Climate characteristics and trends of extreme daily precipitation events associated with cold fronts in the metropolitan region of São Paulo, Brazil

Abstract

The metropolitan region of São Paulo (MRSP), located in southeastern Brazil, is densely populated and economically significant for the country. Cold fronts crossing MRSP are crucial for the precipitation regime and can induce extreme rainfall events. Therefore, this study presents a climatology of the contribution of cold fronts to extreme rainfall events in the MRSP and their associated synoptic patterns, using long term (1960 to 2022) local observations to identify cold fronts and daily rainfall extremes, and a time window of 5 days (from 2 days before and after). The mean circulation patterns associated with these events are analyzed using gridded precipitation and reanalysis for a short period (1980-2022). For 1960-2022, a total of 2535 cold fronts and 1077 extremes rainfall events were identified, with 58% of the daily extremes being associated with cold fronts. These events mainly occur during the austral spring, although their daily mean precipitation is higher in the austral summer. The contribution of cold fronts to extreme precipitation exhibits a statistically significant decrease, contrasting with the increasing trend in daily rainfall extremes. This suggests the influence of other meteorological systemsin increasing rainfall extremes. The synoptic patterns reveal an intense anticyclone located southwest of the main trough in the lower troposphere, intense cyclonic vorticity along the front at low levels, stronger upper troposphere divergence, and strong thickness gradients in the 500-1000 hPa layer affecting MRSP. Most extremes occur on the day before and the day of the cold front due to more favorable dynamic conditions for convective activity.