New perspective on the overturning dynamics in the Indian Ocean

The Indian Ocean Meridional Overturning Circulation (IMOC) is well known for its remarkable seasonal variation, which was attributed to Ekman flow plus its barotropic compensation (Lee and Marotzke, 1998). However, by tracking the isopycnal displacement, I defined a  sloshing MOC streamfunction, which was found highly resembling the Eulerian MOC streamfunction (see the attached figure). It was thus concluded that the IMOC is predominantly a sloshing mode, associated with the isopycnal displacement. Recognizing that these isopycnal signals were dominated by the first-baroclinic long Rossby waves, I found the IMOC strength was determined by the zonally-integrated Ekman pumping anomaly. As a result, the deep inflow into the Indian Ocean also had seasonal variation that could be attributed to this sloshing mode of overturning circulation. This could be partly verified by the cross-basin transect survey across 32^oS that were fully occupied three times in history. The diffusivity dichotomy problem can be also explained by this new perspective. The importance of the Indian Ocean overturning in the global conveyor-belt was therefore challenged. This result has been published in Han (2021, JPO).