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

Abstract

Marine heatwaves (MHWs) are extended periods of anomalously warm ocean temperatures and are often classified based on their intensity and duration (Hobday et al., 2016, 2018). The most prolonged MHW of the past 70 years, occurring during 2013–2015, covered a broad region of the Northeast Pacific, with local maximum warming of 2.8°C (Bond et al., 2015; Di Lorenzo & Mantua, 2016), and did not end until after the development in the tropics of a strong El Niño. Its ecosystem impacts were unprecedented, including massive stranding, entanglement, and mortality of marine species and seabirds (Cavole et al., 2016; Jones et al., 2018; Santora et al., 2020) and prolonged harmful algal blooms that closed major fisheries (McCabe et al., 2016; Ryan et al., 2017; Sanford et al., 2019). This record-breaking event led to enhanced scrutiny of many Northeast Pacific MHW aspects (Frolicher & Laufkotter, 2018; Holbrook et al., 2020), including their severities (Hobday et al., 2016; Jacox et al., 2020; Scannell et al., 2016), tropical and extratropical driving mechanisms (D. J. Amaya et al., 2020; Bond et al., 2015; Di Lorenzo & Mantua, 2016; Holbrook et al., 2019), and predictability (Hu et al., 2017; Jacox et al. 2019). Abstract 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.