FY 1987

On the atmospheric and oceanic environment of the Gulf of Alaska

Schumacher, J.D., and J.G. Wilson

In Proceedings, A Workshop on Comparative Biology, Assessment, and Management of Gadoids from the North Pacific and Atlantic Oceans, Seattle, WA, June 1985, 135–178 (1986)

We review the state of knowledge of meteorology and physical oceanography in the Gulf of Alaska. We focus on processes that regulate transport, a factor of which likely impacts recruitment and is operative during early life stages when fish, some of their predators and all of their prey are planktonic. The passage of low pressure systems dominates the meteorology, with an average of one storm every four or five days during winter. Associated with these storms are winds up to 40 m s, persistent cloud cover, and up to 8 m of precipitation annually in the coastal mountains. Trajectories for low pressure systems are largely determined by the location and strength of three semi-permanent atmospheric features: the Aleutian low and Siberian high pressure systems in autumn, winter, and spring giving way to the east Pacific high pressure system in summer. Interaction between the high coastal mountains which ring the Gulf and the pressure systems results in significant local features including valley-drainage or katabatic winds, down pressure gradient or gap winds, and along-shore coastal wind jets. Both wind (coastal convergence) and freshwater addition (precipitation and runoff) are responsible for the dominant shelf current or the Alaskan Coastal Current. Because there is a longitudinal variation in wind characteristics, there is also a vast difference in upwelling between the coasts of the eastern and western Gulf of Alaska. Oceanographic features also vary east to west. The largest-scale feature is the offshore boundary current, the Alaskan Stream which is relatively wide and slow (~30 cm s) on the east side of the Gulf but narrows to less than 100 km near Kodiak Island and westward and has peak speeds of approximately 100 cm s. On the continental shelf there is a circulation system which is generally separate from the Alaskan Stream. The outer shelf often has weak net flow, but circulation seems to be steered by the bathymetry in some large troughs which transect the shelf. On the east side of the Gulf, the flow tends to be variable but is stronger in winter than summer as a result of local wind forcing. West of about 145°, there is a distinct narrow current flowing westword; it typically has speeds of 20 cm s but may have values in excess of 100 cm s in autumn. This rapid autumn spin-up is accompanied by surface salinities as low as 25 and results from a maximum in freshwater discharge in September-October. Winds act to constrain the reatively dilute flow along the coast. This geostrophic coastal flow enters Shelikof Strait, where the barotropic mode may be important, and continues west along the Alaska Peninsula. These features of coastal circulation are clearly seen in the seasonal cycles of sea level at tide stations. The large interannual changes that result from El Niño also penetrate poleward into the Gulf of Alaska.