We investigate the South Indian tropical gyre based on long-term observations.
The tropical gyre and the SCTR shrink seasonally westward by 20° of longitude.
The Indonesian Throughflow Front contributes to the seasonal shrinkage of the gyre.
The Sverdrup circulation accounts for the westward migration of the SD during winter.
The Chagos Dome is a well-defined feature at thermocline depths only during summer.