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 Niña event occurred following this MHW, while other recorded double‐peak events were associated with El Niño or neutral conditions.

In recent years, the predominant marine heatwaves in Northeast Pacific, the so‐called warm blobs, have exerted substantial environmental and socioeconomic influences on North America and its coastal waters. We select the historical warm blob events from 1951 to 2018 and divide them into double‐peak and single‐peak categories by their seasonal evolutions. The heat budget analyses attribute the leading contributor of double‐peak warm blobs to the surface heat flux at the first wintertime maximum while the vertical entrainment and diffusion take responsibility for the second peak 5 months later. The positive net heat flux anomalies indicate less heat loss from ocean to the atmosphere due to an exceptionally anomalous high as the northern lobe of the North Pacific Oscillation and intensive easterly anomalies, which counteract with the climatological westerlies. For the single‐peak category, the vertical entrainment and diffusion primarily trigger the warm blob and also lead to its termination. Moreover, the single‐peak warm blob induces positive sea surface temperature (SST) anomalies near the Baja California and establishes the Pacific Meridional Mode as the oceanic bridge to convey its influence onto the tropics and trigger the following El Niño around 10 months after the peak. In addition, the subsurface processes, such as the trade wind charging mechanism, also play an essential part by bringing warmer water upwards to the surface and hence cause the positive SST anomalies in equatorial central‐eastern Pacific. However, the interim connection between the double‐peak warm blob and El Niño is not very clear although their intensities appear to be stronger.

Large-scale marine heatwaves in the Northeast Pacific (NEP), identified here and previously as ‘warm blobs’, have devastating impacts on regional ecosystems. An anomalous atmospheric ridge over the NEP is known to be crucial for maintaining these warm blobs, also causing abnormally cold temperatures over North America during the cold season. Previous studies linked this ridge to teleconnections from tropical sea surface temperature anomalies. However, it was unclear whether teleconnections from the extratropics could also contribute to the ridge. Here we show that planetary wave trains, triggered by increased rainfall and latent heat release over the Mediterranean Sea accompanied by decreased rainfall over the North Atlantic, can transport wave energy to the NEP, guided by the westerly jet, and induce a quasi-barotropic ridge there. Our findings provide insights into extratropical teleconnections sustaining the NEP ridge, offering a source of potential predictability for the warm blobs and temperature fluctuations over North America.