Subseasonal variability of ocean currents in the Bay of Bengal

Upper-ocean currents in the Bay of Bengal (BoB) reverse seasonally in response to wind forcing. Less is known about their behavior on the subseasonal (30–60-day) time scale, including by phase of the leading mode of subseasonal atmospheric variability in the tropics, the Madden–Julian Oscillation (MJO). In this study, anomalies of National Aeronautics and Space Administration (NASA) Ocean Surface Current Analysis Real-time (OSCAR) upper-ocean zonal and meridional current components from 1993 to 2020 were composited by phase of the active MJO to examine how the upper-ocean currents vary subseasonally. These ocean current anomalies were then correlated with anomalies of 10-m zonal and meridional wind from the European Centre for Medium-Range Weather Forecasts (ECMWF) 5th Generation Reanalysis (ERA5) to quantify their relationship to the MJO. The strongest correlations between anomalies of upper-ocean currents and surface winds were found when the active MJO was over Africa (in phase 1) during September and October. In that phase, the BoB south of 10° N sees a reduction in zonal currents by as much as − 0.6 standard anomalies. Other important correlations between anomalies of ocean currents and surface winds were found when the active MJO is over the western Pacific (in phase 6) during September and October, over the Indian Ocean (in phase 2) during November and December, and over the Maritime Continent (in phase 4) during January and February. During the summer monsoon months of May to August, no MJO phases had correlations stronger than + 0.3 between anomalies of upper-ocean currents and surface winds. These results highlight the potential importance of the MJO in subseasonal ocean prediction and motivate future work to better understand the processes by which the MJO modulates the upper ocean.