H. A. Scannell, G. C. Johnson, L. Thompson, J. M. Lyman, S. C. Riser

Published in Geophysical Research Letter, NOV 2020

The reappearance of a northeast Pacific marine heatwave (MHW) sounded alarms in late summer 2019 for a warming event on par with the 2013–2016 MHW known as The Blob. Despite these two events having similar magnitudes in surface warming, differences in seasonality and salinity distinguish their evolutions. We compare and contrast the ocean's role in the evolution and persistence of the 2013–2016 and 2019–2020 MHWs using mapped temperature and salinity data from Argo floats. An unusual near‐surface freshwater anomaly in the Gulf of Alaska during 2019 increased the stability of the water column, preventing the MHW from penetrating deep as strongly as the 2013–2016 event. This freshwater anomaly likely contributed to the intensification of the MHW by increasing the near‐surface buoyancy. The gradual buildup of subsurface heat content throughout 2020 in the region suggests the potential for persistent ecological impacts.

FIG. Variations in upper ocean heat content anomalies (a), temperature anomalies and mixed layer pressure (b), and upper ocean stratification anomalies (c) averaged in 35.5–51.5°N, 135.5–154.5°W (black outline in Figure 1). Ocean heat content anomalies are computed over three different 80‐dbar pressure layers between 10 and 90, 100 and 180, and 200 and 280 dbar. These intervals are shown in (b) as vertical colored lines on the right‐hand side corresponding to (a). The mixed layer pressure and 2004–2019 climatology are computed from 19,697 Argo profiles using the Holte and Talley (2009) density algorithm. The bulk upper ocean stratification anomaly (solid lines) in (c) is computed as N² between 2.5 and 200 dbar and is shown with the SST anomaly (dashed lines). Positive values of N² indicate higher water column stability and greater resistance to overturning or vertical displacement.

Scannell, H. A., Johnson, G. C., Thompson, L., Lyman, J. M., & Riser, S. C. (2020). Subsurface evolution and persistence of marine heatwaves in the Northeast Pacific. Geophysical Research Letters, 47, e2020GL090548. https://doi.org/10.1029/2020GL090548