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

Abstract

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 Niño–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.

Interannual Variability of Tropical Atlantic-to-Pacific Moisture Transport Linked to ENSO, Atlantic Niño, and Freshwater Budget in the Northwestern Tropical Atlantic

“…8a) are highly correlated with the TTI in August (r 5 0.93). The pressure gradients can also modulate the Caribbean low-level jet in the Atlantic sector (Wang 2007;Muñoz et al 2008) and the Papagayo gap winds in the Pacific sector (Yang et al 2017). Indeed, we find high and significant correlation between the TTI interannual variability and the Caribbean low-level jet or the Papagayo winds (Fig.…”

“…2d). The fact that the flow is stronger over the Papagayo than the Tehuantepec Isthmus also implies the topographic effect: the east-west-oriented Papagayo Isthmus facilitates the variability of low-level easterly trade winds but north-south-oriented Tehuantepec does not (Yang et al 2017). However, the two cores both reach their maxima at about 850 hPa (Fig.…”

“…This implies anomalous ascending motion over the tropical Pacific forced by the positive phase of ENSO. Giannini et al (2000) and Yang et al (2017) found that in late summer, the ENSO-forced anomalous ascending motions are accompanied by anomalous descending motions over Central America. Our result is consistent with theirs as over Central America the troposphere exhibits baroclinic structure with significant positive geopotential height anomalies in the lower troposphere.…”

“…The anomalous Central American high and the anomalous tropical Pacific low in the lower troposphere constitute strong anomalous northward pressure gradients. Due to the geostrophic relation, the anomalous pressure gradients induce wind anomalies blowing from the Caribbean Sea into the northeastern tropical Pacific (Yang et al 2017). As wind anomalies dominate the TTI interannual variability, this strong pressure gradients contribute to the transbasin moisture transport.…”

The Role of Atmospheric Transport for El Niño‐Southern Oscillation Teleconnections

“…We found a very strong positive anomaly in high-level cloudiness (Figure S3 in Supporting Information S1), consistent with the high-level advection of moisture but suppression of convection, due to subsiding tagged air (not shown here). However, Bravo-Cabrera et al (2017) found that precipitation might be favored during EN in southern Mexico, and Yang et al (2017) mentioned an increase in convection over Southern Central America. Ropelewski and Halpert (1987) found dry conditions from July to October, but no clear signal for DJF.…”