Physical drivers of the summer 2019 North Pacific marine heatwave
The following summarizes work published in Nature Communications.
See also our recent Nature Comment on marine heatwave definitions
Major Results:
The summer 2019 Blob 2.0 was primarily driven by a multi-month weakening of the North Pacific Subtropical High, which weakened surface winds, reduced evaporative cooling, and thinned the mixed layer.
After initiation, the warm SSTs were reinforced by a positive low-cloud SST feedback.
Atmospheric model simulations show that the strength and persistence of the Subtropical High weakening was partially due to SST forcing from the central equatorial and, surprisingly, the subtropical North Pacific.
In summer 2019, the Northeast Pacific saw a rapid resurgence of extremely warm “Blob”-like sea surface temperature (SST) anomalies (Fig. 1). The multi-month persistence of this “Blob 2.0” generated widespread concern among fishery and wildlife managers for sensitive marine ecosystems along the west coast.
Figure 1 The Blob 2.0. Observed sea surface temperature anomalies (˚F) averaged for JJA 2019.
Figure 2 Observed sea level pressure (values) anomalies (shading) averaged for JJA 2019. Black contours represent the summertime climatological SLP values. Red contours are JJA 2019 SST anomalies starting at 1˚C with a 0.5˚C interval.
The summer 2019 Northeast Pacific SST anomalies were the warmest on record (Fig. 1).
During this time period, North Pacific surface winds were significantly weakened, as indicated by negative sea level pressure anomalies overlying the climatological Subtropical High (Fig. 2).
A mixed layer temperature budget shows that strong surface heat fluxes, dominated by latent heat and downward shortwave radiation, mixed over an extremely thin mixed layer, resulting in Blob 2.0.
Satellite observations of low-cloud fraction show a reduction in low-clouds that closely mirror the warmest SST anomalies, indicating a positive low-cloud SST feedback. These air-sea interactions reinforced the Blob 2.0.
Figure 3 Atmospheric model (AGCM) simulations. (a)-(c) AGCM ensemble mean JJA 2019 sea level pressure anomalies (hPA; shading) for Global SST-forced, Tropical SST-forced, and North Pacific SST-forced experiments, respectively. (b)-(f) Single “Best Member” match for each ensemble of simulations based on pattern correlation of model and observations in black box (see Fig. 2).
We force an atmospheric model with the observed summer 2019 SST anomalies in three domains: 1. Globally, the Tropical Pacific (10˚S-10˚N) only, and the North Pacific (>15˚N) only. Each domain consists of an ensemble of 10-members from January 2018-August 2019.
The ensemble means of these experiments show skill in reproducing the spatial extent of the North Pacific Subtropical High weakening, suggesting remote SSTs likely contributed to the intensity and multi-month persistence of these anomalies (Fig. 3a-c).
However, individual ensemble members better capture the magnitude of the observed Subtropical High weakening, indicating that internal variability was the primarily factor (Fig. 3d-f).