Rainy Season Onset in Central America and the Influence of the ENSO Decaying Phase

The description and understanding of physical mechanisms related to delayed rainy season onset in Central America can provide a framework for early warning systems based on a combination of sub-seasonal and seasonal forecasts. This is especially important in regions affected by recurrent rainfall deficits, such as the Central American Dry Corridor. Here, a generalized approach is presented to compute the onset of the rainy season and to analyze physical drivers and their intraseasonal evolution. The independent and joint influences of both the El Niño-Southern Oscillation (ENSO) and the sea-surface temperature of the Tropical North Atlantic (TNA) are discussed by analyzing high-resolution daily rainfall data, moist static stability, vertically-integrated moisture convergence, low-level winds and sea level pressure anomalies for the period 1981–2022. Strongly delayed onsets (more than 7 days) occur over northern Central America in the boreal spring following the peak of El Niño events driven by tropospheric warming due to El Niño, whereas La Niña events tend to modestly hasten the onset due to tropospheric cooling. However, certain regions in northern Central America still experience delays during La Niña. In addition, La Niña events with a warm TNA show significantly delayed onset over the Caribbean side of northern Central America. In this case, onset delay is related to a reduction in surface moisture due to an anomalous low-level circulation likely induced by an extratropical wave-train but not by TNA warming. The results suggest that it is key to consider Pacific-Atlantic basin interactions and their impact on the mid-troposphere when diagnosing rainfall characteristics and rainy season onset in Central America.