On the dynamics of the Alaska coastal current

Abstract

The Alaska Coastal Current (ACC) in the northern Gulf of Alaska is a wind- and buoyancy-driven near-surface jet primarily maintained by the horizontal salinity gradient due to fresh water entering at the coast. It serves as the major source of fresh water to the North Pacific Ocean. The buoyancy driving force is the major focus of this investigation. The study area is situated just "downstream" of Prince William Sound (PWS), a large estuary whose surface outflow is seen to occupy a narrow inshore band after joining the ACC. The effect of this band appears to be the formation of an occasional double maximum in the ACC. The period focused on in this study was selected on the basis of weak windstress but large fresh water input in order to emphasize the buoyancy forcing. The TS characteristics and a water mass tracing technique are used to separate the thermal and haline signals in the buoyancy forcing and to track the origin and fate of the source waters of the study area. The buoyancy driving force is shown to be primarily haline, with temperature playing a secondary, moderating role. Because of the large topographic variability and sloping density interfaces, and in order to exploit the available data, a diagnostic model retaining the baroclinicity and bottom topography terms was chosen to study the dynamics. Model premises are verified by results from hydrographic surveys, moored current meters, and a profiling current meter. The model predicts a midshelf region of negligible sealevel gradient, with a nearshore ($\approx$70 km wide) band over which the sealevel changes by about 25 cm. The sloping surface drives a strong ($\approx$100 cm/s) surface flow, which decreases to zero and reverses below about 100 m due to the opposing baroclinic pressure gradient. The flow splits around a shoal region. The onshore portion joins the outflow from PWS and accelerates downstream forming a double maximum. The offshore segment forms a large meander before rejoining the rest of the ACC, advecting midshelf water shoreward. The momentum balance is dominated by the JEBAT terms, which primarily determine the flow along and across contours of f/H.