Sea surface temperature (SST) and upper ocean heat content (OHC, upper 700 m) in the tropical Indian Ocean underwent rapid warming during 1950–2015, with the SSTs showing an average warming of about 1 °C. The SST and OHC trends are very likely to continue in the future, under different emission scenarios. Climate models project a rise in tropical Indian Ocean SST by 1.2–1.6 °C and 1.6–2.7 °C in the near (2040–2069) and far (2070–2099) future across greenhouse gas (GHG) emissions scenarios RCP4.5 and RCP8.5, relative to the reference period of 1976–2005. Indian Ocean warming has very likely resulted in decreasing trend in oxygen (O2) concentrations in the tropical Indian Ocean, and declining trends in pH and marine phytoplankton over the western Indian Ocean. The observed trends in O2, pH and marine phytoplankton are projected to increase in the future with continued GHG emissions.

This study investigates the relationship between inter‐decadal variation in the number of monsoon depressions (MDs) over the Bay of Bengal (BoB) and the Pacific Decadal Oscillation (PDO). It is shown that there is an out‐of‐phase variation in the number of MDs over the BoB and the PDO, except during 1927–1945. Quantitative estimates of the relative contributions of individual environmental parameters show that the variation in the mid‐tropospheric relative humidity over the BoB is the primary reason for the observed variation in the number of MDs. It is further postulated that the variation in the sea surface temperature in the western equatorial Indian Ocean associated with the PDO could be one of the reasons for the changes in the moisture advection over to the BoB and hence the variation in the number of MDs in inter‐decadal timescale.

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