PLANETARY-SCALE AND SYNOPTIC-SCALE SIGNATURES ASSOCIATED WITH CENTRAL-AMERICAN COLD SURGES

Motivated by outstanding issues from a previous case study of a midlat itude cold surge that affected Mexico and Central America, the climato logy of Central American cold surges is examined in this paper. An ind ependently derived listing of 177 cold-surge events is employed for wh ich the following properties are tabulated: onset date, duration, time between cold-surge events, latitude of maximum equatorward penetratio n (phi(min)), and 48-h maximum surface temperature change at Merida, M exico (Delta T). These data show that 75% of the cold surges have dura tions of 2-6 days, the same timescale as mobile disturbances in the we sterlies. Also, there does not appear to be any relationship between D elta T and the duration of the event, although cold surges that penetr ate to low latitudes (phi(min) = 7 degrees-10 degrees N) have a weak t endency to persist longer than those that do not penetrate to low lati tudes (phi(min) = 15 degrees-20 degrees N). In addition, the Reding da ta indicate that the cold surges tend to reach their most equatorward extent where topographic features impede the progress of equatorward-m oving cold air; the temperature decrease in the postsurge air (as meas ured by Delta T) does not appear to be related to the most equatorward extent. To examine the planetary-and synoptic-scale patterns associat ed with different categories of cold surges, events with similar chara cteristics from this database were composited: COLD (phi(min) less tha n or equal to 10 degrees N and Delta T greater than or equal to 9 degr ees C), COOL (phi(min) less than or equal to 10 degrees N and Delta T = 4 degrees-5 degrees C), and LONG (events lasting at least 8 days). C OLD surges are characterized by a persistent upper-level ridge over th e western United States, 200-hPa confluence over the Gulf of Mexico, a nd the migration of a Canadian lower-tropospheric anticyclone equatorw ard along the Rocky Mountains and the Sierra Madre. In contrast, COOL surges are associated with a progressive, upper-level ridge over the w estern United States, weak 200-hPa confluence over the Gulf of Mexico, and the migration of a North Pacific anticyclone over the intermounta in west and into the southeast United States. LONG surges are associat ed with a slower-moving planetary-scale pattern; 200-hPa confluence ov er the Gulf of Mexico; the occurrence of multiple cold surges, which r einforce the anticyclone over Mexico; and the absence of low-latitude, upper-tropospheric, mobile short-wave troughs to prematurely weaken t he anticyclone. Cold surges (especially COLD) can be associated with a n acceleration of the trade winds over the eastern North Pacific Ocean and play a role in El Nino-Southern Oscillation. The results in this paper are compared to the results of previous studies of North America n, Central American, and east Asian cold surges.