Characterization of regional heatwaves in southern South America

Heat waves (HWs) can cause large socioeconomic and environmental impacts and they have become more intense and frequent across most land regions in the world since 1950s. Additionally, they will further increase in the future even if global warming stabilized at 1.5ºC. Therefore, HWs are one of the climate extremes of major concern. In this study we obtain for the first time a regional catalogue of warm season (WS: October – March). HW events for southern South America (approximately 17–60ºS, 35–80ºW) during 1979 – 2018 using maximum temperature data from 137 conventional weather stations. Moreover, we assess the synoptic patterns associated with regional HWs from daily data of the ERA5 reanalysis.

By applying hierarchical clustering of local HWs at the weather stations, we identify five regions based on stations with high co-occurrence of simultaneous HWs. Regional HW events are defined as WS periods of at least three consecutive days with maximum temperature exceeding the 90^th percentile of the local T_Max distribution on more than 40% of the stations of each region. We also calculate the accumulative maximum temperature anomaly to rank the magnitude of regional HW events.

For all regions, HW events are associated with significant and co-located positive 500 hPa geopotential height (z500) anomalies and with the intensification of the Northwestern Argentinean Low (NAL, The exception is the southernmost region, where HW events are linked to barotropic extratropical systems blocking the westerly flow. HW events affecting northern and central-eastern transitional regions are also associated with positive outgoing longwave radiation (OLR) anomalies, pointing to enhanced subsidence by intensification of the South Atlantic convergence zone (SACZ), as well as an intensification of the northerly low-level flow associated with South American Low-Level Jet (SALLJ) events. The results suggest that small changes in the magnitude and / or location of the aforementioned signatures can shape HWs over different regions of the domain. Finally, the frequency of regional HW days shows a significant increase over central and northern Chile, central Argentina, and northern Argentinian Patagonia. On the whole, except for the two southernmost regions, the most severe regional events have occurred during the 21st century.