Over the past decade, our understanding of the Indian Ocean has advanced through concerted efforts toward measuring the ocean circulation and air–sea exchanges, detecting changes in water masses, and linking physical processes to ecologically important variables. New circulation pathways and mechanisms have been discovered that control atmospheric and oceanic mean state and variability. This review brings together new understanding of the ocean–atmosphere system in the Indian Ocean since the last comprehensive review, describing the Indian Ocean circulation patterns, air–sea interactions, and climate variability. Coordinated international focus on the Indian Ocean has motivated the application of new technologies to deliver higher-resolution observations and models of Indian Ocean processes. As a result we are discovering the importance of small-scale processes in setting the large-scale gradients and circulation, interactions between physical and biogeochemical processes, interactions between boundary currents and the interior, and interactions between the surface and the deep ocean. A newly discovered regional climate mode in the southeast Indian Ocean, the Ningaloo Niño, has instigated more regional air–sea coupling and marine heatwave research in the global oceans. In the last decade, we have seen rapid warming of the Indian Ocean overlaid with extremes in the form of marine heatwaves. These events have motivated studies that have delivered new insight into the variability in ocean heat content and exchanges in the Indian Ocean and have highlighted the critical role of the Indian Ocean as a clearing house for anthropogenic heat. This synthesis paper reviews the advances in these areas in the last decade.

Near‐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.

The southern branch of the South Equatorial Current (SSEC) is the northern limit of the South Atlantic Subtropical Gyre. When this current reaches Brazil around 14°S it bifurcates into a southward flow as the Brazil Current (BC) and the surface portion of the northward flowing North Brazil Undercurrent (NBUC). The SSEC system is a key component of the western boundary supply, influencing the NBUC/BC variability and, therefore, global climate through the Meridional Overturning Circulation. In this study, using altimetry satellite data and reanalyzes outputs (1993–2018), we revisit the SSEC mean state and show this current arriving at the South Atlantic western boundary as a multi‐banded flow with surface signatures resulting from different subsurface cores. These bands have velocities between 0.02 and 0.07 m s⁻¹ and, as shown by ADCP data from the PIRATA project, their signature in synoptic scenarios is obscured by eddies and waves with velocities between 0.1 and 0.3 m s⁻¹. In addition, the SSEC annual cycle analysis shows that the seasonality of the bands is out of phase with each other, presenting westward transport anomalies between 0.4 and 2.6 Sv. Finally, our results show that the seasonality of this multi‐banded flow both defines where the BC is born, and modulates the seasonality of semi‐permanent mesoscale eddies off Brazil.

The mesoscale activity of western boundary currents can induce changes in the oligotrophic conditions of the oceanic subtropical gyres. Off SE Brazil, recurrent meanders in the Brazil Current can modulate the pelagic ecosystem through eddy‐induced upwelling, as well as drive coastal upwelling and cross‐shelf exchanges. Despite their relevance, direct measurements of such meanders are scarce. At the end of the Spring of 2019, we acquired measurements of both physical and biogeochemical properties along a transect through one of these features, with 5–10 km spacing between stations. Displacements of the isopycnals at the meander edges arose from submesoscale filaments along the meander's rim, associated with the signature of shelf water masses. Chlorophyll distributions along the transect indicated phytoplankton responses to the different dynamic regimes in the meander: eddy stirring in the surface mixed layers versus eddy pumping in the subsurface waters, and mesoscale dynamics in the meander center versus submesoscale dynamics in its periphery. Subsurface phytoplankton biomass was enhanced in the meander center, likely due to eddy pumping raising the nutricline into the euphotic zone. Eddy pumping also appears to have contributed to a shift in the phytoplankton community composition to larger organisms. Our results indicate that the physical dynamics create different habitats across the meander, leading to spatial patchiness in the chlorophyll distributions, with different types of organisms in distinct parts of the meander.

This study uses submesoscale‐permitting simulations to explore the formation of temperature inversions at shallow, salinity‐controlled density fronts representative of conditions in the wintertime Bay of Bengal (BoB). Our simulations complement earlier one‐dimensional studies that have largely used mooring records in the BoB to infer mechanisms for causing temperature inversions, by exploring three‐dimensional frontal processes. We use three different initial conditions with the same lateral and vertical gradients in density, with differing lateral contrasts in temperature within the mixed layer. Our simulations evolve unforced for a few inertial periods, are then forced by weak downfront winds for a few more inertial periods to generate an eddy field, after which the winds are turned off. The formation of inversions proceeds via the subduction of low potential vorticity (PV) fluid from the surface during the forced phase. The inversions result from tilting of horizontal temperature gradients into the vertical direction. Tilting is simultaneously the dominant source of negative vertical vorticity in the Lagrangian vorticity budget. Turning off the winds caps the subducted fluid by overlying stratified fluid, creating temperature inversions with O(1–10 km) lateral scales and O(10 m) thickness. The inversions are pycnostads with anticyclonic rotation. The increase in temperature for the strongest inversion in each simulation is comparable to the initial lateral contrast in temperature. The strongest inversions are associated with anticyclonic rotation and low PV. Our results have potential implications for the vertical thermal structure of the upper BoB in coarse‐resolution models that do not resolve subduction at submesoscale fronts.