Abstract The southeast Indian Ocean (SEIO) has witnessed amplifying variability in the 0–700 m heat content during the past decades. More heat pile‐up (HPU) events have been observed than before, leading to increased occurrence of marine heatwaves and threats to local marine ecosystems. Here, we show that most of these HPU events cooccur with multi‐year La Niña (MLN) conditions, and Pacific‐origin downwelling baroclinic waves play a key role in establishing the upper‐layer convergence of the SEIO. However, model experiments and budget analysis for the recent 2020–2023 event reveal that the extra heat in the SEIO is fueled mostly by the heat redistribution over the Indian Ocean, rather than by the Pacific via the Indonesian Throughflow (ITF). Climate models project more HPU events in the SEIO in future, partly associated with the increasing MLN events in a warming climate.

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