Flow variability within the A laska C oastal C urrent in winter

Abstract

Coastal circulation off Kayak Island in the northern Gulf of Alaska was explored in wintertime (October 2012-March 2013) by deploying nine moorings within the Alaska Coastal Current (ACC). Hydrographic, bottom pressure, and velocity observations depicted well the winter variability of the ACC. Atmospheric observations showed a net loss of heat, 30 W m−2 or more, from the ocean to the atmosphere and indicated that storms with downwelling-favorable winds over 10 m s−1 frequently passed over the area. Due to vigorous mixing during storms, the waters were well-mixed or weakly stratified whereas bottom-pressure anomalies were mainly related to surface-elevation fluctuations and indicated that there was also a cross-shelf surface-elevation gradient. Current observations showed along-shelf nearly-barotropic subtidal flow of 40 cm s−1 or more throughout the water column. They also indicated that along-shelf flow was primarily driven by the cross-shelf pressure gradient resulting from the cross-shelf surface-elevation gradient and not by wind stress. Analyses suggested that flow dynamics within the ACC in winter were well-described by vertically-averaged momentum equations and showed a dominance of the cross-shelf pressure gradient that was mainly balanced by the Coriolis term. Observations also showed that when winds relaxed, cold low-salinity waters moved offshore and stratification was reestablished. Consequently, near-shore waters were less dense, i.e., cooler and fresher than offshore waters resulting in the cross-shelf density gradient that may have contributed to the along-shelf flow by generating near-surface currents of ∼ 20 cm s−1. This article is protected by copyright. All rights reserved.