THE 1993 SUPERSTORM COLD SURGE - FRONTAL STRUCTURE, GAP FLOW, AND TROPICAL IMPACT

In the wake of the eastern United States cyclone of 12-14 March 1993, a cold surge, originating over Alaska and western Canada, brought nort herlies exceeding 20 m s(-1) and temperature decreases up to 15 degree s C over 24 h into Mexico and Central America. This paper addresses th e multiscale aspects of the surge from the planetary scale to the meso scale, focusing on 1) the structure and evolution of the leading edge of the cold surge, 2) the reasons for its extraordinary intensity and equatorward extent, and 3) the impact of the surge on the Tropics, spe cifically, on the strength of the trade winds and on the sea surface t emperature in the eastern Pacific. The cold surge was initiated as a d eveloping cyclone over the Gulf of Mexico, and an upstream anticyclone east of the Rockies caused an along-barrier pressure gradient to form , forcing topographically channeled northerlies along the Rocky and Si erra Madre Mountains to transport cold air equatorward. On the mesosca le. the leading edge of the cold surge possessed nonclassical frontal structure. For example, as the cold surge entered Mexico, the coldest air and the strongest wind arrived at about 900 hPa before affecting t he surface, suggestive of a tipped-forward leading edge to the surge. Also, satellite imagery and surface observations indicate that the lea ding edge appeared to be successively regenerated in the warm presurge air. The cold surge had characteristics reminiscent of a Kelvin wave, a tipped-forward cold front, a pressure-jump line, a bore, and a grav ity current, but none of these conceptual/dynamical models was fully a pplicable. Associated with the cold surge, gap winds up to 25 m s(-1) were observed in the Gulfs of Tehuantepec (a tehuantepecer), Fonseca, Papagayo, and Panama, owing to the strong cross-mountain pressure grad ient. In the case of the tehuanrepecer, a rope cloud emanated from the Isthmus of Tehuantepec and turned anticyclonically, consistent with a n inertial oscillation. On the synoptic and planetary scales, the extr aordinary equatorward extent of the cold surge was aided by topographi c channeling similar to cold-air damming, by a low-latitude upper-trop ospheric trough, and by the lower branch of the secondary circulation associated with a confluent jet-entrance region aloft. The cold surge also impacted the tropical atmosphere and ocean, by contributing to th e strengthening of the northeast trade winds over the eastern Pacific Ocean an by inducing local cooling of the sea surface temperature in t he Gulfs of Tehuantepec and Papagayo by about 4 degrees-8 degrees C.