- Email: [email protected]
On the climatological mean circulation over the eastern Bering Sea shelf
ConrinentalSh~lfResearch.
Pergamon
Vol. 16, No. 10. pp. 1297-1305,1996 Published by Elsevier Science Ltd Printed in Great Britain
027tW343(95)00067-4
On the climatological
mean circulation over the eastern Bering Sea shelf
R. K. REED*
and P. J. STABENO*
(Received 22 February 1995; in revised form 16 June 1995; accepted 9 August 1995)
Abstract-We derive climatological mean summer circulation over the eastern Bering Sea shelf. Geostrophic flow (from CTD data, 1975-1989) and drifter velocities (from satellite-tracked buoys, 1986-1994) were used. The following features are shown: (1) in depths >lOO m, a northwestward flowof-4cms-I, which is largely baroclinic; (2) near the 50 m isobath, a flow of -2 cm s-’ , which is only partially baroclinic: and (3) a semi-permanent, convoluted flow of l-2 cm s-l, between the 100 and 50 m isobaths. that was not recognized in earlier analyses. Data from current moorings indicate that there is no significant tidal enhancement of net flow on the shelf as earlier suggested. This new climatology also shows clearly a divergence of the inflow through Unimak Pass, and it suggests that the shelf salinity distribution is influenced by advection as well as diffusion. Published by Elsevier Science Ltd
INTRODUCTION Recent results from NOAA’s Fisheries Oceanography Coordinated Investigations (FOCI) have shown high concentrations of pollock larvae in late spring over the eastern Bering Sea shelf. We have thus become interested in the possible impact of circulation on these larvae during summer following spawning. Consequently, new information on shelf circulation is derived, using both the geostrophic relation and direct measurements. Our intent was to provide a modern climatology of shelf circulation in summer. Schumacher and Kinder (1983) summarized many of the data over the shelf then available; direct current measurements and computed geostrophic flow were examined. Both results were comparable, although the periods of the two types of data were different; the current moorings were deployed year-long, mainly during 1976-1978 but also in 1980-1981, and the hydrocast data used were from summers of 4 yr (1975, 1976, 1980 and 1981). Coachman (1986) employed these data, plus some later results. A point of concern is that the relatively short duration and sparse area1 coverage of these data might result in means or other characteristics that depart from a long-term climatology. The existing analyses (Schumacher and Kinder, 1983; Coachman, 1986) indicated that circulation over the eastern Bering Sea shelf was quite weak. Nearshore, there was an along-isobath flow of -2 cm s-l; offshore of 100 m, net flows were toward the northwest at 2-5 cm s-’ (see Fig. 1). Between these regimes, a region of no net flow was inferred. *National Oceanic 98115. U.S.A.
and Atmospheric
Administration.
Pacific Marine Environmental
1297
Laboratory,
Seattle,
WA
R. K. Reed and P. J. Stabeno
1298
AD O/50 db (dyn . m x 103)
Aleutian Is.
&
1
I
I
I
I
I
I
I
I
I
I
1
I
,
I
I
160” 165’ 170°w Fig. 1. Geopotential topography of the sea surface (in dyn m x lo”), referred to 50 db, during summer (June-September), 1975-1989. The values listed are means for each 0.5” latitude x 1 .O” longitude area.
GEOPOTENTIAL
1
I
1
155
TOPOGRAPHY
We examine the geopotential topography of the sea surface, referred to 50 db (decibars), using climatological CTD (conductivity, temperature, depth) data from the National Oceanographic Data Center. The mean circulation during summer, soon after pollock larvae spawn, is derived. Data and methods Geopotential gradients here are quite small, necessitating use of CTD data only, which have standard errors from the method of Wooster and Taft (1958) of co.1 dyn cm. Because of our interest in summer conditions, data from June, July, August and September were used; geopotential anomalies were then averaged over 0.5” latitude and 1 .O”longitude areas (-50 x 50 km), and the final values were adjusted spatially, if needed, to be appropriate for the center of each area. After removal of casts with doubtful or incomplete data (<5%), a total of 1299 casts, during the period 1975-1989, were available. A reference level of 50 db is used because a substantial part of the area is little deeper than this. Offshore, some use is made of the 100 db level. Previous work here (Schumacher and
Climatological circulation-Bering
Sea shelf
1299
Kinder, 1983; and references therein) has shown the geostrophic relation to yield a good approximation to actual flow. Temporal variability occurs on many scales (see below), but it became clear that each cast is not an “independent” estimate. Although we do not know what the de-correlation scale might be, the following procedure was used: (1) data at time series stations at the same site over two days or less were averaged to yield one value; (2) a single “independent” estimate is an average of the values for each area for each month (June 1976, for example); and (3) at least five independent estimates, in at least three of the four summer months, were required for use of data in an area. The procedure would seem to be conservative and probably yields larger standard errors than really exist. The number of independent estimates for the 49 areas used are shown in Fig. 2(a). The standard errors (based on 1 S.D. ; 67% confidence limits) of the derived geopotential anomalies vary from 0.2 to 0.9 dyn cm, but only four are >O.S dyn cm [Fig. 2(b)]. The typical standard error in the anomaly difference between two areas is -0.5 dyn cm. Hence use of a contour interval of 1 dyn cm, as in Fig. 1, seems appropriate. Variability Both spatial and temporal variability occurs. We envision that temporal variability exists on many scales but may be prominent at daily, seasonal and year-to-year (interannual) intervals. Data at the 16 time series stations (noted above; 55-58”N, 162-168”W) can yield an estimate of approximate daily variability. The ranges of geopotential anomaly (maximum value minus minimum value) varied from 0.1 to 0.8 dyn cm; the mean range (f S.D.) was 0.3 + 0.2 dyn cm. Estimates of the variability during the four calendar months used are shown in Table 1. These are based on the observed geopotential ranges, not gradients, that occurred over various years in individual areas. Thus this variability is about four times greater than that on daily scales. Flow patterns The geopotential topography of the sea surface, referred to 50 db, is shown in Fig, 1. The maximum range (4.6 dyn cm) is across the northward flow near Unimak Pass. The inflow through the pass appears to diverge, with part continuing northward and part turning eastward. This had not been shown previously (Coachman, 1986. for example). The source of this inflow is the Alaska Coastal Current (Royer, 1981). Schumacher etal. (1982) reported a measured speed at 59 m (near bottom) in the pass (from 2.5 months in summer 1980) of 6 cm s-r; our surface geostrophic speed, referred to 50 db, is -5 cm s-‘. This comparison implies that surface flow in the pass may be only -50% baroclinic, with the remainder being barotropic (Schumacher et al., 1982). It should be noted, however, that conditions in 1980 may not have been typical of long-term mean conditions. Surface geostrophic speeds near the 50 m isobath (Fig. 1) are l-2 cm s-l toward the northeast and turning cyclonically toward the northwest, as were synoptic summer geostrophic speeds and limited current measurements at 20 m (Schumacher and Kinder, 1983). Figure 1 also shows a northwestward flow north of the eastern Aleutians and offshore of the 100 m isobath. (Its source appears to be the inflow through Unimak Pass and the northeastward flow along the north side of the Aleutians.) The geostrophic speeds here are 2-3 cm s-‘; use of 100 db as a reference level (not shown) approximately doubles
1300
R. K. Reed and P. J. Stabeno
6O’N
,6~~~6~~6./11~13~12! 9151 i 9 1 d ‘/i2i 9'1 81'101121 9,
55Q
Std. error (dyn m x 103) 6O’N
Bering Sea
t
~ 516 _
41
1
5
a 55"
17o*w
estimates of the O/50 db geopotential anomaly in each 0.5” Fig. 2. (a) Number of independent latitude by 1.0” longitude area, as defined in the text. (b) Standard errors of the 0150 db geopotential anomaly (in dyn m x 1O3: 67% confidence limits) in each area, during summer. 1975 1989.
the surface speed. These latter results are in general agreement with the baroclinic and measured speeds in Schumacher and Kinder (1983). Hence, the flow offshore of 100 m appears to be largely baroclinic. An unexpected feature in Fig. 1 is that shown by the path of the 14 dyn cm contour
Climatological circulation-Bering
Sea shelf
1301
Table 1. Estimates of the variability in geopotential anomaly (O/SOdb). based on data from multiple months, during various years, per area, sorted by calendar month
Month
No. of values
June July Aug. Sep.
43 21 35 30
Mean range f S.D. (in dyn cm) 1.2 + 1.5 f 1.1 f 1.4 f
0.6 0.9 0.8 1.1
between the 50 and 100 m isobaths. Although the maximum geostrophic speed is only -2 cm s-l , there is a clear indication of a flow that moves onshore near the Pribilof Islands, turns eastward, rotates cyclonically and then moves westward near 58”N. (The westward flow is not well resolved by these sparse data, with relatively large standard errors, however.) Although CTD data are generally limited in non-summer months, there were adequate data in February-April in the area 5657”N, 164-166”W to show significant eastward flow. Also, three synoptic surveys in 1976 (Reed, 1978) indicated eastward flow of l-2 cm s- ’near the feature in Fig. 1. This large-scale, albeit weak, circulation suggests a link or connection between the offshore and inshore flow regimes, even though they have been thought to be largely decoupled and to have no net flow between them (Schumacher and Kinder, 1983; Coachman, 1986). Coachman (1986) invoked a relatively large eddy diffusivity (5 x lo6 cm’s -‘) to explain the salinity distribution near the 100 m isobath. Our estimate of salinity advection, using the climatological surface salinity in Reed (1995), yields a value three times the salinity diffusion estimate with the coefficient of 5 x lo6 cm2 S -‘. Hence, observed property distributions seem likely to be partially influenced by cross-shelf advection.
DIRECT
CURRENT
MEASUREMENTS
Data from satellite-tracked drifting buoys over the shelf area are presented discussed. We also briefly examine some results from current moorings.
and
Data and methods Because of data scarcity, the “summer” period was extended, and satellite-tracked drifter data from mid-May to mid-October 19861994 were used. The buoys were drogued at 40 m; during 1986-1988, “holey sock” drogues were used, between 1987 and 1993, “tristar” drogues were employed, and in 1994 “holey sock” drogues were again used. An average of -12 satellites fixes were obtained daily, with a standard position error of -0.2 km. The methods used in deriving vectors are generally like those described by Stabeno and Reed (1994). The exceptions are that here a 4 day period within a grid area was considered an independent estimate, and the vectors have not been adjusted to the center of the areas. Finally, if the derived scalar speed did not exceed the standard error, or there were less than three independent estimates, vectors are not shown in Fig. 3. The number of independent estimates and the standard errors are shown in Fig. 4(a) and (b), respectively.
1302
R. K. Reed and P. J. Stabeno
Net Vectors (cm s-l) 60”N
*
ii
55”
I 0
I 10
cm
5.’
a I
1
I
I
17O”W Fig. 3.
Vector
/
1
I 165”
/
/
1
I
I
/
/
_
,
160”
net flow (according to the speed scale shown) for each 0.5” latitude longitude area, during 15 May-15 October. 198&1994.
155’ X
1.0”
Drifter velocities Much of the shelf area east of 165”W has inadequate data to show climatological net vectors (Fig. 3). On the other hand, there are significant data in Fig. 3 north and west of the CTD data in Fig. 1. Offshore of 100 m, between 55 and 58”N, Fig. 3 shows speeds of are similar to those from the typically 2-5 cm s-‘. Both the speeds and directions geopotential topography, especially when speeds are referred to 100 db (noted above), which again suggests largely baroclinic flow. The consistent flow along the shelf-slope region north of the Pribilof Islands was not shown in Fig. 1 because of a lack of CTD data. The direct measurements, however, do not indicate a westward flow near 58”N. As noted above, this feature is dubious. Data near the 50 m isobath east of 164”W (Fig. 3) yield typical speeds of -2 cm s-’ compared to our geostrophic speeds of l-2 cm s-l (noted above). Since the drogues were at -40 db, it would appear that a nonbaroclinic component of flow may occur near the 50 m isobath. Between the 50 and 100 m isobaths, Fig. 3 shows small and variable vectors that
Climatological
circulation-Bering
1303
Sea shelf
Standard error (cm s-l)
(W
17OQW
165"
160"
155"
Fig. 4. (a) Number of independent estimates, as discussed in Stabeno and Reed (1994) and in the text. (b) Standard error of the scalar net flow (in cm s -’ , 67% confidence limits). Data are during 15 May-15 October, 1986-1994.
are also somewhat larger than the geostrophic, baroclinic some data in Fig. 3 during May and October might be a Kinder (1983) concluded that there is an enhancement of summer months. These direct measurements, however,
speeds in Fig. 1. Inclusion of factor, since Schumacher and flow inshore of 100 m in nonsupport the existence of the
1304
R. K. Reed and P. J. Stabeno
convoluted eastward flow near 57”N in Fig. 1. Because of the various data sets showing this flow. we suspect that it may be somewhat stationary and permanent.
Tidal enhancement of net flows Schumacher and Kinder (1983) suggested that regions of the shelf near the 50 and 100 m by tidal enhancement (or isobaths might have net flows of l-4 cm s-’ generated rectification; Loder, 1980). As we have discussed, geostrophic flow inshore of 100 m is somewhat less than that derived from the drifter data. Hence a tidal enhancement of flow might reconcile this difference. Consequently, we examined the data from the current moorings (Schumacher and Kinder, 1983, plus two subsequent moorings) for any evidence of a tidal effect on the net flow. The hourly current records were demodulated to create tidal constituent (M,) time series. and estimates of correlation were calculated between these series and the low-pass filtered series (from which tides have been essentially eliminated; see Schumacher and Reed, 1992). The results are quite conclusive; only one of the 21 values (with a correlation of 0.32) exceeds the 95% significance level, and it explains only 10% of the correlated variance. One value should reach this level by chance. however, which suggests there is no significant tidal enhancement of net flow in this region.
CONCLUSIONS New climatologies of geopotential topography and direct (drifter) measurements have been presented. Although they substantiate some features found in earlier work, much of which was synoptic rather than climatological (long-term means), they also reveal the following new results: (1) An eastward flow exists between the Pribilof Islands and the 50 m isobath. Thus, the middle shelf is influenced by advection, as well as the inner and outer shelves. (2) Unlike previous work, our analysis indicates that tidal rectification does not generate net flow. The nature of the weak barotropic flow suggested is unclear. (3) The geopotential topography clearly shows a divergence of the Unimak Pass inflow, with part of the flow moving northwestward near the 100 m isobath. (4) The eastward how inshore of the Pribilof Islands suggests that the middle shelf is influenced by advection and that the salinity distribution does not result entirely from diffusive processes. AcknoM~ledgemenfs-We thank J. D. Schumacher for advice and encouragement. D. G. Kachel prepared the CTD data products. We also appreciate the assistance of S. Stillwaugh and others from the National Oceanographic Data Center. Comments by tworeviewers were quite helpful. This is contribution FOCI-B 229 to the Fishcries Coordinated Investigations and is part of the Coastal Ocean Program of NOAA. Contribution No. 1556 from NOAA/PMEL.
REFERENCES Coachman L. K. (1986) Circulation, water masses. and fluxes on the southern Bering Sea shelf. ConfinentalShelf Reseurch, 5, 23-108. Loder J. W. (1980) Topographic rectification of tidal currents on the sides of Georges Bank. Journal ofPhysical Oceanograph_v, 10,1399-1415.
Climatological
circulation-Bering
Sea shelf
1305
Reed R. K. (1978) The heat budget of a region in the eastern Bering Sea, summer 1976. Journul of Geophysical Research. 83.3635-3645. Reed R. K. (1995) Water properties over the Bering Sea shelf: climatology and variations. NOAA Technical Report ERL 452.PMEL 42,lS pp. Royer T. C. (1981) Baroclinic transport in the Gulf of Alaska. Part II. Fresh water driven coastal current. Journal of Marine Research, 39,251-266. Schumacher J. D., C. A. Pearson and J. E. Overland (1982) On exchange of water between the Gulf of Alaska and the Bering Sea through Unimak Pass. Journul of Geophysical Research, 87.5785-5795. Schumacher J. D. and T. H. Kinder (1983) Low-frequency current regimes over the Bering Sea shelf. Juurnulof Physical Oceanography, 13,607-623. Schumacher J. D. and R. K. Reed (1992) Characteristics of currents Over the continental slope of the eastern Bering Sea. Journal of Geophysical Research. 97,9423-9433. Stabeno P. J. and R. K. Reed (1994) Circulation in the Bering Sea observed by satellite-tracked drifters: 1986 1993. Journal of Physical Oceanography. 24,84&854. Wooster W. S. and B. A. Taft (19.58) On the reliability of field measurements of temperature and salinity in the ocean. Journal of Marine Research. 17, 552-566.
Pergamon
Vol. 16, No. 10. pp. 1297-1305,1996 Published by Elsevier Science Ltd Printed in Great Britain
027tW343(95)00067-4
On the climatological
mean circulation over the eastern Bering Sea shelf
R. K. REED*
and P. J. STABENO*
(Received 22 February 1995; in revised form 16 June 1995; accepted 9 August 1995)
Abstract-We derive climatological mean summer circulation over the eastern Bering Sea shelf. Geostrophic flow (from CTD data, 1975-1989) and drifter velocities (from satellite-tracked buoys, 1986-1994) were used. The following features are shown: (1) in depths >lOO m, a northwestward flowof-4cms-I, which is largely baroclinic; (2) near the 50 m isobath, a flow of -2 cm s-’ , which is only partially baroclinic: and (3) a semi-permanent, convoluted flow of l-2 cm s-l, between the 100 and 50 m isobaths. that was not recognized in earlier analyses. Data from current moorings indicate that there is no significant tidal enhancement of net flow on the shelf as earlier suggested. This new climatology also shows clearly a divergence of the inflow through Unimak Pass, and it suggests that the shelf salinity distribution is influenced by advection as well as diffusion. Published by Elsevier Science Ltd
INTRODUCTION Recent results from NOAA’s Fisheries Oceanography Coordinated Investigations (FOCI) have shown high concentrations of pollock larvae in late spring over the eastern Bering Sea shelf. We have thus become interested in the possible impact of circulation on these larvae during summer following spawning. Consequently, new information on shelf circulation is derived, using both the geostrophic relation and direct measurements. Our intent was to provide a modern climatology of shelf circulation in summer. Schumacher and Kinder (1983) summarized many of the data over the shelf then available; direct current measurements and computed geostrophic flow were examined. Both results were comparable, although the periods of the two types of data were different; the current moorings were deployed year-long, mainly during 1976-1978 but also in 1980-1981, and the hydrocast data used were from summers of 4 yr (1975, 1976, 1980 and 1981). Coachman (1986) employed these data, plus some later results. A point of concern is that the relatively short duration and sparse area1 coverage of these data might result in means or other characteristics that depart from a long-term climatology. The existing analyses (Schumacher and Kinder, 1983; Coachman, 1986) indicated that circulation over the eastern Bering Sea shelf was quite weak. Nearshore, there was an along-isobath flow of -2 cm s-l; offshore of 100 m, net flows were toward the northwest at 2-5 cm s-’ (see Fig. 1). Between these regimes, a region of no net flow was inferred. *National Oceanic 98115. U.S.A.
and Atmospheric
Administration.
Pacific Marine Environmental
1297
Laboratory,
Seattle,
WA
R. K. Reed and P. J. Stabeno
1298
AD O/50 db (dyn . m x 103)
Aleutian Is.
&
1
I
I
I
I
I
I
I
I
I
I
1
I
,
I
I
160” 165’ 170°w Fig. 1. Geopotential topography of the sea surface (in dyn m x lo”), referred to 50 db, during summer (June-September), 1975-1989. The values listed are means for each 0.5” latitude x 1 .O” longitude area.
GEOPOTENTIAL
1
I
1
155
TOPOGRAPHY
We examine the geopotential topography of the sea surface, referred to 50 db (decibars), using climatological CTD (conductivity, temperature, depth) data from the National Oceanographic Data Center. The mean circulation during summer, soon after pollock larvae spawn, is derived. Data and methods Geopotential gradients here are quite small, necessitating use of CTD data only, which have standard errors from the method of Wooster and Taft (1958) of co.1 dyn cm. Because of our interest in summer conditions, data from June, July, August and September were used; geopotential anomalies were then averaged over 0.5” latitude and 1 .O”longitude areas (-50 x 50 km), and the final values were adjusted spatially, if needed, to be appropriate for the center of each area. After removal of casts with doubtful or incomplete data (<5%), a total of 1299 casts, during the period 1975-1989, were available. A reference level of 50 db is used because a substantial part of the area is little deeper than this. Offshore, some use is made of the 100 db level. Previous work here (Schumacher and
Climatological circulation-Bering
Sea shelf
1299
Kinder, 1983; and references therein) has shown the geostrophic relation to yield a good approximation to actual flow. Temporal variability occurs on many scales (see below), but it became clear that each cast is not an “independent” estimate. Although we do not know what the de-correlation scale might be, the following procedure was used: (1) data at time series stations at the same site over two days or less were averaged to yield one value; (2) a single “independent” estimate is an average of the values for each area for each month (June 1976, for example); and (3) at least five independent estimates, in at least three of the four summer months, were required for use of data in an area. The procedure would seem to be conservative and probably yields larger standard errors than really exist. The number of independent estimates for the 49 areas used are shown in Fig. 2(a). The standard errors (based on 1 S.D. ; 67% confidence limits) of the derived geopotential anomalies vary from 0.2 to 0.9 dyn cm, but only four are >O.S dyn cm [Fig. 2(b)]. The typical standard error in the anomaly difference between two areas is -0.5 dyn cm. Hence use of a contour interval of 1 dyn cm, as in Fig. 1, seems appropriate. Variability Both spatial and temporal variability occurs. We envision that temporal variability exists on many scales but may be prominent at daily, seasonal and year-to-year (interannual) intervals. Data at the 16 time series stations (noted above; 55-58”N, 162-168”W) can yield an estimate of approximate daily variability. The ranges of geopotential anomaly (maximum value minus minimum value) varied from 0.1 to 0.8 dyn cm; the mean range (f S.D.) was 0.3 + 0.2 dyn cm. Estimates of the variability during the four calendar months used are shown in Table 1. These are based on the observed geopotential ranges, not gradients, that occurred over various years in individual areas. Thus this variability is about four times greater than that on daily scales. Flow patterns The geopotential topography of the sea surface, referred to 50 db, is shown in Fig, 1. The maximum range (4.6 dyn cm) is across the northward flow near Unimak Pass. The inflow through the pass appears to diverge, with part continuing northward and part turning eastward. This had not been shown previously (Coachman, 1986. for example). The source of this inflow is the Alaska Coastal Current (Royer, 1981). Schumacher etal. (1982) reported a measured speed at 59 m (near bottom) in the pass (from 2.5 months in summer 1980) of 6 cm s-r; our surface geostrophic speed, referred to 50 db, is -5 cm s-‘. This comparison implies that surface flow in the pass may be only -50% baroclinic, with the remainder being barotropic (Schumacher et al., 1982). It should be noted, however, that conditions in 1980 may not have been typical of long-term mean conditions. Surface geostrophic speeds near the 50 m isobath (Fig. 1) are l-2 cm s-l toward the northeast and turning cyclonically toward the northwest, as were synoptic summer geostrophic speeds and limited current measurements at 20 m (Schumacher and Kinder, 1983). Figure 1 also shows a northwestward flow north of the eastern Aleutians and offshore of the 100 m isobath. (Its source appears to be the inflow through Unimak Pass and the northeastward flow along the north side of the Aleutians.) The geostrophic speeds here are 2-3 cm s-‘; use of 100 db as a reference level (not shown) approximately doubles
1300
R. K. Reed and P. J. Stabeno
6O’N
,6~~~6~~6./11~13~12! 9151 i 9 1 d ‘/i2i 9'1 81'101121 9,
55Q
Std. error (dyn m x 103) 6O’N
Bering Sea
t
~ 516 _
41
1
5
a 55"
17o*w
estimates of the O/50 db geopotential anomaly in each 0.5” Fig. 2. (a) Number of independent latitude by 1.0” longitude area, as defined in the text. (b) Standard errors of the 0150 db geopotential anomaly (in dyn m x 1O3: 67% confidence limits) in each area, during summer. 1975 1989.
the surface speed. These latter results are in general agreement with the baroclinic and measured speeds in Schumacher and Kinder (1983). Hence, the flow offshore of 100 m appears to be largely baroclinic. An unexpected feature in Fig. 1 is that shown by the path of the 14 dyn cm contour
Climatological circulation-Bering
Sea shelf
1301
Table 1. Estimates of the variability in geopotential anomaly (O/SOdb). based on data from multiple months, during various years, per area, sorted by calendar month
Month
No. of values
June July Aug. Sep.
43 21 35 30
Mean range f S.D. (in dyn cm) 1.2 + 1.5 f 1.1 f 1.4 f
0.6 0.9 0.8 1.1
between the 50 and 100 m isobaths. Although the maximum geostrophic speed is only -2 cm s-l , there is a clear indication of a flow that moves onshore near the Pribilof Islands, turns eastward, rotates cyclonically and then moves westward near 58”N. (The westward flow is not well resolved by these sparse data, with relatively large standard errors, however.) Although CTD data are generally limited in non-summer months, there were adequate data in February-April in the area 5657”N, 164-166”W to show significant eastward flow. Also, three synoptic surveys in 1976 (Reed, 1978) indicated eastward flow of l-2 cm s- ’near the feature in Fig. 1. This large-scale, albeit weak, circulation suggests a link or connection between the offshore and inshore flow regimes, even though they have been thought to be largely decoupled and to have no net flow between them (Schumacher and Kinder, 1983; Coachman, 1986). Coachman (1986) invoked a relatively large eddy diffusivity (5 x lo6 cm’s -‘) to explain the salinity distribution near the 100 m isobath. Our estimate of salinity advection, using the climatological surface salinity in Reed (1995), yields a value three times the salinity diffusion estimate with the coefficient of 5 x lo6 cm2 S -‘. Hence, observed property distributions seem likely to be partially influenced by cross-shelf advection.
DIRECT
CURRENT
MEASUREMENTS
Data from satellite-tracked drifting buoys over the shelf area are presented discussed. We also briefly examine some results from current moorings.
and
Data and methods Because of data scarcity, the “summer” period was extended, and satellite-tracked drifter data from mid-May to mid-October 19861994 were used. The buoys were drogued at 40 m; during 1986-1988, “holey sock” drogues were used, between 1987 and 1993, “tristar” drogues were employed, and in 1994 “holey sock” drogues were again used. An average of -12 satellites fixes were obtained daily, with a standard position error of -0.2 km. The methods used in deriving vectors are generally like those described by Stabeno and Reed (1994). The exceptions are that here a 4 day period within a grid area was considered an independent estimate, and the vectors have not been adjusted to the center of the areas. Finally, if the derived scalar speed did not exceed the standard error, or there were less than three independent estimates, vectors are not shown in Fig. 3. The number of independent estimates and the standard errors are shown in Fig. 4(a) and (b), respectively.
1302
R. K. Reed and P. J. Stabeno
Net Vectors (cm s-l) 60”N
*
ii
55”
I 0
I 10
cm
5.’
a I
1
I
I
17O”W Fig. 3.
Vector
/
1
I 165”
/
/
1
I
I
/
/
_
,
160”
net flow (according to the speed scale shown) for each 0.5” latitude longitude area, during 15 May-15 October. 198&1994.
155’ X
1.0”
Drifter velocities Much of the shelf area east of 165”W has inadequate data to show climatological net vectors (Fig. 3). On the other hand, there are significant data in Fig. 3 north and west of the CTD data in Fig. 1. Offshore of 100 m, between 55 and 58”N, Fig. 3 shows speeds of are similar to those from the typically 2-5 cm s-‘. Both the speeds and directions geopotential topography, especially when speeds are referred to 100 db (noted above), which again suggests largely baroclinic flow. The consistent flow along the shelf-slope region north of the Pribilof Islands was not shown in Fig. 1 because of a lack of CTD data. The direct measurements, however, do not indicate a westward flow near 58”N. As noted above, this feature is dubious. Data near the 50 m isobath east of 164”W (Fig. 3) yield typical speeds of -2 cm s-’ compared to our geostrophic speeds of l-2 cm s-l (noted above). Since the drogues were at -40 db, it would appear that a nonbaroclinic component of flow may occur near the 50 m isobath. Between the 50 and 100 m isobaths, Fig. 3 shows small and variable vectors that
Climatological
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Sea shelf
Standard error (cm s-l)
(W
17OQW
165"
160"
155"
Fig. 4. (a) Number of independent estimates, as discussed in Stabeno and Reed (1994) and in the text. (b) Standard error of the scalar net flow (in cm s -’ , 67% confidence limits). Data are during 15 May-15 October, 1986-1994.
are also somewhat larger than the geostrophic, baroclinic some data in Fig. 3 during May and October might be a Kinder (1983) concluded that there is an enhancement of summer months. These direct measurements, however,
speeds in Fig. 1. Inclusion of factor, since Schumacher and flow inshore of 100 m in nonsupport the existence of the
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R. K. Reed and P. J. Stabeno
convoluted eastward flow near 57”N in Fig. 1. Because of the various data sets showing this flow. we suspect that it may be somewhat stationary and permanent.
Tidal enhancement of net flows Schumacher and Kinder (1983) suggested that regions of the shelf near the 50 and 100 m by tidal enhancement (or isobaths might have net flows of l-4 cm s-’ generated rectification; Loder, 1980). As we have discussed, geostrophic flow inshore of 100 m is somewhat less than that derived from the drifter data. Hence a tidal enhancement of flow might reconcile this difference. Consequently, we examined the data from the current moorings (Schumacher and Kinder, 1983, plus two subsequent moorings) for any evidence of a tidal effect on the net flow. The hourly current records were demodulated to create tidal constituent (M,) time series. and estimates of correlation were calculated between these series and the low-pass filtered series (from which tides have been essentially eliminated; see Schumacher and Reed, 1992). The results are quite conclusive; only one of the 21 values (with a correlation of 0.32) exceeds the 95% significance level, and it explains only 10% of the correlated variance. One value should reach this level by chance. however, which suggests there is no significant tidal enhancement of net flow in this region.
CONCLUSIONS New climatologies of geopotential topography and direct (drifter) measurements have been presented. Although they substantiate some features found in earlier work, much of which was synoptic rather than climatological (long-term means), they also reveal the following new results: (1) An eastward flow exists between the Pribilof Islands and the 50 m isobath. Thus, the middle shelf is influenced by advection, as well as the inner and outer shelves. (2) Unlike previous work, our analysis indicates that tidal rectification does not generate net flow. The nature of the weak barotropic flow suggested is unclear. (3) The geopotential topography clearly shows a divergence of the Unimak Pass inflow, with part of the flow moving northwestward near the 100 m isobath. (4) The eastward how inshore of the Pribilof Islands suggests that the middle shelf is influenced by advection and that the salinity distribution does not result entirely from diffusive processes. AcknoM~ledgemenfs-We thank J. D. Schumacher for advice and encouragement. D. G. Kachel prepared the CTD data products. We also appreciate the assistance of S. Stillwaugh and others from the National Oceanographic Data Center. Comments by tworeviewers were quite helpful. This is contribution FOCI-B 229 to the Fishcries Coordinated Investigations and is part of the Coastal Ocean Program of NOAA. Contribution No. 1556 from NOAA/PMEL.
REFERENCES Coachman L. K. (1986) Circulation, water masses. and fluxes on the southern Bering Sea shelf. ConfinentalShelf Reseurch, 5, 23-108. Loder J. W. (1980) Topographic rectification of tidal currents on the sides of Georges Bank. Journal ofPhysical Oceanograph_v, 10,1399-1415.
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Reed R. K. (1978) The heat budget of a region in the eastern Bering Sea, summer 1976. Journul of Geophysical Research. 83.3635-3645. Reed R. K. (1995) Water properties over the Bering Sea shelf: climatology and variations. NOAA Technical Report ERL 452.PMEL 42,lS pp. Royer T. C. (1981) Baroclinic transport in the Gulf of Alaska. Part II. Fresh water driven coastal current. Journal of Marine Research, 39,251-266. Schumacher J. D., C. A. Pearson and J. E. Overland (1982) On exchange of water between the Gulf of Alaska and the Bering Sea through Unimak Pass. Journul of Geophysical Research, 87.5785-5795. Schumacher J. D. and T. H. Kinder (1983) Low-frequency current regimes over the Bering Sea shelf. Juurnulof Physical Oceanography, 13,607-623. Schumacher J. D. and R. K. Reed (1992) Characteristics of currents Over the continental slope of the eastern Bering Sea. Journal of Geophysical Research. 97,9423-9433. Stabeno P. J. and R. K. Reed (1994) Circulation in the Bering Sea observed by satellite-tracked drifters: 1986 1993. Journal of Physical Oceanography. 24,84&854. Wooster W. S. and B. A. Taft (19.58) On the reliability of field measurements of temperature and salinity in the ocean. Journal of Marine Research. 17, 552-566.