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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. [ABSTRACT FROM AUTHOR]

Persian Gulf Water (PGW) is an oxygenated, high-salinity water mass that originates in thePersian Gulf. Injection of PGW into the Bay of Bengal (BoB) could act to ventilate the BoBoxygen minimum zone, but little is known about when, where and how PGW is transported tothe BoB. Using a combination of models and observations, we show that eddy-like featuresinject PGW into the BoB throughout the year, and we identify the pathways and time scalesfor transport of PGW from the Arabian Sea. Temperature, salinity and oxygen observationsfrom an autonomous ocean glider deployed at 85.5˚ E as part of the Bay of Bengal BoundaryLayer Experiment (BoBBLE) demonstrate the presence of PGW in the southwesternBay of Bengal (BoB); the glider observes an increase in salinity and oxygen at thepotential density of PGW (1026.5 kg m−3, approximately 250 m deep). Output froma NEMO 1/12th degree ocean reanalysis product show that this PGW signal isassociated with a northward-flowing filament of high-salinity water, and backwardparticle tracking experiments confirm that the origin of this water is in the northernArabian Sea. The particle tracking experiments identify two pathways along whichPGW travels from its outflow in the northern Arabian Sea to the southwestern BoB.One pathway follows the coast of India southward in the eastern Arabian Sea; thesecond follows the Omani and Somali coasts southward in the western Arabian Seaand connects to the BoB via the equatorial currents. The shorter, eastern pathwaytakes approximately 3.5 years; the longer, western pathway takes approximately4.5 years. We calculate the transient salinity flux, v′S′, into the BoB across 8˚ Nfrom 11 years of NEMO reanalysis to investigate the interannual and longitudinalvariability of the PGW flux into the BoB. The greatest influx of PGW into the BoB isto the east of Sri Lanka, in which region the time-mean salinity flux is directednorthwards. The PGW filament observed by the glider, however, is anomalous:it occurs in a region of limited salt flux and the filament does not occur as partof an eddy-like feature, which the majority of flux events appear to be. There isno apparent annual cycle in the salinity flux: events occur throughout the year. [ABSTRACT FROM AUTHOR]