How transient winds produce coastal countercurrents along a straight coastline: A process‑oriented modelling study

Coastal countercurrents (CCCs) are swift alongshore currents that flow opposite to the prevailing wind direction. In the world’s major upwelling regions, CCCs often develop after weakening or reversal of upwelling-favourable coastal winds. Using a process-oriented modelling approach, this paper explores the CCC development after a spatially confined upwelling-favourable wind event for a simplified coastal ocean with a straight coastline. Findings show that wind relaxation after a 5-day upwelling event can produce a swift CCC, ~ 5 km in width and ~ 0.3–0.4 m/s in speed, extending ~ 150 km along the coast. The development of such CCCs relies on two dynamical features. Firstly, spatially varying offshore Ekman transport near the upwind margin of wind field creates an alongshore pressure-gradient force that opposes the alongshore wind-stress force. After wind relaxation, the unbalanced alongshore pressure-gradient force provides the initial acceleration of the CCC. Secondly, the gravitational-rotational adjustment of the body of upwelled dense water induces a negative sea-level anomaly that sustains the alongshore pressure-gradient forcing of the CCC. Findings of this paper supplement previous theories on the formation of CCCs and enhance the understanding of the functioning of coastal upwelling systems.