Circulation and hydrography in the northwestern Gulf of Alaska

Abstract

Hydrography and satellite-tracked drifters from the Gulf of Alaska Recirculation Study (GARS) were used to describe the regional circulation from 1986 to 1989 in the northwest Gulf of Alaska. The average baroclinic transport (0/1000 db) from six occupations of a section across the Alaska Stream near the Shumagin Islands (55°N, 160°W) was 7.4 Sv. The seasonal variation in the transport of the Alaska Stream was negligible relative to the seasonal variation of the Sverdrup transport in the Gulf of Alaska as calculated from the wind-stress curl. However, the mean transport agreed with mean annual Sverdrup transport. Anticyclonic mesoscale eddies frequently appeared in the dynamic topography and hydrography along the easternmost sections of the cruise grid (140°W). Drifters released in the Alaska Stream offshore of Kodiak Island usually moved southwestward following the isobaths. However, the trajectories of four drifters from 1988 to 1989 described an anticyclonic meander in the Alaska Stream that propagated southwestward at about 0.022 m s −1. The hydrography confirmed the existence of the meander off Kodiak Island in April 1988. The temperature and salinity characteristics of the anticyclonic meanders and eddies indicated that the water masses at the center of the features were derived from Alaska Current water. Two mechanisms of enhanced vertical mixing in the Gulf of Alaska are suggested by the hydrography. The first one is due to wind mixing of the water column by intense winter storms and subsequent outcropping of the 26.8 δ φ isopycnal surface in the center of the Alaska Gyre: a mechanism originally proposed by Van Scoy et al. (Journal of Geophysical Research, 96, 16,801–16,810, 1991). The second one is associated with fine structure in the temperature and salinity profiles centered at a density of 26.8 δ ϑ , the approximate density of North Pacific Intermediate Water. These mechanisms freshen the water on the 26.8 δ gj isopycnal surface. Subsequent lateral mixing on isopycnal surfaces by mesoscale eddy activity may contribute to the low salinity signature of the North Pacific Intermediate Water.