Characterizing dry and humid heatwaves in Southern South America: regional trends and large-scale climate drivers

This study characterizes the spatiotemporal variability of heatwaves occurring under dry and humid conditions during the warm season (October–March) in southern South America (SSA) between 1979 and 2018. Using daily maximum temperature and precipitation data from 132 meteorological stations, we identified compound extremes based on the co-occurrence (simultaneous or sequential) of heatwaves with anomalously dry or wet conditions, defined by the Standardized Precipitation Index. Four types of compound events were analyzed: simultaneous and sequential dry heatwaves (SIDH, SEDH), and simultaneous and sequential humid heatwaves (SIHH, SEHH). SIDH and SEDH were the most common compound events, particularly in northeastern and central Argentina, while humid events (SIHH and SEHH) were generally less frequent, especially SIHH. Changes in frequency, duration, intensity, and spatial extent of compound heatwaves were examined by comparing two subperiods (1979–1998 and 1999–2018). Our findings show an increase in the frequency and spatial extension of dry events (SIDH and SEDH) and SEHH in recent decades. We also investigated the influence of large-scale climate drivers—El Niño–Southern Oscillation, Pacific Decadal Oscillation (PDO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM)—and their combined phases on compound heatwave occurrence. In particular, the concurrent occurrence of La Niña, negative PDO, negative IOD, and positive SAM phases was associated with an increased occurrence of dry compound heatwaves and a reduction in humid ones. These findings highlight the importance of understanding compound heatwaves, especially in regions like SSA where land–atmosphere feedbacks and climate variability strongly influence extremes.