Coupled air-sea interactions drove and sustained the 2013-2016 North Pacific marine heatwave.

The 2013-2016 marine heatwave (MHW) in the Northeastern Pacific ranks among the most intense extratropical ocean warming events on record. Its intricate evolution has complicated efforts to understand the underlying dynamics crucial for predicting future MHWs. Here, we track the MHW using a closed, three-dimensional Lagrangian heat budget that quantifies the processes driving its evolution. Three-dimensional particle trajectories separate the MHW into northern and southern components and reveal that they are of distinct kinematic origin. We quantitatively demonstrate through a closed budget that advection, rather than surface forcing, plays the dominant role in sustaining the MHW's heat content. The northern component was maintained by a weakening of Ekman heat transport across the North Pacific Current, combined with reduced wintertime surface heat loss. The southern component was driven by weakened Ekman upwelling caused by reduced alongshore winds over the California Current. We further identify a persistent low sea-level pressure anomaly over the central Pacific that altered wind patterns, redistributing heat and moisture poleward to ultimately force both parts of the MHW. Elevated surface temperatures, in turn, amplified this pressure anomaly, completing a positive feedback loop that sustained the event across multiple years.