Dissertation Defense: Extreme precipitation events associated with cold fronts in the metropolitan region of São Paulo: Climatology, climate trends, and synoptic patterns

South America is constantly influenced by the passage of frontal systems, as certain specific regions of the continent have highly favorable characteristics for their development of these systems. The metropolitan region of São Paulo (RMSP) located in southeastern Brazil is an area of high population density and economic importance, and is subject to the abrupt weather changes due to passage of frontal systems. These systems, in turn, are crucial for the region's precipitation regime. Concerns related to the occurrence of extreme precipitation events have increased significantly in recent decades, as such events have important impacts in regions like the RMSP. This study investigated the\ interannual variability of extreme daily precipitation events (EECs) related to the passage of cold fronts in the RMSP and the synoptic patterns associated, as well as possible trends and regime changes in the time series. For the period 1960-2022, local observations of atmospheric variables (wind, temperature, and precipitation) were used to identify cold fronts and EECs, while ERA5 and NCEP2 reanalyses provided the synoptic patterns. A 5-day time window (from 2 days before to 2 days after) was considered to relate EECs with cold fronts in the RMSP. A total of 1077 EECs were identified, with 58% (630) of them associated with cold fronts. These events preferably occur during the austral spring, although the associated daily mean precipitation is higher in the austral summer (the region's rainy season). The relative contribution of cold fronts to extreme precipitation shows a statistically significant monotonic decrease, which contrasts with the increasing trend in the intensity and frequency of EECs. This suggests the influence of other meteorological systems in increasing precipitation extremes. For the EECs associated with the cold fronts, the synoptic patterns reveal in the lower troposphere the presence of troughs over the southsoutheast Brazil allied with an intense anticyclone located southwestward (Argentina and Paraguay), intense anomalous cyclonic vorticity along the front, intensified moisture flux, strong thickness gradients in the 500- 1000 hPa layer. Additionally, these events present a slightly tilted trough from the middle to upper troposphere, and upper-level wind divergence due to the intensified subtropical jet stream. The vertical structure of these cold fronts has stronger baroclinic features at 1000-850 hPa layer and weaker ones at 500-250 hPa layer. Most EECs occur on the days before and on the day of the cold front due to more favorable dynamic conditions for convective activity, such as more intense and extensive low-level cyclonic vorticity and cyclonic structure in moisture flux anomalies. The described synoptic patterns of EECs associated with cold fronts represent a contribution of this work to the weather forecasting of extreme rainfall events in the RMSP.