During the cooler half of the year cold air from the interior of the North American continent frequently penetrates equatorward into Central America. Central American cold surges occur 12-20 times per year and exhibit considerable intraseasonal and interannual variability. Nearly 40% of cold season precipitation occurs in conjunction with 20-40% of all cold surges across eastern Mexico. Cold surge events can be associated with significant precipitation over Mexico and Central America where the cold northerly flow is forced to ascend poleward-facing mountain barriers. Especially heavy precipitation can arise when an extended cloud plume from the tropical eastern Pacific and associated disturbance embedded in the subtropical jetstream crosses Mexico and interacts with a remnant low-level baroclinic zone left behind in the wake of a previous cold surge.
A 21-season (1977-98) climatology of cold surges across Mexico and Central America, stratified by wet and dry events, will be used to help identify possible characteristic precipitation signatures. Data sources include surface land and marine observations, the Comprehensive Oceanic and Atmospheric Data Set (COADS), daily rainfall observations, and outgoing longwave radiation (OLR) measurements. The National Centers for Environmental Prediction (NCEP) reanalyses are used to identify, compare and contrast synoptic signatures associated with wet and dry cold surges events.
Initial investigation of the synoptic setting for surge events producing precipitation shows a tropical connection via a low-level inverted trough between the surge high and the subtropical high. An upper-level trough over Mexico with approaching jet streaks in the subtropical jetstream provided the support for enhanced rising motion in the area of significant precipitation. In contrast, the synoptic setting for dry surge events lacked the low-level support of an inverted trough, instead the subtropical high remained in place over the Caribbean and failed to retreat into the Atlantic. In the dry events the upper-level trough was located farther west over the eastern Pacific, and in some cases was nonexistent with a zonal jetstream across Mexico. The role of the subtropical jetstream during wet and dry cold surge events for a more comprehensive number of events will be presented.