Southern Bay Of Bengal Research Articles

Negative Surface Chlorophyll Concentration Anomalies in the Southeast Arabian Sea During Summer in 2015 and 2019

AbstractSatellite observations revealed two extremely low surface chlorophyll concentration (SCC) events with a warm sea surface temperature anomaly in the southeastern Arabian Sea (SEAS, 6°–15°N, 72°–77°E) during the summer (July–August–September) in 2015 and 2019. We find that the physical processes leading to these two similar low SCC events are remarkably different. The low SCC in the SEAS during summer 2019 is mainly related to the weakened upwelling and deepening of the thermocline depth due to the combined effects of the local wind anomalies and the arrival of westward‐propagating downwelling coastal Kelvin wave driven by easterly anomalies near the eastern Sri Lanka during an extreme positive Indian Ocean Dipole (IOD) event. In summer 2015, a weaker positive IOD‐induced easterly anomalies in the southern Bay of Bengal also drives downwelling coastal Kelvin waves westward, deepening the thermocline in the SEAS. But unlike that in summer 2019, the local wind stress curl anomalies in the SEAS during summer 2015 favors upwelling, which counteracts the downward motion of the coastal Kelvin waves, leading to weaker downward transport (one‐third of that in 2019). Meanwhile, the upper ocean layer in the SEAS experiences extreme warming during summer owing to the development of 2015/2016 super El Niño. This substantial warming enhances upper oceanic stratification, which results in weaker vertical mixing and reduces the SCC to an extremely low level despite the much weaker IOD strength in 2015.

The origin of summer high-salinity water in the southern Bay of Bengal and its interannual variabilities

The origin of summer high-salinity water in the southern Bay of Bengal and its interannual variabilities

Winter monsoon brings substantial terrestrial aerosols to Bay of Bengal and adjacent Indian Ocean: Insights from carbon and nitrogen isotopes

It has been currently recognized that air pollutants (e.g., carbon and nitrogen) from South and Southeast Asia have a significant impact on the coastal waters of the northern Indian Ocean. However, the understanding of dynamics and fates of these pollutants in aerosols to the remote ocean through long-distance transportation still needs to be improved. To address this issue, stable isotopic compositions (δ13C, δ15N) of total carbon (TC) and nitrogen (TN) and other geochemical indexes in aerosols are investigated over the Bay of Bengal (BOB) and the adjacent Indian Ocean during the winter monsoon period. The concentrations of TN, TC and aerosol nutrients exhibited a decreasing trend from the northern BOB (N-BOB) to the southern BOB (S-BOB) and Indian Ocean, reflecting the weakening trend of polluted continental influence from the N-BOB to the ocean. The high concentrations of TC (4.28–18.21 μg m−3) in the aerosols of the N-BOB were mainly influenced by the emissions of coal combustion (68 ± 21%), while the high concentrations of TN (0.69–7.31 μg m−3) in the N-BOB were mainly from biofuel/biomass burning (52 ± 31%). In the S-BOB, the lower concentrations of TC (1.42–2.98 μg m−3) and TN (0.38–2.03 μg m−3) mainly originated from the formation of secondary aerosols and the changes in aerosol chemical composition during long-distance transportation from the continent. The primary marine source was responsible for the low TC and TN concentrations over the Indian Ocean. This study quantifies the nitrogen and carbon source shifts from continental anthropogenic activities in the N-BOB to dominant marine and photochemical transformation sources in the remote ocean, quantitatively deepening the understanding of the impact of terrestrial aerosols on the ocean.

Untangling the dynamics of heavy rainfall events (HRE) in the Northeast monsoon season and exploring its connection to the Indian Ocean warming and the Madden Julian Oscillation

Southeastern Peninsular India witnessed heavy rainfall events (HRE) during the recent El Nino year 2015 and La Nina year 2021 in November and early December. Both these events were associated with enhanced easterly wave activity and active Madden Julian Oscillation (MJO) over the Maritime continent. The MJO was on the fourth (MJO-4) phase for 15 and 17 days in November 2015 and 2021 respectively. A weakening in the positive correlation between El Nino and Northeast monsoon rainfall and a strengthening in the positive correlation between the MJO-4 days and HRE is observed in very recent years. This study revealed that the unequal changes in Sea Surface Temperature (SST) in the Indo-Pacific region i.e., warming over the eastern Indian Ocean (IO) and cooling over the western Pacific Ocean can be favourable for the increased occurrence of MJO-4 days as well as HRE over southern Peninsular India. The easterly wind anomalies towards the southern Bay of Bengal (BoB) and southeastern Peninsular India and westerly wind anomalies over the central equatorial Indian Ocean are significantly correlated with HRE in the recent epoch (2001−2021). Warming in eastern IO brings easterlies from the cool western Pacific and westerlies from the western IO, which helps the MJO to remain active over maritime continent and inhibits its further eastward propagation. The HREs in the recent epoch are associated with the low-pressure systems and cyclonic circulation over southern Peninsular India, in which moisture transport is from the Arabian Sea to Peninsular India. The HRE in the early epoch (1981–2000) is related to the anticyclonic circulation over BoB region and the moisture transport is from BoB and southeastern IO. A proposed mechanism for the HRE in the recent epoch is as follows. The SST gradient in the Indo-Pacific Ocean is conducive for the moistening of MJO-4 region and is favourable for the enhanced easterly wave activity and the formation of low-pressure systems, which get strengthened while moving towards southern Peninsular India due to the presence of strong equatorial westerlies over IO. These strengthened low pressure systems leads to strong ascending wind anomalies over southern Peninsular India and strong descending anomalies over Central IO and HREs over southern Peninsular India.

Research on the Characteristics of Raindrop Spectrum and Its Water Vapour Transport Sources in the Southwest Vortex: A Case Study of 15–16 July 2021

This study investigated the convective weather features, precipitation microphysical characteristics, and water vapour transport characteristics associated with a southwest vortex precipitation event that occurred on the eastern edge of the Qinghai–Tibet Plateau, coinciding with a southwest vortex event, from 15 to 16 July 2021, using conventional observations of raindrop spectra, ERA5 reanalysis data, CMORPH precipitation data, and the HYSPLIT_v4 backward trajectory model. The findings aim to provide theoretical insights for improving the forecasting and numerical simulations of southwest vortex precipitation events. The findings revealed that the precipitation event induced by the southwestern vortex at Emeishan Station on 15–16 July 2021 was characterised by high rainfall intensity and significant precipitation accumulation. The raindrop spectrum exhibited a broad distribution with a notable bimodal structure. Both the Sichuan Basin and the Tibetan Plateau were dominated by the South Asian high pressure at higher altitudes, while a pronounced low-pressure system developed at mid and low altitudes within the basin, establishing a meteorological context marked by upper-level divergence and lower-level convergence. Throughout the event, notable vertical uplift velocities were recorded across the Sichuan Basin and Tibetan Plateau, along with distinct positive vorticity zones in the lower and middle strata of the Sichuan Basin, indicating that the atmosphere was in a state of thermal instability. The majority of moisture was in the mid and lower troposphere with evident convergence movements, which played a crucial role in the southwest vortex’s development. WRF numerical simulations of the Emeishan precipitation event more accurately modelled the weather conditions for this precipitation but tended to overestimate the level of precipitation. It was observed that the region around Emei Mountain primarily received moisture influx from the southern Bay of Bengal and the South China Sea, with moisture transport chiefly originating from the Sichuan Basin and in a south-westward trajectory.

Climate change quadruples flood‐causing extreme monsoon rainfall events in Bangladesh and northeast India

Bangladesh and northeast India are the most densely populated regions in the world where severe floods as a result of extreme rainfall events kill hundreds of people and cause socio‐economic losses regularly. Owing to local high topography, the moisture‐carrying monsoon winds converge near southeast Bangladesh (SEB) and northeast Bangladesh and India (NEBI), which produces significant extreme rainfall events from May to October. Using observed data, we find an increasing trend of 1‐day extreme event (150 mmday) frequency during 1950–2021. The extreme rainfall events quadrupled over western Meghalaya (affecting NEBI) and coastal SEB during this period. Composite analysis indicates that warm Bay of Bengal sea‐surface temperature intensifies the lower tropospheric moisture transport and flux through the low‐level jet (LLJ) to inland, where mountain‐forced moisture converges and precipitates as rainfall during extreme events. To understand the role of climate change, we use high‐resolution downscaled models from Coupled Model Intercomparison Project phase 6 (CMIP6). We find that the monsoon extreme event increase is ongoing and the region of quadrupled events further extends over the NEBI and SEB in the future (2050–2079) compared with historical simulations (1950–1979). A quadrupling of the intense daily moisture transport episodes due to increased LLJ instability, a northward shift of LLJ, and increased moisture contribute to the increased future extreme events. This dynamic process causes moisture to be transported to the NEBI from the southern Bay of Bengal, and the local thermodynamic response to climate change contributes to the increased extreme rainfall events. The CMIP6 projection indicates that more devastating flood‐causing extreme rainfall events will become more frequent in the future.

A 70 kyr Record of Monsoon-induced Marine Productivity Changes, Terrigenous Flux and Weathering Variations in the Southern Bay of Bengal

Abstract The southwest (SW) and northeast (NE) monsoon winds significantly affect the oceanographic and climatic changes over the Bay of Bengal (BoB). We present here detailed geochemical and isotopic records from two sediment cores (SK157 14; 5°11′ N: 90°5′ E and SK157-16; 8°46′ N: 90°18′ E), collected from the southern BoB. In this study, we mainly focus on variations in paleoproductivity and terrigenous fluxes introduced into the BoB since last ~70 kyr: We have reconstructed biogenic barium (Babio), calcium carbonate (CaCO3) content, and total organic content (TOC) in both sediment cores to infer paleoproductivity during the period of study. The distributions of productivity proxies in both these cores indicate that the productivity may have been significantly high during MIS 1 and MIS 3 (warmer interglacial periods), when SW monsoon was intensified. The terrigenous proxies (Al%, Ti%, d(0.9)) show that the southern BoB may have received higher terrigenous input during the MIS 2 and MIS 4 (glacial periods) coinciding with the intensification of NE monsoon. The weathering proxy (K/Al), in contrast, shows no significant changes during MIS 2, 3 and 4. However; a significant shift is seen during glacial to Holocene transition.

Vertical variation of bacterial production and potential role in oxygen loss in the southern Bay of Bengal.

Marine environments wherein long-term microbial oxygen consumption exceeds oxygen replenishment can be associated with oxygen minimum zones (OMZ). The Bay of Bengal OMZ (BOB-OMZ) is one of the most intense OMZs globally. To assess the contribution of bacterial oxygen consumption to oxygen loss in BOB-OMZ, we measured bacterial production (BP), temperature, salinity, and dissolved oxygen (DO) in the whole water column. We then compared the estimated bacterial oxygen demand (BOD) with diapycnal oxygen supply (DOS) at depths of 50-200 m in the southern BOB in January 2020. The average BP was 3.53 ± 3.15 μmol C m-3 h-1 in the upper 200 m of four stations, which was lower than those reported in other tropical waters. The vertical distribution of BP differed between the open ocean and nearshore areas. In the open ocean, temperature and DO were the most important predictors for BP in the whole water column. In the nearshore areas, when DO increased sharply from the suboxic state, extremely high BP occurred at 200 m. The average estimated BOD/DOS could reach up to 153% at depths of 50-200 m, indicating advection and anticyclonic eddies probably are important DO replenishment pathways in the BOB.

Detection of materially coherent eddies from satellite altimetry in the Bay of Bengal

Mesoscale eddies are a well-recognized feature of the global oceans, being of dynamical relevance to various ocean processes and of significance to transport of various species. In the last few decades, advancements in satellite observations of the sea surface have revealed mesoscale and sub-mesoscale eddy features in many parts of the ocean. An objective definition and detection of eddies has, however, remained a challenge, thus resulting in unreliable estimates of eddy formation, evolution and dissipation. In this study, we present an implementation of a Lagrangian averaged vorticity deviation (LAVD) based method to detect and characterize eddies in the Bay of Bengal (BoB), a freshwater-dominated semi-enclosed basin in the northern Indian Ocean. Systematic sensitivity studies are performed to identify parameter values that result in robust eddy detection in the BoB, and finite-time coherence in material transport associated with the LAVD-based eddies is demonstrated. LAVD-based eddy detection is then used to statistically characterize the Sri Lanka dome (SLD) eddy, an annual mesoscale cyclonic eddy of dynamical and physical importance in the southern BoB. Analysing satellite-based ocean surface currents from 27 years (1993–2019) of data, the inter-annual variability in the SLD formation, its spatio-temporal evolution and dissipation are elucidated. On an average, SLD has a lifetime of 15 weeks and a maximum area of 5 square degrees, with the longest lifetime of 38 weeks in 1996 and a maximum area of 12 square degrees in 1998. Long-lived SLDs are shown to spatially excurse into the BoB, potentially influencing large-scale transport of physical and biogeochemical properties. A potential correlation between large SLD lifetime and large variations in the monthly evolution of Dipole Mode Index and NINO3.4 is highlighted. An SSH-based Eulerian eddy detection method is also implemented to understand its efficacy in identifying materially coherent eddies in the BoB region. The paper concludes with a summary of our results, and a brief discussion of potential applications of LAVD-based eddy detection in the northern Indian Ocean.

Observed oceanic response to Tropical Cyclone Amphan (2020) from a subsurface mooring in the Bay of Bengal

Upper ocean responses to the super Tropical Cyclone (TC) Amphan and modulation of energy dissipation by internal tides were investigated using in situ observations from a subsurface mooring in Bay of Bengal (BoB). The observations showed that the entire observed water column (150–800 m) cooled significantly, up to a maximum of 2 ℃, and salinity increased significantly at 150–200 m within one week after the TC, mainly due to the TC-induced upwelling. Dynamic response was characterized by strong near-inertial oscillation above 120 m. The near inertial waves (NIWs) had a maximum velocity and kinetic energy at 112.6 m, about 0.34 m/s and 59.5 J/m3, respectively. The energy spectrum and bicoherence analysis indicated that, the nonlinear wave-wave interaction between NIWs (f) and semi-diurnal tide (D2), as well as the self-interaction of NIWs were enhanced after the passage of TC. The nonlinear wave-wave interaction processes were able to transfer energy to higher mode internal waves, thus accelerating energy attenuation. Hence, the observed e-folding time for near-inertial kinetic energy (6.2 days) was only 41 % of the theoretical e-folding time. This study provided a new evidence for the mechanism of energy dissipation of NIWs and semi-diurnal tide during a TC condition in the southern BoB.

On the Influence of the Bay of Bengal’s Sea Surface Temperature Gradients on Rainfall of the South Asian Monsoon

Abstract The southwest monsoon delivers over 70% of India’s annual rainfall and is crucial to the success of agriculture across much of South Asia. Monsoon precipitation is known to be sensitive to sea surface temperature (SST) in the Bay of Bengal (BoB). Here, we use a configuration of the Unified Model of the Met Office coupled to an ocean mixed layer model to investigate the role of upper-ocean features in the BoB on southwest monsoon precipitation. We focus on the pronounced zonal and meridional SST gradients characteristic of the BoB; the zonal gradient in particular has an as-yet unknown effect on monsoon rainfall. We find that the zonal SST gradient is responsible for a 50% decrease in rainfall over the southern BoB (approximately 5 mm day−1), and a 50% increase in rainfall over Bangladesh and northern India (approximately 1 mm day−1). This increase is remotely forced by a strengthening of the monsoon Hadley circulation. The meridional SST gradient acts to decrease precipitation over the BoB itself, similarly to the zonal SST gradient, but does not have comparable effects over land. The impacts of barrier layers and high-salinity subsurface water are also investigated, but neither has significant effects on monsoon precipitation in this model; the influence of barrier layers on precipitation is felt in the months after the southwest monsoon. Models should accurately represent oceanic processes that directly influence BoB SST, such as the BoB cold pool, in order to faithfully represent monsoon rainfall.

An Energetic Mesoscale Anticyclonic Eddy in the Southern Bay of Bengal in June 2020: A Case Study

AbstractThe southern Bay of Bengal (BoB) is a region of rich mesoscale eddy activity that plays an important role in modulating water exchange between the BoB and the Arabian Sea. Although statistical surface characteristics of mesoscale eddies in the BoB have been sufficiently explored previously, most studies concentrated on the western and central BoB, but rarely in the southern BoB. In this study, an energetic mesoscale anticyclonic eddy (AE) was detected by long‐term mooring observations in the southern BoB during June 2020. The mooring observations revealed that the AE‐induced maximum temperature and salinity anomalies at 100 m were as large as 6.8°C and 0.3 psu, respectively. The available Argo profiles showed that the AE‐induced salinity anomalies had a three‐layer vertical structure, with a positive anomaly (maximum 0.28 psu) deeper than 100 m and (maximum 0.72 psu) shallower than 40 m, and a negative anomaly (maximum −0.86 psu) between 40 and 100 m, indicating that the AE could exert a significant effect on the redistribution of surface freshwater in the southern BoB. The AE formation was closely connected with remote forcing (equatorial Kelvin wave) and local processes (Rossby wave and local wind stress curl). Local energy conversion analysis demonstrated that baroclinic and barotropic instabilities, as well as wind stress work made comparable contributions to the generation of the AE. Given the frequent occurrence of the AEs to northwest of Sumatra, they could have significant effects on surface freshwater redistribution and regional ecosystem productivity in the southern BoB.

Monitoring of monthly scale chlorophyll concentration variability in the Bay of Bengal and Arabian Sea using MODIS Aqua Satellite Data

Study has been carried out to monitor the phytoplankton biomass in Bay of Bengal (BoB) and Arabian Sea (AS) using Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite data. Cloud masking, geometric corrections and subsets generations were performed to retrieve chlorophyll images from MODIS-Aqua data during the periods January - December for the years 2007 and 2008. The two regions (BoB & AS) have been divided into four subsets; subset-1 (Northern Bay of Bengal), subset-2 (Southern Bay of Bengal), subset-3 (Northern Arabian Sea) and subset-4 (Southern Arabian Sea). The results were analyzed and confirmed that chlorophyll concentration mean range was high (0.97-1.89 mg m-3) in northern Arabian Sea during the months of July for both years 2007 and 2008 and low concentration range (0.12-0.35 mg m-3) was obtained during April month for both years in southern Bay of Bengal. This study found to be important as information about the chlorophyll concentration in the Northern Indian Ocean.

The sources and transport pathways of sediment in the northern Ninety-east Ridge of the India Ocean over the last 35000 years

The Ninety-east Ridge (NER) is located in the southern Bay of Bengal in the northeast Indian Ocean and is composed of pelagic and hemipelagic sediments. In addition to contributions from marine biomass, the ridge also contains terrestrially sourced sedimentary material. However, considerable disagreement remains regarding the origin of these terrestrial materials and transport pathways. This paper discusses the collection of seafloor surface sediments and three sediment cores recovered from the northern region of the NER, as well as the analysis of clay minerals, Sr-Nd isotopes, and sediment grain size. The ages of the three core sediments are constrained by AMS 14C dating to better establish the source and transport pathways of the terrestrial materials within NER sediments over the past 35000 years. The research results show that the Qinghai-Tibet Plateau is the predominate source of terrigenous sedimentary material in the NER. In the plateau, the crustal materials were weathered and stripped and then transported to the Andaman Sea via the Irrawaddy River. From there, the material was transported westward by monsoon-driven circulation to the northernmost part of the NER before being transported to the south for final deposition. This transport mode has changed little over the past 35000 years. However, during the rapidly changing climate of the Younger Dryas (12.9~11.5 ka BP), there were some variations in the input amount, grain size, and Sr-Nd isotope value of the source material. The above conclusions are significant for re-evaluating the source of terrigenous sediments, the temporal and spatial changes in transport modes, and the sensitivity of the NER to climatic shifts.

Strong subsurface meridional current forced by monsoon intraseasonal oscillation in the southern Bay of Bengal during summer 2020

The meridional current in the southern Bay of Bengal (BOB) exhibits prominent intraseasonal variability (ISV), which exerts a critical influence on meridional mass and energy exchange. However, its relationship with the northward propagating monsoon intraseasonal oscillation (MISO), which is the predominant variability in the tropical Indian Ocean during summer, is not well understood. Using a one-year mooring deployed at 5.5 °N, 90 °E, a strong ISV of the meridional current is observed between 150 and 250 m during summer, exhibiting amplitudes exceeding 0.4 ms−1 and periods of 30–70 d. Further analysis shows that the summer ISV is forced by a strong MISO event with the following dynamic processes. The MISO first drives the zonal wind stress at the equator, leading to the equatorial Kelvin wave and the reflected equatorial Rossby wave at 5 °N. Then, the MISO propagates northward and generates local wind stress curl-induced Ekman pumping near the mooring site, leading to the subsequent off-equatorial Rossby wave at 8 °N. As a result, the synergy of the remotely- and locally-forced Rossby wave causes strong subsurface ISV in southern BOB. This study provides a new insight into the dynamic relationship between the MISO and the meridional current in the southern BOB, which has important implications for regional climate research and prediction.

Seasonal Features and Potential Mechanisms of Submesoscale Processes in the Southern Bay of Bengal during 2011/12

Abstract This study investigates the seasonal features and generation mechanisms of submesoscale processes (SMPs) in the southern Bay of Bengal (BoB) during 2011/12, based on the output of a high-resolution model, LLC4320 (latitude–longitude–polar cap). The results show that the southern BoB exhibits the most energetic SMPs, with significant seasonal variations. The SMPs are more active during the summer and winter monsoon periods. During the monsoon periods, the sharpening horizontal buoyancy gradients associated with strong straining effects favor the frontogenesis and mixed layer instability (MLI), which are responsible for the SMPs generation. The symmetric instability (SI) scale is about 3–10 km in the southern BoB, which can be partially resolved by LLC4320. The SI is more active during summer and winter, with a proportion of 40%–80% during the study period when the necessary conditions for SI are satisfied. Energetics analysis suggests that the energy source of SMPs is mainly from the local large-scale and mesoscale processes. Baroclinic instability at submesoscales plays a significant role, further confirming the importance of frontogenesis and MLI. Barotropic instability also has considerable contribution to the submesoscale kinetic energy, especially during summer. Significance Statement Submesoscale processes (SMPs) are ubiquitous in the Bay of Bengal (BoB). Affected by the seasonally reversing monsoon, abundant rainfall and runoff, and equatorial remote forcing, the upper circulation in the BoB is complex, featuring active mesoscale eddies and rich submesoscale phenomena, making the BoB a “natural test ground” for submesoscale studies. It is found in this work that characteristics of SMPs in the BoB are quite different from other regions. In the southern bay, SMPs are most active during the summer and winter monsoons due to the frontogenesis, enhanced mixed layer instability (MLI), and symmetric instability. These findings could deepen our understanding on multiscale dynamic processes and energy cascade in the BoB and have implications for the study of marine ecology and biogeochemical processes.

Characteristics of a Seasonal Front in the Southern Bay of Bengal: Dynamics, Mixing, and Water Mass Transport

Abstract The formation of a sharp oceanic front located south-southeast of Sri Lanka during the southwest monsoon is examined through in situ and remote observations and high-resolution model output. Remote sensing and model output reveal that the front extends approximately 200 km eastward from the southeast coast of Sri Lanka toward the southern Bay of Bengal (BoB). This annually occurring front is associated with the boundary between the southwest monsoon current with high-salinity water to the south, and a weak flow field comprised of relatively fresh BoB water to the north. The front contains a line of high chlorophyll extending from the coastal upwelling zone, often for several hundred kilometers. Elevated turbulent diffusivities ∼10−2 m2 s−1 along with large diapycnal fluxes of heat and salt were found within the front. The formation of the front and vertical transports are linked to local wind stress curl. Large vertical velocities (∼50 m day−1) indicate the importance of ageostrophic, submesoscale processes. To examine these processes, the Ertel potential vorticity (PV) was computed using the observations and numerical model output. The model output shows a ribbon of negative PV along the front between the coastal upwelling zone and two eddies (Sri Lanka Dome and an anticyclonic eddy) typically found in the southern BoB. PV estimates support the view that the flow is susceptible to submesoscale instabilities, which in turn generate high vertical velocities within the front. Frontal upwelling and heightened mixing show that the seasonal front is regionally important to linking the fresh surface water of the BoB with the Arabian Sea. Significance Statement Within the ocean, motions span extraordinarily wide ranges of sizes and time scales. In this study we focus on a narrow, intensified feature called a front. This front occurs in the southern Bay of Bengal during the summer monsoon and forms a boundary between fresher water to the north and saltier water to the south. Features such as this are difficult to study, however, by combining observations made from ships and satellites with output from numerical models of the ocean, we are able to better understand the front. This is important because fronts like the one studied here play a role in determining the pathways of heat within the ocean, which, in turn, may feedback into the atmosphere and weather patterns.

Climatology, trend of aerosol-cloud parameters and their correlation over the Northern Indian Ocean

Aerosols are one of the important atmospheric constituents and exert indirect impact on climate through the modification of microphysical and radiative properties of clouds that in turn perturb the precipitation pattern. Thus, the long term quantification of changes in aerosol and cloud characteristics and their interactions on both temporal as well as spatial scale will provide a crucial information for the better assessment of future climate change. In present study, 18 years (2003–2020) MODerate Resolution Imaging Spectro-radiometer (MODIS) derived aerosol-cloud dataset over the Northern Indian Ocean (NIO) were analysed to assess climatology and trend of aerosol, cloud characteristics and their correlation. We found a strong heterogeneity in spatio-temporal variation of aerosol and cloud parameters over the NIO that are more prominent for the coastal region. The climatological mean of aerosol loading is found high (AOD ≥ 0.5) over the outflow region along the Indian sub-continent and low (AOD ≤ 0.2) over the northern equatorial open ocean. The climatological mean of cloud properties shows dominance of optically thicker deep convective (CTP < 600 hPa and CTT < 260 K) clouds over the southern Bay of Bengal (BoB) and thinner shallow (CTP > 700 hPa and CTT > 273 K) over the northwestern Arabian Sea (AS). Similarly, bigger effective radii (>17 µm) observed along the equatorial open ocean whereas smaller CER (<17 µm) were found over Indian sub-continental coastline and western AS. Further, trend analysis reveals an increasing pattern in AOD (0.002 yr−1), CER (0.051 µm yr−1), LWP (0.033 gm−2 yr−1) and CF (0.002 yr−1) while COD, CTT and CTP show negative trend in order of −0.005 yr−1, −0.094 K yr−1 and −1.160 hPa yr−1, respectively. We also perform similar analysis for seven sub-region of interest (R1 to R7) across the NIO and results show a decreasing pattern in AOD (−0.001 yr−1) at R4 against maximum mean AOD (0.44 ± 0.03). However, coastal sub-regions R1 and R5 illustrate maximum increase in aerosol loading (>0.003 yr−1) suggesting a significant impact of sub-continental outflow over the regions. The spatial correlation of cloud properties with respect to AOD shows a positive slope for CER (0.14) and CF (0.48) and a negative for COD (−0.19), LWP (−0.18), CTT (−0.37), CTP (−0.41). The present study provides in-depth information about the aerosol-cloud characteristics for a long term scale over NIO and could be useful in regional aerosol-cloud interaction induced climate forcing estimation.

Formation, maintenance and diurnal variability of subsurface chlorophyll maximum during the summer monsoon in the southern Bay of Bengal

The vertical structure of chlorophyll in stratified oceanic waters exhibits a maximum substantially below the surface, known as the subsurface chlorophyll maximum (SCM). In the southern Bay of Bengal (BoB), a 12-day high-resolution time-series observation at 89°E and 8°N, during the 2016 summer monsoon showed that the SCM was located within the barrier layer (BL) and the euphotic layer. The strong BL (N2=3×10−4 s−2) located between 30 to 70 m, associated with a low salinity (<33.8 psu) water near the surface, maintained the stratification and the SCM. This SCM oscillated diurnally with an amplitude of 0.1 to 0.2 mg m−3. The lowest values of average chlorophyll in the SCM occurred around 06:00 hours (local time) and reached a maximum around 16:00 hours, four hours after the maximum in surface shortwave radiation. A coupled physical–biogeochemical one-dimensional model was used to examine the formation, maintenance and diurnal variability of the SCM in the southern BoB. The wind speed (<10 m s−1) that prevailed in the region during the weak phase of the monsoon was insufficient to break the BL and cause a surface bloom. The model results revealed that the surface shortwave radiation drove the diurnal cycle of the growth rate of phytoplankton at the SCM, hence the diurnal cycle of phytoplankton biomass and chlorophyll. The model’s control simulation showed the dominance of small phytoplankton, and its growth rate was limited by light and nutrients at the SCM. Whereas, in the ML, the phytoplankton growth rate was limited by nutrients. In the model, the regenerated production supported around 60% of the chl-a at the location of SCM. The subsurface high-salinity core, a prominent feature in the southern BoB, did not influence the SCM. This paper offers new insights into understanding the SCM formation, maintenance and diurnal variability in stratified tropical oceans.

Seasonal Diversity of El Niño‐Induced Marine Heatwave Increases in the Bay of Bengal

AbstractThis study explored the characteristics of marine heatwaves (MHWs) and the related dynamics in Bay of Bengal (BOB) associated with El Niño events during 1982–2020. This showed that El Niño can induce more MHW days, which displays remarkable seasonality. The strong influences of El Niño were found in the southern BOB at its peak phase in winter and the following spring, characterized by an increase in MHW days by 106% in winter and 83% in spring. In the proceeding summer, its influences weakened and shifted to the central bay. The more MHW days during winter and spring are primarily caused by the enhanced solar radiation associated with El Niño and partly contributed by the thick barrier layer in winter. In contrast, besides atmospheric heating, the deepening thermocline in the central bay induced by equatorial and local winds also plays an important role in the increase in MHWs during summer.