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Flow Along and Across the Aleutian Ridge

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Flow Along and Across the Aleutian Ridge Journal of Marine Research, 52, 639-648,1994 Flow along and across the Aleutian Ridge by R. K. Reed’ and P. J. Stabeno’ ABSTRACT During a synoptic hydrocast survey in September 1993 near the Aleutian Islands, net northward flow of Alaskan Stream water occurred through deep passesnear 180 and 172W. This inflow ( - 4 x lo6 m3s-l) was the sourceof the eastwardflow in the Bering Sea north of the islands.The eastward flow, however, was weaker and more convoluted than the stream flow ( - 7 x lo6 m3s-l, referred to 1000db) south of the islands. 1. Introduction The ridge formed by the Aleutian Islands has a major effect on ocean circulation in the region. The Alaskan Stream is constrained to flow westward along the south side of the ridge, but branches of it move northward into the Bering Sea through the deeper passes(Amchitka and Amukta, Fig. 1; Reed et al., 1993). We present results from a synoptic hydrocast survey of this region in September 1993. This work was part of the Fisheries Oceanography Coordinated Investigations, an element in the Coastal Ocean Program of NOAA. Our emphasis is on understanding effects of the environment on pollock stocks. In August 1991, a survey of most of the deep Bering Sea basin was conducted; in September 1992, we investigated circula- tion near the western Aleutian Islands. The present study focused on flow near the central Aleutians, with particular emphasis on flow through the deep passes and along the north side of the islands. The work was also in preparation for deploying current moorings at critical sites to monitor flow likely to impinge on the eastern slope-shelf where pollock spawning occurs. During 4-12 September 1993, a total of 68 CTD (conductivity, temperature, depth) castswere taken from the NOAA ship Suweyor in the area shown in Figure 1. A Seabird SBE-9 CTD was used, and data were recorded on disk during the downcast to near bottom or a maximum depth of 1500 m. As determined from samples taken on each cast, no salinity corrections were necessary. Data were averaged over l-m intervals, and these values were used to compute density and geopotential anomaly. Data from two satellite-tracked drifting buoys (made by 1. National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, 7600 Sandpoint Way NE, Seattle, Washington, 981150070, U.S.A. 639 640 Journal of Marine Research 15~4 55"N Bering Sea 52" .,* 51" I I I I I I I I I I I I I 50" i 7aoE 1800 1780 176" 174" 172" 17O"W Figure 1. Location of CTD casts taken during 4-12 September 1993. The 200- and 1000-m isobaths are from National Ocean Survey chart 513. Oceanroutes Seimac, with tristar drogues at 40 m) were also used. The ship did not have an acoustic Doppler current profiler. 2. Physical properties The existence of relatively warm water with low surface salinity is a characteristic of the upper Alaskan Stream in the nearshore, high-speed region (Favorite, 1967; Reed, 1984). Figure 2 shows the depths to which waters warmer than 4°C extend and the distribution of surface salinity. The depth of 4°C waters was > 400 m just south of the Aleutians and west of 173W (Fig. 2a); farther offshore, depths decreased to ~200 m. The deep warm water extended westward into Amchitka Pass, but depths north of the islands, in the Bering Sea, were mostly < 200 m. The abrupt attenuation of warm water north and east of the pass is striking. Finally, the area near Amukta Pass had 4°C water generally deeper than the inflowing source water to the east. The sea-surface salinity distribution (Fig. 2b) had weak horizontal gradients, except south of the Aleutians near 169.5W. Alaskan Stream waters mainly had values near 32.0%0; on part of the western section and in Amchitka pass, however, values were about 32.7%0. North of the islands, salinity was quite uniform at 32.8-32.9%0, except on the easternmost section where values were -32.4%0. Both south and north of the islands, values were - 0.5%0 less than typical conditions (Dodimead et al,, 1963; Sayles et al., 1979). Reed & Stabeno: Aleutian Ridge flow 641 55"N Depth (m) of 4% isotherm 54" 52' 51" 50" 178"E 180" 178" 176" 174" 172" 17O"W 55"N Surface salinity (%o) 52" 51" (W I I I I I I I I I I I I I 50" 178"E 180" 178" 176" 174" 172" 17O"W Figure 2. (a) Depths (m) to which the 4.OO”C isotherm extend and (b) sea-surface salinity (%o) distribution, 4-12 September 1993. Stations 52-55 comprise a section across Amutka Pass, and temperature, salinity (which largely controls density), and geostrophic flow are shown in Figure 3. Figure 3a indicates that the vertical gradient of temperature in Amukta Pass was small; in fact, the decrease of temperature from the surface to the bottom was 2.1-2.6”C, except at station 54 where it was 3.3”C. Figure 3b also shows small vertical salinity changes (O&1.0%0, except 1.6%0 at station 54). The vertical ranges of temperature and salinity at station 54 are nearly the same as those at station 2, on the outer shelf to the east of the pass and the apparent source of pass waters (see 642 Journal of Marine Research 15274 52 53 54 55 0 0 m m (4 Figure 3. Vertical sections of (a) temperature (“C), (b) salinity (SO), and (c) geostrophic flow (cm s-l), referred to the deepest common level, across Amukta Pass, 10 September 1993. On (b), depth intervals where sigma-t density increased downward by <O.OlO units per 10 m, or decreased by <O.OlO units per 10 m, are shown by open or shaded rectangles, respectively. On (c), the shaded areas indicate southward flow; otherwise flow was north- ward. 19943 Reed & Stabeno: Aleutian Ridge Jlow 643 Fig. 4~). We also show in Figure 3b zones of very small vertical gradients of sigma-t density, which imply mixing. The homogenization of water properties in Amukta Pass, except at station 54, is believed to result from tidal mixing. This process also seems to occur in Amchitka Pass (about three times deeper than Amukta Pass), where tidal flow is -40 cm s-i (Reed et al., 1993). Only two tidal current measurements in the vicinity of Amukta Pass are available (U.S. Department of Commerce, 1993); one site was in shallow water on the east side of Seguam Island (see Fig. 3) and the other was 2 km east of Yunaska Island (southeast of station 58, see Fig. 1). The two measurements gave maximum tidal flows of N 70 and 100 cm s-l, respectively, but extrapolating these flows into the pass is dubious. It is not clear why waters at station 54 were little affected by mixing, but elsewhere tidal mixing seems to account for the warm deep waters and relatively high surface salinity seen in Figure 2. 3. Geostrophic flow Figure 4 shows the geopotential topography of the sea surface (referred to 1000 db), of the 300-db surface (referred to 1000 db), and of the sea surface (referred to 300 db). As shown in Figure 4a, the Alaskan Stream was well-developed south of the Aleutians, especially west of 173W. The values of geopotential anomaly were -0.05 dyn m greater than in Dodimead et al. (1963), mainly as a result of the relatively low near-surface salinity noted above, and the relief across the flows south of Amlia and Tanaga islands was also rather large. The maximum geostrophic speed (44 or 50 cm s-l, lOO/lOOO or 100/1500 db, stations ll-12), however, was low compared to most previous data because the gradient across the flow was fairly uniform rather than concentrated on the inshore side (Reed, 1984). A substantial part of the stream shoaler than 1000 m moved northward into Amchitka Pass, but some of this flow turned back to the south. The flow continuing along the north side of the islands was weak and convoluted except on the section off - 171W. We expected a more narrow, continuous, intense flow along the slope based on results from drifter trajectories (Stabeno and Reed, 1994) but the lack of agreement suggests that considerable variability exists. The flow at 300 db (Fig. 4b) was similar in direction to that at the sea surface but was appreciably weaker. At 300 db, the counterclockwise rotation at the surface southwest of Umnak Island was absent, and the strong northward flow tendency north of the island was greatly reduced. To examine flow in Amukta Pass, we used the 300-db surface (Fig. 4~). This map shows a well-developed westward flow that entered the pass; some of this inflow moved back to the east, however. This latter feature explains the high surface salinity at stations 3 and 4 ( > 32.5%0) in Figure 2b and the southward flow in Figure 3c. The clockwise circulation feature near 52N, 169W was a near-surface feature that was absent below 40 db. The Amukta Pass inflow moved eastward and northward, and there was additional northward flow off Unmak Island as a result of low-salinity shelf water moving northward through the i X.\ 55"N : I... I I I I I I I I I I I I AD O/1000 db l.Pj 1 (a), I I I I I I I I I I I I 50" 178"E 180" 178' 176" 174" 172’= 17O”W AD 300/1000 db .
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