Northeast Pacific Marine Heatwaves, Tropical Variability and Climate Change
Tongtong Xu
Georgia Institute of Technology
Tuesday, Sep 28, 2021, 2:00 pm
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Some questions remain concerning the record-breaking 2013–2015 Northeast Pacific marine heatwave (MHW) event: (1) was it exceptional or merely the most pronounced of a group of similar events, (2) was its intensity and multiyear duration driven by internal extratropical processes or did the tropics play an important role, (3) was the severity of this event affected by climate change, and if so was it through changes in the mean or in the variability? 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 composite evolutions are dynamically reproduced by a large ensemble simulation of a Pacific Linear Inverse Model (LIM), thereby providing a greater range of MHW expressions than the short observational record alone. We find that overall the tropics influence MHWs primarily by increasing their duration, with MHW amplitude driven more by extratropical conditions, consistent with the 2013-2015 event’s evolution. To assess the effect of climate change on MHW statistics, we separately analyzed the historical trend and the ocean variability within the HadISST data set over the 1958-2017 period, and in multiple large climate ensembles of a global LIM, which considered either the effect of the trend or the variability change. We find that while under climate change the Northeast Pacific appears to have undergone a notable mean warming, it has not had so notable an increase in variability. Consequently, we find that mean warming in the Northeast Pacific has led to increasing occurrence of intense MHWs, with a lesser impact on their duration. This suggests that the recording-breaking 2013-2015 event was affected by climate change, particularly for its extreme intensity.