The Railroad Switch Effect of Seasonally Reversing Currents on the Bay of Bengal High‐Salinity Core

The Southwest Monsoon Current (SMC) flows eastward from the Arabian Sea into the Bay of Bengal (BoB) during summer, advecting a core of high‐salinity water. This high‐salinity core has been linked with Arabian Sea High‐Salinity Water that is presumed to enter the BoB directly from the Arabian Sea via the SMC. Here we show that the high‐salinity core originates primarily from the western equatorial Indian Ocean, reaching the BoB via the Somali Current, the Equatorial Undercurrent, and the SMC. Years with anomalously saline high‐salinity cores are linked with the East Africa Coastal Current and the Somali Current winter convergence and an anomalously strong Equatorial Undercurrent. Seasonal reversals that occur at the Somali Current and SMC junctions act as railroad switches diverting water masses to different basins in the northern Indian Ocean. Interannual fluctuations of the Equatorial Undercurrent are linked to wind stress and El Niño. Plain Language Summary The northern Indian Ocean experiences a seasonal reversal of currents due to monsoon winds. During the summer, the monsoon current transports high‐salinity water from the Arabian Sea into the Bay of Bengal. This supply of salty water is believed to originate from the eastern Arabian Sea. Here we find that the intrusion of high‐salinity water originates instead from the western Arabian Sea. The origins of the high‐salinity water are traced to the western equatorial Indian Ocean via a seasonal equatorial undercurrent. In the western equatorial Indian Ocean there is a seasonal convergence of currents that is crucial to the supply of this salty water. Variability in the equatorial undercurrent is linked to wind fields and the El Niño–Southern Oscillation. As a result, these findings shed new insight into which large‐scale patterns influence the subsurface salinity in the Bay of Bengal that then modulate the variability of sea surface temperature and the strength of air‐sea coupling in this region. Better representation of interaction between patterns of climate variability and the currents of the equatorial Indian Ocean could improve the representation of the Bay of Bengal in climate models and thus the representation of monsoon processes, including rainfall. Key Points The high‐salinity core observed in the Bay of Bengal during the southwest monsoon originates from the western equatorial Indian Ocean The Somali Current, Equatorial Undercurrent, and Monsoon Current are key to supply and variability of th