National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2007

Idealized three-dimensional modeling of seasonal variation in the Alaska Coastal Current

Williams, W.J., T.J. Weingartner, and A.J. Hermann

J. Geophys. Res., 112(C7), C07001, doi: 10.1029/2005JC003285 (2007)

Seasonal variation of the buoyancy- and downwelling-wind-forced Alaska Coastal Current (ACC) and the fate of freshwater contained in it is considered using idealized analytical and numerical models of the ACC formed from a half-line source of buoyant inflow. The coastal current initially develops two-dimensionally but becomes three-dimensional from a balance between coastal influx of buoyancy and its downstream transport, which leads to a coastal current depth limit Hmax = (2Qf / g′)1/2, where x is along-shelf distance, Q is the line source strength for unit length, f is the Coriolis frequency, and g′ is the reduced gravity of the buoyant inflow. This limit is unchanged under downwelling wind stress and is reached on timescales of less than 1 month for the ACC. The coastal current width is roughly constant in x and increases in time at the same rate as the two-dimensional solution. Imposition of a downwelling wind stress τ results in an approximate balance among wind stress and along- and cross-shelf momentum advection so that the current width is reduced to YwindLD (Qf / τ/ρ0)1/2, where LD is the Rossby radius of deformation, τ is the wind stress and ρ0 is a reference density. Waves/eddying motions eventually grow in the half-line source coastal current with wavelengths proportional to the coastal current width and with a downstream phase speed slower than the maximum current speed. These features cause an offshore flux of buoyant water, a broader coastal current, and further accumulation of buoyancy on the shelf. Increasing downwelling wind stress reduces the effects of the instabilities. Continual accumulation of buoyancy on the shelf occurs during all model runs but is nearly absent under maximum winter downwelling wind stress. It is suggested that freshwater accumulation on the shelf during spring, summer, and fall may be largely lost downstream during winter.

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