Conservative Surface Wave Effects on a Wind-Driven Coastal Upwelling System

Abstract

Upwelling brings deep, cold, and nutrient-rich water to the euphotic zone, enhancing biological primary productivity. Coastal upwelling is affected by various factors, such as winds, topography, and tides. However, it remains unclear how the upwelling is affected by surface waves, particularly the Stokes drift and its related forces, that is, conservative wave effects. Here using a coupled wave–circulation model, we examined how conservative wave effects impact the wind-driven coastal upwelling system over an idealized continental shelf. We showed that conservative wave effects reduce upwelling but enhance downwelling; consequently, the amount of deep cold water brought up to the surface by upwelling is reduced with waves, leading to a weaker upwelling front than that without waves. Conservative wave effects also change the potential vorticity (PV) fluxes across the sea surface/bottom and alter the thickness of surface/bottom negative-PV layers. In addition, conservative wave effects modify the turbulent thermal wind (TTW) associated with the upwelling front, forming a Stokes–TTW balance. Further, we studied sensitivities of the upwelling and downwelling magnitudes to four parameters: wave height, wind stress, shelf slope, and wave incident angle. We combined these parameters into a single nondimensional number that can indicate when conservative wave effects need to be included in the upwelling and downwelling.