Potential drivers of Northwest Pacific marine heatwaves inferred from adjoint sensitivities

Marine heatwaves (MHWs) in the Northwest Pacific (NWP) have intensified in recent decades, yet the mechanisms responsible for persistent upper-ocean warming remain poorly understood. Previous studies primarily focused on the record-breaking July 2022 surface MHW, whereas the warming in 2022 persisted throughout the year and extended well below the mixed layer. Here, I investigate the 2022 annual upper-ocean (0–100 m) MHW east of the Kamchatka Peninsula using the Estimating the Circulation and Climate of the Ocean technology involving the MITgcm adjoint framework. Adjoint sensitivity and perturbation analysis are used to identify the dominant forcing pathways. The results show that thermodynamic forcing, including near-surface turbulent heat fluxes and radiative fluxes, contributes primarily through local air–sea heat exchange on seasonal timescales. In contrast, zonal wind forcing exhibits sensitivity several years prior to the event, indicating the importance of ocean dynamical adjustment, subsurface heat storage, and ocean memory. Perturbation experiments further demonstrate that wind-induced responses extend throughout the upper ocean rather than remaining confined to the surface. Compared with Northeast Pacific MHWs, NWP MHWs exhibit stronger wind sensitivity and greater state dependence. These results suggest that persistent upper-ocean MHWs in the NWP are governed by a combination of thermodynamic forcing and delayed ocean dynamical processes. The findings highlight the importance of subsurface ocean memory for long-duration MHWs and demonstrate the value of adjoint methods for identifying their precursor pathways. The results should, however, be interpreted in light of the coarse model resolution and the linear assumptions inherent in the adjoint framework.