Multidecadal meridional dipole mode in the Indian Ocean subsurface ocean heat content

Abstract

Multidecadal changes in the background state of the Indian Ocean, such as variations in ocean circulation patterns, sea level and heat storage, can act as a carrier wave for the climate change and other variabilities. The long-term (~60 years since 1958) analysis of subsurface ocean heat content (sub-OHC) in the Indian Ocean exhibits the presence of a dominant multidecadal meridional dipole mode in the region. The analysis shows that until the late 1980s, a basin-wide meridional dipole mode is present, followed by the mode embedded in uniform basin-wide patterns. The trends of thermocline depth and sea surface height also exhibit the similar patterns. It is found that two distinct mechanisms are account for the observed patters in the Indian Ocean. Firstly, Local wind forcing is responsible for the meridional dipole patterns. In the off-equatorial southern Indian Ocean region, wind stress and Ekman pumping velocity trends favor downwelling (upwelling), resulting in thermocline depth deepening (shallowing) during 1958-1975 and 1976-1987, respectively. Secondly, the observed basin-wide warming and cooling trends during 1988-2000 and 2001-2014 are explained by the combined effect of local wind forcing and heat transport from the western Pacific through the Indonesian throughflow.