Marine heatwaves and cold spells in the Philippine seas: evolution and drivers of recent extremes

Abstract. Marine heatwaves (MHWs) and marine cold spells (MCSs) are critical ocean thermal extremes with substantial ecological and socioeconomic impacts, yet their regional characteristics and underlying drivers remain poorly understood. The Philippines, a region of high marine biodiversity and exposed to rapidly changing ocean conditions, is particularly vulnerable to these extremes, but recent changes during exceptionally warm years remain insufficiently documented. Here, we investigate the spatiotemporal variability of MHWs and MCSs in the Philippine seas from 1993–2025 using a high-resolution (0.083° × 0.083°) sea surface temperature dataset. Results reveal pronounced spatial and temporal contrasts in the frequency, intensity, and duration of MHWs and MCSs, reflecting the combined influence of local ocean–atmosphere interactions and large-scale climate variability. We find that MHWs have become increasingly frequent, persistent, and intense in recent years, whereas MCSs have substantially declined, consistent with a shift toward a warmer regional mean state. The use of higher-resolution data enables the detection of fine-scale coastal and shelf processes and identifies localized MHW hotspots, particularly along the western coast of Luzon Island, which are not clearly resolved in coarser datasets. The contrasting trends in MHWs and MCSs are associated with a pronounced shift in the SSTA distribution toward warmer conditions, beginning around 2013. Case studies of major events during 2022–2025 further demonstrate that recent MHWs were shaped by the interaction between long-term warming and ENSO-related atmospheric circulation variability. La Niña and El Niño conditions contributed to distinct warming patterns through changes in regional ocean–atmosphere coupling, including persistent anomalous anticyclonic circulation, reduced cloud cover, enhanced shortwave radiation, and weakened ocean cooling processes. These findings highlight how basin-scale climate variability is translated into regional marine thermal extremes in a tropical archipelagic environment and provide important implications for monitoring, prediction, and risk assessment of ocean warming extremes in vulnerable marine regions.