Satellite observations of the wind jets off the Pacific coast of Central America. Part I: Case studies and statistical characteristics

Measurements of near-surface winds by the NASA scatterometer (NSCAT) from October 1996 through June 1997 are analyzed to investigate the three major wind jets along the Pacific coast of Central America that blow over the Gulfs of Tehuantepec, Papagayo, and Panama. Each jet is easily identifiable as locally intense offshore winds in the lee of low-elevation gaps through the Sierra Madre mountain range. The jets have relatively narrow cross-stream width but often extend several hundred kilometers or more into the Pacific. The Tehuantepec and Papagayo jets sometimes merge with the northeast trade winds of the Pacific. The Tehuantepec jet was highly energetic with characteristic timescales of about 2 days. Events were triggered by high pressures associated with cold surges into the Gulf of Mexico that originated over the Great Plains of North America. The Papagayo and Panama jets were much more persistent than the Tehuantepec jets. The winds at both of these lower-latitude locations exhibited a strong seasonal variation with almost exclusively offshore flow from late November 1996 through late May 1997 and periods of onshore flow in October and November during the late stages of the 1996 Central American monsoon season. Superimposed on this low-frequency seasonal variation were events with characteristic timescales of a few days. Based on NSCAT data, the spatial and temporal evolution of major wind events is described in detail for three representative case studies. In December 1996, the jets developed sequentially from north to south, consistent with the notion that wind events in the two lower-latitude jets are associated with cold-air outbreaks that trigger the Tehuantepec jet a day or so earlier. In November 1996 and March 1997, the Papagayo and Panama jets were strongly influenced by tropical phenomena that had little apparent association with the Tehuantepec jet. These latter two case studies, together with the distinction between the statistical characteristics of the three jets, suggest that the Papagayo and Panama jets are predominantly controlled by a mechanism that is very different from the across-gap pressure gradients associated with high pressure systems of midlatitude origin that control the Tehuantepec jet.