Date of Completion
James Edson; Kelly Lombardo
Field of Study
Master of Science
The term ‘wind relaxation’ describes weakening or reversal of the prevailing upwelling-favorable winds. Summertime wind relaxations along the U.S. West Coast exhibit an ‘event cycle’ spanning ~12 days. The winds first relax or reverse off the coast of Oregon. Next, the upwelling-favorable winds strengthen off the coast of central California; the strong winds move poleward and reach Oregon in ~3 days. Then the winds relax off central California. This previously known synoptic scale pattern in the momentum flux from atmosphere to ocean has led to two questions: 1) How does sea-surface temperature (SST) respond on scales of 100s to 1000s of km from the coast during the two wind relaxations, and 2) What drives the SST response? Satellite microwave radiometer data indicate the wind relaxations off Oregon result in anomalously warm SSTs, up to 1C above climatology, with a spatial extent up to 2000 km offshore. To determine whether the net surface heat flux drives the SST response, we analyzed the net surface heat flux and the latent and sensible heat flux components from the Objectively Analyzed air-sea Fluxes (OAFlux) product and the shortwave and longwave radiative flux components from the International Satellite Cloud Climatology Project (ISCCP). During the wind relaxation off Oregon, the warm SST anomaly is largely a result of anomalous heating by the net surface heat flux, specifically from a decrease in cooling from the latent heat flux due to weaker winds. The net surface heat flux accounts for up to 90% of the warm anomaly, depending on spatial location. When the winds next strengthen off of California for ~4 days, the SSTs become anomalously cold. Finally, during the ~5 days between the end of the wind reintensification and the end of the second wind relaxation off central California, the SSTs warm offshore of California, yet a cold SST anomaly persists from the preconditioned cold water. In contrast to the Oregon relaxation, the change in SST during this second wind relaxation is not primarily driven by the net surface heat flux. The wind stress and therefore cooling from the latent heat flux are reduced during the second wind relaxation, yet the net surface heat flux anomaly is small because there is increased cloudiness and reduced solar radiation. These effects (i.e., reductions in both latent cooling and solar warming) tend to cancel each other, so the net surface heat flux only accounts for up to 25% of the change in SST, depending on spatial location. The amount of penetrating solar radiation lost through the base of the mixed-layer is small (~10% of the shortwave radiation at the surface) for both wind relaxations. Estimates of the horizontal and vertical advection and mixing terms in a mixed-layer heat budget suggest that mixed-layer shoaling is the largest contributor to the ocean surface warming during the second wind relaxation. Using satellite vector winds and Argo float data, we determined that upwelling due to wind stress curl anomalies alone cannot explain the observed mixed-layer shoaling signal. Thus, we hypothesize that reduced wind-driven vertical mixing is the primary driver of the warming trend in the SST anomaly during the central California wind relaxation. To test this hypothesis would require long time series of vertical profiles of upper ocean structure with higher temporal resolution than the Argo floats. Future studies should focus on the importance of cloudiness in this region, as well as, in other eastern boundary upwelling systems. Whether the net surface heat flux is the driver of SST anomalies during wind relaxations appears dependent on if cloud coverage increases or decreases, thus affecting the incoming solar radiation.
Flynn, Kayla, "The Causes of Regional Sea-Surface Temperature Anomalies During Wind Relaxation Events Off the U.S. West Coast in Summer" (2016). Master's Theses. 984.