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The northward propagating intraseasonal Oscillation (ISO) is one of the dominant modes of tropical variability during Boreal summers. Several mechanisms have been proposed to explain northward propagation. Yet the factors that decide the ISO rainfall over a particular region remain elusive. This study shows that the ISO rainfall anomalies weaken across the south Bay of Bengal (SBoB) before they re-strengthen over the north Bay of Bengal (NBoB). We use the moisture budget to understand the reason for these weakening-strengthening cycles. We find that the horizontal moisture flux convergence predominantly controls the ISO rainfall anomalies over the two regions. The convergence of background moisture by the ISO wind perturbations decides the ISO rainfall structure. Since past literature suggests that the Planetary Boundary Layer (PBL) convergence is caused by barotropic vorticity, we further conduct the vorticity budget to understand the rainfall structure. We find that though vorticity tilting helps in the generation of the positive tendency of ISO vortex, it is the vorticity stretching that enhances the PBL convergence and hence the ISO rainfall over the NBoB. Further splitting of the stretching term helps us conclude that the convergence due to wind perturbations is the predominant term, representing feedback between dynamics and thermodynamics. We hypothesize that the weaker rainfall anomalies in the SBoB result from the weaker background column relative humidity and moisture, which do not allow the initial dynamic perturbations to grow as fast as they do in an environment with stronger background relative humidity and moisture (NBoB). Our study helps us understand the factors that control the ISO rainfall over a particular region and could help improve the model simulations.

The Irrawaddy (IR) is the largest river discharging into the Andaman Sea and plays an important role in the salinity distribution and the mixed layer physics of the Andaman Sea. This study presents the first report of the IR plume pathways in the Andaman Sea during the summer monsoon and the mechanisms behind them. An ocean circulation model is employed to conduct idealized experiments in which the freshwater forcing, due to rivers other than IR as well as precipitation, are ignored. Our simulations reveal that, during the summer monsoon, the discharge from Irrawaddy spreads as a freshwater jet oriented towards southeast and accumulates over the shelf at the eastern coast of the Andaman Sea. Climatology of Chlorophyll-a concentration measured by satellite and surface currents from global ocean model reanalysis indicates the presence of the Irrawaddy freshwater jet during the summer monsoon. The evolution of surface salinity and currents along the jet suggests that the IR freshwater traps momentum imparted by winds. The momentum balance in the Irrawaddy jet is between Coriolis and wind friction term, indicating that the freshwater jet is completely driven by winds during the summer monsoon. Surface distribution of wind friction term also shows that the northwest-southeast orientation of the Irrawaddy jet is due to the southwesterly orientation of the summer monsoon winds. Further experiments with three different wind forcing scenarios (no winds, winds over the equator only, and winds over the Bay of Bengal only) reveal that the flow of Irrawaddy jet during the summer monsoon is completely controlled by the local winds.

The inflow of high-saline water from the Arabian Sea (AS) into the Bay of Bengal (BoB) and its subsequent mixing with the relatively fresh BoB water is vital for the north Indian Ocean salt budget. During June–September, the Summer Monsoon Current carries high-salinity water from the AS to the BoB. A time series of microstructure and hydrographic data collected from 4 to 14 July 2016 in the southern BoB (8°N, 89°E) showed the presence of a subsurface (60–150 m) high-salinity core. The high-salinity core was composed of relatively warm and saline AS water overlying the relatively cold and fresh BoB water. The lower part of the high-salinity core showed double-diffusive salt fingering instability. Salt fingering staircases with varying thickness (up to 10 m) in the temperature and salinity profiles were also observed at the base of a high-salinity core at approximately 75–150-m depth. The average downward diapycnal salt flux out of the high-salinity core due to the effect of salt fingering was 2.8 × 10−7 kg m−2 s−1, approximately one order of magnitude higher than the flux if salt fingering was neglected.

Response of Ganga-Brahmaputra river plume to wind forcing in the Bay of Bengal is studied using a numerical ocean circulation model. Four different wind forcing scenarios, namely, winds over the entire model domain, no winds anywhere over the model domain, winds over Equatorial Indian Ocean only and winds over Bay of Bengal only, are considered. Model simulations are carried out in an idealized setting where forcing from other rivers and precipitation is ignored. Despite the absence of this forcing, model captures observed phases of Ganga-Brahmaputra river plume, seasonal cycle of sea surface temperature and spatio-temporal structure of East India Coastal Current (EICC) reasonably well. Horizontal structure of the plume is investigated using surface salinity, surface currents and freshwater thickness obtained from the simulation that includes Ganga-Brahmaputra river discharge and winds over the entire model domain. The plume spreads upstream (eastward) but remains confined to the coast in northern bay during spring. During summer monsoon, plume spreads southward along the east coast of India and subsequently southeastward over the open bay and reaches the northern tip of Andaman islands by the end of October. During winter monsoon, the plume flows southward, assisted by EICC along the east coast of India and recedes northward in the central bay. In the absence of winds, the plume flows southward along the coast of India throughout the year. Equatorial winds force the plume farther (compared to no winds case) southward along the coast of India prominently in the winter monsoon. Local winds control the horizontal structure of the plume in the bay as they produce the seasonal structure that closely resembles that produced by the winds over the entire model domain. Momentum balance of the plume reveals that geostrophy controls the westward drift of the freshwater around the river mouth. Wind friction and associated Ekman flow are important in the upstream spreading of the plume during spring and in the eastward expansion over the open bay during summer monsoon.

A good understanding of the general circulation features of the oceans, particularly of the coastal waters, and ability to predict the key oceanographic parameters with good accuracy and sufficient lead time are necessary for the safe conduct of maritime activities such as fishing, shipping, and offshore industries. Considering these requirements and buoyed by the advancements in the field of ocean modeling, data assimilation, and ocean observation networks along with the availability of the high-performance computational facility in India, Indian National Centre for Ocean Information Services has set up a “High-Resolution Operational Ocean Forecast and Reanalysis System” (HOOFS) with an aim to provide accurate ocean analysis and forecasts for the public, researchers, and other types of users like navigators and the Indian Coast Guard. Major components of HOOFS are (i) a suite of numerical ocean models configured for the Indian Ocean and the coastal waters using the Regional Ocean Modeling System (ROMS) for forecasting physical and biogeochemical state of the ocean and (ii) the data assimilation based on local ensemble transform Kalman filter that assimilates in situ and satellite observations in ROMS. Apart from the routine forecasts of key oceanographic parameters, a few important applications such as (i) Potential Fishing Zone forecasting system and (ii) Search and Rescue Aid Tool are also developed as part of the HOOFS project. The architecture of HOOFS, an account of the quality of ocean analysis and forecasts produced by it and important applications developed based on HOOFS are briefly discussed in this article.

Sea surface temperature (SST) is a fundamental driver of tropical weather systems such as monsoon rainfall and tropical cyclones. However, understanding of the factors that control SST variability is lacking, especially during the monsoons when in situ observations are sparse. Here we use a ground-breaking observational approach to determine the controls on the SST variability in the southern Bay of Bengal. We achieve this through the first full closure of the ocean mixed layer energy budget derived entirely from in situ observations during the Bay of Bengal Boundary Layer Experiment (BoBBLE). Locally measured horizontal advection and entrainment contribute more significantly than expected to SST evolution and thus oceanic variability during the observation period. These processes are poorly resolved by state-of-the-art climate models, which may contribute to poor representation of monsoon rainfall variability. The novel techniques presented here provide a blueprint for future observational experiments to quantify the mixed layer heat budget on longer time scales and to evaluate these processes in models.

The structure and variability of undercurrents in the East India Coastal Current (EICC), which is the western boundary current system in the Bay of Bengal (BoB), and the mechanisms of their formation are examined in this study. We used current data collected by Acoustic Doppler current profilers (ADCP) moored off Cuddalore (~ 12oN), Kakinada (~ 16.5oN), Visakhapatnam (~ 17.7oN), and Gopalpur (~ 19.4oN) and simulations for the period 2013–2014 from a high-resolution model configured for the BoB. The undercurrents were observed at all these locations, mainly during summer (June–August) and winter (October–December). Undercurrents were seen at relatively shallow depths (75 m), and their occurrences were more frequent off Cuddalore, whereas they were deep (100–150 m) and less frequent in the northern part of the east coast (off Visakhapatnam and Gopalpur). Numerical simulations showed that the interaction of the westward propagating anticyclonic eddies with the equatorward EICC weakened the strong surface flow and reversed the weak subsurface flow in the northern part of the western BoB. This interaction resulted in the formation of the poleward undercurrent here. Once these mesoscale eddies dissipated due to the interaction with the continental slope, the poleward undercurrents vanished and equatorward flow in the subsurface reappeared. The observed undercurrents near the shelf break region (75–200 m) in the southern part of the coast (off Cuddalore) were associated with small subsurface eddies (diameter of about 20–30 km), which developed due to large zonal gradient in the alongshore component of EICC. Subsurface anticyclonic circulations of larger spatial extent (diameter > 200 km) were responsible for the observed undercurrents in the deeper levels (deeper than 250 m) off Cuddalore. We further show that intraseasonal variability of undercurrents near the shelf break off Cuddalore was directly linked to intraseasonal variability in the strength of surface EICC itself. Results from this study suggest that the undercurrents observed below the EICC were not continuous poleward flow, but they were part of distinct anticyclonic eddies.

From June–September, the summer monsoon current (SMC) flows eastward south of Sri Lanka and bends northeastward to form a swift jet that enters the Bay of Bengal (BoB). This part of SMC serves as the major component of the water exchange between the Arabian Sea and the BoB, maintaining the salt and freshwater of the North Indian Ocean. The processes that determine the evolution, intensification, and meandering of the SMC involve both local and remote forcing by winds. Interactions of the SMC with westward-propagating Rossby waves and eddies are only partly understood. In this study, we investigate these processes using an Indian Ocean general circulation model (MOM4p1) that is capable of simulating the SMC realistically. Because eddies and meanders are smoothed out in the climatology, our analyses focus on a single year of 2009, a period when a strong anticyclonic bend in the SMC was observed. An eddy-kinetic-energy budget analysis shows the region to be a zone of significant eddy activity, where both barotropic and baroclinic instabilities are active. Based on the analysis, we classify the evolution of SMC into stages of onset, intensification, anticyclonic bend, anticyclonic vortex formation, meandering, and termination. In addition, analysis of eddy-potential-vorticity flux and eddy-enstrophy decay reveal when, where, and how the eddies tend to drive the mean flow. Rossby waves and westward-propagating eddies arriving from the east energize the SMC in June and accelerate the mean flow through an up-gradient eddy-potential-vorticity flux. At the same time, local winds also strengthen the flow, by increasing its mean near-surface kinetic energy and raising isopycnals, the latter building up available potential energy (APE). The baroclinic instability that takes place in late July and early August releases APE, thereby generating the SMC meanders.

The Southwest Monsoon Current (SMC) flows eastward from the Arabian Sea into the Bay of Bengal (BoB) during summer, advecting a core of high salinity water. This high salinity core has been linked with Arabian Sea High Salinity Water that is presumed to enter the BoB directly from the Arabian Sea via the SMC. Here we show that the high salinity core originates primarily from the western equatorial Indian Ocean, reaching the BoB via the Somali Current, the Equatorial Undercurrent and the SMC. Years with anomalously saline high salinity cores are linked with the East Africa Coastal Current and the Somali Current winter convergence, and an anomalously strong Equatorial Undercurrent. Seasonal reversals that occur at the Somali Current and SMC junctions act as ‘railroad switches’ diverting water masses to different basins in the northern Indian Ocean. Interannual fluctuations of the Equatorial Undercurrent are linked to wind stress and El Niño.

Chlorophyll has long been known to influence air–sea gas exchange and CO2 drawdown. But chlorophyll also influences regional climate through its effect on solar radiation absorption and thus sea surface temperature (SST). In the Bay of Bengal, the effect of chlorophyll on SST has been demonstrated to have a significant impact on the Indian summer (southwest) monsoon. However, little is known about the drivers and impacts of chlorophyll variability in the Bay of Bengal during the southwest monsoon. Here we use observations of downwelling irradiance measured by an ocean glider and three profiling floats to determine the spatial and temporal variability of solar absorption across the southern Bay of Bengal during the 2016 summer monsoon. A two-band exponential solar absorption scheme is fitted to vertical profiles of photosynthetically active radiation to determine the effective scale depth of blue light. Scale depths of blue light are found to vary from 12 m during the highest (0.3–0.5 mg m−3) mixed-layer chlorophyll concentrations to over 25 m when the mixed-layer chlorophyll concentrations are below 0.1 mg m−3. The Southwest Monsoon Current and coastal regions of the Bay of Bengal are observed to have higher mixed-layer chlorophyll concentrations and shallower solar penetration depths than other regions of the southern Bay of Bengal. Substantial sub-daily variability in solar radiation absorption is observed, which highlights the importance of near-surface ocean processes in modulating mixed-layer chlorophyll. Simulations using a one-dimensional K-profile parameterization ocean mixed-layer model with observed surface forcing from July 2016 show that a 0.3 mg m−3 increase in chlorophyll concentration increases sea surface temperature by 0.35 ∘C in 1 month, with SST differences growing rapidly during calm and sunny conditions. This has the potential to influence monsoon rainfall around the Bay of Bengal and its intraseasonal variability.

The most important factor determining the interannual variation of the Indian summer monsoon rainfall (ISMR) is El Nino Southern Oscillation (ENSO). The equatorial Indian Ocean oscillation (EQUINOO), which is considered to be the atmospheric component of the coupled Indian Ocean Dipole Mode, also plays an important role. However, the coupling between the oceanic component of Indian Ocean Dipole and EQUINOO is not strong and while the index of EQUINOO is well correlated with ISMR, the index of the oceanic component, Dipole Mode Index, is not. Climate models are now able to generate reasonable predictions of ENSO and can also simulate ENSO impact on the monsoon well. Hence, seasons in which the contribution of ENSO to ISMR dominates that of EQUINOO, the monsoon predictions generated by the models are reasonable. However, large errors occur in the prediction of ISMR when the EQUINOO contribution dominates because the models, in general, are not capable of simulating the link between EQUINOO and ISMR. For further progress, deeper understanding and improved prediction of EQUINOO and its link to ISMR is essential.

Persian Gulf Water (PGW) is an oxygenated, high‐salinity water mass that has recently been detected in the Bay of Bengal (BoB). However, little is known about the transport pathways of PGW into the BoB. Ocean glider observations presented here demonstrate the presence of PGW in the southwestern BoB. Output from an ocean re‐analysis product shows that this PGW signal is associated with a northward‐flowing filament of high‐salinity water. Particle tracking experiments reveal two pathways: one in the eastern Arabian Sea that takes a minimum of two years, and another in the western Arabian Sea that takes a minimum of three years. The western pathway connects to the BoB via equatorial currents. The greatest influx of PGW occurs between 82 and 87°E during the southwest monsoon. We propose that injection of PGW to the BoB OMZ contributes to keeping oxygen concentrations in the BoB above the level at which de‐nitrification occurs. Plain‐language summary The Persian Gulf is a hot, shallow sea that acts like a vast salt pan. Consequently, water flowing out of the Gulf has a very high salt concentration; it also has a relatively high dissolved oxygen concentration. This high‐salt, high‐oxygen signal is distinct to Persian Gulf Water and is largely preserved as Persian Gulf Water spreads in the ocean's interior. In observations collected by an ocean glider, we identify the remnants of this high‐salt, high‐oxygen signal in the southwestern Bay of Bengal, a region that is notably lacking in dissolved oxygen. Using an ocean model, we identify two pathways taken by Persian Gulf Water between the northern Arabian Sea and the Bay of Bengal: one in the eastern Arabian Sea that takes a minimum of two years; and one in the western Arabian Sea that takes a minimum of three years. Persian Gulf Water arrives in the Bay of Bengal throughout the year, but particularly during the southwest monsoon (June to September). Persian Gulf Water brings oxygen to the Bay of Bengal and potentially plays a role in keeping dissolved oxygen levels in the Bay above the level at which its ecological functioning would be significantly altered.

The tropical Indian Ocean has been warming at an alarming rate in the recent decade with the southern tropical Indian Ocean (STIO; 15oS-35oS) being the hotspot storing more than 40% of the heat abs...

The strong stratification of the Bay of Bengal (BoB) causes rapid variations in sea surface temperature (SST) that influence the development of monsoon rainfall systems. This stratification is driven by the salinity difference between the fresh surface waters of the northern bay and the supply of warm, salty water by the Southwest Monsoon Current (SMC). Despite the influence of the SMC on monsoon dynamics, observations of this current during the monsoon are sparse. Using data from high-resolution in situ measurements along an east–west section at 8°N in the southern BoB, we calculate that the northward transport during July 2016 was between 16.7 and 24.5 Sv (1 Sv ≡ 106 m3 s−1), although up to ⅔ of this transport is associated with persistent recirculating eddies, including the Sri Lanka Dome. Comparison with climatology suggests the SMC in early July was close to the average annual maximum strength. The NEMO 1/12° ocean model with data assimilation is found to faithfully represent the variability of the SMC and associated water masses. We show how the variability in SMC strength and position is driven by the complex interplay between local forcing (wind stress curl over the Sri Lanka Dome) and remote forcing (Kelvin and Rossby wave propagation). Thus, various modes of climatic variability will influence SMC strength and location on time scales from weeks to years. Idealized one-dimensional ocean model experiments show that subsurface water masses advected by the SMC significantly alter the evolution of SST and salinity, potentially impacting Indian monsoon rainfall.

The multiscale synoptic circulation in the southern Bay of Bengal region is simulated using a high-resolution Feature-Oriented Regional Modeling and Simulation (FORMS) approach. First, the region’s prevalent ocean circulation features are identified and mapped onto a FORMS template using a literature survey, and analysis of in-situ and remote sensing data. These features include the Southwest Monsoon Current (SMC), Sri-Lanka Dome, Anti-cyclonic Eddy, and Subsurface Baroclinic Eddy to the east of the SMC. New data-driven feature models are developed for the SMC and Subsurface Baroclinic Eddy using observational transects collected during the Bay of Bengal Boundary Layer Experiment (BoBBLE) of July 2016. Specifically, a new elliptical feature model for the SMC’s high salinity core is developed and utilized. The developed temperature and salinity feature models are placed on the FORMS template, and pseudo observation profiles are sampled for use in an objective analysis initialization scheme. The background for the objective analysis is taken to be the NEMO global analysis model data on the day of the synoptic initialization. The FORMS derived initialization fields on 04 July 2016 are used with the MIT-MSEAS Primitive Equation modeling system to simulate the ocean circulation in the region from 04 July to 15 July 2016. The forecast skill of the model is quantified by comparison to the time-series observations collected during BoBBLE (but not used in the FORMS development) and ARGO data available in the region during the simulation. Four experimental short-term synoptic simulations at a 3 km horizontal resolution and 70 vertical levels, with and without the different components of the feature models are showcased. Results are utilized to calibrate and improve the feature model, including the addition of a Subsurface Baroclinic Eddy and the development of a new surface bias-correction scheme to obtain a better match with validation datasets. The present data-driven feature modeling approach improves the representation and forecast skill for several critical dynamical features: (i) the initial high salinity core at the time-series location (ii) evolution and expansion of the sub-surface high salinity core; and (iii) salinization and freshening events at the time-series location between 3 m to 20 m. The MSEAS model simulation from the final experiment match the BoBBLE temperature and salinity data (total RMSE of 0.14 psu in salinity and 0.57 C in temperature in between 60-120 m depths and total RMSE of 0.17 psu in salinity and 0.33 C in temperature in between 3-60 m depths) and the ARGO measured temperature and salinity (total RMSE of 0.16 psu in salinity and 0.66 C in temperature), showcasing the model’s success in providing excellent spatio-temporal representation of the ocean in the region of interest.

Earlier studies suggest that the mixed-layer (ML) temperature or sea surface temperature (SST) of the Bay of Bengal during the summer monsoon is determined primarily by air–sea fluxes. We use an oceanic general circulation model (OGCM) to show that oceanic processes also play a significant role. Model heat-budget computations show that horizontal and vertical advection, constituting the direct role of dynamics, contribute significantly to the SST tendency in the western bay. The eastern limit of this direct-role regime is determined by downwelling Rossby waves, which are forced largely from the equatorial Indian Ocean, but are modified by alongshore winds at the eastern boundary and Ekman pumping along their westward path across the bay. The current and, as a result, advection weaken behind the Rossby waves. To the east of the Rossby wavefront, the thermocline is deep, permitting a deeper ML, but the actual ML depth (MLD) depends on the wind speed, with salinity also playing a role in the northern bay. Yet, there is negligible change in the SST even when MLD changes significantly because the deep thermocline decouples the changes in MLD and SST. In contrast, the shallower thermocline in the western bay limits the potential MLD, leading to larger changes in SST. The upwelling (downwelling) Rossby wave essentially conditions the upper ocean by decreasing (increasing) the potential depth of the mixed layer. SST variation weakens only when the thermocline deepens during downwelling events, which occur later in the western bay because Rossby waves propagate westward. The significant, but subtle, role of the Rossby waves in decoupling the changes in MLD and SST via downwelling is indirect, unlike the direct role of advection estimated in a standard heat-budget computation, and has been largely ignored in earlier studies.

During the summer monsoon (June–September), the influence of cyclonic curl of local winds causes the formation of a thermocline dome called the Sri Lankan Dome (SLD) in the southern Bay of Bengal (BoB). In addition, the subsurface flow associated with the Summer Monsoon Current (SMC) brings Arabian Sea-High Salinity Water mass to the southern BoB. We show that both these oceanographic features are enriched in dissolved CO2 with the upper boundary shoaling very close to the surface. We observed episodic deep mixing events leading to entrainment of CO2-rich subsurface water with the mixed layer. CO2 disequilibrium within the top 45 m reached as high as + 404 µatm. Our estimated mixed layer ventilation rates ranged between 2.4 and 4.6 days between sampling stations suggesting equilibration with upwelled waters were still evolving. We also encountered a patch of Arabian Sea-High Salinity Water mass with low aqueous pCO2 suggesting past ventilation. Our modest estimates suggest a grid area of 2° latitude × 2° longitude can trigger a mean release of 0.78 Gg C day−1, which is significantly higher than the estimated new production rates due to upwelled nutrients. Our study illustrates that upwelled water associated with the SLD in conjunction with the barrier layer erosion accompanied with the flow of SMC has the potential to occasionally ventilate in southern BoB. We believe that these processes can negate the region's benefits by acting as a CO2 source which underscores the need for detailed investigation.

Circulation in the Bay of Bengal (BoB) is driven not only by local winds, but are also strongly forced by the reflection of equatorial Kelvin waves (EKWs) from the eastern boundary of the Indian Ocean. The equatorial influence attains its peak during the monsoon-transition period when strong eastward currents force the strong EKWs along the equator. The Andaman Sea, lying between the Andaman and Nicobar island chains to its west and Indonesia, Thailand, and Myanmar to the south, east, and north, is connected to the equatorial ocean and the BoB by three primary passages, the southern (6°N), middle (10°N), and northern (15°N) channels. We use ocean circulation models, together with satellite altimeter data, to study the pathways by which equatorial signals pass through the Andaman Sea to the BoB and associated dynamical interactions in the process. The mean coastal circulation within the Andaman Sea and around the islands is primarily driven by equatorial forcing, with the local winds forcing a weak sea-level signal. On the other hand, the current forced by local winds is comparable to that forced remotely from the equator. Our results suggest that the Andaman and Nicobar Islands not only influence the circulation within the Andaman Sea, but also significantly alter the circulation in the interior bay and along the east coast of India, implying that they need to be represented accurately in numerical models of the Indian Ocean.

Operational oceanography can be described as the provision of routine oceanographic information needed for decision-making purposes. It is dependent upon sustained research and development through the end-to-end framework of an operational service, from observation collection to delivery mechanisms. The core components of operational oceanographic systems are a multi-platform observation network, a data management system, a data assimilative prediction system, and a dissemination/accessibility system. These are interdependent, necessitating communication and exchange between them, and together provide the mechanism through which a clear picture of ocean conditions, in the past, present, and future, can be seen. Ocean observations play a critical role in all aspects of operational oceanography, not only for assimilation but as part of the research cycle, and for verification and validation of products. Data assimilative prediction systems are advancing at a fast pace, in tandem with improved science and the growth in computing power. To make best use of the system capability these advances would be matched by equivalent advances in operational observation coverage. This synergy between the prediction and observation systems underpins the quality of products available to stakeholders, and justifies the need for sustained ocean observations. In this white paper, the components of an operational oceanographic system are described, highlighting the critical role of ocean observations, and how the operational systems will evolve over the next decade to improve the characterization of ocean conditions, including at finer spatial and temporal scales.

During the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series of microstructure measurements were obtained at 8°N, 89°E from 4 to 14 July 2016. These observations captured events of barrier layer (BL) erosion and reformation. Initially, a three-layer structure was observed: a fresh surface mixed layer (ML) of thickness 10–20 m; a BL below of 30–40-m thickness with similar temperature but higher salinity; and a high salinity core layer, associated with the Summer Monsoon Current. Each of these three layers was in relative motion to the others, leading to regions of high shear at the interfaces. However, the destabilizing influence of the shear regions was not enough to overcome the haline stratification, and the three-layer structure was preserved. A salinity budget using in situ observations suggested that during the BL erosion, differential advection brought high salinity surface waters (34.5 psu) with weak stratification to the time series location and replaced the three-layer structure with a deep ML (~60 m). The resulting weakened stratification at the time series location then allowed atmospheric wind forcing to penetrate deeper. The turbulent kinetic energy dissipation rate and eddy diffusivity showed elevated values above 10−7 W kg−1 and 10−4 m2 s−1, respectively, in the upper 60 m. Later, the surface salinity decreased again (33.8 psu) through differential horizontal advection, stratification became stronger and elevated mixing rates were confined to the upper 20 m, and the BL reformed. A 1D model experiment suggested that in the study region, differential advection of temperature–salinity characteristics is essential for the maintenance of BL and to the extent to which mixing penetrates the water column.

The Bay of Bengal (BoB) is considered to be a region of low biological productivity, owing to nutrient limitation, caused by strong salinity stratification induced by the freshwater influx from rivers and precipitation. Satellite and in situ observations, however, reveal the presence of prominent regional blooms in the bay in response to monsoonal forcings. Bloom dynamics of the BoB are presumably determined by freshwater as well as the local and remote effect of winds and remain to be explored in detail. Using a coupled physical-ecosystem model, we have examined the oceanic processes controlling productivity in the northwestern BoB during the summer monsoon. The region exhibits a prominent bloom lasting for a period of about 2 months, supporting major fishing zones along the northeast coast of India. The ecosystem model simulates the spatial and temporal evolution of the surface bloom in good agreement with Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) observations. Vertical distribution of upper ocean physical and biological tracers and a nitrate budget analysis reveal the dominant role of coastal upwelling induced by alongshore winds in triggering the bloom. Horizontal advection plays a secondary role by supplying nutrients from coastal to offshore regions. The bloom decays with the weakening of winds and upwelling by the end of summer monsoon. The simulated bloom in the northwestern bay remains largely unaffected by the freshwater effects, since the peak bloom occurs before the arrival of river plumes.

The intense winter phytoplankton bloom during November–February in the northeastern Arabian Sea (NEAS) was thought, until recently, to be controlled only by a convective deepening of the mixed layer (ML) owing to cool and dry northeasterlies. But a recent study has shown that the deepening of the ML in the southern NEAS is inhibited by the poleward advection of low-salinity water from the south by the West India Coastal Current (WICC). Using an Ocean General Circulation Model coupled with an ecosystem model, we investigate the consequences of the inhibition of mixed-layer deepening for winter phytoplankton bloom in the NEAS. We show that, during the winter monsoon, the shallow ML inhibits the entrainment of nutrients in the southern NEAS. Strong (weak) positive nitrate tendency in the northern (southern) NEAS seen in the model during the winter monsoon is maintained by strong (weak) entrainment. As a result, the chlorophyll integrated to 200 m depth from the surface is lower in the southern NEAS than in the northern NEAS. The inhibition of mixed-layer deepening in the south affects the size-based distribution of small and large phytoplankton, nutrient limitation terms and growth rate, and their elemental composition. The WICC, which inhibits the deepening of the ML and affects the winter bloom in the NEAS, is driven by coastal Kelvin waves generated by remote winds. This paper demonstrates a mechanism by which remotely forced coastal Kelvin waves impact the biology in the north Indian Ocean.

The high-salinity water masses that originate in the North Indian Ocean are Arabian Sea High-Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW). Among them, only ASHSW has been shown to exist in the Bay of Bengal. We use CTD data from recent cruises to show that PGW and RSW also exist in the bay. The presence of RSW is marked by a deviation of the salinity vertical profile from a fitted curve at depths ranging from 500 to 1000 m; this deviation, though small (of the order of similar to 0.005 psu and therefore comparable to the CTD accuracy of 0.003 psu), is an order of magnitude larger than the similar to 0.0003 psu fluctuations associated with the background turbulence or instrument noise in this depth regime, allowing us to infer the existence of RSW throughout the bay. PGW is marked by the presence of a salinity maximum at 200-450 m; in the southwestern bay, PGW can be distinguished from the salinity maximum due to ASHSW because of the intervening Arabian Sea Salinity Minimum. This salinity minimum and the maximum associated with ASHSW disappear east and north of the south-central bay (85A degrees E, 8A degrees N) owing to mixing between the fresher surface waters that are native to the bay (Bay of Bengal Water or BBW) with the high-salinity ASHSW. Hence, ASHSW is not seen as a distinct water mass in the northern and eastern bay and the maximum salinity over most of the bay is associated with PGW. The surface water over most of the bay is therefore a mixture of ASHSW and the low-salinity BBW. As a corollary, we can also infer that the weak oxygen peak seen within the oxygen-minimum zone in the bay at a depth of 250-400 m is associated with PGW. The hydrographic data also show that these three high-salinity water masses are advected into the bay by the Summer Monsoon Current, which is seen to be a deep current extending to 1000 m. These deep currents extend into the northern bay as well, providing a mechanism for spreading ASHSW, PGW, and RSW throughout the bay.

Individual and combined effects of rainfall and river discharge on the Bay of Bengal (BoB) is investigated using an Ocean General Circulation Model. A set of four sensitivity experiments, forced with same air-sea heat flux, but retaining either river runoff or rainfall or both is carried out. These experiments show that the river water is exported out of the bay along the western boundary during winter and rain water along the eastern boundary during summer. Runoff leads to a large (>3 psu) decrease in salinity in the northern bay during summer and along the western boundary during winter, with a weaker contribution from rainfall. The sea surface temperature response to freshwater forcing shows large spatial variations with eastern bay showing higher differences. The northwestern bay warms by similar to 1.5 degrees C in the presence of freshwater during summer, due to greater heat absorption within a shallow mixed layer (ML). This warming is caused in nearly equal proportions by rain and river water in early summer, but the contribution by river water dominates during peak and withdrawal phases of the summer monsoon. Northeastern bay, in contrast, is cooler by 1.5-3 degrees C in the presence of freshwater, caused primarily by river runoff, owing to the winter cooling over a thin ML. Temperature inversions form due to surface cooling of a river stratified layer during winter in the northwestern bay and due to radiation penetrating below the ML during summer in the northeastern bay.

Seasonal variability of the vertical structure of chlorophyll-a (chl-a) in the central Arabian Sea is described using seasonal climatology from bio-Argo floats. A quarter-degree resolution coupled OGCM-ecosystem model is employed to explain the physical and biogeochemical processes that determine the observed seasonality of the chl-a profiles. The most prominent feature of the chl-a is that seasonal surface bloom (chl-a > 0.25 mg m−3) occurs during the winter (November–February) and summer (June–September) monsoons. A sub-surface maximum (SCM) in chl-a occurs at a depth of about 60 m during the spring (March–May) and fall (October) oligotrophy. The SCM is absent during the peak of winter and summer bloom. The model simulated the observed seasonal evolution of chl-a and physical variables. Nitrate budget analysis, using the model simulation, reveals that vertical entrainment of nutrients plays a vital role in supplying nutrients into the surface mixed layer (ML) during the summer forced by wind mixing and winter due to convective cooling at the surface. Mixing is critical than Ekman pumping during the summer and winter monsoons. Further, the horizontal and vertical advection of nitrate is weak. The model showed the dominance of small phytoplankton in the central Arabian Sea. Analysis of the growth rate equation in the model shows that during the seasons of bloom, nutrients are surplus in the ML, and the euphotic depth that is shallower than the mixed layer depth drives the vertical structure of chl-a. The limiting nutrient above the euphotic depth during the winter (summer) is iron (phosphate). During oligotrophy, the growth rate of phytoplankton in the ML (SCM region) is determined by nitrogen (iron) limitation.

The northern Bay of Bengal receives a large amount of fresh water through river runoff and rainfall during the Indian summer monsoon (June–September). This fresh water spreads offshore and can modulate the upper-ocean chlorophyll via two processes: advection of river nutrients and inhibition of vertical supply of subsurface nutrients by increasing the stratification. We address these two processes during the summer monsoon of 2012 using hydrographic observations and a coupled physical-biogeochemical model. The observations show an increase in surface chlorophyll associated with the advection of a freshwater plume on the shelf. Near the slope, but over a cross-shore distance of ~ 70 km, a thin layer separates this elevated surface chlorophyll from the subsurface chlorophyll maximum layer (SCML). This separation disappears as the freshwater plume reaches the open ocean. The SCML shoals toward the coast and is absent on the shelf. Time-series observations from the open ocean also show an increase in chlorophyll in concurrence with the arrival of a freshwater plume. Simulations with a coupled physical-biogeochemical model show, however, that it is the wind-induced vertical processes that cause the increase in chlorophyll in the open ocean and not the nutrients brought by the horizontal advection of freshwater plumes. The increase in stratification only limits and does not completely inhibit the vertical supply of nutrients from the subsurface layers.

The region of relatively cooler water, sitting in the shadow zone of monsoon rainfall, around southern tip of India and around Sri Lanka, is known as the cold pool. A ship-board observations program to the east of Sri Lanka during the summer monsoon of 2009 captured two cooling events within the core of the Summer Monsoon Current (SMC). Time-series of temperature profiles within the cold pool showed a cooling of the sea surface temperature (SST) by about 0.3°C during 22–25 July 2009 and about 0.4°C during 29 July to 2 August 2009. During the cooling periods, both wind speed and mixed layer depth (MLD) increased, but the air-sea heat gain by the ocean remained positive. Efforts were made to estimate the temperature advection term by means of an Operation Advection strategy using CTD data at four locations about 6.5 km away from the time-series location and ship-board acoustic Doppler current profile measurements. Estimates of horizontal temperature advection suggest that these cooling events are induced by the advection of cooler water by the SMC. Simulation using an Indian Ocean model captures such cooling events and the model temperature equation shows that cooling events through the season are driven either by advection or by entrainment. Thus observations and model simulation concurrently demonstrate that cold pool is maintained through the summer monsoon by the SMC in spite of the ocean gaining heat from the atmosphere.

Signatures of Rossby waves that are evident in satellite ocean colour data in the southeastern Arabian Sea (SEAS) are more prominent during the winter than the summer monsoon. During the winter, the sea level is high, and the conditions are less conducive for the injection of nutrients through the eddy-pumping mechanism. In this study, we use satellite observations and a physical-ecosystem model to examine the role of horizontal advection associated with Rossby waves in enhancing chlorophyll concentration in the SEAS. Zonal advection dominates over meridional advection, and its effect is more prominent in the south of Indian subcontinent and west of the Maldives. Except along the coast and around the islands, the vertical processes are weak and do not lead to a substantial increase in chlorophyll. Model simulations show that the currents associated with the Rossby wave also transport nutrients and other physicochemical properties of water along the path of the wave. The advected water decreases the concentration of Arabian Sea High Salinity water mass and replenishes the oxygen in the oxygen minimum zone in the SEAS.

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.