We have generated a high resolution coral Δ14C record spanning the last 50 years to document the seasonal and interannual redistribution of surface waters in the western tropical Pacific. Prebomb (1947--1956) Δ14C values average -63? and have a total range of 30?. Values begin to increase in 1957, reaching a maximum of 137? in mid-1983. Large interannual variability of up to 80? closely follows the El Ni¿o-Southern Oscillation (ENSO). During each ENSO warm phase, Δ14C values begin to increase, reflecting the reduction of low-14C water upwelling in the east and the invasion of subtropical water into the western equatorial tropical Pacific. Maximum Δ14C values are in phase or lag the corresponding sea surface temperature maxima in the eastern tropical Pacific, whereas the rapid return to more negative Δ14C is in phase with eastern Pacific ENSO indices. The highest-amplitude excursions occur during the 1965/1966 and 1972/1973 events, when the 14C contrast is highest between the eastern Pacific and subtropics. The 1982/1983 El Ni¿o, although a larger ENSO event, has a lower Δ14C amplitude, reflecting the penetration of bomb radiocarbon into the equatorial undercurrent and the reduced contrast in Δ14C between thermocline and subtropical surface waters at that time. This coral record demonstrates the potential for using similar radiocarbon time series for documenting variability in Pacific shallow circulation over interannual and decadal timescales. ¿ 1998 American Geophysical Union