Synergistic modulation of multi-layer marine thermal extremes in the northwestern Pacific by coupled climate modes and its implications for typhoon intensification

Marine thermal extremes in the Northwestern Pacific (NWP) are intensifying under climate change, yet their vertical structure and modulation by coupled climate modes remain insufficiently understood. This study characterizes the spatiotemporal evolution of multi-layer marine thermal extremes from 1993 to 2023, including surface marine heatwaves (MHWs), upper-100-m mean temperature extremes (T100), and upper-ocean heat content extremes (UOHC; integrated from the sea surface to the depth of the 26 °C isotherm). Results reveal significant intensification of marine thermal extremes across the East China Sea (ECS), South China Sea (SCS), and WP, with the WP showing the strongest increase in cumulative thermal exposure. While surface MHWs and T100 exhibit strong coupling ( r = 0.7–0.9), surface MHWs are much weaker correlated with UOHC ( r = 0.2–0.5), indicating that surface-based metrics alone are insufficient to represent typhoon-relevant subsurface heat reservoirs. Climate-phase stratification further shows that coupled climate modes modulate these extremes in a region- and altitude-dependent manner. The La Niña/PDO− combination produces the clearest multi-layer amplification in the WP, with non-additive enhancement of thermal extremes by 40–80% relative to the linear sum of individual mode effects. In contrast, IOD–PDO interactions exert more regionally concentrated effects, particularly in the SCS. Co-occurrence analysis indicates that La Niña/PDO− conditions are associated with stronger spatial overlap between major typhoon pathways and multi-layer thermal extremes, suggesting enhanced typhoon-relevant upper-ocean thermal exposure during specific compound climate states. These findings demonstrate that surface MHWs, T100, and UOHC provide complementary but non-equivalent information on upper-ocean thermal hazards.