South american cold surges: Impact on regional climate

Episodic incursions of cold, mid-latitude air into subtropical and tropical latitudes are frequently observed along the east side of the Andes cordillera. They are best defined and intense from fall to spring, but also observed during the warm season. Extreme wintertime episodes produce freezing conditions from central Argentina to southern Brazil, which has motivated a number case studies that focus in the synoptic aspects of the so-called South American cold surges.

These transient incursions inject cold and dry air into the lower tropical troposphere, and therefore they represent an energy sink for the tropics. Because of the recurrence of cold surges over South America, we speculate that they may have a sizable effect upon the regional climate. To test this hypothesis the poleward transport of heat and moisture by transient eddies was calculated using data from NCEP/NCAR reanalysis. In the lower troposphere, both fields exhibit an elongated subtropical maximum to the east of the Andes, which is largely produced by cold surges. At the equatorward flank of these maxima there is significant cooling/drying associated with the meridional divergence of the heat/moisture flux. We found these values comparable with the rate of warming/moistening produced by the mean meridional circulation. Furthermore, the divergence of moist energy flux evaluated in a box with vertical walls at 20°S and 5°N, is about a 15% of the surface energy input at the height of the austral winter.

Thus, South American cold surges not only have a transient impact, but also play a significant role in the maintenance of the heat balance over the central part of the continent. It is concluded that a proper representation of their aggregated effect is necessary for a realistic modeling of the regional climate.