Radiocarbon (14C) in the skeletal aragonite of annually banded corals track radiocarbon concentrations in dissolved inorganic carbon (DIC) in surface seawater. As a result of nuclear weapons testing in the 1950s, oceanic uptake of excess 14C in the atmosphere has increased the contrast between surface and deep ocean 14C concentrations. We present accelerator mass spectrometric (AMS) measurements of 14C/12C ratios (Δ14C) in Porites corals from the Mentawai Islands, Sumatra (0¿S, 98¿E) and Watamu, Kenya (3¿S, 39¿E) to document the temporal and spatial evolution of the 14C zonal gradient in the tropical Indian Ocean. The rise in Δ14C in the Sumatra coral, in response to the maximum in nuclear weapons testing, is delayed by 2--3 years relative to the rise in coral Δ14C from the coast of Kenya. Kenya coral Δ14C values rise quickly because surface waters are in prolonged contact with the atmosphere. In contrast, wind-induced upwelling and rapid mixing along the coast of Sumatra entrains 14C-depleted water from the subsurface, which dilutes the effect of the uptake of bomb-produced 14C by the surface ocean. Bimonthly AMS Δ14C measurements on the Mentawai coral reveal mainly interannual variability with minor seasonal variability. Singular spectrum analysis of the Sumatra coral Δ14C record reveals a significant 3-year periodicity. These results lend support to the concept that interannual variability in Indian Ocean upwelling and sea surface temperatures is related to ENSO-like teleconnections over the Indo-Pacific basin.