Leading Dynamical Processes of Global Marine Heatwaves

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. Yet, 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 in simulating MHWs is still unclear. In this study, we make use of Argo floats 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 overall 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 advective and diffusive convergence of heat, play crucial roles in the equatorial regions, specific extra-tropical zones (e.g., western boundary currents such as the Gulf Stream and Kuroshio), and the Southern Ocean. Regional analyses in the Northeast Pacific, Southwest 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.