Leading dynamical processes of global marine heatwaves in an ocean state estimate

Abstract. Marine heatwaves (MHWs) have emerged as a very active area of research due to the devastating impacts of these events on marine ecosystems across different trophic levels. However, a clear understanding of the local drivers of these extreme ocean conditions is still limited at a global scale. Observations of the terms needed to constrain ocean heat budgets are very sparse, ocean reanalysis products are generally non-conservative and inadequate to conduct accurate heat budget analyses, and the fidelity of climate models with respect to simulating MHWs is still unclear. In this study, we make use of Argo float observations, a satellite-based sea surface temperature product, and the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate to assess MHW characteristics over the global ocean. ECCO is then used to evaluate local MHW drivers. ECCO assimilates observations using the adjoint methodology, which optimizes the system trajectory given the observational constraints in a conservative fashion, making it an ideal product for the estimation of heat budgets. The representation of MHWs in ECCO is generally consistent with observations, although ECCO tends to underestimate MHW frequency and intensity and overestimate duration, relative to the observational products. Atmospheric forcing emerges as the dominant contributor to MHW onset and decline across most regions, while ocean dynamics, including the advective and diffusive convergence of heat, play crucial roles in the equatorial regions, specifically in the extratropical zones (e.g., western boundary currents, such as the Gulf Stream and Kuroshio) and the Southern Ocean. Regional analyses in the northeastern Pacific, southwestern Pacific, and Tasman Sea show diversity in leading dynamical mechanisms for MHW onset and decline both across regions and across events in the same regions: while air–sea exchanges of heat may contribute most frequently to MHW onset and decline, other mechanisms can also often provide dominant contributions and, at times, be the main driver. A more complete understanding of MHWs and their drivers is crucial for predicting their initiation, duration, intensity, and decline, to ultimately inform the development of mitigation and adaptation strategies for affected communities.