Large-scale climate drivers of the interannual variability of marine heatwaves in the western North Pacific and its marginal seas

Marine heatwaves (MHWs) are anomalously warm seawater events that can persist for months, harming marine ecosystems and biodiversity. The western North Pacific (WNP) and its marginal seas host productive coastal ecosystems and densely populated coasts that have already experienced substantial ecological and socio-economic impacts from MHWs. This study examined the interannual variability of MHWs in the WNP and its marginal seas using 41 years of satellite-based sea surface temperature and reanalysis data. Spectral analysis identified interannual variability (< 6 years) distinct from the long-term warming trend. Nearly half of the interannual peaks coincided with El Niño to La Niña transition periods, during which MHWs persisted throughout the year over the WNP. During the mature phase of El Niño (September-January), southerly wind anomalies associated with the Philippine Sea Anticyclone and Kuroshio Anticyclone intensified downward turbulent heat fluxes, triggering MHWs. As El Niño weakened and La Niña developed (June-September), positive temperature anomalies in the WNP thermocline weakened the vertical temperature gradient, resulting in positive anomalies in the entrainment and vertical diffusion terms and sustaining MHWs during summer. These subsurface temperature anomalies were maintained by negative wind stress curl associated with a westward-extended Western North Pacific Subtropical High and by baroclinic eddies propagating westward along the Subtropical Countercurrent. In contrast, during peak years not associated with an El Niño to La Niña transition, MHWs appeared primarily in summer (July-September), the East Asian rainy season, over the WNP and its marginal seas. An anticyclonic circulation embedded within the circum-global teleconnection pattern may suppress convective activity, thereby enhancing downward shortwave radiation and leading to MHW occurrences. This study provides an integrated understanding of how MHWs in the WNP and its marginal seas are influenced by various large-scale climate drivers on the interannual timescale.