Meeting Documents

Investigating the Change and Variability in the Indian Ocean Meridional Overturning Circulation

Abstract

The Indian Ocean Meridional Overturning Circulation (IMOC) is a key component of the global thermohaline circulation, regulating heat transport, ocean–atmosphere interactions, and long-term climate variability. Compared to other basins, however, its dynamics are less well understood, especially across seasonal to multi-decadal timescales. This study examines the structure, variability, and long-term changes of the Shallow Meridional Overturning Circulation (SMOC) and the Deep Meridional Overturning Circulation (DMOC), using multiple reanalysis products (ECCOv4r4, ORAS4, ORAS5, SODA3.15.2, CGLORS, GLORYS2v4) and CMIP6/SMILE simulations. Results reveal distinct seasonal patterns, with monsoon-driven reversals in the Subtropical Cell (STC) and Cross-Equatorial Cell (CEC). Both cells also show strong interannual to decadal variability linked to zonal wind stress. Model comparisons demonstrate high agreement (≥80%) in capturing these seasonal features. The STC, located between 5°S–20°S, remains anticlockwise year-round, confined to the upper 400 m, and upwells to the Seychelles Dome thermocline ridge. The CEC, connecting both hemispheres, upwells to ~10°N off Somalia, reverses from December–March, and is anticlockwise during the rest of the year. A clockwise subtropical gyre at ~20°S and a northward-flowing equatorial roll between 5°S–5°N are also observed. For the DMOC, reanalysis products consistently show two overturning cells: an anticlockwise cell south of 20°S and a clockwise cell centered on ~10°S during boreal winter, both reversing in boreal summer with the monsoon. CMIP6 historical simulations agree with these patterns, reproducing SMOC and DMOC structures with ≥80% multi-model mean agreement. A comparison between 1980–1989 and 2005–2014 highlights notable decadal variability. By advancing understanding of IMOC variability and long-term change, this study provides new insights into Indian Ocean warming, sea-level rise, and regional ecosystem impacts.

Plain-language Summary: The Indian Ocean plays a major role in moving heat, salt, and water between hemispheres through the Indian Ocean Meridional Overturning Circulation (IMOC). This circulation influences monsoons, sea surface temperatures, and long-term climate patterns, but is less understood than in other oceans because it is strongly shaped by seasonal monsoons. Using multiple ocean reanalysis datasets and climate model simulations, we examined variability in both the shallow (SMOC) and deep (DMOC) overturning circulation. Results show clear seasonal cycles: the subtropical cell (STC) remains active year-round, while the cross-equatorial cell (CEC) reverses direction with the monsoon. At deeper levels, two large cells also switch direction between winter and summer. These patterns appear consistently across datasets, with over 80% agreement. A comparison of the 1980s and 2000s reveals clear decadal changes, highlighting the IMOC’s role in heat storage, warming, sea-level rise, and ecosystem impacts.