Winter circulation in the Gulf of Alaska

The steady-state winter circulation in the Gulf of Alaska is investigated and related to the main thermohaline features. The vertical stratification consists of three principal layers: a cold and low-salinity surface layer in the upper 100 meters, a warmer halocline layer between 100 and 200 meters, and a deep layer below. The halocline layer is noted for a complicated temperature and velocity structure. In the west-central gulf, upward motion of cold and saline water is caused by a strong divergence of the Ekman transport. Vertical velocities at the top of the halocline are of the order of 10−3 cm sec−1. Total mass transports between the surface and the bottom vary around 106 g cm−1 sec−1. In the semi-enclosed northern part of the gulf, counterclockwise eddies are found, with westward transports through Shelikhof Strait. Between latitudes 50° and 55°N, total transports are predominantly westward and represent return flow from eastward transports further south. The geostrophic flow in winter is strongly influenced by the wind, the barotropic and baroclinic modes being of the same order of magnitude. Surface flow has characteristic speeds between 10 and 20 cm sec−1 and is counterclockwise when an atmospheric low pressure dominates the gulf. The velocity structure with depth often shows a maximum in the halocline. This provides a mechanism for the formation of warm or cold water cores, which are frequently observed there. The assumptions and limitations of the underlying method are discussed.