In late 2023, Southeast Asia’s oceans were unusually warm. Near-surface ocean temperatures in the Singapore Strait were nearly 2°C warmer than the long-term average in November—a spike in temperatures not seen in at least four decades. The warming breached the ocean surface down to a depth of 40 metres.
Sea-surface temperatures near Singapore show an unusually warm spike in November 2023, which has not been observed since 1982. All individual years apart from 2023 are shown as grey lines. Blue lines indicate the long-term mean temperatures, averaged over 1982 – 2010 and 1991 – 2020 (Source: Figure 3a of the publication)
In a new study published in Scientific Reports, we explain what caused this unusual warming and highlight the importance of sustained ocean monitoring in the region.
Our research draws on 41 months of near-continuous ocean temperature data from the Singapore Strait—one of the few long-term in-situ records available in this data-scarce region. These were collected using sensors deployed off Sentosa and nearby islands by researchers from the Earth Observatory of Singapore and other groups. We used these in-situ measurements together with satellite data to understand what drove the ocean warming in 2023.
The Build-Up: The migration of ocean heat
The unusual warming emerged in July 2023 near Luzon, north of the Philippines. In the months that followed, the elevated ocean temperatures moved steadily southward — reaching Singapore by October and peaking in November, before moving farther south to the Java Sea by December. This ocean warming coincided with the development of the 2023 El Niño, which began in June and intensified through November.
The unusually high sea-surface temperatures around Singapore in 2023 (wiggles) coincide with the peak of the El Niño (red bars). ONI: Oceanic Niño Index; SILO: Siloso Point; SJI: St. John’s Island; MESN: Singapore Marine Environment Sensing Network (Source: Figure 4a of the publication)
A clear signal from El Niño
The southward migration of warm temperatures closely tracked the patterns of ocean currents, which were affected by El Niño at that time. From October to November 2023, the warm patch within the South China Sea expanded and moved south together with the clockwise ocean circulation in its location. Later, the warm temperatures migrated further south, accompanied by strengthened southward currents within the Indonesian seas in November and December 2023. The strengthened currents in the Indonesian seas stemmed from the enhanced intrusion of warm Pacific Ocean waters during the El Niño event.
Why This Matters
The unusual warming event in 2023 offers a clear example of how large-scale climate patterns like El Niño, which develops in the equatorial Pacific, can directly influence Southeast Asia’s shallow seas. Even though the 2023 El Niño was not the strongest on record, the spike in ocean temperatures observed during this time was larger than earlier stronger El Niño events of 1997/98 and 2015/16. To prepare for future ocean temperature extremes and mitigate their impacts on regional marine ecosystems, it is therefore critical to continue monitoring in-situ ocean temperatures within Southeast Asia.