Extreme Mountain Gap Wind Events in the Isthmus of Tehuantepec

The gap flow winds in the Tehuantapec region have been studied because of their impacts in fisheries by decreasing the sea surface temperature and enhancing this activity. Thanks to these winds it has been possible to build wind farms in the region, at the same time they may be a threat to incoming ships in the Gulf of Tehuantepec. Also play a fundamental role in the dispersion and advection of pollutants from the oil refinery in the city of Salina Cruz into the Eastern Pacific and acts as a main driver of convection in the region. Their potential role in tropical cyclogenesis is a matter of current studies.

The climatology of the winds in the Isthmus of Tehuantepec region is fairly understood. These are mainly modulated by mid-latitude troughs during winter and by the extension of the Azores-Bermuda High during summer, in both cases a meridional pressure gradient is observed between the Gulf of Mexico and the Gulf of Tehuantepec, being generally weaker in summer than in winter. The winter period has been studied by several authors contrasting with the number of authors that have examined the summer. In the present work, using the CFS reanalysis data from 1979 to 2016 we propose an index that represents the mountain gap wind variability defined as the standardized anomaly of the mass averaged wind speed in the region multiplied by the sign of the meridional wind, to derive a climatology of the mountain gap wind. It was observed that the winds are more intense during January and possess a secondary maximum in July, coinciding with maximum vertical extension and intensity of the Caribbean Low Level Jet. The wind is mainly northerly during winter and becomes easterly during summer. However, during summer the dispersion variability in the wind direction is larger in summer than in winter.

Furthermore, the index allows us to define extreme events which are considered as the local maxima that are above the monthly 95 percentile. The extreme events follow the same intraseasonal variability as the wind in general, but the maximum in winter is more notorious. Related to the wind direction, with a peak during July in summer the wind is mainly easterly. These extreme events exhibit a well defined cycle with a periodicity of seven.

Composites of the stream function anomaly averaged from 850 to 700 hPa associated with these extreme events show that during winter and spring, cold surges associated with midlatitude troughs originating west of the Rocky Mountains are responsible of these events. For the month of March it is observed that origin of these extreme events corresponds well with the path of Northeasters. In contrast, during the months from June to August, it appears that the main driver of the extreme events is associated with the propagation of subtropical ridges originating in the west flank of the Azores-Bermuda High. In September, easterly waves may play a significant role in enhancing the mountain gap winds, during these events a midlatitude trough interacts with an Easterly wave and the latter moves northward. This northward shift of the Easterly Wave enhances a northerly flow into the mountain gap.