Observed upper ocean currents south of Sri Lanka exhibit large, irregular fluctuations with periods of days to weeks. An ocean model driven by daily surface winds is able to reproduce the observed fluctuations. We find from model experiments that low frequency (30--50 day) intraseasonal variability (ISV) arises when Rossby waves radiated from the eastern boundary are amplified by hydrodynamic instability in the eastern and central Indian Ocean. High frequency (10--15 day) ISV is forced directly by ISV of the wind field in the eastern Indian Ocean. In spite of the contribution from instability, the ocean circulation south of Sri Lanka is a deterministic response to wind forcing. ¿ 2001 American Geophysical Union