Primary Drivers of Marine Heatwaves in the Northwest Atlantic

As the frequency and intensity of marine heatwaves (MHWs) around the world is increasing, it is becoming more important that techniques for rapidly inferring the physical drivers of MHWs be developed and implemented. We show here the efficacy of an already tested technique, self-organising maps (SOM), for the detection of the primary drivers of hundreds of historic MHWs recorded in the shelf regions of the Northwest Atlantic Ocean. The SOM is given mean synoptic states of several important mixed-layer heat budget variables over the study area during the occurrence of all recorded MHWs over the 1993 – 2018 period. The output of the SOM is then interpreted as a typology of MHW events, and used to deduce what the primary drivers of each MHW type in this region are. The three primary patterns associated with MHWs were a Gulf Stream related warm slope water and air temperature anomaly pushing up from the South, warm air sitting still over the study area for an abnormally long time, and warm easterly air anomalies pushing into the study area. Furthermore, the occurrence of MHWs was usually associated with a clear cyclonic storm system or anticyclonic blocking pattern. The importance of these patterns, as well as the smaller patterns observed are discussed. Advice on the implementation of this technique at an operational level is also provided.