JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. C9, PAGES 18,243-18,254,SEPTEMBER 15, 1994 A census of mesoscale eddies in Shelikof Strait, Alaska, during 1989 Steven J. Bograd • Joint Institute for theStudy of theAtmosphere and Ocean, University of Washington, Seattle PhyllisJ. Stabeno andJames D. Schumacher NOAA Pacific MarineEnvironmental Laboratory, Seattle, Washington Abstract. Over a 5-monthperiodin the spring and summer of 1989, 12 mesoscale eddies were identified crossing a section of moored current meters in theShelikof sea valley. This represents the first census of mesoscale eddies on the northwestern Gulf of Alaska continen- tal shelf. The majority of eddies (7) wereobserved in May and June. Anticyclonic eddies outnumbered cyclonic eddies two to one,but all had characteristics similarto eddies previously observed in theregion. Analysis of the current meter time series revealed thatthe mean speeds, volume transport, and the structure of theflow at thepresent location were similar to current meter and hydrographic measurements during other years in thesea valley. We suggest that 1989wasa typical yearin terms of eddy formation aswell. The proliferation of mesoscale eddies in thisregion in spring has an impact on thelarvaldispersal andeventual recruitment of walleye pollock. Introduction Mesoscale eddies have been observed within coastal current systems on numerous continental shelves, including within the Benguela Current [Bang, 1973], the FloridaCurrent [Leeand Mayer, 1977], the NorwegianCoastal Current[Johannessen et al., 1989], andat theIrishShelf front[Huang et al., 1991]. Their length and timescales vary geographically but are typically between 10 and50 km and 1 and4 weeks, respectively. Meso- scale eddiescan contribute to the retentionor dispersal of planktonic organisms [e.g., Denman and Powell, 1984; Lobel andRobinson, 1986]and thebiological productivity of coastal regions [e.g., Owen, 1981; Rey, 1981 ]. On the Gulf of Alaska continental shelf, mesoscale eddies have been observed in satellite imagery, water property data and satellite-tracked buoy trajectories [Schumacher et al., 1991; Vastano et al., 1992; Schumacher et al., 1993]. Their dimen- sions and physicalcharacteristics are similar to other shelf eddies [Schumacher et al., 1993]. High concentrations of walleye pollock (Theragra chalcogramma) larvae have been observed within these eddies [Schumacheret al., 1993]. An objective of National Oceanic and Atmospheric Admini- stration's (NOAA) Fisheries Oceanography Coordinated Investi- gations (FOCI [Reedet al., 1988; Schumacher and Kendall 1991 ]) is to understand how features of the physical environ- ment, includingeddies, affect recruitment into the walleye pollock fishery in the northwestern Gulf of Alaska. •Now atDepartment of Oceanography, University of British Columbia, Vancouver, Canada. Copyright 1994by theAmerican Geophysical Union. Paper number 94JC01269. 0148-0227/94/94JC-01269505.00 The relatively small dimensions of the eddies (10-20 km radius) previously observed in thevicinityof Shelikof Strait, the generally large-scale waterproperty grids occupied each spring as part of FOCI's field operations, and the limited remote- sensing coverage of the region (dueto cloud cover)suggest that eddies are grossly undersampled here. Eddyobservations have reliedon serendipity, andit remains unknown how manyeddies form each year.FromApril 26 to September 23, 1989,a section of current meter moorings was in the Shelikof sea valley (Figure 1). Thepurpose of this paper is to use these data to count and describe themesoscale eddies crossing themooring section, thus providingthe first census of mesoscale eddieson the northwestern Gulf of Alaskacontinental shelf. We first provide a background description of the springtime environment in the region, and then describe the characteristics of the flow and provide theeddy census. Finally,we discuss thepossible effects of eddies on volume transport andlarval dispersal. OceanographicSetting The Shelikof seavalley refersto the straitof water between Kodiak Islandandthe Alaska Peninsula, andthe conduit (40-80 km wide and >200 m deep) that extends to the shelf break (Figure 1). The dominant circulation featureon the shelf is the Alaska Coastal Current(ACC), a narrow(<25 km), vigorous, low-salinity current driven by a line source of freshwater input along the Gulf of Alaska coastline [Royer, 1981]. The mean annual transport of the ACC in Shelikof Strait is just under 1 x 10 6 m 3 s -1 to thesouthwest, withabout 75%of thetotal volume either remaining on or recirculating backonto the shelf, and therest joiningtheoceanic Alaskan Stream [Schumacher et al., 1989]. During spring, the meanbaroclinic transport is -•0.6 x 10 6 m 3s -1[Reed and Bograd, 1994]. Winds can modify transport of the ACC, resultingin pulses which can exceed 3 x 10 6 m 3 s -1. Theflowin thesea valley is a two-layer estuarinelike circulation, with the ACC in the upper150 m and 18,243