Synergistic effects of heatwaves on the surface urban heat Island in the São Paulo metropolitan region

This study presents the first investigation of the synergistic effects of heatwave (HW) on the urban heat island in a tropical region of South America. The objective is to analyze the spatial and temporal interaction between HWs and the Surface Urban Heat Island (SUHI) over the São Paulo Metropolitan Region (SPMR) during the austral summer, with a focus on the underlying physical processes. A total of 24 HWs affecting the SPMR between 2002 and 2022 were identified. Most episodes were associated with the South Atlantic Convergence Zone (SACZ) and persistent anticyclonic systems, which contributed to enhanced extreme temperatures. During these events, SUHI amplification was observed in both intensity and spatial extent, particularly in the eastern and northeastern sectors of the urban area. The results reveal that HW conditions modify the urban surface energy balance and local atmospheric structure in ways that reinforce SUHI. Urban areas exhibited increased sensible heat flux and net radiation, while suburban regions showed higher latent heat flux. The Urban Boundary Layer (UBL) became deeper and more stable, often characterized by thermal inversions that limited vertical mixing and enhanced heat retention. A case study from February 2014 supports these findings, highlighting reduced wind speeds, elevated temperatures, and the role of sea breezes in modulating nocturnal SUHI intensity during HW conditions. These results provide process-based evidence of the interaction between large-scale HW conditions and local urban processes, supporting a synergistic interpretation of the HW–SUHI relationship.