Are interdecadal sea level changes along the Indian coast influenced by variability of monsoon rainfall?

Abstract

The Mumbai (Bombay) tide gauge data, the only century-long record in the Indian Ocean, show that interdecadal changes in sea level mimic those in rainfall over the Indian subcontinent. We propose that the link between rainfall and sea level arises from changes in salinity in coastal waters. Rivers fed by southwest monsoon (June-September) rainfall bring a large fraction of the runoff to the Bay of Bengal, from where it is transported to the west coast of India by an equatorward East India Coastal Current, which is triggered partly by the withdrawal of the southwest monsoon. The West India Coastal Current carries the low-salinity water from the bay as well as the runoff from local rivers northward. The advection of the riverine inflow to Mumbai occurs within a season, but the slow mixing in the ocean forces changes in the cross-shore density gradient on longer timescales. This density gradient forces a two-layer geostrophic circulation, with a surface current, which flows with the lighter water on its right, and an undercurrent. Lower (higher) salinity at the coast implies higher (lower) coastal sea level and a rise (fall) of the pycnocline at the coast. Thus the interdecadal variability of sea level along the Indian coast can be linked directly to the variability of the monsoon, the major aspect of the climate of the region, but by a mechanism that is different from those generally proposed to link sea level to climate change; these hypotheses usually invoke a change in volume because of global warming.