Long-Term Changes of Surface Total Alkalinity and Its Driving Mechanisms in the North Indian Ocean

This study examines long-term changes in surface total alkalinity (TA) in the North Indian Ocean (NIO) by developing a machine learning-based data product (INCOIS_TA) using ship-based observations collected from different sources during the period 1978–2019 and a reanalysis data product. We identify three sub-regions within the NIO exhibiting significantly increasing TA trends, which are south of 7°N (0.81$\pm$0.38 $\mu$mol ${\text{kg}}^{-1}$ ${\text{yr}}^{-1}$), southeastern coast of the Arabian Sea (AS) (1.16$\pm$0.42 $\mu$mol ${\text{kg}}^{-1}$ ${\text{yr}}^{-1}$), and the southwestern region of the Bay of Bengal (BoB) (0.47$\pm$0.26 $\mu$mol ${\text{kg}}^{-1}$ ${\text{yr}}^{-1}$). The increasing trend in surface salinity primarily drives the rise in TA in these three regions. In other regions, upper ocean warming that increases stratification and weakens vertical mixing results in a reduced vertical supply of nutrients and high sub-surface TA, which may, in turn, lead to a decline in surface TA. However, this expected decrease is found to be insignificant, likely due to substantial atmospheric deposition of nutrients. Analysis of the annual mean excess alkalinity spatial pattern, after normalizing TA, indicates that the AS may be favorable for calcification, while the BoB may not. At interannual time scales, both the climatic modes (El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD)) have significant and equal impact on the surface TA in the BoB, whereas in the AS, the changes in surface TA are dominated only by ENSO. Further, we find that strong El-Niño (La-Niña) and positive (negative) IOD years show a decrease (increase) in surface TA in the NIO.