Volume transport in the Alaska Coastal Current

James D. Schumacher, Phyllis J. Stabeno, and Andrew T. Roach

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Continental Shelf Research, 9(12), 1071-1083 (1989)
Not subject to U.S. copyright. Published in 1990 by Pergamon Press.

Introduction

During the past decade, knowledge of the physical oceanography of the Gulf of Alaska has grown substantially. Discovery of the Alaska Coastal Current (ACC), a narrow coastal jet extending more than 1000 km along the coast (Fig. 1), was one of the most important advances. This is a vigorous coastal current with speeds as large as 175 cm s . Transport is driven by the large flux of freshwater along the coast of Alaska (Royer, 1982). The alongshore wind perturbs this flow through both confinement of the freshwater and alteration of coastal sea level (Schumacher and Reed, 1980; Royer, 1981; Reed and Schumacher, 1981). Between Kodiak Island and the mainland, differential Ekman pumping generates fluctuations in transport (Reed and Schumacher, 1989). The maximum freshwater input is in autumn (Royer, 1982). Concomitantly, speeds in the ACC increase markedly and volume transport is 2-3 times as great as observed during summer, often exceeding 1.9 × 10 m s . After leaving the northeastern coast of Alaska, most of the ACC flows through Shelikof Strait, with a portion continuing westward to Unimak Pass (Schumacher and Reed, 1986). To date, all transport estimates have been based on hydrographic data which are sensitive to the selection of a level of no motion.

Figure 1. Study area setting. Positions of the nine moorings (dots) are indicated. Mooring numbers are consecutive but only outer ones are labeled. Shown in the insert is the regional circulation. Depths are in meters.

In this paper we present the first estimates of transport in the ACC computed using current and bottom pressure records. These records were collected between August 1984 and July 1985 in the western Gulf of Alaska (Fig. 1). This experiment was part of Fisheries Oceanography Coordinated Investigations (FOCI), a continuing NOAA program. The goal of FOCI is to understand biological and physical processes influencing recruitment of pollock (Theragra chalcogramma) in Shelikof Strait, Alaska. The objective of the research component presented here was to answer basic questions regarding characteristics of transport, including: what fraction of the transport is through the sea valley along the Alaska peninsula, and can transport be monitored with bottom pressure observations?

A description of current and bottom pressure data obtained during this experiment, together with surface winds, is presented in Roach et al. (1987). Reed et al. (1987) present a comprehensive analysis of hydrographic data collected in March and July 1985. The focus of this paper is to describe transport, to examine mechanisms causing fluctuations, and to investigate geostrophy.

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