El Niño was a key driver of anomalous ocean warming in Southeast Asia in 2023.

In 2023, global ocean heat content reached unprecedented values since records began in 1960. The translation of global ocean heat into regional and local-scale ocean warming remains poorly understood because of limited observational data, particularly within Southeast Asia. Here, we investigate the 2023 ocean warming event in Southeast Asia using near-continuous 41-month in-situ ocean temperature observations from the Singapore Strait, satellite sea surface temperature (SST) measurements, and high-resolution reanalysis products. We document anomalous ocean warming across the Singapore Strait and surrounding South China Sea and Indonesian Seas to depths of at least 40 m. Peak SSTs of 1.8 °C above the climatological mean were recorded in the central Sunda Shelf in November 2023 for the first time in > 40 years. Concurrent anomalous freshening of the Singapore Strait was observed, with average salinity below the climatological mean from October to December. We identify a southward migration of warm temperature anomalies beginning with the onset of the El Niño in July 2023 near the Luzon Strait. This occurred alongside southward shifts in mean sea-level pressure and near-surface ocean currents in the region. We attribute these observations to the southward shift of the North Equatorial Current bifurcation latitude, which permitted the intrusion of Pacific western boundary currents into the South China Sea and Indonesian seas. Compared to the oceanic drivers, atmospheric forcings played a limited role in driving the ocean warming in 2023. Our study highlights El Niño as the key driver of the ocean warming in Southeast Asia in 2023, and emphasises the need for expanded continuous, in-situ ocean temperature monitoring to enhance understanding of evolving ocean-atmosphere dynamics and impacts in Southeast Asia under a warming climate.