HF Radar Observations of Pressure-Driven Coastal Flows Opposing the Prevailing Winds

The coastal ocean circulation at the northern end of the Southern California Bight and along the central California coast is strongly wind forced. Winds are predominantly from the northwest with seasonally dependent mean velocities in the range of 7 – 10 m/s, setting up equatorward flows. Recent studies have documented intermittent poleward flows along the coast of the northern bight and central coast during wind relaxation events. The existing studies are largely descriptive.

This work quantifies alongshore pressure gradients set up by the generally strong equatorward winds. The work is based primarily on observations from an array of high frequency (HF) radars measuring ocean surface currents and from moored current meters. The HF radars provide the extensive spatial coverage required for observing the evolving current patterns as the winds relax. Alongshore pressure gradients are quantified with an array of bottom pressure sensors and regional tide gauges. The poleward relaxation flows are shown to be pressure driven. The more interesting result is the pressure force set up and the associated relaxation flow structure. The along coast pressure force at the time of a poleward relaxation flow is related to the equatorward wind strength during the 2-3 day period prior to the relaxation. Exceptionally strong equatorward winds set up a sufficiently strong pressure gradient as to force offshore flow at Pt. Conception, that is driven by inertia. Preconditioning by relatively weaker winds gives rise to a weaker pressure gradient that results in poleward coastal flows that round Pt. Conception, California and stay closer to the coastline. The pressure driven poleward flows can oppose the wind force in both cases. The work has implications for larvae and pollutant transport, and shows the importance of the local wind field for getting intermittent coastal poleward flows correct in numerical model simulations.