A Transbasin Mode of Interannual Variability of the Central American Gap Winds: Seasonality and Large-Scale Forcing

A transbasin mode (TBM) is identified as the leading mode of interannual surface wind variability over the Intra-Americas Seas across Central America based on empirical orthogonal function analysis. The TBM is associated with variability in Central American gap winds, most closely with the Papagayo jet but with considerable signals over the Gulfs of Tehuantepec and Panama. Although El Nino-Southern Oscillation (ENSO) is the main large-scale forcing, the TBM features a distinct seasonality due to sea level pressure (SLP) adjustments across the Pacific and Atlantic. During July-September, ENSO causes meridional SLP gradient anomalies across Central America, intensifying anomalous geostrophic winds funneling through Papagayo to form the TBM. During wintertime, ENSO peaks but imparts little anomalous SLP gradient across Central America with a weak projection on the TBM because of the competing effects of the Pacific-North American teleconnection and tropospheric Kelvin waves. Besides ENSO, tropical Atlantic sea surface temperature anomalies make a weak contribution to the TBM in boreal summer by strengthening the cross-basin gradient. ENSO and the Atlantic forcing constitute a cross-basin seesaw pattern in SLP, manifested as an anomalous Walker circulation across the tropical Americas. The TBM appears to be part of the low-level branch of the anomalous Walker circulation, which modulates Central American wind jets by orographic effect. This study highlights the seasonality of gap wind variability, and calls for further research into its influence on regional climate.