Regional Wind Patterns and the Spatial Structure of Marine Heat Waves off the Western United States

Melanie R Fewings, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States and Kevin Scott Brown, Oregon State University, Department of Pharmaceutical Sciences and School of Chemical, Biological, and Environmental Engineering, Corvallis, OR, United States
Abstract:
Marine heat waves (MHWs) in the northeast Pacific during 2014–2016 caused major economic and ecological damage. The strength of the sea-surface temperature (SST) anomalies showed strong spatial variability within the California Current System (CCS). Similar regional variability in MHW intensity is observed along coastlines worldwide, but is not well understood.

Our previous work showed that the coastline shape dictates regional wind patterns that can either alleviate or intensify a MHW. The spatial pattern of SST anomalies during the July 2015 “split MHW” off the western continental U.S., previously described as “flummoxing” climate scientists, was similar to the spatial pattern of the preceding wind stress anomalies. This relationship between wind stress and SST anomalies existed because (i) where SST warmed, the net air-sea heat flux anomalies were small due to increased cloudiness and (ii) the wind stress anomalies were unusually persistent, presumably due to the atmospheric ridging associated with the large-scale MHW. Because the regional wind pattern is a quasi-dipole that extends along the CCS, the result was a stronger MHW offshore of California than offshore of Washington/Oregon during July 2015. This is in contrast to regional variability off Baja California that other researchers have shown was due to changes in cloud cover, and variability in the Southern California Bight that was affected by oceanic along-shelf advection.

Here, we discuss (1) when and where the above relationships between wind, air-sea heat flux, and SST anomalies hold during other MHW events in the CCS region, (2) the differences between typical and extreme wind dipole events, and (3) the contribution of short and long time scale SST anomalies to the occurrence of a MHW. We diagnose terms in a surface mixed layer heat budget using satellite ocean vector wind data and reanalysis products, objectively analyzed air-sea fluxes from SeaFlux and OAFlux, and radiative forcing from CERES.

SST anomalies related to wind variability are among the aspects of CCS MHWs least well predicted by existing models (Jacox et al., 2019). The relationships between coastline shape, wind, and SST anomalies we discuss here may guide efforts to better predict the spatial pattern of future MHWs in the CCS.

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