermal inversions during the winter seasons of 2002–06. Despite the existence of a large low-salinity water intrusion into the Lakshadweep Sea, there was an unusually lower number of near-surface thermal inversions during the winter 2005/06 compared to the other winters. The possible causative mechanisms are examined. During the summer monsoon of 2005 and the following winter season, unusually heavy rainfall occurred over the southwestern Bay of Bengal and the Lakshadweep Sea compared to other years in the study. Furthermore, during the winter of 2005, both the East India Coastal Current and the Winter Monsoon Current were stronger compared to the other years, transporting larger quantities of low salinity waters from the Bay of Bengal into the Lakshadweep Sea where a relatively cooler near-surface thermal regime persisted owing to prolonged upwelling until November 2005. In addition, the observed local surface wind field was relatively stronger, and the net surface heat gain to the ocean was weaker over the Lakshadweep Sea during the postmonsoon season of 2005. Thus, in winter 2005/06, the combination of prolonged upwelling and stronger surface wind field resulting in anomalous net surface heat loss caused weaker secondary warming of the near-surface waters in the Lakshadweep Sea. This led to a weaker horizontal sea surface temperature (SST) gradient between the Lakshadweep Sea and the intruding Bay of Bengal waters and, hence, a reduced number of thermal inversions compared to other winters despite the presence of stronger vertical haline stratification. [ABSTRACT FROM AUTHOR]

Mountain weather changes in the Nepal Himalaya were intensively examined during the 2003 monsoon onset using in situ datasets, observed by multiple automatic weather stations (AWS) distributed across the Coordinated Enhanced Observing Period (CEOP) Himalaya reference site at locations with significant differences in altitude. Onset of monsoon rainfall characterized as nighttime precipitation was initiated simultaneously at all stations with the occurrence and migration of a monsoon depression in the north of the Bay of Bengal. Satellite infrared images detected evident suppression of diurnal cloud development after the onset. About two weeks prior to the onset, the mountain weather abruptly changed such that the daytime valley wind at lower elevations weakened associated with morning clouds and the nighttime southerly wind began at upper elevations. The timing corresponded with the weakening of the westerly wind over the Himalaya due to the northward shift of the upper subtropical jet stream. The time lag between the precipitation onset and the change in the mountain weather was confirmed by 9 yr of in situ AWS data. The mechanism of nighttime southerly winds at high elevations is also discussed in relation to large-scale monsoon flow and local circulation. [ABSTRACT FROM AUTHOR]

The seasonal evolution of the 20–90-day intraseasonal variability (ISV) of the convection in the Indo-Pacific region presents intriguing features related to monsoon dynamics: (i) a sharp ISV maximum in May for the southern Bay of Bengal and in June for the eastern Arabian Sea, (ii) a maximum ISV over the west Pacific in July–September when the ISV over the northern Indian Ocean is weaker, (iii) a persistent ISV north of Australia from December to March, and (iv) a weak ISV over continental regions. The source of these behaviors is investigated from time series of satellite observations, meteorological reanalysis, and from an ocean mixed layer depth (MLD) climatology. For the northern Indian Ocean, sharp ISV maxima in May and June, when the MLD is still small (∼20–30 m), are related to an abrupt surface cooling associated with the setting of the monsoon low-level jet. The ISV of the convection is smaller over these regions during July–September when the MLD is deepened (∼60–70 m) by the monsoon low-level wind forcing. At this time, the low-level wind is weak over the west Pacific, the MLD is small, and the amplitude of the ISV remains large. North of Australia, and also over the south equatorial Indian Ocean, there is no particular reinforcement of the ISV near the onset. A weak low-level jet and a small MLD prevail during the whole monsoon season in good agreement with a more uniform ISV. These results support the hypothesis that a moderately shallow (∼20–30 m) ocean mixed layer is important for maintaining locally the ISV of the convection, when the conditions (e.g., SST, large-scale circulation) for such an ISV to exist are satisfied. The seasonal variability of the MLD, related in part to the monsoon low-level jet variability over the tropical oceans, thus gives a link between the seasonal march of monsoon circulations and the seasonal evolution of the ISV of the convection. This has implications for a better understanding of the origin of the ISV and for the predictability of its amplitude, especially near the monsoon onset date. [ABSTRACT FROM AUTHOR]

The behavior of convective activity over the Bay of Bengal during the 1988 and 1999 monsoon seasons is examined using 3-hourly satellite infrared data. More organized convective activity, spreading farther south into the bay, occurred in 1988 than in 1999. A distinct spatial grouping of convective systems by size is found. The east side of the bay experiences most of the rainfall over water, and here the convective systems are relatively small, short lived, and frequent. At the northwest side of the bay near most of the land-based rainfall, convective activity is organized into much larger and longer-lived systems. The diurnal cycle of all the systems over the bay, regardless of size, shows a 0600 local time (LT) maximum in very cold cloud tops (infrared brightness temperature <210 K), with genesis occurring between 2100 and 0300 LT (2100 LT for the larger, longer-lived systems). The cloud systems dissipate after sunrise, with the larger systems lasting until the afternoon. The land-water interface is important for the convection genesis and thereby affects the spatial distribution of convection. Offshore nocturnal convection begins near shore, with later convection occurring farther out over the bay and attaining a larger size. The preference for nocturnal initiation times differs markedly from the afternoon initiation times typical of the tropical western Pacific Ocean, but the time of maximum cloud area extent and dissipation are similar. The strength of the diurnal cycle varies greatly with location, with the northwest side of the bay experiencing both the highest amount of very cold cloudiness and the strongest diurnal cycle. The Joint Air-Sea Monsoon Interaction Experiment (JASMINE) research cruise experienced the only multiday sequence of large, diurnally repeating, southward moving disturbances at 11 °N, 89°E in the 2 yr examined, but both the convective diurnal cycle and propagation direction were consistent with climatology. An interesting... [ABSTRACT FROM AUTHOR]

Assimilated analysis fields from the South China Sea Monsoon Experiment and the outgoing longwave radiation data from the National Center for Atmospheric Research (NCAR) have been employed to describe the large-scale and synoptic feamres of the subtropical circulation during the Bay of Bengal (BOB; 6°-20°N, 80°-100°E) and South China Sea (SCS; 7°-20°N, 110°-120°E) monsoon onsets in 1998. The results show that the Asian monsoon onset during May 1998 exhibited a typical eastward development from the BOB region to the SCS domain. The weakening and retreat of the subtropical anticyclone from the SCS were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the SCS (NSCS; 15°-20°N, 110°-120°E). As the vertical shear of zonal wind changes in sign, the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the SCS. Based on these observational results, version 4 of the NCAR climate model (CCM3) is used to investigate the physical link between the convection associated with the BOB monsoon vortex and the SCS summer monsoon onset, as well as the mechanism of the evolution of the low-level subtropical anticyclone over the SCS. Introduction of heating over the BOB results in vigorous convection over the BOB, and the BOB monsoon onset, as well as the development of westerlies and vertical ascent over the NSCS region due to an asymmetric Rossby wave response. Together with the low-level moisture advection, convection is induced over the NSCS. It is the condensation heating over the NSCS that causes the overturning of the meridional gradient of temperature over the SCS. Consequently the subtropical anticyclone in the lower troposphere over the SCS weakened gradually. Eventually as convection develops over the entire SCS domain, the subtropical anticyclone moves out of the region. [ABSTRACT FROM AUTHOR]

The Indian southwest monsoon directly affects the lives of over one billion people, providing almost 90% of the annual precipitation to the Indian subcontinent. An important characteristic of the southwest monsoon is variability on subseasonal timescales, with "active" periods of heavy rain interrupted by drier "break" periods. Both the number of monsoon breaks in a season and the timing of these breaks profoundly impact agricultural output from the Indian subcontinent. Most research on monsoon breaks has emphasized possible atmospheric mechanisms. However, new satellite data reveal large-amplitude basin-scale subseasonal sea surface temperature (SST) variability in the Bay of Bengal (BOB), in which northern BoB cooling precedes monsoon breaks by about 1 week. The relationship is statistically significant at the 95% level over the 3 yr examined, and so offers a potential statistical predictor for short-term monsoon variability. The basinwide averaged amplitude of SST changes is 1°-2°C and local changes can exceed 3°C over 2 weeks; these changes are as large as those seen in the local climatological seasonal cycle. This raises the possibility that air-sea interaction may be a significant factor in monsoon variability; the SST variability is coherent with monsoon variability with a phase relationship consistent with a coupled oscillation. A schematic coupled air-sea oscillator mechanism is offered for further study, in which oceanic changes play a dynamical role in monsoon variability. [ABSTRACT FROM AUTHOR]

A nonlinear, 4½-layer reduced-gravity ocean model with active thermodynamics and mixed layer physics is used to investigate the causes of sea level interannual variability in the Bay of Bengal, which may contribute to flooding and cholera outbreaks in Bangladesh. Forcing by NCEP–NCAR reanalysis fields from 1958 to 1998 yields realistic solutions in the Indian Ocean basin north of 29°S. Controlled experiments elucidate the roles of the following forcing mechanisms: interannual variability of the Bay of Bengal wind, equatorial wind, river discharges into the bay, and surface buoyancy flux including precipitation minus evaporation (heat fluxes + P - E). Sea level changes in the bay result largely from wind variability, with a typical amplitude of 10 cm and occasionally 10–25 cm at an interannual timescale. Near the eastern and northern boundaries, sea level anomalies (SLAs) are predominantly caused by equatorial wind variability, which generates coastal Kelvin waves that propagate into the bay along the eastern boundary. Near the western boundary the bay wind has a comparable influence as the equatorial wind, especially during the southwest monsoon season, owing to the counterclockwise propagation of coastal Kelvin waves forced by the large-scale alongshore wind stress in the bay. In the bay interior, SLAs are dominated by the equatorial wind forcing in the central bay, result almost equally from the equatorial and the bay wind in the southwestern bay, and are dominated by the bay wind forcing in the southwestern bay during the southwest monsoon. The westward intensification of the bay wind influence is associated with the westward propagation of Rossby waves forced by the large-scale wind curl in the interior bay. The effect of heat fluxes + P - E is generally small. Influence of interannual variability of river discharges is negligible. SLAs caused by the equatorial wind at the equator and that caused by the bay wind along the northe... [ABSTRACT FROM AUTHOR]