The Continuum of Northeast Pacific Marine Heatwaves and Their Relationship to the Tropical Pacific

Some questions remain concerning the record-breaking 2013-2015 Northeast Pacific marine heatwave (MHW) event: was it exceptional or merely the most pronounced of a group of similar events, and was its intensity and multiyear duration driven by internal extratropical processes or did the tropics play an important role? By analyzing the statistical behavior of the historical MHWs within the ERSST.v3 data set over the 1950-2019 period, we find that Northeast Pacific MHWs occurred over a continuum of intensities and durations, suggesting that these events are a recurrent Pacific phenomenon. These statistics and corresponding composite evolution are dynamically reproduced by a large ensemble simulation of a Pacific Linear Inverse Model, thereby providing a greater range of MHW expressions than the short observational record alone. Consistent with the 2013-2015 event's evolution, we find that overall the tropics influence MHWs primarily by increasing their duration, while MHW intensity is related to the initial extratropical anomalies. Plain Language Summary Marine heatwaves (MHWs), analogous to heatwaves on land, describe when the ocean temperature is abnormally warm for a prolonged period. In the Northeast Pacific during 2013-2015, an exceptionally strong and long-lasting MHW occurred causing destructive ecosystem impacts including massive mortality of fish and birds. Given the unusual nature of the event, it is of considerable interest to ask whether it occurred largely naturally or whether it might have been impacted by climate change. To address this question, we analyze historical MHWs over the 1950-2019 period and, since there may have been too few observed MHWs for comprehensive scientific understanding, additionally construct a statistical model mimicking how Pacific ocean surface temperatures evolve over time. In this manner, we create 2,000 alternate histories of what could have happened over the 1950-2019 period to provide enough MHWs for further analysis. Our findings suggest that Northeast Pacific MHWs are an irregularly recurring natural phenomenon, whose duration is largely dependent on interactions with the remote tropical Pacific but whose intensity depends primarily on local conditions within the North Pacific. A 2013-2015 event could well have occurred without climate change, but our model cannot entirely anticipate its prolonged severity. Key Points . Prolonged and intense marine heatwaves (MHWs) are a recurrent Northeast Pacific phenomenon, well captured by a simple Linear Inverse Model Northeast Pacific MHW duration appears greatly affected by tropical interactions, while MHW intensity stems primarily from the extratropics The 2013-2015 Northeast Pacific MHW was an extreme example of this phenomenon that may have been further strengthened by climate change

Automated summary

Prolonged and intense marine heatwaves (MHWs) are a recurrent phenomenon in the Northeast Pacific, with duration greatly affected by tropical interactions and intensity primarily stemming from the extratropics. The 2013-2015 Northeast Pacific MHW was an extreme example of this phenomenon that may have been further strengthened by climate change.