Gulf of Maine Marine Heatwaves in Eddy‐Resolving and Non‐Eddy Resolving Simulations

Abstract

Marine heatwaves (MHWs), prolonged periods of anomalously high ocean temperatures, have major ecological and socioeconomic consequences. Predicting these events requires knowing which atmospheric and ocean processes control their onset, persistence, and intensification. Here we analyze two major MHWs in the Gulf of Maine (2011–12 and 2015–16) using a pair of forced ocean‐sea ice (FOSI) simulations from the Community Earth System Model (CESM). One simulation has nominal 0.1° ocean resolution, which is sufficient to resolve mesoscale eddies in the Gulf of Maine region (HR‐FOSI), while the other uses CESM’s standard nominal 1° resolution ocean (LR‐FOSI). The mean state of the Gulf of Maine in LR‐FOSI is warmer and has deeper mixed layers. These biases result from a diffuse and northward‐displaced Gulf Stream and show that, although MHWs are defined relative to each simulation’s climatology, mean‐state differences can cause earlier MHW detection and weaker intensification in the LR‐FOSI simulation compared to its HR‐FOSI counterpart. The HR‐FOSI better captures the observed timing and magnitude of both MHWs. Surface heat fluxes initiate warming in both simulations, but LR‐FOSI’s deeper mixed layer reduces the temperature response to its stronger surface forcing, resulting in similar mixed‐layer heating. Differences in ocean heat transport therefore contribute to the different MHW responses. Stronger horizontal heat transport in HR‐FOSI arising from both mean and eddy advection sustains and intensifies the events, while parameterized eddy effects in LR‐FOSI produce cooling. These results show that simulated MHW evolution depends on both atmospheric forcing and the representation of ocean circulation and mixing.