Record-Breaking Summertime Terrestrial and Marine Heat Waves in Southeast Asia Driven by Internal Variability during 2020–22

Abstract Consecutive triple-dip La Niña events occurred from 2020 to 2022, with global impacts on climate and extreme weather events. Located in the Indo-Pacific warm pool, Southeast Asia (SEA; 90°–150°E, 10°S–20°N) witnessed severe terrestrial and marine heat waves during these years, posing substantial threats to regional human health, marine fisheries, and the ecosystem. Using a wide range of observations and atmospheric model simulations, we show that the severe SEA heat waves—record breaking for land in 2021 and for ocean in 2020—exhibited distinct spatial patterns across these three summers, despite a similar La Niña background. The influences of the triple-dip La Niña and other air–sea coupled modes, such as the Indian Ocean dipole, Indian Ocean basin mode, and Atlantic Niño events, were relatively limited. We estimate that more than 17% of the SEA summer heat waves during 2020–22 can be attributed to the regional ocean–atmospheric internal variability unrelated to the aforementioned modes and the long-term trend. The first leading internal mode features an anomalous anticyclone in the lower troposphere over the tropical northwest Pacific, primarily associated with equatorial western Pacific sea surface temperature (SST) cooling and atmospheric internal dynamics unrelated to SST forcing, accounting for over 7% of total contributions in the summers of 2020 and 2022. The second internal mode is attributed to local SST warming, explaining more than 4% of total contributions in the summer of 2021. We uncover two regional internal climate modes driving SEA heat waves, with great implications for the prediction of regional extreme events.