A Persistent and Intense Marine Heatwave in the Northeast Pacific During 2019-2020

Motivated by increasing marine heatwaves (MHWs) and their dramatic climate effects, we analyze the persistent 2019-2020 MHW, which showed significant positive sea surface temperature (SST) anomalies in the Northeast Pacific. Four SST peaks were identified during its evolution, which appeared in November 2019, April, July, and November 2020. Positive temperature anomalies were mostly located within the mixed layer for the first-year winter peak. However, the warm core was centered around 50 m below (at the bottom of) the mixed layer for the summer (second-year winter) peak. The dominant factor for the two wintertime peaks was a surface heat flux anomaly, with reduction in evaporative cooling due to the easterly anomaly. The heat flux and potentially the vertical entrainment combined to induce the peak in spring. In the tropical Pacific, a La Nina event occurred following this MHW, while other recorded double-peak events were associated with El Nino or neutral conditions.