The impact of spatial wind variations on freshwater transport by the Alaska Coastal Current

摘要

The Alaska Coastal Current (ACC) is located in a region with prevailing downwelling-favorable winds, flows over a long stretch of coastline (over 2000 km), and is driven by Multiple sources of freshwater discharge totaling 24000 m(3) s(-1) along its length. 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. The model domain represents a fraction of the ACC region and periodic boundary conditions are applied to allow propagation of the buoyant flow from upstream. The model is forced by Multiple freshwater sources in the central part of the domain and by both constant and spatially-varying, predominantly downwelling-favorable, winds. Freshwater flux gain in the coastal current (as opposed to spreading offshore) is calculated by taking a 30-day averaged difference between freshwater fluxes at the downstream and upstream edges of the buoyancy forcing region. Model runs are split into two categories: relatively high gains (50-60 of total discharge) were observed under moderate wind stress (similar to 0.05 Pa) or no wind conditions while lower gains (35-45) were observed under light average wind stresses (similar to 0.025 Pa), especially when wind varied alongshore. The offshore freshwater, transport is eddy-driven and is enhanced in the areas of converging wind forcing. Eddy generation is associated with the wind-induced deepening of the buoyant layer near the coast. When the surface boundary layer is thin under light wind conditions, this deepening translates into enhanced vertical shear of the alongshore current through the thermal wind balance, Reversal of alongshore wind to upwelling-favorable wind effectively blocks the downstream freshwater transport and spreads the buoyant layer offshore.