Long-term monitoring is an essential component of primate conservation, and much of this research is explicitly concerned with how primates respond to and cope with diverse forms of environmental change. Here, I synthesize over four decades of data on environmental change in the Santa Rosa sector the Costa Rica’s Área de Conservación Guanacaste, to stimulate new research on the impacts of environmental change beyond seasonality on Santa Rosa’s primates. Focusing on climate variables and landscape-scale vegetation phenology, I describe and quantify typical seasonal patterns, interannual variability, and long-term trends. Santa Rosa’s highly seasonal rainfall patterns show marked interannual variability that is largely driven by the El Niño Southern Oscillation (ENSO). The wettest and driest periods on record have occurred in association with powerful cold ENSO episodes (La Niña) and warm ENSO episodes (El Niño), respectively. Start dates for the wet season can vary by 40 days, but no long-term linear trend was evident in the wet season start dates or in total annual rainfall. Temperature anomalies in Santa Rosa are also strongly associated with ENSO conditions over a backdrop of long-term warming. The annual cycle of plant phenology is dominated by large-scale leaf shedding during the long dry season. The timing and degree of seasonal phenological peaks show complex relationships with rainfall. Long-term data, in combination with the site’s natural environmental variability, provide uniquely quantitative context for understanding primate adaptations to changing environments – a framework that can be extended to ecological forecasting under future environmental change.