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Introduction Three low-elevation gaps in the Sierra Madre mountains of Central America create unique meteorological conditions that result in intense wind jets that blow offshore over the eastern tropical Pacific at irregular time intervals. The three wind jets have a profound effect on the upper-ocean temperature and the distributions of nutrients, phytoplankton and zooplankton off the Pacific coast of Central America. Understanding of the evolution and dynamics of these jets is incomplete owing to the sparse distribution of direct observations of oceanic winds in these regions. Near-surface winds were observed over the global ocean with unprecedented spatial and temporal coverage by the NASA scatterometer (NSCAT) during the 9-month period October 1996 through June 1997. This dataset has been analyzed to investigate the statistical characteristics of the three major wind jets and regional relationships between the jets and the wind and pressure fields in the Gulf of Mexico, Caribbean Sea and eastern tropical Pacific. The details are presented in a pair of manuscripts submitted to Monthly Weather Review (Chelton et al., 1998a; 1998b). The results and conclusions are summarized here.

NSCAT observations of surface winds for the 2-day period March 6-7, 1997. (Enlarged image size: 129 K)

The Conventional View Cold air masses originating over the North American Great Plains move south and east across the Gulf of Mexico during the fall, winter and spring. High pressure behind each cold front creates a large sea level pressure difference across the Isthmus of Tehuantepec which generates northerly winds through Chivela Pass and an intense wind jet over the Gulf of Tehuantepec. According to the conventional view (the generality of which is questioned below), the cold front then continues to move southeastward, creating relatively high surface pressure a day or two later in the southwestern Caribbean Sea. The pressure difference between the Caribbean and the Pacific coastline triggers a wind jet that blows across the Nicaraguan lake district, resulting in strong easterly surface winds that can extend far into the eastern tropical Pacific west of the Gulf of Papagayo. High surface pressure over the southwestern Caribbean Sea also generates northerly surface winds across the Isthmus of Panama and over the Gulf of Panama in the eastern tropical Pacific.

Map of the geographical region of interest. (Enlarged image size: 55.1 K)

Animation of NSCAT Winds The evolution of each of the three major wind jets and the relationships between the jets and the surrounding wind field are easily seen from an animation of 2-day composite averages of NSCAT winds on a 1-degree grid at daily intervals from November 11, 1996 through June 29, 1997.

Results and Conclusions from Analyses of NSCAT Data

1. Statistical analyses of the winds in the core of each jet reveal that the winds are highly anisotropic in all three jets. The Tehuantepec jet is much stronger and more variable with time scales of variability that are three times shorter than in the other two jets.

2. Case study examples reveal that the Tehuantepec jet is driven by pressure variations in the Gulf of Mexico, in accord with the conventional view. However, in contradiction to the conventional view, the Papagayo and Panama jets in two of the three case studies were strongly influenced by tropical circulations that had little or no influence on the Tehuantepec jet.

3. The correlation structures of the winds in each of the major jets reveal that variations of the Papagayo and Panama jets are not significantly correlated with the Tehuantepec jet at any time lag. The Papagayo and Panama jets are significantly correlated with each other and coupled to zonally coherent tradewinds extending from the Caribbean Sea to the eastern tropical Pacific.

4. The detailed structures of the wind fields in the jets show that:

• The Tehuantepec and Panama jets turn anticyclonically to the west in a manner that is consistent with jets that are inertially balanced at the coast and become progressively more geostrophically balanced with increasing distance from the coast. There is no evidence of anticyclonic turning of the Papagayo jet.
  
  
• There is a divergent fanning away from the axis in all three jets.
  
  
• Systematic differences between the divergence of surface winds in the NSCAT observations and the ECMWF operational analyses suggest significant errors in the parameterizations of boundary-layer processes in the ECMWF first-guess fields.