Two types of warm blobs in the Northeast Pacific and their potential effect on the El Niño

Abstract

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.

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

“…We present detailed evolution of the 2019–2020 MHW (black curve), and compare it with the double‐peak events (color curves) identified in Chen et al. (2021) (Figure 1b). Four peaks are clearly seen during its lifetime, in November 2019, April 2020, July 2020, and November 2020.…”

“…Note that the well‐known MHW during 2013–2015 included two events (i.e., 2013–2014 and 2015) due to the three‐month interval in between, as documented by Chen et al. (2021).…”

“…Following Chen et al. (2021), we define a persistent MHW when the blob index is greater than 1.0 (accounting for around 17.5% of the total time series) and lasts for five months or longer with no more than one month interruption. The peak indicates the month when the blob index reaches a maximum value during the evolution of MHW.…”

“…Quantitatively, some studies revealed that the MHWs were generated primarily through surface heat flux and ocean advection anomalies (Bond et al., 2015; Liang et al., 2017), while Chen et al. (2021) reported the dominant role of surface heat flux in boreal winter but vertical entrainment in boreal summer, respectively. Nevertheless, specific contributions of atmospheric and oceanic processes to the most recent 2019–2020 MHW remain unclear.…”

“…After identifying a salient amplification in summer 2015 during the 2013–2015 MHW (Hu et al., 2017; Liang et al., 2017), Chen et al. (2021) classified the historical MHWs in the NE Pacific into double‐peak and single‐peak categories, and revealed their different evolutionary features, particularly during the winter and summer. However, the characteristics of these MHWs in the boreal spring are not clearly demonstrated.…”