Strong subsurface meridional current forced by monsoon intraseasonal oscillation in the southern Bay of Bengal during summer 2020

Abstract

The meridional current in the southern Bay of Bengal (BOB) exhibits prominent intraseasonal variability (ISV), which exerts a critical influence on meridional mass and energy exchange. However, its relationship with the northward propagating monsoon intraseasonal oscillation (MISO), which is the predominant variability in the tropical Indian Ocean during summer, is not well understood. Using a one-year mooring deployed at 5.5 °N, 90 °E, a strong ISV of the meridional current is observed between 150 and 250 m during summer, exhibiting amplitudes exceeding 0.4 ms−1 and periods of 30–70 d. Further analysis shows that the summer ISV is forced by a strong MISO event with the following dynamic processes. The MISO first drives the zonal wind stress at the equator, leading to the equatorial Kelvin wave and the reflected equatorial Rossby wave at 5 °N. Then, the MISO propagates northward and generates local wind stress curl-induced Ekman pumping near the mooring site, leading to the subsequent off-equatorial Rossby wave at 8 °N. As a result, the synergy of the remotely- and locally-forced Rossby wave causes strong subsurface ISV in southern BOB. This study provides a new insight into the dynamic relationship between the MISO and the meridional current in the southern BOB, which has important implications for regional climate research and prediction.