Attribution of upper gyre's variability in the western Bay of Bengal boundary through Indian Ocean climate modes

Abstract

The Bay of Bengal (BOB), a marginal sea of the northern Indian Ocean, experiences significant monsoon influences. Satellite observations show considerable inter-annual variability in the seasonal oceanic gyre and the summertime offshore jet extending from the Western Boundary Currents (WBC). While the Indian Ocean Dipole (IOD) mode is well-known, the recently identified Indian Ocean Tripole (IOT) mode has also attracted attention. However, the impact of the IOT mode on BOB circulation remains underexplored. This study examines the effects of the boreal autumn-matured IOD and the summer-matured IOT on BOB circulation using analyses of merged satellite data and outputs from a 1.5-layer reduced-gravity numerical model. Results indicate that both the IOD and IOT modes can influence BOB ocean circulation by generating local wind anomalies over the BOB. Unlike the IOD mode, the IOT mode induce less equatorial wind anomalies that can remotely affect the northern BOB circulation. Further analyses indicate that upper seawater warming during boreal spring leads to land-sea breezes over the western BOB, driving the western boundary offshore jet to higher latitudes. Unexpectedly, under scenario of northward offshore jet shift, the gyre response in the northern BOB and near the western boundary is more strongly attributed to wind anomalies associated with the IOT mode than the IOD mode. This study enhances our understanding of land-sea-atmosphere interactions in the marginal sea of monsoon region.