What Caused the Warm Anomalies at Depth in the Northern Gulf of Alaska in 2019?

A marine heat wave (MHW) developed in the northeast Pacific Ocean in the summer of 2019. This event is receiving considerable attention from the climate and fisheries-oceanography communities. Our interest is in the northern Gulf of Alaska near the shelf break, and in particular, the very warm temperatures at depths ranging from roughly 100 to 300 meters. These temperatures actually appear to exceed those that occurred during the recent extreme MHW of 2014-16, and imply a degraded habitat for Pacific cod. The present study uses a combination of station data from fishery trawl surveys (which include temperature observations on the outer shelf), and gridded data from Argo and NOAA’s Global Ocean Data Assimilation System (GODAS), to diagnose the cause(s) of the anomalous warmth at depth. Our focus is on determining whether horizontal temperature advection can account for the observed temperatures, and the aspect(s) of the atmospheric forcing that are related. The results are considered in the context of the GODAS record extending back to 1980.