Flow Variability within the Alaska Coastal Current in winter

Nine moorings were deployed on the shelf in the northern Gulf of Alaska in October 2012 and recovered in March 2013. All moorings were located southeast of Kayak Island and placed within the Alaska Coastal Current (ACC) which is a major circulation feature on the Alaskan shelf. The ACC propagates over hundreds of kilometers along the coast before entering the Bering Sea. Hydrographic, bottom pressure, and velocity observations from the moorings depicted well the winter variability of the ACC. Atmospheric observations from Kayak Island and an offshore buoy showed a net loss of heat from the ocean to the atmosphere and indicated that storms frequently passed over the area. As a result of the intense surface cooling and vigorous wind mixing, the waters were weakly vertically stratified. On occasions when downwelling-favorable winds associated with storms relaxed, fresh water moved offshore and stratification was re-established. Consequently, near-shore waters were less dense, i.e., cooler and fresher than offshore waters resulting in a cross-shelf density gradient that contributed to the along-shelf flow generating near-surface currents of ~ 10 cm/s. Bottom pressure anomalies were mainly related to water-level fluctuations and indicated that there was also a cross-shelf gradient that frequently generated currents over 20 cm/s. Current observations often showed down-shelf nearly-barotropic subtidal flows with speeds of ~ 40 cm/s throughout the water column with occasional full or partial water-column direction reversals lasting for several hours. Observations also indicated that along-shelf flow fluctuations were primarily driven by the cross-shelf pressure gradient and not wind stress. Evaluated terms of vertically averaged momentum equations, which fairly well approximated flow dynamics within the ACC in winter, showed a dominance of the cross-shelf pressure gradient that was mainly balanced by the Coriolis term.