The Effects of Spatially Variable Wind Forcing on Freshwater Transport in a Buoyancy-Driven Coastal Current

J. Rogers-Cotrone, A. Yankovsky
Department of Geological Sciences, University of South Carolina, Columbia, South Carolina, USA

T. J. Weingartner
Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska, USA

The Alaska Coastal Current (ACC) is located in a region with prevailing downwelling favorable winds, flows over a long stretch of coastline (over 1000 km), and is driven by multiple sources of freshwater discharge totaling 23000 m3 s-1 along its length. Previous studies of wind effects on buoyancy-driven coastal currents have focused on single sources of freshwater and spatially-uniform winds. Using the Regional Ocean Modeling System (ROMS) we attempt to determine how spatially variable winds affect the downstream transport of freshwater along a long coastline with nearly continuous sources of freshwater.

Our model domain is 500 km long and extends 80 km offshore with a bottom topography representative of the ACC region. Ten sources of freshwater are evenly spaced at 25 km intervals along the middle 225 km of the domain, each with a discharge of 200 m3 s-1. This domain represents a fraction of the total length of the ACC and we use periodic boundary conditions, allowing water flowing through the downstream boundary to re-enter the domain at the upstream boundary, to mimic a continuous buoyant flow from outside of our domain. Both constant and spatially varying, predominantly downwelling favorable winds are applied over the domain. The spatial variations of wind are introduced as one period of a harmonic function. Freshwater gain in the coastal current through the buoyancy forcing region is calculated by taking a 30-day averaged difference between freshwater fluxes at the downstream and upstream edges of this buoyancy forcing region.

Results from several runs are split into two categories based on this freshwater gain. Values of gain are relatively high (between 1000 m3 s-1 and 1300 m3 s-1) for all runs with moderate average wind stress (~ 0.05 pa), regardless of its spatial variations, as well as for the case of no wind forcing. Values of gain are nearly 50% lower (ranging from 500 m3 s-1 to 800 m3 s-1) for runs with average wind stresses of about 0.025pa, especially when wind varied spatially along the coast line. Thus light and variable downwelling favorable winds can result in substantially lower freshwater gain than under no wind conditions. The reversal of wind to upwelling favorable conditions over limited fraction of coastline effectively blocks the downstream freshwater transport as expected. Possible mechanisms for this reduction in freshwater gain under lighter wind conditions are identified.