The interannual variability of shallow meridional overturning circulation and its association with the south-west Indian Ocean heat content variability

Abstract

The present study examines the interannual variability of the Indian Ocean Shallow Meridional Overturning Circulation (SMOC) using century-long reanalysis data. The strength of the transport associated with SMOC is calculated using meridional overturning stream function. The highest SMOC variability is found between the latitudes 5° S and 15° S, which displayed enhancement in amplitude after 1940s. Strong and weak SMOC years are identified using standard deviation threshold. Strong SMOC years composite displayed excess southward transport (~ 2.5 Sv) and subduction over the South Indian Ocean. The associated southward heat transport (~ 0.25 PW) reduced the sea level and upper 200 m ocean heat content (OHC) over the Southwest Indian Ocean (SWIO). On the other hand, weaker SMOC years composite displayed weak heat transport, increased sea level and OHC over the SWIO. Tide gauge and satellite observations also displayed similar variation in sea level for respective phases of SMOC. Further analysis reveals that the SMOC variability is primarily driven by changes in the zonal wind stress south of the equator and displays close association with the Southern Oscillation Index. The Ocean model-based sensitivity experiments confirm that the OHC variability over SWIO is closely associated with the SMOC variability and is primarily driven by local wind forcing as a response to El Niño Southern Oscillation. However, the role of remote forcing from Pacific through oceanic pathway over SWIO is not evident. The present study provides a comprehensive understanding of the interannual variability of SMOC and its linkage to sea level and OHC variability over SWIO during the last century.