Northeast Pacific Marine Heatwave Mechanism Inferred from Adjoint Sensitivities

Abstract A classic example of a marine heatwave (MHW) was the 2014–16 warm event that spread across the northeastern Pacific (NEP) Ocean. We use an adjoint sensitivity approach to shed new light on potential causes for such reoccurring NEP MHW events. The study is based on the Massachusetts Institute of Technology General Circulation Model (MITgcm) and its adjoint, for which the mean top 100-m potential temperature during different target years was set as the objective function, separately for the two target regions (145°–160°W, 48°–56°N) and (130°–145°W, 40°–48°N). Resulting adjoint sensitivities show that during MHW years, local turbulent surface heat flux is the dominant atmospheric driver, with air temperature, specific humidity, and longwave radiation leading to up to 80% of the temperature anomaly of the NEP; during normal years, this is only about 60%. In contrast, increased wind typically does not lead to an MHW occurrence as it is associated with a deepening of the mixed layer. We find the horizontal temperature advection, i.e., the impact of the basinwide ocean circulation, to be less important during an MHW year, but it could act as a preconditioning of MHW through its role in climate oscillations. Response analysis shows that atmospheric forcing anomalies occurring within 3 months (from October to December) prior to an MHW year play a critical role in driving the MHW. The reconstruction using various sensitivity periods suggests that the leading 6-month atmospheric conditions should have potential predictive skills for the next year. Reconstruction that includes leading 36-month atmospheric conditions performs better than persistence.