A review of the Indian Ocean carbon dynamics, acidity, and productivity in a changing environment

The Indian Ocean dynamics is governed by the seasonal reversal of monsoon winds and the associated ocean currents. The relatively deep thermocline along the equator due to a lack of steady easterlies, low-latitude connection to the neighbouring Pacific, and a lack of northward heat export due to the position of the Asian continent are important factors in regulating the ocean state. These features make it a unique ecosystem among the world's tropical oceans and determine key features of potential air -sea interaction at different time scales. The pCO(2) shows a large seasonal variation linked with monsoon circulation. The Indian Ocean's northwestern part acts as an atmospheric CO2 source, whereas the northeastern part acts as a net atmospheric CO2 sink. The region between the latitudes of 15S-50S in the Indian Ocean is a major subduction zone because of positive wind stress curl. The subducted water masses are transported to the northern Indian Ocean by the crossequatorial cell (a shallow meridional overturning circulation). Based on the regional studies carried out on the carbonate system in the Indian Ocean, the area north of 15S is a source of atmospheric CO2, while the area between 15 degrees S and 50 degrees S is a sink. A recent synthesis of models (observational climatology) over different spatial scales provides an estimate of the mean value of CO2 in the north of 37.5S of the Indian Ocean as - 0.19 +/- 0.1 PgC/yr (-0.07 +/- 0.14 PgC/yr) during 1985-2018. The estimated decrease in pH (acidification) using model outputs in the Indian Ocean basin is 0.0675 units during 1961-2010, in which the contribution of dissolved inorganic carbon and surface temperature is 69.3 % and 13.8 %, respectively. The range of the Indian Ocean's annual primary production based on satellite estimates over the last two decades (1998-2018) is 7.72-8.70 Gt C/ yr, whereas the climatological mean is 8.24 +/- 0.30 Gt C/yr. This paper consolidates the current state of understanding of the Indian Ocean carbon fluxes, acidification, and productivity using available field and satellitebased measurements, model simulations, and re-constructed datasets. It provides an overview of the functioning of the Indian Ocean's air -to -sea CO2 exchange and highlights its influence on global climate. Finally, it aims to highlight the grey areas of the Indian Ocean carbon cycle that need to be understood.