The varied monthly precipitation data of stations across Costa Rica are assigned to one of five regions, each of which displays a distinct precipitation regime. Peaks and troughs in each regional regime reflect the relative contribution and timing of the major precipitation-bearing winds flowing from the Pacific, the Caribbean, and continental North America. Lag cross-correlations with a Southern Oscillation index reveal different regional associations, and at differing times, with fluctuating atmosphere-ocean conditions in the tropical Pacific. Patterns of correlations are coherent within each region, but display interregional differences that are more marked than previously suggested. The Pacific coast experiences drought shortly before and during the onset of an ENSO event. The Caribbean coast receives increased summer precipitation but decreased winter rains during these episodes. The interior regions of the country display a mixed response to these events. In the years following a warm phase ENSO the signs change abruptly as the cold ENSO phase sets in, indicating a more complex sequence of environmental changes than a simple, single drought. This is most pronounced along the Caribbean coast, where the summer and winter seasonal rainfalls exhibit opposing associations to the same atmosphere-ocean conditions.