Dynamics of Wave Propagation Leading to Frost Over South America: Extratropical Latitude versus Tropical Latitude

MÜLLER GABRIELA V; AMBRIZZI TÉRCIO

Melbourne, Australia

Conferencia; 9th International Conference on Southern Hemisphere Meteorology and Oceanography; 2009

American Meteorological Society (AMS)

This study presents a new vision on the extreme cold events which affect South America considering the dynamics of wave train propagation which control the systems causing such events. Through a composite analysis of meridional wind at 250 hPa for some frost event types observed, the differences leading to extreme cold situations due to extratropical latitudes influence with respect to tropical latitudes are studied. The results show that for extratropical latitudes a double wave train propagates through the southern hemisphere following the polar and subtropical jet streams positions. In the days previous to the frost event affecting central and northeastern Argentina these trains coincide in phase west of the continent giving place to an extended and intense polar air advection in the whole troposphere. In turn, the extreme cold events observed in tropical latitudes are associated with a single wave pattern propagating in the Pacific Ocean rapidly driving the low level anticyclone from the southwest of the continent to low latitudes. This propagation involves a southern circulation due to the meridional wind penetration and consequently polar advection causing temperatures to drop below 0ºC. However intense cold events -but with T>0ºC- are associated to a subtropical wave train propagating through the Pacific Ocean which couples with a high frequency wave train in the south Atlantic subpolar latitudes. The net zonal propagation leads to the rapid entrance of the anticyclone through the west of the continent, going further north than in the extreme frost cases. This anticyclone is reinforced by the extended upstream cyclone, a configuration that produces southern winds over central-southeastern Brazil and consequently the observed temperature decrease.