Equatorial Circulation in the Western Indian Ocean During Onset of the 2018 Summer Monsoon and Links to the Bay of Bengal

Abstract

Cross-equator transects occupied by an underwater glider and a research vessel in the western Indian Ocean captured the evolution of equatorial circulation during onset of the boreal summer monsoon in 2018. At the end of the winter monsoon in March, surface currents were westward, while the equatorial undercurrent carried salty Arabian Sea High-Salinity Water eastward. As winds transitioned from westward to eastward during April, an eastward near-surface Wyrtki Jet developed, while the equatorial undercurrent weakened, vanishing by May. A first-mode baroclinic Kelvin wave propagated through the survey region after westward winds relaxed. However, the vertical structure of the evolving circulation was inconsistent with the first baroclinic mode, suggesting the influence of higher modes in setting observed vertical structure. The strong equatorial undercurrent at the end of the winter monsoon allowed high-salinity waters from the western equatorial Indian Ocean to reach the southern Bay of Bengal in summer 2018.Plain Language Summary Circulation throughout the Indian Ocean is strongly influenced by the twice-per-year reversal of the monsoon winds over the basin. New observations collected using both an autonomous underwater glider and a research vessel in 2018 captured the reversal of layered currents in the upper ocean as the northeast (winter) monsoon gave way to the southwest (summer) monsoon. From March to May, near-surface currents switched form westward to eastward, while the eastward equatorial undercurrent weakened and vanished. Equatorial planetary waves play a key role in propagating signals, such as those associated with changing monsoon winds, through the equatorial oceans. An eastward propagating Kelvin wave moving at a speed consistent with the first baroclinic mode was detected by satellite, but the observed vertical structure of the changing currents during the monsoon transition was inconsistent with the first baroclinic mode. Higher-mode waves are suspected to play a role in the evolution of equatorial currents during the monsoon transition. Relatively strong subsurface flow at the end of the winter monsoon carried salty waters originating in the Arabian Sea eastward, leading to a salty subsurface feature in the Bay of Bengal in summer 2018.