Structure and Seasonal Variation of the Indian Ocean Tropical Gyre Based on Surface Drifters

Using the Gauss-Markov decomposition method, this study investigates the mean structure and seasonal variation of the tropical gyre in the Indian Ocean based on the observations of surface drifters. In the climatological mean, the clockwise tropical gyre consists of the equatorial Wyrtki Jets (WJs), the South Equatorial Current (SEC), and the eastern and western boundary currents. This gyre system redistributes the water mass over the entire tropical Indian Ocean basin. Its variations are associated with the monsoon transitions, featuring a typical clockwise pattern in the boreal spring and fall seasons. The relative importance of the geostrophic and Ekman components of the surface currents as well as the role of eddy activity were further examined. It was found that the geostrophic component dominates the overall features of the tropical gyre, including the SEC meandering, the broad eastern boundary current, and the axes of the WJs in boreal spring and fall, whereas the Ekman component strengthens the intensity of the WJs and SEC. Eddies are active over the southeastern tropical Indian Ocean and transport a warm and fresh water mass westward, with direct impact on the southern branch of the tropical gyre. In particular, the trajectories of drifters reveal that during strong Indian Ocean Dipole or El Nino-Southern Oscillation events, long-lived eddies were able to reach the southwestern Indian Ocean with a moving speed close to that of the first baroclinic Rossby waves. Plain Language Summary Similar to its peers in the tropical Pacific and Atlantic, the tropical gyre in the Indian Ocean occupies the equatorial and southern tropical ocean, featuring a clockwise circulation pattern in climatology. This basin-wide gyre consists of the Wyrtki Jets, the South Equatorial Current (SEC), and the eastern and western boundary currents. Equatorial westerly and trade winds are major factors forcing the gyre, whereas the monsoon dominates its seasonal variations. The dynamic features of the tropical gyre are mainly determined by the geostrophic component, whereas the Ekman component reinforces the strength of the Wyrtki Jets and SEC. In the southeastern Indian Ocean, eddy activity has a direct impact on the SEC, influencing the transport of warm and fresh water.