JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. C7, PAGES 14,113-14,126, JULY 15, 1994

Circulation of Prince William Sound, Alaska

H. J. Niebauer, Thomas C. Royer, and Thomas J. Weingartner Institute of Marine Science, University of Alaska, Fairbanks

Abstract. The circulation of Prince William Sound, Alaska, is described using hydrographic (1974-1989), current meter (1977-1979), and acoustic Doppler current profiler (1986-1990) observations from both the sound and the adjacent Gulf of Alaska. Ancillary data include data for winds, freshwater runoff, and satellite-tracked drifters. Prince William Sound is a small inland sea in that it is wide enough to have appreciable horizontal cyclonic circulation. It is also a fjord in that it has basin depths to 700 m but is stilled at 180 m to the open ocean. The general circulation pattern is deftned by a portion of the westward flowing Alaska Coastal Current on the Gulf of Alaska shelf that enters Prince William Sound through Hinchinbrook Entrance and transits the sound from east to west before exiting through Montague Strait and rejoining the coastal current. However, there is much variability in this circulation, especially in the transport through Hinchinbrook Entrance. In addition, some of the water entering the sound becomes involved in the cyclonic circulation in the northern sound and so has a longer residence time. The circulation is strongly mediated by seasonal and interannual variations in winds and freshwater runoff as well as by local topography both inside and outside the sound. In winter, the strong cyclonic winds over the Gulf of Alaska cause coastal downwelling and strong flow in the upper layers into Prince William Sound through Hinchinbrook Entrance and out through Montague Strait. In summer, the downwelling ceases, allowing subsurface denser water to rise above the sill and flow into the sound through the bottom layers of Hinchinbrook Entrance. We conclude that the best transport data came from Montague Strait, from which we estimate that -40% of the volume of Prince William Sound is flushed in summer (May-September). This estimated volume rises to about 200% in winter (October-April).

Introduction and Background

Prince William Sound (PWS) is located along the southem coast of Alaska (Figure 1) in the North Pacific Ocean. Central PWS is about 60 km by 90 km with maximum depths of >350 m (one area is >700 m deep). PWS is surrounded by mountains (up to 4-km elevation) of the Chugach and Kenai Mountains, which are the northern extensions of the Cascade Range. The climate is maritime with moderate air temperatures for the subarctic, high rates of precipitation, and strong winds associated with the storms that migrate onshore from the Aleutian low to the west. Maximum cyclonic winds over the Gulf of Alaska occur from late fall to early spring (October-March) generating on-shore Ekman transport and resulting in coastal convergence and coastal downwelling outside PWS. These storms are also responsible for the high rates of precipitation and freshwater runoff via orographic effects from the mountains. For example, annual precipitation in excess of 8 m has been recorded within the sound [Royer, 1979]. In winter, the precipitation is typically stored in the snow pack, which melts and runs off in the spring and summer. Relaxation of the winds, resulting in relaxation of coastal downwelling, occurs in summer.

The coastline of the sound is rugged, with many Oords and tidewater glaciers. One of the fjords opening into PWS is Port Valdez (Figure 1), an ice-free port that is the terminus of the Trans-Alaska Pipeline. All of the oil from the Alaska Pipeline is

Copyright 1994 by the American Geophysical Union.

Paper number 94JC00712. 0148-0227/94/94JC-00712505.00

shipped through PWS, out Hinchinbrook Entrance, and into the north Pacific. It was from Port Valdez that the oil tanker T/V

Exxon Valdez sailed on March 23, 1989, only to go aground on Bligh Reef just outside Port Valdez but still in PWS just after midnight on March 24, causing North America's largest oil spill.

The Gulf of Alaska is the ocean boundary for PWS. In the Gulf of Alaska off PWS, the continental shelf is 100-150 km wide (Figure 1) and has numerous topographic features that af- fect circulation. These features include islands, such as Kayak and Middleton, as well as shelf bathymetric troughs which serve as conduits for shelf-deep water exchange. However, these troughs are silled by glacial moraines which restrict deeper circulation. Circulation between PWS and the gulf is further restricted by Montague and Hinchinbrook Islands and numerous other smaller islands (Figure 1). The major passages connecting PWS with the Gulf of Alaska, that is Hinchinbrook Entrance

(---11 km wide by •-320 m deep) and Montague Strait (---8 km wide by ---240 m deep), are on either side of Montague Island. Note that the continental shelf outside the sound is shallower

(180 m) than either Hinchinbrook Entrance (HE) or Montague Strait (MS) (Figure 1). Considering the freshwater input to the sound and the 180-m-deep shelf/sills guarding 700-m interior depths, PWS can be classified as a large, complex, t•ord-type estuarine system [Schmidt, 1977; Muench and Heggie, 1978]. However, PWS also exhibits characteristics of a small inland sea with appreciable horizontal circulation.

In order to describe and understand the circulation of PWS, we have assembled ocean and atmosphere data sets collected over the last --20 years, and we present an analysis of the circu- lation of PWS and its interaction with the adjacent North Pacific.

14,113 .

14,114 NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA

1490 1480 1470 1460 1450 2642 .

Valdez

Reef

Anchorage

Prince William Sound

Kntght Is Passage

Seward Htnchtnbrook Entrance

Resurrection Latouche Bay Is.

Valdez

61 ¸

61o

Cordova

Hinchinbrook• Is.

600 ...,• 60 ̧

100 ddleton

1490 1480 1470 1460 1450 Figure 1. Charts of Prince William Sound and the adjacent Gulf of Alaska in the north Pacific Ocean. Inset is from Royer et al. [ 1990]. Stars in Hinchinbrook Entrance and Montague Strait on either side of Montague Island indicate locations of current meter moorings. Heights and depths are in meters.

The data include hydrographic surveys from 1974-1989, current meter data from moorings in Hinchinbrook Entrance and Montague Strait in 1977-1979, and acoustic Doppler current profiler (ADCP) observations collected from 1986-1990. Ancillary data are on winds, freshwater runoff, and satellite-tracked drifters.

Methods

Hydrography

Hydrographic data were collected on 59 cruises to PWS and the adjacent Gulf of Alaska shelf between 1974 and 1989. The

hydrographic data consist of salinity/conductivity and tempera- ture averaged over 1-m intervals from various Plessey salinity- temperature-depth/conductivity-temperature-depth (model 9040 series) and Neil Brown conductivity-temperature-depth systems. These data were calibrated by in situ water samples and reversing thermometers.

Acoustic Doppler Current Profiler

Between April and November 1989, following the grounding of T/V Exxon Valdez, four cruises of approximately 8 days duration each were conducted on the R/V Alpha Helix. ADCP data were collected throughout the sound on three of these

NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA 14,115

149 ø 148 ø 147 ø

LONG, ITUDE

61.0 ø

149 ø 148 ø 147 ø LONGITUDE

Figure 2. Acoustic Doppler current profiler current vectors for (a) April 1989, (b) May 1989, and (c) October 1989. Vectors are 20-m values minus 100-m values as explained in text.

cruises (April, May, and October-November). Additional ADCP data are available from four earlier cruises in PWS (March and December 1986, December 1989, and October 1990), but the coverage is limited to sections in Hinchinbrook Entrance and Montague Strait.

The R/V Alpha Helix shipboard ADCP is an RD Instruments 300-kHz hull-mounted profiler. Ensemble-averaged velocity profiles were obtained at vertical intervals of 4 m (bin size) by averaging pings over 120-intervals, yielding a precision of-•0.01 m s 4 (RD Instruments). The depth distribution of the velocity estimates extended from 6 m to a maximum of 280 m (the latter usually depending upon sea state). In practice, we rejected data at depths of less than 14 m in order to minimize hull-induced turbulence. We also rejected data from deeper in the water column when the data return became less than 30% good. Prior to analysis, the individual ensembles were subjectively screened,

and those that were erratic or of questionable quality were discarded. These ensembles were typically associated with vessel accelerations during approaching, leaving, or repositioning while on station. Finally, ensembles were averaged in various sized blocks (depending upon ship speed and location). These block sizes ranged from a minimum of 2 en- sembles to a maximum of about 40 (while the ship was on station). These data are not corrected for tides.

In constructing the vector plots (Figure 2), velocities from the four depth bins centered at 94, 98, 102, and 106 m were averaged (resulting in a velocity estimate at-•100 m) and subtracted from the average velocity calculated from estimates for the four depth bins centered between 14 and 26 m (resulting in an estimated velocity at-•20 m). Our reasons for displaying the ADCP velocities at 20 m relative to those at 100 m rather than the

absolute velocities are several. First, the barotropic component

14,116 NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA

60.5 ø

59.50 149 148 ø 147 ø

LONGITUDE

Figure 2. (continued)

of the tide is reduced or eliminated. This procedure was necessary because tidal velocities were expected to vary greatly over relatively short distances owing to the complex bottom topography. For example, the amplitude of the M 2 tide (the dominant tidal constituent in the Gulf of Alaska) ranges from 0.3 to 0.4 m s 4 in HE and MS to about 0.05 m s 4 on the shelf. We

expect a similar variability throughout the interior of PWS. Second, the time spent in some transcots, especially transcots with hydrographic stations, often encompassed a large fraction of the tidal cycle, which leads to difficulty in interpreting the results. Third, bottom tracking was lost over the deeper basins within PWS. Without bottom tracking, we must depend on ship's navigation to extract absolute water velocity. Fourth, the selection of 100 m is consistent with the reference level selected

for determining the baroclinic geostrophic flow in the rest of the paper.

Current Meter Moorings

Current velocities were measured from current meter moor-

ings spanning 15 months (November 1977-February 1979) with one mooring each in Hinchinbrook Entrance (HE) and Montague Strait (MS) (Figures 1 and 3, Table 1). There were six mooring deployments over the 15 months with a total of 28 current meters spread over the moorings. The moorings were suspended above the bottom with the uppermost subsurface floats about 20 m below the surface. Each mooring consisted of four or five Aanderaa RCM-4 current meters with in-line flotation and an

acoustic release connected to the anchor. At 30-min intervals, each current meter recorded current speed and direction, temperature, conductivity and pressure (depth). These data were filtered with a 2.9-hour filter to suppress the short-period fluctuations for presentation of the current meter statistics in Table 1. The data were also filtered with a 35-hour filter to

suppress the short, inertial, and tidal-period fluctuations for subtidal analyses.

To estimate the monthly mean transport through Hinchinbrook Entrance and Montague Strait (Figure 4, Table 2), the daily average speed in the direction of the estimated principal axis of flow in HE (-345øT) and MS (-220øT) from each of the

current meters was multiplied by the cross-sectional area of HE or MS centered on that current meter. Flushing rates (Table 2) for the various depth intervals were calculated by dividing the estimate of the volume of PWS contained within the depth interval by the transport through that depth interval. A critical assumption in these calculations is that there is no lateral variability in HE and MS for the flow into and out of PWS. This assumption appears to be reasonable for MS but is not always true for HE, as discussed in the following sections on ADCP and hydrography. A further assumption is that there are no vertical displacements of water between depth intervals within PWS. In many situations in PWS this is undoubtedly not true.

Results and Discussion

Hydrography

Repeated though infrequent hydrographic surveys since the early 1970s are available for the PWS region including the ad- jacent Gulf of Alaska and the Alaska Coastal Current. There were actually 59 cruises between 1974 and 1989, although most of PWS was not fully covered during these cruises, so that the entire flow field cannot be determined for each cruise. We focus

on two periods when more intense sampling took place: 1977-1979, which coincides with the current meter moorings in Hinchinbrook Entrance and Montague Strait (Figure 3), and 1989 after the Exxon Valdez oil spill, a period for which there is also PWS-wide ADCP coverage (Figure 2).

Circulation on the adjacent shelf. The dominant coastal circulation feature in the northern Gulf of Alaska that has an

impact on PWS is the Alaska Coastal Current (ACC) [Schumacher and Reed, 1980; Royer, 1981a, b; Schumacher et al., 1990]. The nearshore flow in this current is westward past PWS, though there is usually a small reversal on the offshore edge of this current but still on the shelf (note that the ACC is not a shelf break current). The ACC has been studied extensively since 1983, with repeated sections taken to determine its seasonal signal [Johnson et al., 1988]. The mean 0/100 dbar baroclinic geostrophic transport is about 0.2 Sv (1Sv=106 m 3 s -1) with a seasonal fluctuation of about 0.1 Sv. Current velocities are quite

•',IIEBAUER ET AL.' CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA 14,117

14,118 NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA

Table 1. Statistics for Hinchinbrook Entrance and Montague Strait Current Meter Moorings

Depth, m Average Maximum Net Average Average Speed, cm/s Speed, cm/s Velocity, cm/s •øT Temperature, øC Salinity, ø/oo

HE

A* 30 26 124 11•346 5.9 31.5 A 100 26 89 7•346 5.2 32.1 A 200 25 95 3•347 5.5 33.0 A* 270 9 25 1•232 6.1 34.1

B* 30 6 85 2•038 7.9 30.9 B 50 32 114 5•047 6.9 31.7 B 100 29 99 1•359 5.9 31.1 B 200 29 91 3•164 5.5 32.6 B 320 33 100 15•358 5.2 32.8

C 30 48 163 24•008 7.8 31.0 C 50 42 160 23•002 7.8 31.0 C 100 34 97 15•015 8.3 32.3 C 200 29 93 3•103 6.0 32.5 C 300 31 97 6•272 5.7 33.0

MS

A 30 36 105 33•219 5.1 31.4 A 50 29 95 24•219 5.2 31.7 A 100 21 81 7•214 5.3 31.9 A* 200 27 87 2•131 6.0 .... A* 220 31 93 7•071 6.0 32.6

B 30 20 77 1•091 8.2 30.5 B 50 19 69 5•213 6.6 31.4 B 100 19 61 7•224 5.4 32.1 B 200 18 56 2•083 5.4 31.8 B 225 17 58 2•037 5.4 32.5

C 50 30 101 21•216 7.4 31.2 C 100 21 86 6•231 6.9 32.0 C 200 21 76 4•030 5.9 .... C 225 23 81 7•063 5.8 33.0

All data are 2.9-hour filtered. A, November 1977-April 1978; B, April-September, 1978; C, September 1978-1979.

* Short record.

variable but are usually at a minimum in spring of less than 0.25 m s -1, coinciding with the minimum transport, whereas maximum velocities usually occur in fall, when they can exceed 1.5 m s -1 [Johnson and Royer, 1986].

The ACC, as well as PWS, responds strongly to both wind forcing and freshwater discharge forcing. Direct wind meas- urements without topographic interference are not available for this region on a routine basis, so the wind proxy that we use is upwelling indices [Bakun, 1973, 1975], which are calculated from a 3øx3 ø gridded monthly sea level atmospheric pressure grid (e.g., Figure 5). Luick et al. [1987] have shown that though these geostrophic wind computations have been reduced by 30% and rotated counterclockwise by 15 ø to compensate for frictional effects, an additional 11% reduction and 44 ø counterclockwise rotation (59 ø total rotation) are necessary to best approximate the coastal winds between Prince William Sound and Seward,

Alaska (Figure 1). Within the confines of PWS, the "corrections" for the geostrophic winds are expected to be even greater than those for the open coast.

The upwelling indices shown in Figure 5 are large negatively (indicating downwelling) when the Aleutian low is over the Gulf of Alaska, which is the dominant case for winter. In this

situation, there is often a high-pressure cell over the interior of Alaska that, combined with the Aleutian low, produces an off- shore pressure gradient over the sound and the gulf. There is a tendency for ageostrophic winds to be offshore through the bays and passes bordering the northern Gulf of Alaska when the storm systems are offshore. Thus, in PWS during intense coastal downwelling periods, the winds are often strongly southward through HE and southwestward through MS. These complicated local wind structures require a mesoscale atmospheric model for their proper resolution. Since this is beyond the scope of this analysis, we simply use the upwelling indices as a measure of the atmospheric forcing.

The freshwater discharge (Figure 6) is estimated using the hydrology model of Royer [1982], which determines the fresh- water input to the coastal Gulf of Alaska using the National Weather Service monthly mean precipitation and temperature data for southeast and south coast Alaska. The rate of freshwater

discharge from Southeast Alaska is lagged by a month to represent the transit time of those waters as they traverse the northeastern Gulf of Alaska.

As pointed out above, the Alaska Coastal Current responds strongly and seasonally to both wind forcing and freshwater

NIEBAUER ET AL.' CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA 14,119

0.4 ..... HINCHINBROOK ENTRANCE

> • o.• (A)

[• o.z O o.l

• o.o

.• -o.1

•-• -0.2 [1977 1978 -0.3 1979

ND JFMAM JJAS 0ND JF

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U3 o.o

.• -o.1

• -o.2

-0.3

..... MONTAGUE STRAIT

(B)

1971] 1979

N D J FMAM JJAS 0 ND J F

Figure 4. Monthly mean estimated total transport (in sverdrups) for (a) Hinchinbrook Entrance and (b) Montague Strait from current meter data as outlined in text. Positive values are directed into Prince William Sound.

discharge [dohnson et al. 1988; Schumacher et al., 1990]. The phasing of the two forcing functions is important, since the maximum freshwater discharge occurs in the fall (Figure 6), whereas the maximum wind forcing is from December through February (Figure 5). Therefore the maximum amount of fresh- water is in the coastal system prior to the spinning up of the maximum wind stress. Coastal convergence associated with downwelling constrains the buoyancy-influenced flow close to the coast, causing the high baroclinic geostrophic velocities in fall.

Circulation in Prince William Sound. The 0/100 dbar

topography for June 1976 (Figure 7a) hints at inflow into the sound through HE and a sense of counterclockwise or cyclonic flow within the interior of the sound. The September 1978 topography (Figure 7d) also suggests cyclonic flow. In December 1976 (Figure 7b), there is a suggestion of flow out of the sound through MS into the Alaska Coastal Current. Variations of these patterns are seen in the dynamic topographies for August and September 1978 (Figures 7c and 7d). In August and September 1978 (Figures 7c and 7d), the ACC is plainly visible outside of PWS, with strong flow along the southern or seaward side of Montague Island. The majority of dynamic topographies indicate some splitting and inflow of the ACC northward through Hinchinbrook Entrance with outflow through Montague Strait in the western sound rejoining the Alaska Coastal Current as it moves westward along the coast. Other dynamic topography (e.g., August 1978; Figure 7c) shows a portion of the ACC looping into and then back out of HE.

The circulation of PWS is actively influenced by freshwater discharges within the sound. After a portion of the Alaska Coastal Current enters the sound through HE, it is often further diluted as it circulates through the sound. This process is illus- trated in a temperature-salinity diagram for December 1979 (Figure 8) in which the dilutions at stations inside PWS (dashed

curves) are obvious in comparison with the higher-salinity water (solid curves from stations just outside PWS) entering the sound at HE. There are other times when the dilution is not as well

defined, such as August 1980 (not shown), when dynamic heights indicated a flow reversal with inflow through MS.

Baroclinic Geostrophic Flow Through Hinchinbrook Entrance. Though the mean direction of flow is in through HE and out through MS, there exist periods of reversal, especially in Hinchinbrook Entrance. The unsteadiness of the HE flow was

initially surprising. This misconception has been fostered by satellite-tracked drifting buoys in the coastal Gulf of Alaska that so far have always gone into PWS through HE [Royer and Emery, 1987] but have never come out through HE.

For the 23 hydrographic sections across Hinchinbrook Entrance, the "entrance" to PWS, collected since 1974, the baroclinic geostrophic transport was determined in the upper layers from the surface to the 50- and 100-dbar pressure levels (Table 3). For the flow relative to that at 100 dbar, the transport was inward on 10 occasions and outward 13 times, with the average being outward at 0.013 Sv (note that the data are biased in time away from winter). The standard deviation for these estimates was 0.050 Sv, or three or four times the mean, which indicates that we are uncertain as to whether the baroclinic geo- strophic flow relative to that at 100 dbar was into or out of PWS through HE. The average flow relative to that at 50 dbar was also out of the sound at 0.013 Sv with a standard deviation of

0.028 Sv, or approximately two times the mean. The transport was about the same for both reference levels, but the standard deviation was less for the upper 50 dbar.

Baroclinic geostrophic flow through Montague Strait. The baroclinic geostrophic flow through the upper layers of MS was calculated for 16 sections obtained from December 1976 to

December 1982. Relative to that at 100 dbar (Table 3), the flow was outward for -80% of the sections. The mean transport was outward at 0.016 Sv with a standard deviation of 0.037 Sv, so we are more confident that the flow is typically out of the sound through MS. From Table 3 it can be seen that the upper layer baroclinic geostrophic inflow into the sound through HE is best defined from February through May. In MS, the upper layer baroclinic geostrophic outflow is most clear in late fall/winter through April, with outflow also occurring in July.

The unsteady nature of the baroclinic geostrophic flow through HE and MS has several possible causes, including tides and violations of the assumptions that go into the geostrophic approximation. For example, geostrophic equilibrium is ques- tionable in constricted coastal regions such as HE and MS, as well as in PWS, where friction and inertial effects are important. There can be errors due to the placement of the hydrographic stations, and the hydrographic sections may not be orthogonal to the flow. Errors in the salinity (+0.02 practical salinity unit) and temperature (+0.02øC) measurements contribute to transport errors, while the choice of the reference level might be incorrect and probably changes over time and space. In addition, it is possible for the strength of the Alaska Coastal Current to have an inverse influence on the flow into PWS through Hinchinbrook Entrance. For example, increased coastal freshwater discharge in fall causes an increase of the alongshore flow in the northern Gulf of Alaska. The westward flow around the tip of Hinchinbrook Island may increase such that inertial effects would force the flow across the entire entrance before it could

turn into the sound (e.g., Figures 7c and 7d). A decrease of the internal Rossby radius of deformation due to decreased stratification probably mitigates the inertial effect. Calculations

14,120 NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA

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NIEBAUER ET AL.' CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA 14,121

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-4O

-60

-80

-lOO

-12o

-140

-160

-180

-200

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60N, 14.6W ß

(A)

I r MA M J J A S 0 N O J F' M• MJ J w• 0 N 0 977 1978

10

0

-10

-20

-30

-40 -,50 -60

-7'0 -80 -90

-IO0

-110

-120

-150

1989 1990

Figure 5. Upwelling index [Bakun, 1973] for positions in the northern Gulf of Alaska for (a) 1977-1978 and (b) 1989-1990, as determined from sea level atmospheric pressure data from Fleet Numerical Oceanographic .Center in Monterey, California. Negative values indicate downwelling.

of the internal Rossby radius of deformation suggest values in the 5-km range from January to April. Stratification typically begins forming in May and disappears in December, with maximum Rossby radii of 40-70 km in August-October. The width of HE (-11 km) is similar to the Rossby radius during some of the year, so stratification changes are probably important. Finally, northerly winds associated with the winter storm systems could also force upper layer flow southward, out of the sound during the fall and winter storms. Given the above, the unsteadiness of the HE flow is reasonable, though the exact causes are uncertain.

Acoustic Doppler Current Profiler

Near-surface current field. Figures 2a through 2c are plots of the horizontal ADCP velocity vectors at 20 m relative to those at 100 m collected in April, May, and October-November 1989. The data span the region from HE in the east to the Cape Fairfield Line (running along 148.8øW) in the west (Figures 1 and 2). There is spatial heterogeneity of these flow fields, which is not surprising given the complex bathymetry of the sound and the variable wind field associated with the channeling effect of the coastal landforms. However, most of the time the relative ADCP flow at 20 m is directed into PWS through HE, though the flow is not uniformly distributed across the channel. The pattern of vectors suggests that a substantial portion of the upper 20-m inflow relative to the 100-m inflow enters on the western side of

HE adjacent to Montague Island and then veers northward and northeastward across the mouth of the channel (Figures 2a through 2c). While the seaward extent of these data sets does not encompass the ACC, the data sets are consistent with a westward flowing ACC that splits near Montague Island. One branch tums northeastward into the sound through HE, and the remainder continues southwestward seaward of Montague Island. This interpretation is also consistent with coastal zone color scanner imagery of the suspended sediment patterns within the ACC [Royer et al., 1990], with the 0/100 dbar dynamic topography (Figure 7), and with satellite-tracked drifter paths [Royer and Emery, 1987].

Within PWS to the east and southeast of Naked Island (Figure 1) in May and October (Figures 2b and 2c) is a surface cyclonic circulation cell similar to that seen in the 0/100 dbar dynamic topography (Figure 7). North of this cell, the flow is more variable, and no consistent pattern emerges from the data. In the western passages of the sound, there is southward flow through Knight Island Passage. Some of this flow exits the sound through the narrow passes to the west of Montague Strait (April and October in Figures 2a and 2c) and some rejoins the flow in Montague Strait, where it either exits onto the shelf or recirculates northeastward into the interior of the sound.

The ADCP measurements along the Cape Fairfield Line (148.8øW) are consistent with previous descriptions of the seasonal variations within the ACC [Johnson et al., 1988]. ADCP sections (e.g., Figure 2) show minimum westward trans- port and speeds in spring (when winds are weak and coastal discharge is small) and maximum westward transport and speed in fall (when winds are strong and coastal discharge attains its annual maximum). Within the sound, the ADCP data (Figure 2) suggest that the surface (20 m relative to 100 m) circulation of the cyclonic gyre is also more intense in fall than in spring.

Montague Strait and Itinchinbrook Entrance. A total of 11 ADCP transects were made across HE, while 14 transects were made across MS during seven cruises from 1986 through 1990. Velocity cross sections for December 1986 (Figure 9) are used as examples to describe the flow fields in HE and MS as well as to show the difference between the two channels. (Note that these data are relative to Earth and not to flow at 100 m, as are the ADCP data in the previous section. Also, these data are not corrected for tides. However, each section was collected over a period of 20 min or less. Finally, these sections are not shore to shore across HE and MS but rather cover about 50% of the

actual area of HE and MS.) The isotachs of the along-channel and cross-channel velocity component in MS (Figure 9a) are typical of other sections obtained in Montague Strait in that (1) the along-channel velocity component tends to vary uniformly

14,122 NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA

1930-89 MEAN * 50

40

-•. 3o

• 20~

10

• 1977 1•j78

MONTH MONTH

50..

4O

• 3o

o

1930-89 MEAN *

MONTH

Figure 6. Estimated freshwater discharge into the Gulf of Alaska using the hydrology model of Royer [1982], which utilizes National Weather Service monthly mean precipitation and temperature data for southeast and south coast Alaska. Estimates are plotted for (a) 1977-1978 and (b) 1989-1990 along with the 1930-1989 mean.

across the channel, with maximum along-channel speeds observed on the right side of the channel when facing in the direction of flow, and (2) the vertical shears of the current components are nearly constant over the upper 100 m of the water column. The net transport in the top panel of Figure 9a is -0.23 Sv directed out of PWS. However, as noted above, the ADCP data cover only half the area of Montague Strait.

In contrast, the isotachs of the north and east velocity com- ponents collected in Hinchinbrook Entrance on the same cruise (Figure 9b) show that (1) flow reversals occur in the along-channel (north-south) velocity component, and (2) strong cross-channel (east-west) flows and reversals also occur. All of our ADCP transects indicate that flows through Hinchinbrook Entrance are characterized by swift currents with reversals oc-

curring over short (1-2 km) spatial scales. The net transport in the top panel of Figure 9a is -0.26 Sv directed out of PWS. Note that this transport is similar to that of MS given above as well as being in the same outward direction. The point again is that MS transport is probably fairly predictable in time and space, while HE transport is not, although again, the area covered by this section is half the actual area of Hinchinbrook Entrance.

These transects allow us to evaluate the assumption of uni- form channel flow that is used to estimate flushing times pre- sented in the next section as derived from single current meter moorings deployed in Hinchinbrook Entrance and Montague Strait. In the calculations to follow, estimates of the volume fluxes through these channels are made by assuming that the velocity recorded at a mooring site is representative of the

NIEBAUER ET AL.' CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA 14,123

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14,124 NIEBAUER ET AL.' CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA

SALINITY (psu) 26 27 28 29 30 31 32 33 34

10 .... : ..... : .... :', .... ,: .... ::::::::::::::::::::: . .

, ,

,

,

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.

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25 26

Figure 8. Temperature/salinity plots from stations in Hinchinbrook Entrance (solid curves) and interior Prince William Sound (dashed curves) showing dilution of water as it transits the sound.

cross-sectional average. The ADCP results, as well as the geostrophic calculations presented earlier, suggest that this assumption is reasonable for Montague Strait but is probably not good for Hinchinbrook Entrance.

Current Meter Moorings

Given the caveats in the previous sections, the current meter vector plots (Figure 3) and transports (Figure 4, Table 2) suggest that to a first approximation, there is mean flow into PWS through HE and mean outflow through MS. These data suggest that PWS has characteristics of a "flowthrough system" with the

mean flow through PWS in the same direction as the shelf cir- culation outside of PWS (that is, east to west). This visualization is, in a qualitative sense, in agreement with most of the ADCP, hydrographic, and drifter data presented above.

Superimposed on the mean flow are significant flow vari- ations with time scales of days to seasons. Very strong flow events with time scales of days (Figure 3) are especially evident in winter and are undoubtedly due to storms spun out of the Aleutian low. Longer-period current and transport variations lasting from months to seasons (Figures 3 and 4, Tables 1 and 2)

,

show at least three distinct periods' (1) strong and persistent flow out of PWS through MS in the upper 100 m during win- ter-spring 1977-1978 and fall-winter 1978-1979 (net velocities of 0.21-0.33 m s -• with maximum speeds of 1.05 m s-l), (2) strong and persistent deep flow into PWS at 320 m in HE during summer 1978 (net velocities of 0.15 m s -• with maximum speeds of 1.0 m s'l; note that, unfortunately, neither the ADCP nor the dynamic height calculations could measure this), and (3) strong currents into PWS through HE in the upper 100 m during fall-winter of 1977-1978 and 1978-1979 (net velocities of 0.11-0.24 m s -• with maximum speeds of 1.24-1.63 m s-•).

Hinchinbrook Entrance. Net inflow into PWS is estimated

for the total transport each month from May to December 1978 for HE (Figure 4, Table 2). Again, this transport is qualitative as determined from the single current meter moorings assuming uniform flow across the entrance, which we have shown (from hydrography and ADCP) is often not the case. The average winter transport is five to eight times the average summer transport. During the time of these measurements, the summer transport in HE was dominated by the deep inflow between 250 m and the bottom (Figure 4, Table 2) while in winter, the main inflow was in the upper 150 m (Figure 4, Table 2). We note again that the ADCP and dynamic topography did not reach below 250 m.

Minimum inflow through Hinchinbrook Entrance occurred at the beginning and end of summer, in May and September. rl'hc May-September period encompasses the season of weak up- welling outside PWS (Figure 5) but also includes increased

Table 3. Monthly Mean Estimates ofBaroclinic Geostrophic Transport into and out of Prince William Sound

Hinchinbrook Entrance Montague Strait

0/50 dbar 0/100 dbar 0/100 dbar

TRANS SD N TRANS SD N TRANS SD N

Feb. 0.005 0.006 2 0.029 0.035 2 -0.034 0.017 3 March .........

April 0.017 0.006 3 0.041 0.008 3 -0.010 0.0001 2 May 0.027 0.044 2 0.058 0.120 2 0.008 - 1 June -0.010 0.051 2 -0.043 0.028 2 - - -

July -0.061 0.043 2 -0.079 0.051 2 -0.032 0.094 2 Aug. -0.056 0.021 3 -0.064 0.039 3 0.034 0.035 2 Sept. -0.007 0.064 2 -0.055 0.134 2 0.024 - 1 Oct. -0.025 0.034 2 0.030 0.004 2 - - -

Nov. -0.006 0.041 2 0.004 0.147 2 -0.074 0.074 2 Dec. -0.015 0.014 3 -0.048 0.036 3 -0.043 0.015 3

Data (in Sverdrups) are for the upper 100 m of the sound from July 1974 through November 1989. Positive values represent inflow and negative values represent outflow. SD, standard deviation; TRANS, transport; N, number of estimates.

NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA 14,125

Montague Is. - Latouche Is. Hinchinbrook Entrance

December 19, 1986 01:58- 02:10 (A) December 18. 1986 15:05 - 15:25 (B) Along-channel Component (cm/s) Nodh Component

Along-track distance (kin) Along-track distance (krn) 0 1 2 3 o 1 2 3 4 5

-z0 • : : / ' ! : I •-' '•--':' -----"•' -zo--- --4O ' " "' -? $•" -

-'ø E-"- : -"----.•-"-• :' .... :?

-,•o . k.... •' ' ' '" -•o

2.2.0

Montague Is. - Lotouche Is. December 19, 1986 01:58 - 02:10 Cross-channel Component

Hinchinbrook Entrance December 18, 1986 15:05 - 15:25 Eosf Component (cm/s)

Along-track distance (krn) Along-frock distance (kin) 0 I 2 3 0 I 2 3 4 5 6

o/ •_ ' .--:'•' • '1. "'"---'-•--'-••

Figure 9. Acoustic Doppler current profiler isotachs of (top) along-channel flow (positive values directed into Prince William Sound) and (bottom) cross-channel flow (positive values directed to the left) in (a) Montague Strait on December 19, 1986, and (b) Hinchinbrook Entrance on December 18, 1986.

freshwater discharge along the Alaska coast (Figure 6). During summer there is moderate inflow into PWS with an average of about 0.04 Sv and a range of 0.01 to 0.07 Sv (Table 2).

The estimated summer transports through HE would be suf- ficient to flush -40% of the sound from May through September at an average of-8% per month. Individual monthly estimated flushings range from 1% in May to 16% for August (Table 2). Summertime exchange of water between the sound and the shelf is concentrated in the deeper layers (>70 m), with moderate inflow and outflow in upper layers. For example, for June 1978, flow between 40 and 70 m was estimated at -0.02 Sv, which in that time would flush about one third of the volume contained in

this depth interval (this depth interval makes up about 12% of the volume of the sound (Table 2)). Note that this flushing is out of PWS. In contrast, during this same period, flow between 250 m and the bottom was estimated at 0.12 Sv inward, which would flush nearly 100% of the layer between 250 m and the bottom (about 25% of the volume of PWS) during June 1987. This type of deep summer flushing is typical of many Alaskan fjords [Muench and Heggie, 1978].

The summer flow patterns coincide with an increase in freshwater discharge along the Alaska coast (Figure 6) which increases the quantity of less dense surface water seaward of HE

and may serve to drive surface water into PWS. However, this period of potential inflow into PWS also coincides with a period of small offshore Ekman transport and upwelling (actually a relaxation of downwelling) on the shelf outside the entrances to PWS (Figure 5). This apparently causes upper layer transport out of PWS (Figures 3 and 4, Table 2) to a depth of 150 m in June and deeper in July. The relaxation of coastal downwelling allows deep, dense water to rise above the 180 m sill (i.e., shelf), allowing deep transport into PWS.

In fall and winter the situation changes. Maximum inflow occurred from October to at least January (no data beyond January). The average transport into PWS through HE is about 0.3 Sv from October to at least the end of December, which is five or six times the inflow average for May-September. The 0.3 Sv would flush about 65% of PWS each month (Table 2). If the January-April transports are similar to the October-December estimates, we estimate that the sound is potentially flushed about four times between October and April.

An explanation for the winter increase is that during fall and winter, east-to-west coastal winds intensify (winter intensifica- tion of the Aleutian low and associated North Pacific storms) while freshwater discharge increases to a maximum in fall (Figure 6). The winds cause strong onshore Ekman transport and

14,126 NIEBAUER ET AL.: CIRCULATION OF PRINCE WILLIAM SOUND, ALASKA

coastal downwelling on the shelf adjacent to PWS. This forces the upper layer water into PWS through HE coincident with cutting off the dense near-bottom water from flowing into PWS through the deepest part of HE. The downwelling depresses the isopycnal surfaces below the 180 sill, cutting off the deep summer inflow over the sill.

Montague Strait. In contrast to the flow into PWS through HE, the estimated total transport in MS (Figure 4, Table 2) is directed out of PWS in all months except May (the May estimate is -0). Except for the winter magnitude, the MS total transports are approximately mirror images of the HE transports; that is,

processed the hydrographic and current meter data. RosemaD, Ruff and Jane Nanto prepared this manuscript. The smnpling was conducted with support from the National Oceanographic and Atmospheric Administration/Outer Continental Shelf Environmental Assessment Program and from the National Science Foundation through grants OCE8208306 and OCE8608125. The NSF also supported the R/V Alpha Helix cruise after the Exxon Valdez spill. The Exxon Production Research Company supported the analysis and publication of these results. We also thank two anonymous reviewers for their comments. Institute of Marine Science, University of Alaska, Fairbanks, contribution 999.

moderate transport out of MS occurs in summer, with minima at References the beginning (May) and end (September) of summer followed by maximum transport out of PWS in fall and winter.

The average summer transport (May-September) is about -0.04 Sv, with that in June-August being-•-0.07 Sv (the minus signs indicate outflow). These transports are equivalent to flushing 9-10% of PWS per month. Individual summer flushings range from 1% in May to 21% for July (Table 2). These estimates of outflow are similar to but slightly larger than those tbr the HE inflow, suggesting that more water may be flowing out of PWS than is flowing in during the summer. While a portion of this difference may be due to melting and runoff in PWS during the summer, some of this difference is undoubtedly due to assumptions associated with the transport estimations made l¾om a single current meter mooring in HE and another in MS. In addition, and probably just as important, differences between the flows in HE and MS are also made up by inflow and outflow in the various passages to the northeast of HE and to the northwest of MS (Figure 1). Finally, the current shear over depth is less in MS then in HE (e.g., Figures 3 and 9).

In contrast to the seasonal variability in HE, MS transports did not change as drastically from summer to winter. The daily mean transport out of PWS through MS is about -0.11 Sv from October to at least January. This is -2.5 times the outflow for May-September (versus 5-8 times in HE). This rate (-0.11 Sv) is equivalent to flushing about 25% of PWS each month (Table 2). Extrapolation of these rates over the entire October-April period implies that PWS was flushed fewer than two times over this period.

From the above calculations and assumptions (e.g., Table 2), and using the HE and MS estimates, we estimate that about 40% of PWS is flushed from May though September. During summer, the inflow through HE might balance the outflow through MS, although at different depths. In fall and early winter, we obtain two different P WS flushing rates from the HE and MS transports. We estimate that PWS is flushed two (MS) to four (HE) times from October-April, with the MS rate being more reliable. The flushing of PWS surface layers (-100 m) appears to be most rapid in winter, whereas the bottom waters of PWS appear to be better flushed in summer.

Acknowledgments. We are indebted to many people who helped gather and analyze these data. Andy Bakun generously provided the upwelling index data. Captain Ken Turner and the crew of the R/V Acona helped with deployment and recovery of the current meter moorings and collection of hydrographic data. Captain Tom Callahan and the crew of the R/V Alpha Helix helped with the ADCP and the collection of hydrographic data. Chirk Chu

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H. J. Niebauer, T. C. Royer, T. J. Weingartner, Institute of Marine Sciences, University of Alaska Fairbanks, Fairbanks, AK 99775-7220.

(Received May 11, 1993; revised March 8, 1994; accepted March 11, 1994.)