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Multidecadal Changes of the Upper Indian Ocean Heat Content during 1965–2016

Ocean heat uptake is the primary heat sink of the globe and modulates its surface warming rate. In situ observations during the past half century documented obvious multidecadal variations in the upper-ocean heat content (0–400 m; OHC400) of the Indian Ocean (IO). The observed OHC400 showed an increase of (5.9 ± 2.5) × 1021 J decade−1 during 1965–79, followed by a decrease of (−5.2 ± 2.5) × 1021 J decade−1 during 1980–96, and a rapid increase of (13.6 ± 1.1) × 1021 J decade−1 during 2000–14. These variations are faithfully reproduced by an Indo-Pacific simulation of an ocean general circulation model (OGCM), and insights into the underlying mechanisms are gained through OGCM experiments. The Pacific wind forcing through the Indonesian Throughflow (ITF) was the leading driver of the basin-integrated OHC400 increase during 1965–79 and the decrease during 1980–96, whereas after 2000 local wind and heat flux forcing within the IO made a larger contribution. The ITF heat transport is primarily dictated by Pacific trade winds. It directly affects the south IO, after which the signatures can enter the north IO through the meridional heat transport of the western boundary current. The prevailing warming of the western-to-central IO for 2000–14 was largely induced by equatorial easterly wind trends, Ekman downwelling off the equator, and northeasterly wind trends over the west Asia–East Africa coastal region. The increasing downward longwave radiation, probably reflecting anthropogenic greenhouse gas forcing, overcame the decreasing surface shortwave radiation and also made a significant contribution to the rapid upper-IO warming after 2000.