Large‐scale anomalies in sea‐surface temperature and air‐sea fluxes during wind relaxation events off the United States West Coast in summer

Abstract

In summertime along the U.S. West Coast, the winds exhibit a three‐stage cycle spanning ∼12 days. The prevailing upwelling‐favorable winds weaken (relax) or reverse off the Pacific Northwest, then reintensify, then weaken off central California. We study the sea‐surface temperature (SST) response to these “northern” and “southern” wind relaxations. (1) Satellite data indicate northern wind relaxations result in SST anomalies O(+1°C) extending ∼2000 km offshore. Surface heat flux reanalyses indicate the warm anomaly is mainly from decreased latent cooling. (2) After the winds reintensify, SST becomes anomalously cold along central and southern California. (3) During the southern wind relaxations, the cold SST anomaly persists but the SST warms with time. This warming is not driven by surface heat flux. The latent cooling is reduced, yet unlike during the northern relaxation, this change is canceled by a decrease in solar radiation due to increased cloudiness. In the region south of Point Conception, reduced southward advection of cold water and increased northward advection of warm water by the coastal countercurrent could explain the warming. Reduced Ekman pumping likely contributes to the warming trend during the southern relaxations, and reduced wind‐driven entrainment at the base of the mixed layer likely contributes to the warming during both relaxations. Whether the net surface heat flux is the main driver of SST anomalies during wind relaxation depends on the regional response of clouds. Southern wind relaxations follow episodes of enhanced surface cooling, which may contribute to greater cloudiness during southern than northern wind relaxations.

Regional Structure in the Marine Heat Wave of Summer 2015 Off the Western United States

“…The regional wind relaxations and intensifications described above are accompanied by changes in SST. In particular, the wind quasi-dipole pattern is associated with dipole anomalies in SST trends (Flynn et al, 2017). During a typical "southern relaxation, " the SST anomaly off central California increases by ∼0.25-0.5 • C for several days, with a lag of ∼2 days following the onset of the wind relaxation.…”

“…During the typical regional wind relaxations, the net air-sea heat flux anomalies are too small to explain the SST warming trend off California (Flynn et al, 2017). There is not a strong positive air-sea heat flux anomaly during the southern relaxation events because the events are accompanied by reduced shortwave radiation, due to increased cloudiness.…”

“…During a typical "southern relaxation, " the SST anomaly off central California increases by ∼0.25-0.5 • C for several days, with a lag of ∼2 days following the onset of the wind relaxation. However, the actual SST anomaly usually remains negative or near zero because southern wind relaxations are typically preceded by wind intensifications, i.e., the opposite phase of the quasi-dipole, and those wind intensifications create negative SST anomalies (Flynn et al, 2017). The increase in SST anomaly during the following wind relaxation then restores the SST from a negative anomaly to near the climatological value (zero anomaly).…”

“…There is not a strong positive air-sea heat flux anomaly during the southern relaxation events because the events are accompanied by reduced shortwave radiation, due to increased cloudiness. A scaling analysis of other terms in the 3-D heat budget indicates that offshore of the ∼200km wide coastal upwelling region, the warming in SST during the wind relaxations off California is likely from shoaling of the ocean surface mixed layer (ML) due to reduced wind-driven vertical mixing and Ekman pumping (Flynn et al, 2017). The ML can them warm because the (climatological) air-sea heat flux heats a shallower ML, and because entrainment at the base of the ML is presumably reduced during wind relaxations.…”

Marine Boundary Layer Clouds Associated with Coastally Trapped Disturbances: Observations and Model Simulations

“…The flow at 850 hPa becomes increasingly offshore near Northern California during this transition; the MBL cloud deck usually clears (e.g., Kloesel 1992;Crosbie et al 2016), and the alongshore pressure gradient weakens. During these cases, the nearsurface wind and sea surface temperature (SST) fields go through a three-stage cycle (Fewings et al 2016;Flynn et al 2017) whereby the northerly flow eventually diminishes (''relaxes''). If the alongshore pressure gradient reverses, the flow becomes southerly in an event known as a coastally trapped disturbance (CTD; e.g., Dorman 1985;Mass and Bond 1996;Nuss et al 2000;Parish et al 2008).…”