Climatology and the time interval of cold fronts passage over South America

Abstract:

Cold fronts affect the weather over South America throughout the year. The south-central region of South America has been identified as highly favorable to the formation and intensification of frontal systems.

The fronts usually move from southwest to northeast over the continent and the Atlantic Ocean. During their passage important changes in weather conditions are observed: spinning wind, rainfall, decreasing on atmospheric pressure and temperature. After the passage of a cold front it is usually observed a sharp drop of temperature, pressure increase and wind gusts.

There are several studies in the literature about South America cold fronts climatology. However, none of them were performed in pre-defined regions over South America and that have defined the pass interval in each region, which is the goal of the present research.

Daily data of temperature on 925hPa, meridional wind in 925hPa and sea level pressure from the ERA Interim reanalysis from ECMWF, with spatial resolution of 1.5 ° x 1.5 ° for the period 1982-2009 were used. The criteria to identify the each cold front were: a drop in temperature, change in wind direction (north to south) and an increase in pressure from day 0 to day +1 for an average of four points for each of the 19 regions of South America.

During the year, about 40 frontal systems pass in southern South America and this average is reduced to 10 to the northern regions of the continent. Winter is the season with the highest number of passage of frontal systems, summer is the season that presents lower number and spring and autumn have an intermediate number of fronts of the summer and winter.

Analyzing the time interval of cold fronts passage, it was found an average of 8 days in the length of a passage between two fronts in southern Brazil. For the regions further north the interval can reach 26 days during the year. Summer is the season with the longer interval between the passage of fronts and winter and spring shows the smaller intervals.