Synoptic forcing of wind relaxations at Pt. Conception, California

Over the California Current upwelling system in summer, the prevailing upwelling-favorable winds episodically weaken (relax) or reverse direction for a few days. Near Pt. Conception, California, the wind usually does not reverse, but wind relaxation allows poleward oceanic coastal flow with ecological consequences. To determine the offshore extent and synoptic forcing of these wind relaxations, we formed composite averages of wind stress from the QuikSCAT satellite and atmospheric pressure from the North American Regional Reanalysis (NARR) using 67 wind relaxations during summer 2000–2009. Wind relaxations at Pt. Conception are the third stage of an event sequence that repeatedly affects the west coast of North America in summer. First, 5–7 days before the wind weakens near Pt. Conception, the wind weakens or reverses off Oregon and northern California. Second, the upwelling-favorable wind intensifies along central California. Third, the wind relaxes at Pt. Conception, and the area of weakened winds extends poleward to northern California over 3–5 days. The NARR underestimates the wind stress within ∼200 km of coastal capes by a factor of 2. Wind relaxations at Pt. Conception are caused by offshore extension of the desert heat low. This synoptic forcing is related to event cycles that cause wind reversal as in Halliwell and Allen (1987) and Mass and Bond (1996), but includes weaker events. The wind relaxations extend ∼600 km offshore, similarly to the California-scale hydraulic expansion fan shaping the prevailing winds, and ∼1000 km alongshore, limited by an opposing pressure gradient force at Cape Mendocino.