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

Abstract:

The 2014-2015 multi-year Northeast Pacific marine heatwave (MHW) is considered a record-breaking event. It remains unclear if this event was an exception to the large-scale Pacific phenomenology and how its spatiotemporal evolution is linked to the Tropical Pacific. By analyzing the statistical behavior of the historical MHWs in ERSST.v3 over the past 70 years (e.g., Figure 1a), we find that Northeast Pacific warming events can occur over a continuum of intensities and durations (Figure 1a), with the evolution of more extreme conditions similar to that of the 2014-2015 MHW, suggesting that these events are a recurrent phenomenon of the North Pacific dynamics. Using a 40,000-year simulation of a Pacific Linear Inverse Model (LIM), we reproduce the statistics of the historical MHWs, both the relative frequencies of events with different duration and intensity (Figure 1b) and the composite evolution of these events, including the 2014-2015 event. Moreover, we obtain a much broader range of simulated MHW expressions than attainable from the limited duration of observational record. This very large ensemble of realizations is then used to diagnose the influence of the tropical Pacific on MHW evolution. We find that the more persistent MHWs are linked to a stronger La Niña signal during early stages of MHW evolution, and to a delayed development of El Niño in the Tropical Pacific. This indicates a greater impact of the Tropical Pacific on the more prolonged MHWs and suggests that the Tropical Pacific may have played a significant role in shaping the 2014-2015 marine heatwave event.

Figure 1. Intensity-Duration-Frequency (IDF) map of the MHW index (determined by area-averaging SST anomalies within the gray box region in the inset map) from (a) ERSST and (b) from the LIM simulation. The IDF plot is obtained by calculating the number of events that exceeds each intensity and duration pair. Dots mark where the frequencies of intensity and duration pairs from ERSST are significantly different from that of the ensemble LIM simulation at a 95% significance level.