Cyclone/Anticyclone Tracks and Climate Variability: Synoptic Climatology of Cold Waves in Subtropical South America.

PEZZA, ALEXANDRE; AMBRIZZI, TÉRCIO; MÜLLER GABRIELA V

Cruzeiro

Workshop; 4th Prosur Meeting of the Principal Investigators of the PROSUR ? IAI, CRN ? 055. PROgram for the study of regional climate variability, their prediction and impacts, in the mercoSUR area.; 2003

Interamerican Institute for Global Changes Research (IAI).

From a synoptic point of view, the propagation of cold waves can be monitored by the movement (tracks) of migratory extratropical cyclones and anticyclones near the surface. In the present work, an automated procedure developed by Murray and Simmonds (1991) for finding and tracking high and low pressure centers has been applied. We are using mean sea level pressure data set every 12 hours from the NCEP/NCAR Reanalysis, and cold surges with temperatures below 2.5ºC in São Paulo city, with and without frost, have been analyzed for the 1950-2000 period. The results focusing on polar outbreaks show that a strong surge in tropical regions needs two systems of pressure (cyclone and anticyclone) simultaneously, with a strong cold advection preceding a period of marked radiational cooling. Through a superposition technique, climatological ?clouds? showing all tracks on the same map are produced, adding some new insights to the synoptic patterns of propagation and improving the Southern Hemisphere climatology. For all composites the mean cold front cross the Equator, and the extratropical cyclones also play an important role in favoring frost occurrence in São Paulo. The lagged composites indicate that most of the cold events may be tracked up to nine days before their occurrence, with a persistent upper level signal in the eastern Pacific.