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 1993near the Aleutian Islands, net northward flow of Alaskan Streamwater occurred through deep passesnear 180and 172W. This inflow ( - 4 x lo6 m3s-l) wasthe sourceof the eastwardflow in the Bering Seanorth of the islands.The eastwardflow, however, wasweaker and more convoluted than the stream flow ( - 7 x lo6 m3s-l, referred to 1000db) southof 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 circulation 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

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

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 .

178'E 180" 178" 176' 1740 172" 17O"W

55"N

54"

52"

51°

50" 178”E 180" 178" 176" 174" 172" 17O"W Figure 4. Geopotential topography (dyn m) of (a) the sea surface, referred to 1000 db (decibars); (b) the 300-db surface, referred to 1000 db; and (c) the sea surface, referred to 300 db, 4-12 September 1993. 19941 Reed & Stabeno: Aleutian Ridge flow 645

Table 1. Volume transports above 1000 db, referred to 1000 db, 4-11 September, 1993. Westward transport Northward (southward) Eastward transport Stations ( lo6 m3 s-l) transport ( lo6 m3 s-r) ( lo6 m3 s-l) 3-8 4.9 10-14 7.3 Pi-20 7.4 23-27 27-28 (X) 29-33 1.5 33-39 0.6 52-53 (0.5)* 53-57 1.3* 61-64 3.0 *referred to 300db shallow pass west of the island (Fig. 2b). Examining flow at different surfaces with different reference levels clarifies the relevant features. The baroclinic flow in Amukta Pass is of interest, especially because such data have been lacking. In Figure 3c, geostrophic speeds vary considerably. Between stations 52 and 53, maximum southward speed was 31 cm s-l at 20 m, and flow was quite weak below 200 m. The next station pair had weaker flow with a reversal in direction at 100 m. Between stations 54 and 55, the maximum speed was 33 cm-l at 130 m, with weaker speeds above and relatively strong flow and shear near the bottom. Although northward flow of stream water in Amukta Pass was well developed during this cruise, it appears to be intermittent (Schumacher and Reed, 1992). We are aware of the deficiencies of use of the lOOO-db surface as a reference level in the Alaskan Stream. Although near-surface speeds are not greatly in error, upper-ocean transport may be only about one-third of values referred to the bottom (Reed, 1984; Warren and Owens, 1988). While use of the 1500-db surface would give larger transports, this surface is too deep for use in Amchitka Pass, and appreciable flow is often present inshore of 1500 m. We prefer to use a constant reference level and to avoid extrapolation of isopycnals. The reader is advised, however, to consult the comprehensive deep data and thorough analysis in Warren and Owens (1988). Also, recent deep data have been obtained near 180 (G. Roden, University of Washington, and K. Ohtani, Hokkaido University, personal communication). Table 1 gives computed volume transport, referred to 1000 db, except for the stations in Amukta Pass which were referred to 300 db (near bottom). Two of the Alaskan Stream sections had very similar transports (7.3 and 7.4 x lo6 m3 s-l), but the eastern section had a smaller value (4.9 x lo6 m3 s-i). The flow on this section was reduced by the return flow from Amukta Pass (Fig. 4c), but not enough to account for the entire difference in transport. At any rate, none of our values are significantly different than the mean (6.2 x lo6 m3 s-l) for this region of the stream from Favorite (1974). The northward flow through Amchitka Pass was 4.1 x 646 Journal of Marine Research [52,4

lo6 m3 s-l; some of this (1.3 x lo6 m3 s-l) recirculated to the south on the west side of the pass, giving a net northward flow of 2.8 x lo6 m3 s-l. To the north and east of the pass (stations 29-33 and 33-39), the total flow was 2.1 x lo6 m3 s-l. Of the four remaining sections north of the islands, only the section at - 171W (stations 61-64) actually crossed all of the convoluted eastward flow. The value there was 3.0 x lo6 m3 s-r, which is quite close to the net inflow at Amchitka Pass and is, with the addition of the Amutka Pass flow, also similar to the transport along the eastern slope of the Bering Sea (Schumacher and Reed, 1992). Finally, the net northward transport through Amukta Pass was 0.8 x lo6 m3 s-l. We have now obtained three Amchitka Pass sections, with essentially the same station spacing, in summers of 1991, 1992, and 1993. All three, plus two at 52N reported by Favorite (1974) showed the same pattern: a northward flow on the eastern side of the pass, and a southward flow on the western side. The net transports varied widely, however, with values of 2.8 x lo6 m3 s-i southward (Reed et al, 1993) 0.8 x lo6 m3 s-l northward (Reed and Stabeno, 1993) and 2.8 x lo6 m3 s-l northward (present study). It is also interesting that only the southward branch in 1991 had well-developed flow deeper in the water column than - 600 m (Reed et al., 1993). The peak geostrophic speeds that we have observed in Amchitka Pass were 30-40 cm s-l.

4. Drifter trajectories Two satellite-tracked drifter trajectories in the area of the study are shown in Figure 5. Drifter 35 was released by the Surveyor during this cruise, and drifter 38 was released near 156W on 27 April 1993 as part of another project. Drifter 38 initially moved southwestward near depths of 1000 to 1500 m at typical speeds of 50 cm s-l. In late May, it departed the slope and then moved to the northwest toward Amukta Pass. It is difficult to reconcile the northward drifter path with Figure 4, but the data are -3 mo apart. In early June, drifter 38 entered Amukta Pass and stayed in or northeast of the pass until 14 June, when it lost its drogue. The four clockwise rotations had an average period of - 2 days. Thus flow of stream water through Amukta Pass was present in June as well as during the cruise in September. Drifter 35 was launched in the stream, but it moved onshore and remained south of Atka Island for over 5 days. The well-developed clockwise rotating loops are of diurnal tidal period, the predominant period there (U.S. Department of Commerce, 1993). During 17-21 September, the drift speed was -40 cm ssl. In general, observed speeds in Amchitka Pass were - 25 cm s-l, which is between the weakest and strongest near-surface speeds on our section. The southward branch of flow on the western side of the pass was clearly present in agreement with Figure 4. A feature not apparent in Figure 4 is the eastward flow just south of our CTD section, which is perhaps a counterflow inshore of the Alaskan Stream (see Reed and Stabeno, 1993). Reed & Stabeno: Aleutian Ridge flow 647

52"

51"

I I I I I I I I I I I I I I ' 50" 178"E 180" 178" 176" 174" 172" 17O"W Figure 5. Trajectoriesof two satellite-trackeddrifting buoysduring 29 May-8 November1993. Both drifters lost their droguesjust after the last datesshown. The circlesindicate positions at four-day intervals, with month-day dates. Typically, - 12 satellite fixes per day were obtained,with a position error of - 0.2 km. Finally, the last week of the drifter path, although 2 mo later, was similar to the geopotential topography.

5. Conclusions Results from our synoptic data set along both the south and north sides of the Aleutians, as well as across the two deep passes, are generally definitive and unambiguous. The surface waters were less saline than normal, which led to relatively large values of geopotential anomaly. The subsurface warm water in the Alaskan Stream was also quite deep. The geopotential gradient across the stream, however, was fairly uniform, and maximum geostrophic speedswere only - 50 cm s-l. Geostrophic transports, referred to 1000 db, of 5, 7, and 7 x lo6 m3 s-l were not significantly different than a mean of previous data (Favorite, 1974). The flow across Amchitka Passwas northward on the eastern side and southward on the western side, which seems to be the typical pattern. The section across Amukta Pass showed water properties strongly modified by tidal mixing and a net northward flow of modified Alaskan Stream water. Low-salinity water also intruded northward through a shallow pass. The sum of the net inflows through the passes ( - 4 x lo6 m3s-l) is similar to typical transport along the eastern slope of the Bering Sea. Flow along the north side of the Aleutians in September 1993 was weak and convoluted, unlike the narrow, intense flows often revealed by drifter data (Stabeno and Reed, 1994). 648 Journal of Marine Research 15% 4

Acknowledgments. We thank the officers and crew of the NOAA ship Surveyor. We also thank J. Schumacher and C. Pease. We appreciate conversations with G. Roden (University of Washington). The data were taken by C. Dewitt, L. Lawrence, and C. Hadden. This is contribution FOCI-B206 to the Fisheries Oceanography Coordinated Investigations and is part of the Coastal Ocean Program of NOAA. Contribution number 1509 from NOAA, PMEL.

REFERENCES Dodimead, A. J., F. Favorite and T. Hirano. 1963. Review of oceanography of the subarctic Pacific region. Int. N. Pac. Fish. Comm. Bull., 13, 195 pp. Favorite, F. 1967. The Alaskan Stream. Int. N. Pac. Fish. Comm. Bull., 21, 20 pp. - 1974. Flow into the Bering Sea through Aleutian Island passes, in Oceanography of the Bering Sea with Emphasis on Renewable Resources, D. W. Hood and E. J. Kelley, eds., Inst. of Mar. Sci., Univ. of Alaska, Fairbanks, 3-37. Reed, R. K. 1984. Flow of the Alaskan Stream and its variations. Deep-Sea Res., 31, 369-386. Reed, R. K., G. V. Khen, P. J. Stabeno and A. V. Verkhunov. 1993. Water properties and flow over the deep Bering Sea basin, summer 1991. Deep-Sea Res. 40, 232.5-2334. Reed, R. K. and P. J. Stabeno. 1993. The recent return of the Alaskan Stream to Near Strait. J. Mar. Res. 51, 515-527. Sayles, M. A., K. Aagaard and L. K. Coachman. 1979. Oceanographic Atlas of the Bering Sea Basin. Univ. of Washington Press, 158 pp. Schumacher, J. D. and R. K. Reed. 1992. Characteristics of currents over the continental slope of the eastern Bering Sea. J. Geophys. Res., 97, 9423-9433. Stabeno, P. J. and R. K. Reed. 1994. Circulation in the Bering Sea observed by satellite- tracked drifters: 1986-1993. J. Phys. Oceanogr., 24, 848-854. Warren, B. A. and W. B. Owens. 1988. Deep currents in the central subarctic Pacific Ocean. J. Phys. Oceanogr., 18, 529-551. U.S. Department of Commerce. 1993. Tidal current tables 1994, Pacific coast of North America and Asia. U.S. Govt. Print. Off., 213 pp.

Received: I February, 1994; accepted: 24 February, 1994.