Isthmus of Tehuantepec Wind Climatology and ENSO Signal.

The statistical characteristics of the winds at the Isthmus of Tehuantepec and their seasonal and interannual variability are studied through the analysis of several datasets and a reconstruction of the winds for a 31-yr period. Observations show that the long-term monthly mean wind speeds and frequency of occurrence of northerly winds have a strong seasonal signal, with maximum values during December-January, minimum during May-June, and a relative maximum in July. The frequency distribution of wind speed is bimodal, a feature that is closely related to the wind direction, with northerly winds being stronger. Based on these results and the close relationship between the across-Isthmus pressure differences and the local winds, a statistical model is developed to get a reconstruction of 12-hourly winds through the Isthmus of Tehuantepec for 1964-95. The model reproduces fairly well the main characteristics of the observed winds: the bimodal distribution of the wind speed and the seasonal signal in the wind speed and frequency of occurrence of northerly winds. Reconstructed winds show that the high frequency of northerly winds in July is associated with weaker winds than those observed in winter. The summer maximum seems to be related with the westward displacement and strengthening of the Bermuda high during this time of the year. Based on the model results, the long-term monthly mean wind speeds show larger values during El Niño years compared with La Niña years. During La Niña years winds are significantly weaker than in neutral years for February-March, June-September, and November, and the percentage of occurrence of northerly winds is significantly lower than in neutral years from June to November. The larger occurrence of northerly winds during El Niño years compared with neutral years is statistically significant only for May and September.