North Bay Of Bengal Research Articles

A study on radial characteristics of North Indian Ocean tropical cyclones and associated energy indices through numerical modeling

The structural characteristics of North Indian Ocean (NIO) Tropical Cyclones (TCs) are emphasized in this study by adopting Weather Research and Forecasting (WRF) numerical modeling framework. TCs that occurred during 2001–2020, were simulated using WRF model in the CTRL, and scatterometer wind vortices from four satellites were considered for assimilation purpose in the WRFDA experiment. Considering the results from both types of simulations, best-track data sets, and ERA5 global analyses, several aspects including radial parameters, the thermodynamic and dynamic properties, horizontal structure, etc., were analyzed besides developing some robust empirical relationships. In the process, model performance was evaluated too. The study emphasized on sub-basin, sectorial, intensity-based, range-based, and frequency-based analysis for the radial parameters. Also, yearly, and seasonal variations were considered, inter-comparisons were carried out, the inter-relationships with numerous environmental and storm-related characteristics have been explored. The study provides valuable insights into the TC radial characteristics over NIO, and associated thermodynamic and dynamic behavior. It upholds that the relatively greater intensity TCs occurring during October–November months are mostly observed to have genesis over the East Arabian Sea (EAS), Middle Bay of Bengal (MBOB), and North Bay of Bengal (NBOB) sectors. The EAS region exhibits higher values for the energy indices and maximum sustained wind (MSW), compared to the Western Arabian Sea (WAS), while the maximum convective potential values show an opposite trend. The assimilation of scatterometer wind data has a limited impact on model results, although it is more effective over the Arabian Sea (AS) than the Bay of Bengal (BOB). WRFDA could provide a relatively better estimate for most of the TC radial features over both AS and BOB. The extensive analysis of TC radial features facilitated developing several empirical relationships involving radial characteristics, MSW and minimum central pressure, which are valid for NIO region.

Role of Anomalous Ocean Warming on the Intensification of Pre‐Monsoon Tropical Cyclones Over the Northern Bay of Bengal

AbstractThe northern Indian Ocean, typically the Bay of Bengal (BoB), has experienced several stronger tropical cyclones during the pre‐monsoon season in the last few years. Similarly, in 2021, cyclone Yaas reached the Very Severe Cyclonic Storm (VSCS) stage despite forming above 14°N. In comparison to the other three cyclones that formed in 2007 (Akash), 2009 (Aila), and 2017 (Mora) with a similar cyclogenesis location and track, the pre‐cyclonic condition in 2021 is more favorable. It was found that from 2007 to 2021, the area associated with Sea Surface Temperature greater than 31°C increased from 0.1 percent to 29 percent. The Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction buoy at 15°N also recorded an increasing trend of 0.1°C per year in both average and maximum temperatures during May. The warming in 2021 is primarily linked with Marine Heat Wave events, which extensively cover the north BoB for a longer duration starting from the first week of May. Such warming extends even to the subsurface, providing the heat required to convert the depression into VSCS within 2 days. The weaker western boundary current during 2021 made the northern BoB fresher and favorable for cyclone, intensification. The associated anticyclonic eddy along the track extended the abnormal distribution of low saline water even up to the subsurface (∼100m), which is vital in increasing the ocean heat content. These simultaneous pre‐cyclonic oceanic conditions can be incorporated into the weather model to understand and predict the rapid intensification within a shorter duration.

Influence of boreal summer monsoon intraseasonal oscillations on the occurrences of Marine Heatwave events over the North Bay of Bengal

Influence of boreal summer monsoon intraseasonal oscillations on the occurrences of Marine Heatwave events over the North Bay of Bengal

Variability and predictability of the Northeast India summer monsoon rainfall

AbstractThe mesoscale orography over one of the wettest regions of the world makes the Northeast India (NEI) vulnerable to hydrological disasters while sustaining a biodiversity “hotspot.” The monsoon rainy season over the NEI is known to be longer than June–September (JJAS), but an objective definition has been lacking. Understanding the drivers and predictability of rainfall variability over the region is key for sustainable development planning and adaptation to increasing disasters in the backdrop of a warming climate but remained poor due to lack of a working definition of “summer monsoon season” over NEI. Here, using the relationship between rainfall over the region and winds at 850 hPa over North Bay of Bengal (BoB), we provide an objective definition of the monsoon season over NEI and argue that the “summer monsoon rainy season” over the NEI (NEIR) is from May to September (MJJAS). In contrast to a significant negative relationship between JJAS Central India (CI) rainfall and a JJAS El Niño–Southern Oscillation (ENSO) index, the MJJAS rainfall over the NEI has no relationship with MJJAS or JJAS ENSO. Instead, we unravel that (a) the tropical Northwest‐Pacific (TNWP) sea surface temperature (SST), (b) the South Equatorial Indian Ocean Dipole (SEIOD) and (c) the Atlantic Zonal mode (AZM) are potential drivers of the NEIR variability. Using a causal inference algorithm, namely the Peter and Clark Momentary Conditional Independence (PCMCI) method, we show that SEIOD directly influences NEIR while TNWP SST has a two‐way connection with the NEIR. While “internal variability” may be higher over the NEI, significant modulation of the variances of the subseasonal fluctuations by predictable drivers like the Atlantic Niño and North Atlantic water temperature provides optimism for seasonal prediction of the NEIR.

Eddy‐Freshwater Interaction Using Regional Ocean Modeling System in the Bay of Bengal

AbstractEddy‐freshwater interaction is studied in the north Bay of Bengal (BoB) with a high‐resolution simulation using the Regional Ocean Modeling System. Following observations, the model simulates the trapping and homogenization of river water by a cyclonic mesoscale eddy on a submonthly time scale from October to November 2015. As fresh river water is trapped in the eddy, it is characterized by strong vertical and lateral gradients in salinity. Within a few weeks, these gradients relax along with the progressive homogenization of freshwater within the eddy. A mixed‐layer salinity budget shows the importance of ageostrophic vertical advection in addition to lateral advection during the evolution of salinity within the eddy. An analysis of the eddy kinetic energy (EKE) budget in the upper ocean indicates the development of barotropic and baroclinic instabilities. The vertical profiles of EKE conversion terms reveal that the surface freshwater was involved in the evolution of baroclinic instability within the mixed layer. In addition, an eddy available potential energy (EPE) budget shows that the entrainment of the river water raises the EPE, which is due to an increase in lateral salinity gradients across the eddy during the trapping event. Subsequently, the salinity homogenization leads to a decrease in the EPE, and its rate of decay is modulated by a correlation between surface buoyancy fluxes and density anomalies. Finally, reanalysis data show similar trapping and homogenization of freshwater events across multiple years, highlighting the importance of this mechanism on the subseasonal time scale in the BoB.

Variability of the thermal front and its relationship with Chlorophyll-a in the north Bay of Bengal

The spatial and temporal variability of the thermal fronts and their relationship with chlorophyll-a in the north Bay of Bengal (BOB) was studied using time series of MODIS daily Sea Surface Temperature (SST) and weekly chlorophyll-a concentration to investigate the locations of intense fish catch activities in the continental shelf and slope as inferred from NOAA Visible Infrared Imaging Radiometer Suite (VIIRS) nighttime boat detection products. An edge-detection algorithm operated on each SST image to identify thermal fronts in which a gradient of 0.3 °C/pixel (∼0.075 °C/km) was chosen following the sensitivity analysis. These thermal fronts at the daily scale were aggregated weekly to estimate the thermal front density. A least square-based harmonic analysis procedure was adopted to estimate mean climatology and seasonal harmonics of thermal front density and chlorophyll-a data sets. It shows persistent and coherent spatial features between the datasets in their climatology and seasonal cycles. They exhibit distinct locations of maxima co-occurring between the 10 and 50 m isobaths in the continental shelf and slope region coincided with the areas of the VIIRS mapped fishing vessels cluster sites. The annual frequency dominates the seasonal cycles of TFD with a positive phase during November–March (winter and spring seasons) and peaks in January. Mean (residual) circulation, as simulated from a regional ocean model, shows coherent spatial features congruent to the persistent thermal fronts and steered by the bathymetry. These locations with a high probability of thermal fronts and high concentrations of Chlorophyll-a can be considered as the perennial potential fishing zone, with the winter season being a more favorable period for fishing.

Recurrence of wintertime SST anomalies in the Bay of Bengal: characteristics and causes

This study investigates the winter-to-winter recurrence (WWR) of sea surface temperature anomaly (SSTA) in the Bay of Bengal (BOB) and its causes. During 1948–2016, 36% years show WWRs, which occurred over the BOB north of 13° N. The WWR of net surface heat flux (Qnet), especially the latent heat flux (Qlat) associated with change of winter monsoon wind speed, is essential for all the SSTA WWR events. The interdecadal wintertime warming (cooling) of BOB SST before (after) 1976, which is induced by the weakened (enhanced) monsoon wind and latent heat flux associated with the Interdecadal Pacific Oscillation (IPO) and decadal variability of the Indian Ocean, are the causes for the observed interdecadal variability of warm (cold) SSTA WWR. However, nearly half (48%) of the WWR events are significantly contributed from the subsurface processes, which is equivalent to ~ 26% (42%) contribution to Qnet for the warm (cold) SSTA WWR events. Different from the mid-latitude subsurface re-emergence mechanism that depends critically on wintertime mixed layer deepening, the BOB subsurface effects on WWR of SSTA are associated with shoaling/deepening thermocline, forced remotely by winds along the equator and Ekman pumping within the BOB.

Quasi‐Biweekly Mode of the Asian Summer Monsoon Revealed in Bay of Bengal Surface Observations

AbstractAsian summer monsoon has a planetary‐scale, westward propagating “quasi‐biweekly” mode of variability with a 10–25 day period. Six years of moored observations at 18°N, 89.5°E in the north Bay of Bengal (BoB) reveal distinct quasi‐biweekly variability in sea surface salinity (SSS) during summer and autumn, with peak‐to‐peak amplitude of 3–8 psu. This large‐amplitude SSS variability is not due to variations of surface freshwater flux or river runoff. We show from the moored data, satellite SSS, and reanalyses that surface winds associated with the quasi‐biweekly monsoon mode and embedded weather‐scale systems, drive SSS and coastal sea level variability in 2015 summer monsoon. When winds are calm, geostrophic currents associated with mesoscale ocean eddies transport Ganga‐Brahmaputra‐Meghna river water southward to the mooring, salinity falls, and the ocean mixed layer shallows to 1–10 m. During active (cloudy, windy) spells of quasi‐biweekly monsoon mode, directly wind‐forced surface currents carry river water away to the east and north, leading to increased salinity at the moorings, and rise of sea level by 0.1–0.5 m along the eastern and northern boundary of the bay. During July–August 2015, a shallow pool of low‐salinity river water lies in the northeastern bay. The amplitude of a 20‐day oscillation of sea surface temperature (SST) is two times larger within the fresh pool than in the saltier ocean to the west, although surface heat flux is nearly identical in the two regions. This is direct evidence that spatial‐temporal variations of BoB salinity influences sub‐seasonal SST variations, and possibly SST‐mediated monsoon air‐sea interaction.

The impact of Indian Ocean dipole on tropical Indian Ocean surface wave heights in ERA5 and CMIP5 models

AbstractThe present study examines the relationship between the tropical Indian Ocean (TIO) significant wave height (SWH) and Indian Ocean dipole (IOD) during boreal summer season (June through August) in the latest version of European Centre for Medium‐Range Weather Forecasts reanalysis (ERA5) and wave simulations forced with surface winds and sea‐ice fields from the Coupled Model Intercomparison Project version‐5 (CMIP5) models. The interannual variability of SWH shows a significant negative correlation with the IOD over TIO. SWH anomalies display meridional tripole pattern with significant negative (positive) anomalies over eastern equatorial Indian Ocean caused by anomalous easterlies (westerlies), and positive (negative) anomalies over southeastern TIO and the north Bay of Bengal during positive (negative) phase of IOD. The strong wave heights along the east coast of India during positive IOD and the south and southwest coast of India during negative IOD are noticed. CMIP5 models GFDL‐CM3, MRI‐CGCM3 and the multi‐model mean display considerable skill in capturing these teleconnections with substantial magnitude differences. A thorough understanding of the teleconnections between IOD and TIO wave heights is a significant prerequisite for the accurate forecast of surface waves in the Indian Ocean. Hence, this study advocates the importance evaluating the ability of models in representing the SWH and IOD interactions and its implications on Indian coastal regions in the form of inundation, coastal flooding and other vulnerabilities in a changing climate scenario.

Evaluation of Different Heat Flux Products Over the Tropical Indian Ocean

AbstractNet heat flux (Qnet) and its components from four reanalysis (NCEP‐2, CFSR, ERA5, and MERRA) and two blended products (OAFlux & TropFlux) are compared with in situ observation (two Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction buoys and one Woods Hole Oceanographic Institution buoy) over the north Indian Ocean to quantify their uncertainties in daily, seasonal, and annual scales. These comparisons provide the present status of Qnet error in most state of the art reanalysis/blended products. The root‐mean‐square error (RMSE) remains similar to the RMSE a decade earlier, despite more observation and improved models and reanalysis methods. However, there is a clear separation of flux quality from the older generation of reanalysis (NCEP‐2) to the newer production of reanalysis (MERRA, CFSR, and ERA5). While individually ERA5 provides the best estimate, the ensemble mean (i.e., average of ERA5, CFSR, MERRA, TropFlux, and OAFlux) is very close to ERA5 both in terms of correlations and RMSE and provides the most reliable estimate by virtue of removal of some of the uncertainties in estimation of flux by each of the flux products. A significant reduction of RMSE in Qnet estimates from 100 W/m2 (in NCEP‐2) to 45 W/m2 (in the ensemble mean) is considerable progress. It is noteworthy that all the recent flux products estimate the increasing trend of Qnet in the north Bay of Bengal and subseasonal fluctuations with significant fidelity. Also, in the south of equator location vigorous subseasonal fluctuations in boreal winter are well captured. We believe that this is significant progress in the estimation of Qnet over the Indian Ocean.

Response of a high-resolution ocean circulation model to winds from different sources in simulating summer monsoon freshening in the North Bay of Bengal: A case study

A very high-resolution model for the Bay of Bengal is configured to simulate submesoscale flow features, which are otherwise absent in coarse resolution models. In this manuscript, we present the response of this model to simulate summer monsoon freshening in the northern Bay of Bengal when forced with two different winds, one based on weather forecast model output from National Centre for medium range weather forecast (NCMRWF) and the other based on satellite-derived SCATSAT ocean surface winds for 2017 as a case study. Our results reveal that the spatio-temporal evolution of lateral freshwater dispersal in the northern Bay of Bengal is sensitive to wind vorticity and eddy formation/evolution. A reasonably realistic decrease of 2–3 psu in the sea surface salinity at the mooring location of 18oN, 90oE towards the end of September 2017 is simulated in the model when forced with the SCATSAT winds, whereas significantly weaker freshening occurs at the mooring location when the model is forced with the NCMRWF winds. A filament of strong positive vorticity connecting two cyclonic eddies in the north central Bay in the NCMRWF wind forced simulations prevents the direct southward advection of freshwater, resulting in higher salinity at the mooring location. A similar mechanism that would prevent southward advection of fresh water is absent in the SCATSAT forced model run. Westward moving Rossby waves in the SCATSAT forced model run are found to play an important role in the evolution of freshening events at various locations in the northern Bay of Bengal. Lagrangian Coherent Structures (LCS) derived from the model currents suggest that the background conditions were favourable to advect freshwater from Ganges-Brahmaputra towards the mooring location in the northern Bay of Bengal in the SCATSAT forced simulations.

Characteristics of Deep Cloud Systems under Weak and Strong Synoptic Forcing during the Indian Summer Monsoon Season

AbstractSynoptic-scale weather systems are often responsible for initiating mesoscale convective systems (MCSs). Here, we explore how synoptic forcing influences MCS characteristics, such as the maximum size, lifespan, cloud-top height, propagation speed, and triggering over the Indian region. We used 30-min interval infrared (IR) data of the Indian Kalpana-1 geostationary satellite. Cloud systems (CSs) in this data are identified and tracked using an object tracking algorithm. ERA-Interim 850-hPa vorticity is taken as a proxy for the synoptic forcing. The probability of CSs being larger, longer lived, and deeper is more in the presence of a synoptic-scale vorticity field; however, the influence of synoptic forcing is not evident on the westward propagation of CSs over land. There exists a linear relationship between maximum size, lifespan, and average cloud-top height of CSs regardless of the nature of synoptic forcing. Formation of CSs peaks around 1500 LST over land, which is independent of synoptic forcing. Over the north Bay of Bengal, CSs formation is predominantly nocturnal when synoptic forcing is strong, whereas, 0300 and 1200 LST are the preferred times when synoptic forcing is weak. Long-lived CSs are preferentially triggered in the western flank of the 850-hPa vorticity gradient field of a monsoon low pressure system. Once triggered, CSs propagate westward and ahead of the synoptic system and dissipate around midnight. Formation of new CSs on the next day occurs in the afternoon hours in the wake of previous day’s CSs and where vorticity gradient is also present. Formation and westward propagations of CSs on successive days move the synoptic envelope westward.

Thermal fronts and attracting Lagrangian Coherent Structures in the north Bay of Bengal during December 2015–March 2016

We investigate two-dimensional advective stirring by satellite-derived ocean surface currents in the north Bay of Bengal during December 2015–March 2016 using the framework of Lagrangian Coherent Structures (LCS). The forward and backward finite-time Lyapunov exponent (fFTLE and bFTLE, respectively) fields indicate that freshwater parcels from the river run-off are subjected to intense stirring by the ocean surface currents, which contain both geostrophic and wind-forced Ekman currents. Large-scale thermal fronts present in the satellite-based sea surface temperature (SST) fields are then shown to closely resemble attracting LCS (aLCS) that govern horizontal advective stirring on subseasonal time scales. Statistical estimates of the angle between the thermal fronts (of all spatial scales present in the SST data) and the aLCS from the entire north Bay of Bengal show a preference to align with each other. Significantly, in the presence of sufficiently strong aLCS in their neighbourhood, thermal fronts are shown to have a strong preference to align with them. Finite-time advection of fluid particles over around a week to compute the bFTLE fields results in the strongest alignment between the thermal fronts and neighbouring aLCS. The robustness of our results are established by presenting analyses of (i) geostrophic currents from altimeter data, and (ii) a reanalysis-based fine-scale salinity field. Attracting LCS, identified as sufficiently strong ridges in the bFTLE fields, are argued to represent reasonable estimates of the location and orientation of fronts (thermal or salinity) formed by advective stirring over subseasonal time scales.

Coupled Ocean–Atmosphere Summer Intraseasonal Oscillation over the Bay of Bengal

This study shows the active role of subsurface ocean temperature in the evolution of coupled intra-seasonal oscillation (ISO) in the Bay of Bengal (BoB) using multi-satellite observations and ocean analysis product. Satellite-derived humidity profiles obtained from the atmospheric infrared sounder (AIRS) show that intense rains over the BoB are associated with the moistening (drying) in the lower and mid-troposphere during the active (break) phase of summer intraseasonal oscillation (ISO). Anomalous moistening in the mid-troposphere up to 500 hPa ahead of the maximum precipitation band over north BoB gives a precursor signal for the northward movement of the rain band. During the active (break) phase, the upper-tropospheric positive (negative) temperature anomaly ahead of the maximum rain band also sets a precondition by heating the mid- to upper troposphere. Daily subsurface temperature from Global Ocean Data Assimilation System (GODAS) analysis show that during the active phase, tropospheric moistening (drying) coincides with the subsurface warm (cold) temperature up to 200-m depth. The upper ocean warms uniformly by ~ 1 °C during the active phase as compared to the break phase in the entire BoB. The presence of a thin warm layer below the maximum rain band creates an environment conducive to sustaining the active phase on the ISO time scale. A positive sea surface temperature (SST) anomaly along with upper ocean warming ahead of a rain band in the north BoB in association with lower and mid-tropospheric moistening sets a precondition for the northward movement of the rain band. The anomalous warming (cooling) in the thermocline is associated with deeper (shallower) thermocline depth [23° isotherms (D23)] and coincides with the mixed-layer warming.

Vertical distribution of the Asian tropopause aerosols detected by CALIPSO

Characterizing the vertical distribution of aerosol optical properties is crucial to reduce the uncertainty in quantifying the radiative forcing and climate effects of aerosols. The analysis of four-year (2007–2010) Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar measurements revealed the existence of tropospheric aerosol layers associated with the Asian summer monsoon. The measurements of five typical aerosol optical and microphysical parameters were used to explore the properties, spatial/vertical distributions, annual evolution of tropopause aerosols over the South Asia region. Results extracted from various latitude-height and longitude-height cross sections of aerosol extinction coefficient at 532 and 1064 nm, backscatter coefficient at 532 nm, and depolarization ratio at 532 nm demonstrated that a large amount of aerosols vertically extended up to the tropopause (12 km) during the monsoon season over the north Arabian Sea, India, north Bay of Bengal, and equatorial Indian Ocean, finally reaching the southeast of the Tibetan Plateau. Convective transport associated with Asian summer monsoon is an important factor controlling the vertical distribution of tropopause aerosols. The evolution of aerosol scattering ratio at 532 nm indicated that from equatorial Indian Ocean to South Asia, there exists an upward tilting and ascending structure of the aerosols layer during the monsoon season, which typically indicates enhanced aerosols over the Asian monsoon region. Information on aerosol size distribution and detailed composition are needed for better understanding the nature and origin of this aerosol layer. Enhancement of the tropopause aerosols should be considered in the future studies in evaluating the regional or global climate systems. Further satellite observations of aerosols and in-situ observations are also urgently needed to diagnose this aerosol layer, which likely originate from anthropogenic emissions.

Simulation of the diurnal cycle of rainfall during Indian summer monsoon season using mesoscale model

Monsoon rainfall exhibits the prominent diurnal variability. It is essential to capture the diurnal behaviour of the rainfall on the spatiotemporal scale by mesoscale models for its accurate prediction. The present study appraises the diurnal variability of simulated rainfall by Weather Research and Forecasting (WRF) model over the South Asian region during Indian summer monsoon seasons of the years 2008–2012. For examining the amplitude, phase, and variance of the simulated diurnal cycle of rainfall, harmonic analysis of model outputs are carried out and compared it with that of TRMM rainfall estimates to quantify errors. The model is overall successful in simulating the pattern of diurnal variation of rainfall, but underestimates its amplitude compared to the observed one especially over the western Himalayas, northeast India, central India, and the north Bay of Bengal (BoB). However, the amplitudes of the diurnal variation match well over south BoB. The simulated phases of the diurnal cycle either leads (over central India and NW India by 3 h) or lags (over the sub-Himalayan region by 12 h) than the observed phase angle. The simulated time of maximum rainfall matches over south Peninsula, while over the west coast, neither observation nor forecast shows pronounced diurnal cycle. The north BoB shows morning maxima in rainfall in both observation and forecast; however, over south BoB, the time of maximum simulated rainfall leads by 3 h. Even though the model is overall successful in representing the diurnal variation of rainfall, it requires improvement for proper representation of phase and sub-diurnal variation appropriately.

Shear Flow Instabilities and Unstable Events Over the North Bay of Bengal

AbstractA year‐long mooring data are used to study the upper ocean unstable events and instabilities at 18°N 89°E, which is a climatologically important region in the North Bay of Bengal. Near‐surface stability is studied from the context of the buoyancy frequency normalized shear (Vz/N) and reduced shear (S2−4N2) which are convenient measures to quantify flow stability, compared to the more widely used Richardson number (Ri). The analysis is carried out across three contrasting time periods, the monsoon, postmonsoon, and the winter of year 2012. Although it is well known that the flow stability changes from stable to unstable at Ri = Ricr=0.25, the relative importance of the perturbations of shear and buoyancy frequency in driving the unstable events is not well studied over the open oceans and more particularly over the Bay of Bengal. At 18°N, 89°E both higher than average shear and lower than average buoyancy frequency perturbations are crucial in driving the unstable events during the summer and premonsoon period. However, at increasing depths, the influence of shear perturbations becomes more dominant. Invoking the Miles‐Howard criteria for flow instability, it is seen that during the postmonsoon period, the buoyancy frequency perturbations are more critical than shear perturbations in driving the unstable events. In winter, the unstable events are influenced by both the buoyancy frequency and shear perturbations.

Subseasonal Dispersal of Freshwater in the Northern Bay of Bengal in the 2013 Summer Monsoon Season

AbstractNear‐surface salinity in the north Bay of Bengal (BoB) is very low for nearly three seasons (July–January), due to freshwater input from summer monsoon rainfall and seasonal discharge from several major rivers, including the Ganga‐Brahmaputra‐Meghna and the Irrawady. The low‐salinity surface layer leads to a very shallow density mixed layer, profoundly influencing air‐sea interaction on diurnal to seasonal time scales. We use mooring and satellite observations to study the mechanisms responsible for lateral dispersal of river water in the north BoB on subseasonal time scales (days to weeks) during the summer monsoon season of 2013. A new ocean current data set, the BoB near‐surface current and advection estimation (BoBcat), is developed to account for the influence of near‐surface stratification on directly wind‐forced currents. A salt balance based on Aquarius sea surface salinity, BoBcat currents and moored observations shows that subseasonal sea surface salinity variability is mainly due to lateral advection. A shallow Ekman current, forced by enhanced wind stress during an active spell of the monsoon in mid‐August, plays a dominant role in moving freshwater from the western boundary to the interior. During subseasonal spells of weak monsoon winds, stirring by mesoscale eddies is the main mechanism of dispersal of low‐salinity water. The dispersal of river water in the BoB is very sensitive to surface wind stress.

Investigation on the Bay of Bengal branch of summer monsoon during normal and delayed onset over Gangetic West Bengal

The regional features of Bay of Bengal (BOB) branch of summer monsoon (SM) are examined to identify the causes of delayed onset over Gangetic West Bengal (GWB) in the years having normal onset over Kerala coast. The normal onset over both GWB and Kerala is designated as Normal–Normal (NN) years, while delayed onset over GWB and normal onset over Kerala is termed Normal–Delayed (ND) years. The temperature gradient (TTg), winds at 850 and 150 hPa pressure levels, sea-surface temperature (SST), outgoing long wave radiation (OLR), low-level moisture convergence, instability, and rainfall rate (RR) are analyzed in this study using National Centers for Environmental Prediction and National Oceanic and Atmospheric Administration reanalysis dataset during the period from 1981 to 2015. The result shows that TTg over BOB plays a significant role in controlling the movement of BOB branch of SM. Warm SST is observed to prevail over north BOB during NN years. The divergence at 150 hPa and convergence at 850 hPa pressure levels are found to influence the propagation of BOB branch of SM during both NN and ND years. The winds at 850 hPa level converge over BOB and GWB during NN years, whereas winds converge more over eastern BOB and Indo-Chinese peninsula during ND years. Result depicts abundance of low-level (850–1000 hPa) moisture over eastern BOB and Indo-Chinese peninsula during ND years, whereas moisture is observed to converge over north and north-eastern BOB during NN years. The RR is observed to be slightly higher during NN than ND years. However, it may not be concluded from the analysis that delayed onset over GWB will be responsible for less RR over the study region.

Role of cloud feedback in regulating the “pool of inhibited cloudiness” over the Bay of Bengal

Cloud-circulation feedback by latent heating (LH) is a major contributor for driving atmospheric circulation. This study examines the potential role of spatial variations of LH in driving a mini-circulation for generating the “pool of inhibited cloudiness”, where cloud formation in the lower and middle troposphere is significantly inhibited over the southwest Bay of Bengal (BoB) during the Indian summer monsoon. Spaceborne observations show that LH rates in the deep convective regions of the head BoB are as high as 0.2 K/h, while that over the “pool” are about 5 times smaller. This differential heating results in modulating the mean circulation. 20 years of reanalysis data show good correlation between (a) upper troposphere (300 hPa) convergence and lower tropospheric (1000 hPa) divergence at the “pool” and (b) lower tropospheric divergence at the “pool” and lower tropospheric convergence at the head BoB as well as at the west coast of India. Besides the orographic influence, a portion of the air mass descending at the “pool” arises from deep convective regions in the north BoB and equatorial trough. This subsidence is responsible for the persistent inhibition to cloud formation at the “pool”. This case forms a classic example of the cloud-circulation feedback.