Variability of the Shallow Overturning Circulation in the Indian Ocean

Variability of the Indian Ocean (IO) shallow overturning circulation during 1958-2017 was investigated, which consisted of a cross-equator cell (CEC) and a southern subtropical cell (SSTC). Structures of these shallow cells were examined through the velocity sections and the transport steamfunction patterns based on the newly released Ocean Reanalysis System 5 data set, and the results showed that the returning flows, at the subsurface, of the shallow cells mainly presented at the western boundary while absented in the ocean interior. Strengths of the IO shallow cells were then determined according to the annual-mean meridional streamfunction profiles. Both the CEC and the SSTC exhibited significant variability on interannual to decadal timescales during 1958-2017, and this variability mainly resulted from changes in both meridional Ekman transports and meridional geostrophic flows in the upper layer, and each component could dominate the shallow cells' variations in certain years. Contributions of the geostrophic currents to the shallow cells were likely opposite to that of the Ekman transport in the South Indian Ocean during years when the latter reached the extremes. The CEC and the SSTC were negatively correlated during the research period. The CEC exhibited a decadal weakening during 2000-2015, contributing to the decadal increasing of the IO heat content, while the SSTC only experienced very little strengthening. Here this paper presents a comprehensively study of the interannual to decadal variability of the IO shallow cells and the corresponding reasons and influences and tried to link these variations with other variations in the IO. Plain Language Summary Studying ocean circulation is of great importance in understanding the changes and processes of the ocean and atmosphere. Meridional flows play an important role in balancing the heat in the ocean, and thus, their changes have great influences on climate change. The shallow overturning circulation is the wind-driven circulation in the upper few hundred meters along the north-south direction. This work examined the structure and time series of the Indian Ocean shallow overturning circulation and the results shown obvious variability on different timescales. This paper further explored the reasons for this variability and its influences on ocean heat redistributions. Our findings promote the understanding of the oceanic circulation and its role in the sea-atmosphere system and climate changes.