A Climatology of Extreme South American Andean Cold Surges

Cold surges represent one of several phenomena by which midlatitude features can modulate the atmosphere, both dynamically and thermodynamically, deep into the tropics. This study involves the construction of a climatology of the strongest South American cold surges that follow along the Andes Mountains to quantify the extent to which these surges modulate the atmosphere from the midlatitudes to the tropics. Cold surges occurring during June-September (austral winter) from 1980 to 2017 are considered. In this study, cold-surge events are identified using standardized anomalies of 925-hPa meridional wind and 925-hPa temperature. As compared with previous cold-surge investigations, the use of standardized anomalies better enables spatial variation in cold-surge intensity and impacts to be quantified. A strong cold surge is defined as one in which the 925-hPa temperature is at least 3 standardized anomalies below 0 and the 925-hPa meridional wind is at least 3 standardized anomalies above 0 on the meso-alpha scale or larger. Using these criteria, 67 events are identified. The composite cold surge is characterized by highly anomalous cold, southerly flow that originates in northern Argentina and progresses northward, significantly modulating lower-tropospheric kinematic and thermodynamic fields across the entire Amazon basin over a period of 2 to as many as 8 days.