- Email: [email protected]
Potential radionuclide transport pathways from seafloor dumpsites: Kamchatka region of the North Pacific Ocean
Pergamon PII: S0025-326X(97)00091-X
Marine Pollution Bulletin, Vol. 35, Nos 7-12. pp. 353-364. 1997 © 1997 Elsevier Science Ltd. All rights rese~'ed Printed in Great Britain 0025-326X/97 $I 7,00+0.110
Potential Radionuclide Transport Pathways from Seafloor Dumpsites: Kamchatka Region of the North Pacific Ocean MARK D. MOREHEAD*, ROBIN D. MUENCHt, ROBERT BACASTOW:~ and RICHARD K. DEWEY§
*Institute of Arctic & Alpine Research, University of Colorado, 1560 30th Street Boulder, CO 80309-0450, USA tEarth & Space Research, 1910 Fairview Avenue East, Suite 102, Seattle, Washington 98102-3620, USA ~Scripps Institute of Oceanography, University of California, La Jolla, CA 92093-0220, USA §Centre for Earth & Ocean Research, University of Victoria, P.O. Box 1700, MS 4015, Victoria, BC V8W 2Y2, Canada
Encapsulated nuclear waste materials, dumped by Russia, are present at two deepwater seafloor locations in the offshore north-west Pacific Ocean, south-east of the Kamchatka Peninsula. This paper assesses potential pathways by which these wastes might, if released from their containers, disperse away from the dumpsites and through the surrounding ocean. A review of large-scale ocean circulation theory and of field and model results suggests that mean abyssal currents are north-eastward to eastward from the dumpsite locations and would advect leaking materials toward the north-eastern Pacific. Results of advective and diffusive horizontal plume transport models are consistent with this sense of flow. Trajectory speeds are, however, subject to considerable uncertainty. Our results suggest that as little as 5 years or as long as 100 years might be required for material to be transported from the dump sites to the north-east Pacific. Dilution by 4 or 5 orders of magnitude is predicted during this transit. Vertical mixing or upwelling are necessary in order to transport contaminants upward from north-east Pacific abyssal waters to the near-surface layers before they can potentially impact productive coastal regions, such as those off Alaska. Information concerning such upweiling mechanisms is inadequate for estimation of rates or to identify geographical areas that might be at risk. © 1997 Elsevier Science Ltd. All rights reserved
Keywords: North Pacific; abyssal ocean circulation; abyssal trajectories; radionuclides; dispersion; Russian radioactive wastes.
*Author for correspondencee-mail:[email protected]
A number of Russian nuclear waste dumpsites exist on the seafloor in the north-west Pacific Ocean, the Sea of Okhotsk and the Sea of Japan (Fig. 1). This work assesses potential transport pathways from those sites situated in the north-west Pacific Ocean south-east of the Kamchatka Peninsula, as there is particular concern that radionuclide leakage from these sites might impact the coastal and shelf ecosystem in the Bering Sea and the Gulf of Alaska. The materials at the sites consist, insofar as known, of encapsulated solid and liquid wastes. This paper briefly reviews the present knowledge of circulation in the abyssal North Pacific Ocean and uses models to assess potential pathways from the Kamchatka sites. The North Pacific Ocean (Fig. 2) is one of the deeper portions of the world's oceans and can be divided into the central Pacific Basin (from about 10°S to about 20°N), the north-west Pacific Basin (north of 30°N, bounded on the north-west by the Kamchatka Peninsula and Japan, and on the east and north-east by the Hawaiian Islands and the Emperor Seamount Chain), and the north-east Pacific Basin (north of about 30°N, bounded on the west by the Hawaiian Islands and the Emperor Seamount Chain and on the east by the North American continent). Both dumpsites are situated in the north-west Pacific Basin. The westernmost site is on the edge of the Kamchatka-Kuril trench in over 6000 m of water. The eastern site is located on a relatively flat portion of the abyssal plain where the water is slightly less than 6000 m deep. This article briefly reviews our present understanding, based on conceptual and numerical model results and on field studies, of abyssal circulation in the North Pacific Ocean. Results are then presented from advective and diffusive plume trajectory models, discussed within the context of regional circulation, and used to estimate possible material trajectories leading away from the dumpsites. 353
Marine Pollution Bulletin 132 °
144 °
156 °
168 °
180 °
56 °
48 °
40 °
132 °
144 °
156 °
Fig. 1 K n o w n R u s s i a n r a d i o a c t i v e m a t e r i a l s e a - b e d d u m p s i t e s in t h e north-western Pacific Ocean. This study evaluates potential t r a j e c t o r i e s f r o m sites 4, 7 a n d 8.)
Abyssal Circulation Theories Transport of radionuclides from the Russian nuclear dumpsites to the areas of concern off Japan and Alaska is dependent on the horizontal abyssal circulation, while transport from abyssal depths to the biologically active upper layers depends upon vertical mixing and upwelling. Stommel (1958) and Stommel and Arons (1960a,b) developed the original abyssal circulation model upon which our present understanding has been built. The Stommel-Arons (Stommel and Arons, 1960a,b) model assumes that a basin-wide oceanic heat balance is maintained. Heat sinks are provided by cold, dense water which originates from the Antarctic and North Atlantic and enters the Pacific from the south ocean as deep water. Heat sources, required in order to maintain a constant temperature, are present as atmospheric and solar heating of the ocean surface. Heat diffuses slowly down from the surface, and upwelling of the cold deep waters through horizontal advection, modified by the Coriolis force, offsets the diffusion to maintain the balance. As the ensuing slow interior flow impinges on the ocean boundaries, currents are set up to satisfy the boundary conditions. In particular, western boundary currents are required to satisfy the mass balance. The Stommel-Arons (Stommel and Arons, 1960a,b) conceptualization has interesting ramifications for the North Pacific, which contains no local source of cold deep water, within the context of possible transport of 354
radionuclides from the Kamchatka region. It predicts a deep northward western boundary current which supplies water north across the equator into the deep North Pacific. The model also predicts a weak, southward flowing deep western boundary current in the northernmost part of the North Pacific. No deep southward return flow across the equator is predicted to occur. In addition, depending on the basin configuration, a northern boundary current flowing toward the west may develop along the Aleutian Island Arc. Stommel and Arons estimated total transport in the world abyssal circulation to be 15-90x 106 m 3 s - i , vertical velocities under the thermocline to be on the order of 0.5-3.0 cm day -1, and replacement time for the deep water to be on the order of 300-1800 years. In developing their model, Stommel and Arons (1960a,b) assumed a flat bottomed ocean with simple meridional boundaries, and steady state conditions. Density and dissolved property fields were not explicitly treated. More recent investigations have incorporated the effects on model results of more realistic assumptions. Warren (1976) modelled the structure of the deep western boundary currents to determine why these currents are wider than their upper ocean counterparts. For this model, he treated the boundary currents as flow corrections to the interior fields. A flat bottom was assumed. The model used linear equations, with continuously stratified conditions and lateral diffusion of density and momentum. The study concluded that the magnitude of the deep current speeds is limited by the weak abyssal density stratification, therefore, a correspondingly broader current is required to obtain the necessary mass balance. Kawase (1993) utilized a numerical model to study the effect of a concave or bowl shaped basin on the upwelling-driven abyssal circulation, and found that the sloping topography acted as a beta plane, changing the relative vorticity of the boundary currents. When the model was run with a flat bottom it produced the typical Stommel-Arons (Stommel and Arons, 1960a,b) flow regime, with a weak northward interior flow and a strong western boundary current. The same model run with a concave bottom, a closed abyssal basin, and contained completely in one hemisphere produced a basin-wide cyclonic circulation of greater strength than the flat bottom case and with less western intensification. This modelling effort did not contain realistic basin configurations (i.e. trenches or ridges), but it did predict some of the effects that may arise with the inclusion of realistic bottom topography. Pedlosky (1992) included the oceanic density field and used a linear baroclinic model to investigate the abyssal circulation. This model followed the Stommel and Arons conceptualization but utilized spatially varying upwelling from the abyss upward into the main thermocline. In the vicinity of the upwelling, barotropic meridional velocities were driven through the Sverdrup
Volume 35/Numbers 7-12/July-December 1997 I
I
t
~
I
Alaska
60 Sea o f
"~Alaska~i
55 50
II Northeast
45
"~" Pacific
Basin
Northwest
40
Pacific/j~ Basin
f
35 %
30 f
f
25 20 15 sln
10
140
150
160
170
180
190
~200
210
220
Fig. 2 Bathymetry and geographical locations for the North Pacific Ocean, indicating locations of the material dumpsites (black rectangles) that are addressed in this study. The sense of the regional abyssal ocean circulation can be determined through reference to Fig. 5.
relation (a balance between northward mass transport and the surface wind stress scaled by one over beta)• Upwelling may be locally stronger in the eastern basins, as suggested by a shallow thermocline in those regions. Pedlosky's model showed that the temperature anomalies created by upwelling in the eastern part of a basin can propagate westward as Rossby Waves, which can in turn force baroclinic meridional velocities. This baroclinic model was a natural next step in transforming the barotropic Stommel and Arons model into a tool for more accurately predicting abyssal circulation• Pedlosky and Chapman (1993) further advanced the baroclinic model by addressing the influence of meridional topography on circulation patterns• It was found that a north-south bottom slope had little effect on the interior flow but had a substantial effect on the eastern boundary flow. An e a s t e r n boundary current appeared for all bottom slopes with the anti-beta sense (down to the north, in the northern hemisphere). The
stronger the slope, the narrower and thicker the deep eastern boundary current• It was also found that the interior meridional velocities could change sign with longitudinal variations in the upwelling into the main thermocline, as in the previous models. However, for all cases studied a mass transporting western boundary current existed• Our understanding of abyssal circulation in the world's oceans is still evolving• As presently conceptualized, this circulation is dominated by deep western boundary currents which satisfy the mass balance, upwelling over most of the basin but especially in the eastern portion, and topographic features modifying the interior and boundary flows• The deep western boundary currents are broader, as compared to their upper ocean counterparts, and have correspondingly lower speeds. The western boundary current may contain two separate branches in the North Pacific, with a southward branch along the Kamchatka 355
Marine Pollution Bulletin Peninsula in the northern portion of the ocean, and a northward branch flowing in from the South Pacific across the equator. The interior flow is dominated by upwelling that is probably concentrated in the eastern basin and by a northward barotropic flow in the vicinity of the upwelling.
Abyssal Circulation Field Results Direct measurements of abyssal currents are difficult due to the weakness and high variability of these currents. Most studies have attempted to determine abyssal circulation indirectly using the distributions of density and chemical tracers. Abyssal circulation Wooster and Volkmann (1960) and Knauss (1962) first compiled historic hydrographic data and used it to estimate abyssal flow patterns in the Pacific. The deep flow patterns which they derived from water properties conform with the predictions of the Stommel and Arons model. The deep flow originates far to the south in the Antarctic and Indian Oceans via the South Pacific Basin. In passing northward through the Central Pacific Basin it bifurcates into two streams, one flowing into the north-east Pacific Basin and one into the north-west Basin. The flow volume into the north-west Pacific Basin may be the larger of the two, as there is some indication of water flowing eastward out of the northern end of this basin into the north-east Pacific Basin. This finding was consistent with the prediction, based on model results, of more intense upwelling in the northeast than elsewhere. Based on the temperature and oxygen distributions, Wooster and Volkmann (1960) hypothesized an abyssal flow pattern which they depicted as an 'arbitrary flow line' at 5000 m depth along which the in situ temperature slowly increases from 0.5°C in the southern Pacific to more than 1.5°C in the north-eastern Pacific (Fig. 3). The warming is presumed to be due to mixing with overlying warmer waters and to heating through the ocean floor. A slight salinity decrease along the flow line was presumed due to mixing with the lower salinity overlying water. Dissolved oxygen content decreased along the flow line, from 5.0-3.4mll -1, due to consumption by oxidative processes and mixing with the overlying water. Knauss (1962) estimated overall net northward speeds for this flow to be on the order of 0.05-0.1 cm s-1 with a corresponding volume transport of 15-25×106 m 3 s -1. In the North Pacific, this flow line suggests northward flow in the west turning gradually east toward the north-east Pacific. There is no measured or hypothesized abyssal return flow across the equator, therefore it is presumed that north-flowing water must upwell in the North Pacific. Upwelling is probably most intense in the eastern portion. Based on water temperature and oxygen content, the north-east Pacific Basin contains water 356
which passes by the Aleutian Trench, through the Emperor Seamount Chain near Midway Island, and from the central Pacific Basin, that is longest removed from ventilation with the atmosphere (Wooster and Volkmann, 1960; Knauss, 1962). This conclusion is, again, consistent with the north-east Pacific being the final destination of abyssal water originating in the Southern Ocean and North Atlantic and the likely site for upwelling. Mantyla and Reid (1983) conducted a survey using more recent and higher quality abyssal hydrographic data and confirmed many of these hypotheses. The horizontal distribution of potential temperature at 5000 m for the World Ocean and for the North Pacific illustrates the major features discussed above (Fig. 4). This figure shows the coldest (and presumably newest, assuming a cold source in the Antarctic) water surrounding the Antarctic and the warmest (and presumably oldest) water to occur in the north-east Pacific Basin. Even though the temperature gradients in the North Pacific Ocean are relatively small, the hypothesized flow patterns are supported as the water warms during its passage along Wooster and Volkmann, 1960 'arbitrary flow line' (Fig. 3). Boundary circulation: the Kamchatka-Kuril and Aleutian trenches Several regional field studies have addressed the deep boundary currents in the North Pacific Ocean. Particular efforts have been put into studying the Japan, Kamchatka-Kuril and Aleutian trenches. Nan'niti and Akamatsu (1966) found a southward flow between 2000 and 3000 m in the Japan Trench using drifters. Reed (1970) collected hydrographic and mooring data across the Aleutian Trench and detected a 'stream' of anomalous water south of the Aleutian Islands. Reed (1970) hypothesized that this water originated from a source west of the Emperor Seamounts, thereafter flowing northward along the Seamounts into the Aleutian Trench after which it flowed east along the Trench. Warren and Owens (1988) observed, using hydrographic and current data, two nearly zonal deep currents in the Aleutian Trench which increased in speed from the bottom to near surface maxima. North of the Trench, and close to the Aleutian Island Arc, was the westward flowing deep extension of the Alaskan Stream extending to 4000-4500 m depths. Warren and Owens (1988) hypothesized that this current is the boundary current predicted by a Stommel-Arons (Stommel and Arons, 1960a,b) type circulation. Since the North Pacific does not have a strong separation between deep and shallow flows, the deep boundary current extends upward nearly to the surface. Warren and Owens (1988) found a second current flowing eastward along the south side of the Aleutian Trench and extending onto the Aleutian Rise. This is presumably the current suggested by Reed (1970). They
Volume 35/Numbers 7-12/July-December 1997 Scripps [] Albatross 0 Sncnius V Carnegie • Discovery
Fig. 3 In Situ abyssal temperatures (°C, upper numbers) and dissolved ] oxygen (ml 1-, lower number, when available) at 5000 m depth in the Pacific Ocean (Wooster and Volkmann, 1960). Dashed line indicates the 'arbitrary flow line' that represents a trajectory for north-flowingwaters.
hypothesize that this current is forced by the topography of the Aleutian Rise, which slopes downward towards the Aleutian Trench. Their maps of potential temperature indicate that this current may extend as far west as the Zenkevich Rise. They predict that this current flows eastward into the Gulf o f Alaska, at which point the current recirculates and flows west as part o f the Alaskan Stream. This continuity in the
current is consistent with potential temperature patterns at 2000 and 4000 m, as isotherms in the vicinity of the Aleutian Trench connect with those in the vicinity of the Kamchatka-Kuril Trench (Warren and Owens, 1988). Talley e t al. (1991) measured hydrographic and chemical parameters along zonal and meridional deep ocean transects along 47°N and 152°W, respectively. Isotherms in the deep portion of the N o r t h Pacific Basin 357
Marine Pollution Bulletin
P o t e n t i a l T e m p e r a t u r e (*C) at 5 0 0 0 m I
I
I
I
I
I
--
I
80
60 40 20 e) -IP
0 e~
-20 -40 -60 -80 0
I
I
I
50
i00
150
-0.5
0
r
200 Longitude
0.5
1
1.5
I
I
I
250
300
350
2
2.5
P o t e n t i a l T e m p e r a t u r e (*C) a t 5 0 0 0 m in the N o r t h Pacific 60
50
40
"m 3o ell
20
10
0
130
140
150
0.8
160
170
0.9
180 190 Longitude
1
200
1.1
210
1.2
Fig. 4 Abyssal potential temperatures (°C) at 5000 m depth from the NODC data archives for the World and the North Pacific Oceans, showing the warmest, hence oldest, water occurring in the north-east Pacific. 358
220
230
Volume 35/Numbers 7-12/July-December 1997 slope down to the east, indicative of a northward flow in the interior, and are consistent with a Stommel-Arons (Stommel and Arons, 1960a,b) flow pattern. Flow was shown, based on water properties (low potential temperature, silica, phosphate and nitrate; high salinity, density, and oxygen), to be northward in the deeper portions of the Kuril-Kamchatka Trench (below 4500 m) between 42°N and 49°N. The trend continues eastward to the Warren and Owens (1988) section discussed above. The hydrographic section crossing the Aleutian Trench at 152°W shows a very similar structure to that of the Warren and Owens (1988) section and suggests westward flow above and north of the Aleutian Trench and eastward flow south of the trench and above the Aleutian Rise. The flow becomes more complex immediately above the Kamchatka-Kuril Trench, and the circulations inferred from water properties and geostrophic equations are less reliable than in the regions of simpler bathymetry. A narrow southward flowing boundary current at about 2000 m depth may be the extension of the westward flowing boundary current in the Aleutian Trench. Northward flow was present near 3000 m above the center and west side of the trench. There is some indication of southward flow above the eastern portion of the trench and northward flow again just east of the trench, above the Zenkevich Rise. There is no clear connection between these currents and the deep currents in the Aleutian Trench. Recently, the first direct measurements of the western boundary currents were made in the vicinity of the Kamchatka-Kuril Trench using RAFOS floats (S. Riser pers. comm. 1995). The floats were deployed just south of the Talley et al. (1991) 47°N hydrographic line, along a transect perpendicular to the trench at 3000 m depths. The easternmost floats (deployed over the Zenkevich Rise) were entrained into a deep portion of the Kuroshio and were transported eastward into the north-west Pacific Basin (south of the dump sites). The floats near and just east of the axis of the trench moved south at speeds less than 2 cm s - ' . The floats along the western edge of the trench, over its eastern portion, and at the start of the Zenkevich Rise flowed north at speeds from 1-3 cm s -1. The floats showed consistent speeds with very little temporal variation and little dispersion. This flow pattern agrees with the general trends found in the hydrographic data collected by Talley, but with a stronger northerly component. An analysis of the silica, oxygen, temperature and salinity distributions, assuming these flow directions, indicates that the younger water is flowing northward and the older water is flowing southward, in general agreement with a Stommel-Arons (Stommel and Arons, 1960a,b) type model. The hydrographic data from Talley et al. (1991) and other researchers show the density distribution along the trench to be relatively constant, therefore, it is plausible that the current distribution measured by these
floats continues along the length of the trench and up to the junction with the Aleutian Trench. The boundary currents near the Aleutian Trench have a different distribution than those around the Kamchatka-Kuril Trench, with one current flowing westward on the north side of the trench, that extends to the bottom and is the Alaskan Stream. The second flows eastward on the south side of the trench and over the rise to the south of the trench. In the far north-west corner of the Pacific these currents turn, mix, and recombine with those of the Kamchatka-Kuril system and those from the Bering Sea flowing south through the Kamchatka Strait. This area has not been sampled adequately to define the flow patterns. The area may in addition, have strong temporal variability which would complicate the analysis. Aleutian pass currents
In order to impact the Bering Sea, any contaminants present in the North Pacific must flow north through channels in the Aleutian Island Arc. All of these channels save the westernmost, the Kamchatka Strait, have shallow sills and allow only upper layer exchange between the Bering Sea and the North Pacific Ocean. Sayles et al. (1979) report the depths as 430 m for Amukta Pass, 1155 m for Amchitka Strait, 640 m for Buldir Pass, 2000 m for Near Strait and 4420 m for the unsilled Kamchatka Strait. Kamchatka Strait, because of its lack of a sill, is the only one of these channels through which abyssal transport of contaminants into the Bering Sea from the radionuclide dump sites might occur. Reed et al. (1993) interpreted the temperature distribution from a transect across Kamchatka Strait as being consistent with southward near-surface currents, reflecting the influence of the Kamchatka Current which flows out of the Bering Sea and into the north-west Pacific. A northward current, in the proper sense to transport contaminants from the North Pacific into the Bering Sea, was inferred at depth from the temperature, salinity and silica distributions. We do not, however, know the speeds associated with this flow.
Particle Diffusion Models Two numerical models have been used to study the dispersion of a tracer released near the dump sites. The first is a Lagrangian dispersion model which uses currents and bathymetry derived from the Semtner and Chervin (1992) 0.5 °, 20-level Parallel Ocean Circulation Model (POCM). This model (Dewey and Stegen, 1995a,b) tracks neutrally buoyant particle locations as they are advected by interpolated monthly mean POCM derived currents. Mixing, needed to provide dispersion of the advected material at higher frequencies and smaller scales than provided by the monthly mean currents, is provided in the model through locally scaled stochastic turbulence. 359
Marine Pollution Bulletin
This Lagrangian dispersion model is a realistic oceanic diffusion model in which each particle or ‘cloud’ of material is injected into the water column at a specified location (longitude, latitude and depth) and subsequently advected by a time dependant, turbulent ocean. Local particle densities are later converted back into local material concentrations. The model has been used in other instances to evaluate the dispersion characteristics of various sites throughout the global ocean, where control of the geographic location, depth, injection rate and turbulent intensity over the resulting material plume can be evaluated. The neutrally buoyant particles are tracked as they are advected by the oceanic velocity interpolated to the exact location of each particle according to Eqn (l), [u, v, WI=[(U(x,_Y,z, t)
+ V’(X,Y,z, Q), (Vx>Y,z,
t) + ~‘(XJV, Q)l> (1)
where [U, V, w] are the temporally and spatially interpolated large scale velocities at the particle locations derived from the monthly POCM velocity data (1/2”x 1/2”x20 vertical layers), and [u’,v’,w’]are the locally estimated stochastic turbulent velocities. The large scale velocity field is gridded at the resolution set by the POCM, however, the velocities of each particle are spatially and temporally interpolated to the exact position of the particle. Although the POCM data have global coverage, a limited dispersion model domain (25”x25”) is established around each injection site. The monthly POCM data were generated by a seasonally forced run of the POCM using bi-harmonic horizontal diffusivities (Semtner and Chervin, 1992). The stochastic components of the velocity field represent the influence of sub-grid scale turbulence not resolved by the POCM. In stochastic models where the mean flow is weak in comparison to the turbulent velocities, the fluctuating components can be modelled as a first order Markov process (Sawford, 1984; Bennett, 1987) using the Langevin equation. The horizontal components (u’,v’) can be obtained using
(2) where At is the dispersion model time step, TL is the Lagrangian integral time scale of the turbulence, 02 is the variance (energy density) of the local turbulent velocities, and 5 is a random number drawn independently from a Gaussian distribution with zero mean and unit variance. The vertical component of the stochastic velocity field (w’) is derived separately by scaling the square root of the horizontal turbulent energy density by the local Richardson number (Rt), 360
where the Ri is calculated from the time dependant stratification and shear contained within the POCM data using, g 8P Ri =
-“” (Z)
+(ig2
This first order approximation for suppressing the vertical turbulent components in the stratified ocean allows for more active vertical mixing in stronger shear zones and within weakly stratified regions. We also note that w’ does not contain any ‘memory’, since the Lagrangian integral time scale for vertical motions are much shorter than the model time step (424 h). The Lagrangian time scale for the horizontal turbulence is set equal to the local inertial adjustment period. This can be approximated by one-half the local inertial period,
where R is the rotation rate of the Earth (=7.292x10w5 s-l) and 4 is the local latitude. However, if this Lagrangian integral time scale is longer than the dispersion model time step, then the ratio (At/TL) is set equal to zero (no memory) and the stochastic forcing becomes a simple random walk. Flow components with integral time scales longer than onehalf the inertial period are strongly influenced by the Earth’s rotation, and are not considered ‘turbulence’ by this formulation since they must obey geophysical dynamics as contained within the underlying POCM. Finally, the energy density of the local turbulent velocities (a:) is parameterized to be a fixed fraction of the local variance in the mean velocity field. In other words, the local turbulent kinetic energy (TKE) is scaled to 5% of the local mean kinetic energy (MKE). The value of 5% (0.05) is consistent with in situ observations of oceanic turbulence (Dewey et al., 1991). Plume structures are dominated by the time, space, and divergence characteristics of the ‘mean’ advecting velocity field. The stochastic components act to slowly migrate particles across streamlines in the mean flow, providing a mechanism for cross-flow dispersion. The inherent divergence in the mean velocity field governs and dominates the fate of the injected particles. Model particles are reflected from the POCM boundaries (i.e. the ocean surface and bottom), and ‘lost’ once they leave the limited (25”x25”) domain of the dispersion model. The August 15 currents from the particle dispersion model show typical features of the region with enhanced speeds near the boundaries and steering by the local
Volume 35/Numbers 7-12/July-December 1997
bottom topography (Fig. 5). Current speeds in the middle of the basin are on the order of 1-2 cm s-~, while speeds in the boundary regions are closer to 35 cm s - t . These are roughly consistent with speeds derived from the limited field data summarized above. After 10 years the modelled particles have been advected southward along the Emperor Seamount chain and passed into the east Pacific Basin over deep sills (Fig. 6). The source site was chosen as 50°N, 160°E and 5000 m depth. The concentrations indicate a relative concentration of a tracer, with the white areas indicating zero concentration. Once in the east Pacific the particles moved toward the north-east and the Gulf of Alaska. The modelled dispersion has a number of stagnation points where particles tended to cluster (indicated by the highest concentrations on the figure). The 0.5 ° grid size of the Semtner and Chervin (1992) model prevented flow through the Aleutian Trench and the Kamchatka Strait at these depths.
The second model is based on the circulation field from a 15 level ocean general circulation model developed at the Max-Planck Institut fur Meterologie, Hamburg, Germany for biochemical modelling (Bacastow and Maier-Reimer, 1990; Maier-Reimber, 1993). Mixing is through numerical diffusivity (related to the finite box size), a small explicit horizontal diffusivity and convective adjustment. The 72 x 72 grid is staggered, with a nominal resolution of 2.5°N-S by 5°E-W. The model uses a yearly averaged circulation field, and the tracer is injected with each time step in the cell centered at 48.75°N and 163.75°W. This model is useful for showing the gross features of dispersion over long time scales (greater than 10 years). The results showed dispersion and time scales similar to those of the Lagrangian model, but with a smoother distribution due to the grid size (Fig. 7). The two models show extremes, with the Lagrangian having low diffusivity and a large amount of advection, and the
15 August POCM Velocities at 5000m, Plotted Every 1°
60
50
~
./
/
J/('
~
-
40 -
:
-
:
~ ' , ' : : : : : i
-
" ~/~-~-~
~
,
~--'"~-~-~
r.~
~
~..'4.'~r",','.--T:-
"\-
j
'~
L/~
~". :-. . . . . . . .
/: : : - : . : : -. ," ', ." .':.": ." . -- - - : ' : ; ,
,y~,.4, -. . ,.
s/e..,.../~,J.*.
: - , :_ : : : : .: : - . : . . . .
~ -
:~,
( , , / ~ 4-- Q , -","" . , , ' , , "',' , ,
,'~(ta v
~o
-
J I _/~
~
)
~ ~ : _ ' - : ; : - : : : . - - . " , : ' . : , • ~
--
~,~., . . .". . "
\~
~, '' ,
....... .
.
.
-:;~,
~,,
.
~ _ _
['~.~....
,
. ~.~.
, ,, ~,
I, . . . . . " ' , ' ~ ' , ' 1
. . . . . . . .
z,.
;. • \ - _ ~y,~ ".... ... . . .~., ..' ; .. ~.. .. . .. . .. ~. . . . .... ... . . ~-.'~." . . ,~,...
~.
',
."
......... t-:~;';:
. . . . . . . . ., . . .
,,
j ~.L~__.,
'{~
',~
c"--~"%. , ;--(~
~r.-~.",
l,-t-, I Z/~
~';
.
"~
.
,
-,'k'(:::::::
,
-' '..:-'--.
. . . . .
"t
. . . . . . . . . . . . . . ;,''7"
.....................
,,
1
. . . . . .
" ~ - - ,," . - ". ~. . . '. . . ~. . . .~. . . -. . . - ~ / / ' ' ,": . . . . .
.z, ..............
I
/
.'.','-';:::.
,', . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
'-:--~-~ . . . . .
,m
I
• . , ~ . . . . . . . . . . . . . . . . . . . . .
. . . . . .
"~',." ; ............
-
~:
,...::
: ,',,
• ._.~
),,:..,
.....
~,--~.,
..
._"..,,~
'
I.~..A
~'~,k'.-a~---./
"~_i
.,,,
.
,,...,.-~,/.,,
~,_.,~_- ..~ ~
" : <
~
~ ."/'.,L'".
%.. -10
I'-
%
,~J ~ . , . ' .
; z_-:7-". ,
, ~ 'E
" 130
I
I
I
140
150
160
•
I 170
~
I
180 190 Longitude (W)
.t/~ , % "/__.
K'<-
:"
I
I
I
I
200
210
220
230
Fig. 5 Monthly mean currents on August 15 at 5000 m depth from the POCM Model (Semtner and Chervin, 1992), as used by the Lagrangian Dispersion Model (Dewey and Stegen, 1995a, b). Note the prominant north-eastward abyssal flow in the northwest Pacific off Kamchatka.
361
Marine Pollution Bulletin Concentration
(units)
50°
% %
40
155
160
165
170
175
180
Longitude (E) Fig. 6 Vertically integrated concentrations (relative to the assumed release concentration) of a tracer released at 5000 m depth at 50°N and 160°E, computed from the Lagrangian Dispersion Model after 10 years (Dewey and Stegen, 1995a,b). Lateral dispersion of the tracer has presumablynot stopped (the model run was stopped after a 10-year model run), but has significantly slowed due to constraints imposed on the abyssal circulation by the bottom topographic features bounding the north-east Pacific Basin(see Fig. 2).
second model having high diffusivity and a low amount of advection.
Conclusions The conceptual and numerical model results, field results and trajectory model output presented above suggest four possible flow pathways from the dumpsites: • advection northward along the Kamchatka Trench, through the Kamchatka Strait (4420 m) and into the Bering Sea; • advection northward along the Kamchatka Trench, or advection by the abyssal currents in the north-west Pacific Basin, to the Aleutian Trench, then eastward along the southern edge of the Aleutian Trench to the Gulf of Alaska; • eastward advection in the north-west Pacific Basin through gaps in the Emperor Seamount chain (sill depth of about 5000 m) into the north-east Pacific Basin, then northward to the Aleutian Trench and continuing on to the Gulf of Alaska; and, finally, 362
• slow upwelling to the mid-depth circulation and then flow at mid-depth towards Alaska with potential upper layer outcropping. The first three pathways would transport radionuclides into the abyssal north-east Pacific Ocean or Bering Sea bordering Alaska. Upwelling of this deep water would then be required to transport radionuclides from the abyss into the biologically active upper ocean. Additionally, upwelling would be required for the second and third pathways before passage of dissolved materials through the Aleutians into the Bering Sea. While this occurs according to the theory summarized above, which holds that such upwelling may in fact be strongest in the north-east Pacific Ocean, available observational and modelling results are inadequate to predict an upwelling rate. The above pathways involve generally slow advection speeds (1~, cm s - I in the boundary currents and 13 cm s-~ in the interior flows), and significant mixing may occur along the pathway. The actual spreading rates may, however, be considerably less than implied by these numbers. Doney and Jenkins (1994) researched
Volume 35/Numbers 7-12/July-December 1997 Log(Concentration). D e p t h = 4 0 0 0 m 80 70!
60
,-, 40 Z gl
,~
30 ¸ 2O
10 0
-10 -20
250
200
150
100
Longitude (W)
-9
- 8
-7
-6
-5
-4
Logl0 Tracer Concentration Fig. 7 Concentration of a tracer after 10 years injected at 48.75°N and 163.75°E at 4000 m depth with a local bottom depth of 4785 m. Concentrations are shown for level 14, with the model interfaces for this depth level at 3500 and 4500 m (Bacastow and Maier-Reimer, 1990). The tracer has not become isolated in the strict sense of the word. However, after first spreading eastward it has not advected significantly outward from the north-east Pacific Basin which contains the oldest water in the Pacific Ocean (see also Fig. 4).
the spreading rate o f tritium and 3He in the deep western boundary currents in the N o r t h Atlantic and found the spreading rates of the tracers to be slower than the measured currents. Directly measured currents in the area ranged from 4-10 cm s - t , whereas tracer spreading rates were measured to be 1-2 cm s - 1. Doney and Jenkins (1994) hypothesized that the current has recirculating cells that allow the higher current speeds and lower spreading rates. Whatever the mechanism, if it acts similarly in the abyssal currents of the Pacific, then the transport rates o f the radionuclides might be slower by a factor o f 2 to 4 than suggested above. At the high-end advection rate o f 4 cm s -~, about 5 years would be required for materials to reach the north-east Pacific Ocean from the dumpsites, while at the low-end rate o f 1 cm s - t, decreased still farther by a factor of 5
to allow for Doney and Jenkins (1994) results, about 100 years would be required. Our information on regional scale abyssal advective processes is at present inadequate to provide a significantly better estimate of this time scale. The numerical simulations show 4-5 order of magnitude decreases in particle concentrations along the pathway from their initial locations at the dumpsites to their final abyssal locations, yielding dilutions of 104105 .
The authors wish to thank Bert Semtner for use of the POCM data and Ernst Maier-Reimer for providing their 15 level circulation field. This work was supported by the ONce of Naval Research under contract # N00014-93-C-0222with Science Applications International Corporation. 363
Marine Pollution Bulletin Bacastow, R. and Maier-Reimer, E. (1990) Ocean-circulation model of the carbon cycle. Climate Dynamics 4, 95-125. Bennett, A. F. (1987) A Lagrangian analysis of turbulent diffusion. Reviews in Geophysics 24, 799--822. Dewey, R. K., Mourn, J. N., Paulson, C. A., Caldwell, D. R. and Pierce, S. D. (1991) Structure and dynamics of a coastal filament. Journal of Geophysical Research 96, 14885-14907. Dewey, R. K. and Stegen, G. R. (1995a) A global ocean 3d plume dispersion model. Proceedingsof the lOth Conferenceon Engineering Mechanics, Boulder, CO, May 22, Vol. 1, pp. 207-210. Dewey, R. K. and Stegen, G. R. (1995b) A 3-dimensional ocean plume model using stochastic dispersion. Proceedings for Comptational Fluid Dynamics, Banff, AL, June 25-27, Vol 1. Doney, S. C. and Jenkins, W. J. (1994) Ventilation of the deep western boundary current and abyssal western North Atlantic: estimates from tritium and He 3 distributions. Journal of Physical Oceanography 24, 638--659. Kawase, M. (1993) Effects of a concave bottom geometry on the upwelling-driven circulation in an abyssal ocean basin. Journal of Physical Oceanography 23, 400-405. Knauss, J. A. (1962) On some aspects of the deep circulation of the Pacific. Journal of Geophysical Research 67, 3943--3954. Mantyla, A. W. and Reid, J. L. (1983) Abyssal characteristics of the world oceans. Deep-Sea Research 30, 805-833. Maier-Reimber, E. (1993) Geochemical cycles in an ocean general circulation model: preindnstrial tracer distributions. Global Biochemical Cycles 7, 645--677. Nan'niti, T. and Akamatsu, H. (1966) Deep current observations in the Pacific Ocean near the Japan Trench. Journal of the Oceanography Society of Japan 22, 154-160. Pedlosky, J. (1992) The Baroelinic Structure of the Abyssal Circulation. Journal of Physical Oceanography 22, 652-659. Pedlosky, J. and Chapman, D. C. (1993) Baroclinic structure of the
364
abyssal circulation and the role of meridional topography. Journal of Physical Oceanography 23, 979-991. Reed, R. K. (1970) On the anomalous deep water south of the Aleutian Islands. Journal of Marine Research 28, 371-372. Reed, R. K., Khen, G. V., Stabeno, P. J. and Verkhunov, A. V. (1993) Water properties and flow over the deep Bering Sea basin, summer 1991. Deep-Sea Research 140, 2325--2334. Sawford, B. L. (1984) The basis for, and some limitations of, the Langevin Equation in atmospheric relative dispersion modelling. Atmospheric Environment 18, 2405-2411. Sayles, M. A., Aagaard, K. and L. K. Coachman (1979) Oceanographic Atlas of the Bering Sea Basin. University of Washington Press. Semtner, A. J. Jr and Chervin, R. M. (1992) Ocean general circulation from a global eddy-resolving model. Journal of Geophysical Research 97, 5493-5550. Stommel, H. (1958) The Abyssal Circulation. Deep-Sea Research 5, 580-582. Stommel, H. and Arons, A. B. (1960a) The Abyssal Circulation of the World Ocean I. Deep-Sea Research 6, 140-154. Stommel, H. and Arons, A. B. (1960b) The Abyssal Circulation of the World Ocean II. Deep-Sea Research 6, 217-233. TaUey, L. D., Joyce, T. M. and DeSzoeke, R. A. (1991) Transpacific sections at 47°N and 152°W: distribution of properties. Deep-Sea Research 38(Suppl.), 63-82. Warren, B. A. (1976) Structure of deep western boundary currents. Deep-Sea Research 23, 129-142. Warren, B. A. and Owens, W. B. (1988) Deep currents in the central subarctic Pacific Ocean. Journal of Physical Oceanography 18, 529551. Wooster, W. S. and Volkmann, G. H. (1960) Indications of deep Pacific circulation from the distribution of properties at five kilometers. Journal of Geophysical Research 65, 1239-1249.
Marine Pollution Bulletin, Vol. 35, Nos 7-12. pp. 353-364. 1997 © 1997 Elsevier Science Ltd. All rights rese~'ed Printed in Great Britain 0025-326X/97 $I 7,00+0.110
Potential Radionuclide Transport Pathways from Seafloor Dumpsites: Kamchatka Region of the North Pacific Ocean MARK D. MOREHEAD*, ROBIN D. MUENCHt, ROBERT BACASTOW:~ and RICHARD K. DEWEY§
*Institute of Arctic & Alpine Research, University of Colorado, 1560 30th Street Boulder, CO 80309-0450, USA tEarth & Space Research, 1910 Fairview Avenue East, Suite 102, Seattle, Washington 98102-3620, USA ~Scripps Institute of Oceanography, University of California, La Jolla, CA 92093-0220, USA §Centre for Earth & Ocean Research, University of Victoria, P.O. Box 1700, MS 4015, Victoria, BC V8W 2Y2, Canada
Encapsulated nuclear waste materials, dumped by Russia, are present at two deepwater seafloor locations in the offshore north-west Pacific Ocean, south-east of the Kamchatka Peninsula. This paper assesses potential pathways by which these wastes might, if released from their containers, disperse away from the dumpsites and through the surrounding ocean. A review of large-scale ocean circulation theory and of field and model results suggests that mean abyssal currents are north-eastward to eastward from the dumpsite locations and would advect leaking materials toward the north-eastern Pacific. Results of advective and diffusive horizontal plume transport models are consistent with this sense of flow. Trajectory speeds are, however, subject to considerable uncertainty. Our results suggest that as little as 5 years or as long as 100 years might be required for material to be transported from the dump sites to the north-east Pacific. Dilution by 4 or 5 orders of magnitude is predicted during this transit. Vertical mixing or upwelling are necessary in order to transport contaminants upward from north-east Pacific abyssal waters to the near-surface layers before they can potentially impact productive coastal regions, such as those off Alaska. Information concerning such upweiling mechanisms is inadequate for estimation of rates or to identify geographical areas that might be at risk. © 1997 Elsevier Science Ltd. All rights reserved
Keywords: North Pacific; abyssal ocean circulation; abyssal trajectories; radionuclides; dispersion; Russian radioactive wastes.
*Author for correspondencee-mail:[email protected]
A number of Russian nuclear waste dumpsites exist on the seafloor in the north-west Pacific Ocean, the Sea of Okhotsk and the Sea of Japan (Fig. 1). This work assesses potential transport pathways from those sites situated in the north-west Pacific Ocean south-east of the Kamchatka Peninsula, as there is particular concern that radionuclide leakage from these sites might impact the coastal and shelf ecosystem in the Bering Sea and the Gulf of Alaska. The materials at the sites consist, insofar as known, of encapsulated solid and liquid wastes. This paper briefly reviews the present knowledge of circulation in the abyssal North Pacific Ocean and uses models to assess potential pathways from the Kamchatka sites. The North Pacific Ocean (Fig. 2) is one of the deeper portions of the world's oceans and can be divided into the central Pacific Basin (from about 10°S to about 20°N), the north-west Pacific Basin (north of 30°N, bounded on the north-west by the Kamchatka Peninsula and Japan, and on the east and north-east by the Hawaiian Islands and the Emperor Seamount Chain), and the north-east Pacific Basin (north of about 30°N, bounded on the west by the Hawaiian Islands and the Emperor Seamount Chain and on the east by the North American continent). Both dumpsites are situated in the north-west Pacific Basin. The westernmost site is on the edge of the Kamchatka-Kuril trench in over 6000 m of water. The eastern site is located on a relatively flat portion of the abyssal plain where the water is slightly less than 6000 m deep. This article briefly reviews our present understanding, based on conceptual and numerical model results and on field studies, of abyssal circulation in the North Pacific Ocean. Results are then presented from advective and diffusive plume trajectory models, discussed within the context of regional circulation, and used to estimate possible material trajectories leading away from the dumpsites. 353
Marine Pollution Bulletin 132 °
144 °
156 °
168 °
180 °
56 °
48 °
40 °
132 °
144 °
156 °
Fig. 1 K n o w n R u s s i a n r a d i o a c t i v e m a t e r i a l s e a - b e d d u m p s i t e s in t h e north-western Pacific Ocean. This study evaluates potential t r a j e c t o r i e s f r o m sites 4, 7 a n d 8.)
Abyssal Circulation Theories Transport of radionuclides from the Russian nuclear dumpsites to the areas of concern off Japan and Alaska is dependent on the horizontal abyssal circulation, while transport from abyssal depths to the biologically active upper layers depends upon vertical mixing and upwelling. Stommel (1958) and Stommel and Arons (1960a,b) developed the original abyssal circulation model upon which our present understanding has been built. The Stommel-Arons (Stommel and Arons, 1960a,b) model assumes that a basin-wide oceanic heat balance is maintained. Heat sinks are provided by cold, dense water which originates from the Antarctic and North Atlantic and enters the Pacific from the south ocean as deep water. Heat sources, required in order to maintain a constant temperature, are present as atmospheric and solar heating of the ocean surface. Heat diffuses slowly down from the surface, and upwelling of the cold deep waters through horizontal advection, modified by the Coriolis force, offsets the diffusion to maintain the balance. As the ensuing slow interior flow impinges on the ocean boundaries, currents are set up to satisfy the boundary conditions. In particular, western boundary currents are required to satisfy the mass balance. The Stommel-Arons (Stommel and Arons, 1960a,b) conceptualization has interesting ramifications for the North Pacific, which contains no local source of cold deep water, within the context of possible transport of 354
radionuclides from the Kamchatka region. It predicts a deep northward western boundary current which supplies water north across the equator into the deep North Pacific. The model also predicts a weak, southward flowing deep western boundary current in the northernmost part of the North Pacific. No deep southward return flow across the equator is predicted to occur. In addition, depending on the basin configuration, a northern boundary current flowing toward the west may develop along the Aleutian Island Arc. Stommel and Arons estimated total transport in the world abyssal circulation to be 15-90x 106 m 3 s - i , vertical velocities under the thermocline to be on the order of 0.5-3.0 cm day -1, and replacement time for the deep water to be on the order of 300-1800 years. In developing their model, Stommel and Arons (1960a,b) assumed a flat bottomed ocean with simple meridional boundaries, and steady state conditions. Density and dissolved property fields were not explicitly treated. More recent investigations have incorporated the effects on model results of more realistic assumptions. Warren (1976) modelled the structure of the deep western boundary currents to determine why these currents are wider than their upper ocean counterparts. For this model, he treated the boundary currents as flow corrections to the interior fields. A flat bottom was assumed. The model used linear equations, with continuously stratified conditions and lateral diffusion of density and momentum. The study concluded that the magnitude of the deep current speeds is limited by the weak abyssal density stratification, therefore, a correspondingly broader current is required to obtain the necessary mass balance. Kawase (1993) utilized a numerical model to study the effect of a concave or bowl shaped basin on the upwelling-driven abyssal circulation, and found that the sloping topography acted as a beta plane, changing the relative vorticity of the boundary currents. When the model was run with a flat bottom it produced the typical Stommel-Arons (Stommel and Arons, 1960a,b) flow regime, with a weak northward interior flow and a strong western boundary current. The same model run with a concave bottom, a closed abyssal basin, and contained completely in one hemisphere produced a basin-wide cyclonic circulation of greater strength than the flat bottom case and with less western intensification. This modelling effort did not contain realistic basin configurations (i.e. trenches or ridges), but it did predict some of the effects that may arise with the inclusion of realistic bottom topography. Pedlosky (1992) included the oceanic density field and used a linear baroclinic model to investigate the abyssal circulation. This model followed the Stommel and Arons conceptualization but utilized spatially varying upwelling from the abyss upward into the main thermocline. In the vicinity of the upwelling, barotropic meridional velocities were driven through the Sverdrup
Volume 35/Numbers 7-12/July-December 1997 I
I
t
~
I
Alaska
60 Sea o f
"~Alaska~i
55 50
II Northeast
45
"~" Pacific
Basin
Northwest
40
Pacific/j~ Basin
f
35 %
30 f
f
25 20 15 sln
10
140
150
160
170
180
190
~200
210
220
Fig. 2 Bathymetry and geographical locations for the North Pacific Ocean, indicating locations of the material dumpsites (black rectangles) that are addressed in this study. The sense of the regional abyssal ocean circulation can be determined through reference to Fig. 5.
relation (a balance between northward mass transport and the surface wind stress scaled by one over beta)• Upwelling may be locally stronger in the eastern basins, as suggested by a shallow thermocline in those regions. Pedlosky's model showed that the temperature anomalies created by upwelling in the eastern part of a basin can propagate westward as Rossby Waves, which can in turn force baroclinic meridional velocities. This baroclinic model was a natural next step in transforming the barotropic Stommel and Arons model into a tool for more accurately predicting abyssal circulation• Pedlosky and Chapman (1993) further advanced the baroclinic model by addressing the influence of meridional topography on circulation patterns• It was found that a north-south bottom slope had little effect on the interior flow but had a substantial effect on the eastern boundary flow. An e a s t e r n boundary current appeared for all bottom slopes with the anti-beta sense (down to the north, in the northern hemisphere). The
stronger the slope, the narrower and thicker the deep eastern boundary current• It was also found that the interior meridional velocities could change sign with longitudinal variations in the upwelling into the main thermocline, as in the previous models. However, for all cases studied a mass transporting western boundary current existed• Our understanding of abyssal circulation in the world's oceans is still evolving• As presently conceptualized, this circulation is dominated by deep western boundary currents which satisfy the mass balance, upwelling over most of the basin but especially in the eastern portion, and topographic features modifying the interior and boundary flows• The deep western boundary currents are broader, as compared to their upper ocean counterparts, and have correspondingly lower speeds. The western boundary current may contain two separate branches in the North Pacific, with a southward branch along the Kamchatka 355
Marine Pollution Bulletin Peninsula in the northern portion of the ocean, and a northward branch flowing in from the South Pacific across the equator. The interior flow is dominated by upwelling that is probably concentrated in the eastern basin and by a northward barotropic flow in the vicinity of the upwelling.
Abyssal Circulation Field Results Direct measurements of abyssal currents are difficult due to the weakness and high variability of these currents. Most studies have attempted to determine abyssal circulation indirectly using the distributions of density and chemical tracers. Abyssal circulation Wooster and Volkmann (1960) and Knauss (1962) first compiled historic hydrographic data and used it to estimate abyssal flow patterns in the Pacific. The deep flow patterns which they derived from water properties conform with the predictions of the Stommel and Arons model. The deep flow originates far to the south in the Antarctic and Indian Oceans via the South Pacific Basin. In passing northward through the Central Pacific Basin it bifurcates into two streams, one flowing into the north-east Pacific Basin and one into the north-west Basin. The flow volume into the north-west Pacific Basin may be the larger of the two, as there is some indication of water flowing eastward out of the northern end of this basin into the north-east Pacific Basin. This finding was consistent with the prediction, based on model results, of more intense upwelling in the northeast than elsewhere. Based on the temperature and oxygen distributions, Wooster and Volkmann (1960) hypothesized an abyssal flow pattern which they depicted as an 'arbitrary flow line' at 5000 m depth along which the in situ temperature slowly increases from 0.5°C in the southern Pacific to more than 1.5°C in the north-eastern Pacific (Fig. 3). The warming is presumed to be due to mixing with overlying warmer waters and to heating through the ocean floor. A slight salinity decrease along the flow line was presumed due to mixing with the lower salinity overlying water. Dissolved oxygen content decreased along the flow line, from 5.0-3.4mll -1, due to consumption by oxidative processes and mixing with the overlying water. Knauss (1962) estimated overall net northward speeds for this flow to be on the order of 0.05-0.1 cm s-1 with a corresponding volume transport of 15-25×106 m 3 s -1. In the North Pacific, this flow line suggests northward flow in the west turning gradually east toward the north-east Pacific. There is no measured or hypothesized abyssal return flow across the equator, therefore it is presumed that north-flowing water must upwell in the North Pacific. Upwelling is probably most intense in the eastern portion. Based on water temperature and oxygen content, the north-east Pacific Basin contains water 356
which passes by the Aleutian Trench, through the Emperor Seamount Chain near Midway Island, and from the central Pacific Basin, that is longest removed from ventilation with the atmosphere (Wooster and Volkmann, 1960; Knauss, 1962). This conclusion is, again, consistent with the north-east Pacific being the final destination of abyssal water originating in the Southern Ocean and North Atlantic and the likely site for upwelling. Mantyla and Reid (1983) conducted a survey using more recent and higher quality abyssal hydrographic data and confirmed many of these hypotheses. The horizontal distribution of potential temperature at 5000 m for the World Ocean and for the North Pacific illustrates the major features discussed above (Fig. 4). This figure shows the coldest (and presumably newest, assuming a cold source in the Antarctic) water surrounding the Antarctic and the warmest (and presumably oldest) water to occur in the north-east Pacific Basin. Even though the temperature gradients in the North Pacific Ocean are relatively small, the hypothesized flow patterns are supported as the water warms during its passage along Wooster and Volkmann, 1960 'arbitrary flow line' (Fig. 3). Boundary circulation: the Kamchatka-Kuril and Aleutian trenches Several regional field studies have addressed the deep boundary currents in the North Pacific Ocean. Particular efforts have been put into studying the Japan, Kamchatka-Kuril and Aleutian trenches. Nan'niti and Akamatsu (1966) found a southward flow between 2000 and 3000 m in the Japan Trench using drifters. Reed (1970) collected hydrographic and mooring data across the Aleutian Trench and detected a 'stream' of anomalous water south of the Aleutian Islands. Reed (1970) hypothesized that this water originated from a source west of the Emperor Seamounts, thereafter flowing northward along the Seamounts into the Aleutian Trench after which it flowed east along the Trench. Warren and Owens (1988) observed, using hydrographic and current data, two nearly zonal deep currents in the Aleutian Trench which increased in speed from the bottom to near surface maxima. North of the Trench, and close to the Aleutian Island Arc, was the westward flowing deep extension of the Alaskan Stream extending to 4000-4500 m depths. Warren and Owens (1988) hypothesized that this current is the boundary current predicted by a Stommel-Arons (Stommel and Arons, 1960a,b) type circulation. Since the North Pacific does not have a strong separation between deep and shallow flows, the deep boundary current extends upward nearly to the surface. Warren and Owens (1988) found a second current flowing eastward along the south side of the Aleutian Trench and extending onto the Aleutian Rise. This is presumably the current suggested by Reed (1970). They
Volume 35/Numbers 7-12/July-December 1997 Scripps [] Albatross 0 Sncnius V Carnegie • Discovery
Fig. 3 In Situ abyssal temperatures (°C, upper numbers) and dissolved ] oxygen (ml 1-, lower number, when available) at 5000 m depth in the Pacific Ocean (Wooster and Volkmann, 1960). Dashed line indicates the 'arbitrary flow line' that represents a trajectory for north-flowingwaters.
hypothesize that this current is forced by the topography of the Aleutian Rise, which slopes downward towards the Aleutian Trench. Their maps of potential temperature indicate that this current may extend as far west as the Zenkevich Rise. They predict that this current flows eastward into the Gulf o f Alaska, at which point the current recirculates and flows west as part o f the Alaskan Stream. This continuity in the
current is consistent with potential temperature patterns at 2000 and 4000 m, as isotherms in the vicinity of the Aleutian Trench connect with those in the vicinity of the Kamchatka-Kuril Trench (Warren and Owens, 1988). Talley e t al. (1991) measured hydrographic and chemical parameters along zonal and meridional deep ocean transects along 47°N and 152°W, respectively. Isotherms in the deep portion of the N o r t h Pacific Basin 357
Marine Pollution Bulletin
P o t e n t i a l T e m p e r a t u r e (*C) at 5 0 0 0 m I
I
I
I
I
I
--
I
80
60 40 20 e) -IP
0 e~
-20 -40 -60 -80 0
I
I
I
50
i00
150
-0.5
0
r
200 Longitude
0.5
1
1.5
I
I
I
250
300
350
2
2.5
P o t e n t i a l T e m p e r a t u r e (*C) a t 5 0 0 0 m in the N o r t h Pacific 60
50
40
"m 3o ell
20
10
0
130
140
150
0.8
160
170
0.9
180 190 Longitude
1
200
1.1
210
1.2
Fig. 4 Abyssal potential temperatures (°C) at 5000 m depth from the NODC data archives for the World and the North Pacific Oceans, showing the warmest, hence oldest, water occurring in the north-east Pacific. 358
220
230
Volume 35/Numbers 7-12/July-December 1997 slope down to the east, indicative of a northward flow in the interior, and are consistent with a Stommel-Arons (Stommel and Arons, 1960a,b) flow pattern. Flow was shown, based on water properties (low potential temperature, silica, phosphate and nitrate; high salinity, density, and oxygen), to be northward in the deeper portions of the Kuril-Kamchatka Trench (below 4500 m) between 42°N and 49°N. The trend continues eastward to the Warren and Owens (1988) section discussed above. The hydrographic section crossing the Aleutian Trench at 152°W shows a very similar structure to that of the Warren and Owens (1988) section and suggests westward flow above and north of the Aleutian Trench and eastward flow south of the trench and above the Aleutian Rise. The flow becomes more complex immediately above the Kamchatka-Kuril Trench, and the circulations inferred from water properties and geostrophic equations are less reliable than in the regions of simpler bathymetry. A narrow southward flowing boundary current at about 2000 m depth may be the extension of the westward flowing boundary current in the Aleutian Trench. Northward flow was present near 3000 m above the center and west side of the trench. There is some indication of southward flow above the eastern portion of the trench and northward flow again just east of the trench, above the Zenkevich Rise. There is no clear connection between these currents and the deep currents in the Aleutian Trench. Recently, the first direct measurements of the western boundary currents were made in the vicinity of the Kamchatka-Kuril Trench using RAFOS floats (S. Riser pers. comm. 1995). The floats were deployed just south of the Talley et al. (1991) 47°N hydrographic line, along a transect perpendicular to the trench at 3000 m depths. The easternmost floats (deployed over the Zenkevich Rise) were entrained into a deep portion of the Kuroshio and were transported eastward into the north-west Pacific Basin (south of the dump sites). The floats near and just east of the axis of the trench moved south at speeds less than 2 cm s - ' . The floats along the western edge of the trench, over its eastern portion, and at the start of the Zenkevich Rise flowed north at speeds from 1-3 cm s -1. The floats showed consistent speeds with very little temporal variation and little dispersion. This flow pattern agrees with the general trends found in the hydrographic data collected by Talley, but with a stronger northerly component. An analysis of the silica, oxygen, temperature and salinity distributions, assuming these flow directions, indicates that the younger water is flowing northward and the older water is flowing southward, in general agreement with a Stommel-Arons (Stommel and Arons, 1960a,b) type model. The hydrographic data from Talley et al. (1991) and other researchers show the density distribution along the trench to be relatively constant, therefore, it is plausible that the current distribution measured by these
floats continues along the length of the trench and up to the junction with the Aleutian Trench. The boundary currents near the Aleutian Trench have a different distribution than those around the Kamchatka-Kuril Trench, with one current flowing westward on the north side of the trench, that extends to the bottom and is the Alaskan Stream. The second flows eastward on the south side of the trench and over the rise to the south of the trench. In the far north-west corner of the Pacific these currents turn, mix, and recombine with those of the Kamchatka-Kuril system and those from the Bering Sea flowing south through the Kamchatka Strait. This area has not been sampled adequately to define the flow patterns. The area may in addition, have strong temporal variability which would complicate the analysis. Aleutian pass currents
In order to impact the Bering Sea, any contaminants present in the North Pacific must flow north through channels in the Aleutian Island Arc. All of these channels save the westernmost, the Kamchatka Strait, have shallow sills and allow only upper layer exchange between the Bering Sea and the North Pacific Ocean. Sayles et al. (1979) report the depths as 430 m for Amukta Pass, 1155 m for Amchitka Strait, 640 m for Buldir Pass, 2000 m for Near Strait and 4420 m for the unsilled Kamchatka Strait. Kamchatka Strait, because of its lack of a sill, is the only one of these channels through which abyssal transport of contaminants into the Bering Sea from the radionuclide dump sites might occur. Reed et al. (1993) interpreted the temperature distribution from a transect across Kamchatka Strait as being consistent with southward near-surface currents, reflecting the influence of the Kamchatka Current which flows out of the Bering Sea and into the north-west Pacific. A northward current, in the proper sense to transport contaminants from the North Pacific into the Bering Sea, was inferred at depth from the temperature, salinity and silica distributions. We do not, however, know the speeds associated with this flow.
Particle Diffusion Models Two numerical models have been used to study the dispersion of a tracer released near the dump sites. The first is a Lagrangian dispersion model which uses currents and bathymetry derived from the Semtner and Chervin (1992) 0.5 °, 20-level Parallel Ocean Circulation Model (POCM). This model (Dewey and Stegen, 1995a,b) tracks neutrally buoyant particle locations as they are advected by interpolated monthly mean POCM derived currents. Mixing, needed to provide dispersion of the advected material at higher frequencies and smaller scales than provided by the monthly mean currents, is provided in the model through locally scaled stochastic turbulence. 359
Marine Pollution Bulletin
This Lagrangian dispersion model is a realistic oceanic diffusion model in which each particle or ‘cloud’ of material is injected into the water column at a specified location (longitude, latitude and depth) and subsequently advected by a time dependant, turbulent ocean. Local particle densities are later converted back into local material concentrations. The model has been used in other instances to evaluate the dispersion characteristics of various sites throughout the global ocean, where control of the geographic location, depth, injection rate and turbulent intensity over the resulting material plume can be evaluated. The neutrally buoyant particles are tracked as they are advected by the oceanic velocity interpolated to the exact location of each particle according to Eqn (l), [u, v, WI=[(U(x,_Y,z, t)
+ V’(X,Y,z, Q), (Vx>Y,z,
t) + ~‘(XJV, Q)l> (1)
where [U, V, w] are the temporally and spatially interpolated large scale velocities at the particle locations derived from the monthly POCM velocity data (1/2”x 1/2”x20 vertical layers), and [u’,v’,w’]are the locally estimated stochastic turbulent velocities. The large scale velocity field is gridded at the resolution set by the POCM, however, the velocities of each particle are spatially and temporally interpolated to the exact position of the particle. Although the POCM data have global coverage, a limited dispersion model domain (25”x25”) is established around each injection site. The monthly POCM data were generated by a seasonally forced run of the POCM using bi-harmonic horizontal diffusivities (Semtner and Chervin, 1992). The stochastic components of the velocity field represent the influence of sub-grid scale turbulence not resolved by the POCM. In stochastic models where the mean flow is weak in comparison to the turbulent velocities, the fluctuating components can be modelled as a first order Markov process (Sawford, 1984; Bennett, 1987) using the Langevin equation. The horizontal components (u’,v’) can be obtained using
(2) where At is the dispersion model time step, TL is the Lagrangian integral time scale of the turbulence, 02 is the variance (energy density) of the local turbulent velocities, and 5 is a random number drawn independently from a Gaussian distribution with zero mean and unit variance. The vertical component of the stochastic velocity field (w’) is derived separately by scaling the square root of the horizontal turbulent energy density by the local Richardson number (Rt), 360
where the Ri is calculated from the time dependant stratification and shear contained within the POCM data using, g 8P Ri =
-“” (Z)
+(ig2
This first order approximation for suppressing the vertical turbulent components in the stratified ocean allows for more active vertical mixing in stronger shear zones and within weakly stratified regions. We also note that w’ does not contain any ‘memory’, since the Lagrangian integral time scale for vertical motions are much shorter than the model time step (424 h). The Lagrangian time scale for the horizontal turbulence is set equal to the local inertial adjustment period. This can be approximated by one-half the local inertial period,
where R is the rotation rate of the Earth (=7.292x10w5 s-l) and 4 is the local latitude. However, if this Lagrangian integral time scale is longer than the dispersion model time step, then the ratio (At/TL) is set equal to zero (no memory) and the stochastic forcing becomes a simple random walk. Flow components with integral time scales longer than onehalf the inertial period are strongly influenced by the Earth’s rotation, and are not considered ‘turbulence’ by this formulation since they must obey geophysical dynamics as contained within the underlying POCM. Finally, the energy density of the local turbulent velocities (a:) is parameterized to be a fixed fraction of the local variance in the mean velocity field. In other words, the local turbulent kinetic energy (TKE) is scaled to 5% of the local mean kinetic energy (MKE). The value of 5% (0.05) is consistent with in situ observations of oceanic turbulence (Dewey et al., 1991). Plume structures are dominated by the time, space, and divergence characteristics of the ‘mean’ advecting velocity field. The stochastic components act to slowly migrate particles across streamlines in the mean flow, providing a mechanism for cross-flow dispersion. The inherent divergence in the mean velocity field governs and dominates the fate of the injected particles. Model particles are reflected from the POCM boundaries (i.e. the ocean surface and bottom), and ‘lost’ once they leave the limited (25”x25”) domain of the dispersion model. The August 15 currents from the particle dispersion model show typical features of the region with enhanced speeds near the boundaries and steering by the local
Volume 35/Numbers 7-12/July-December 1997
bottom topography (Fig. 5). Current speeds in the middle of the basin are on the order of 1-2 cm s-~, while speeds in the boundary regions are closer to 35 cm s - t . These are roughly consistent with speeds derived from the limited field data summarized above. After 10 years the modelled particles have been advected southward along the Emperor Seamount chain and passed into the east Pacific Basin over deep sills (Fig. 6). The source site was chosen as 50°N, 160°E and 5000 m depth. The concentrations indicate a relative concentration of a tracer, with the white areas indicating zero concentration. Once in the east Pacific the particles moved toward the north-east and the Gulf of Alaska. The modelled dispersion has a number of stagnation points where particles tended to cluster (indicated by the highest concentrations on the figure). The 0.5 ° grid size of the Semtner and Chervin (1992) model prevented flow through the Aleutian Trench and the Kamchatka Strait at these depths.
The second model is based on the circulation field from a 15 level ocean general circulation model developed at the Max-Planck Institut fur Meterologie, Hamburg, Germany for biochemical modelling (Bacastow and Maier-Reimer, 1990; Maier-Reimber, 1993). Mixing is through numerical diffusivity (related to the finite box size), a small explicit horizontal diffusivity and convective adjustment. The 72 x 72 grid is staggered, with a nominal resolution of 2.5°N-S by 5°E-W. The model uses a yearly averaged circulation field, and the tracer is injected with each time step in the cell centered at 48.75°N and 163.75°W. This model is useful for showing the gross features of dispersion over long time scales (greater than 10 years). The results showed dispersion and time scales similar to those of the Lagrangian model, but with a smoother distribution due to the grid size (Fig. 7). The two models show extremes, with the Lagrangian having low diffusivity and a large amount of advection, and the
15 August POCM Velocities at 5000m, Plotted Every 1°
60
50
~
./
/
J/('
~
-
40 -
:
-
:
~ ' , ' : : : : : i
-
" ~/~-~-~
~
,
~--'"~-~-~
r.~
~
~..'4.'~r",','.--T:-
"\-
j
'~
L/~
~". :-. . . . . . . .
/: : : - : . : : -. ," ', ." .':.": ." . -- - - : ' : ; ,
,y~,.4, -. . ,.
s/e..,.../~,J.*.
: - , :_ : : : : .: : - . : . . . .
~ -
:~,
( , , / ~ 4-- Q , -","" . , , ' , , "',' , ,
,'~(ta v
~o
-
J I _/~
~
)
~ ~ : _ ' - : ; : - : : : . - - . " , : ' . : , • ~
--
~,~., . . .". . "
\~
~, '' ,
....... .
.
.
-:;~,
~,,
.
~ _ _
['~.~....
,
. ~.~.
, ,, ~,
I, . . . . . " ' , ' ~ ' , ' 1
. . . . . . . .
z,.
;. • \ - _ ~y,~ ".... ... . . .~., ..' ; .. ~.. .. . .. . .. ~. . . . .... ... . . ~-.'~." . . ,~,...
~.
',
."
......... t-:~;';:
. . . . . . . . ., . . .
,,
j ~.L~__.,
'{~
',~
c"--~"%. , ;--(~
~r.-~.",
l,-t-, I Z/~
~';
.
"~
.
,
-,'k'(:::::::
,
-' '..:-'--.
. . . . .
"t
. . . . . . . . . . . . . . ;,''7"
.....................
,,
1
. . . . . .
" ~ - - ,," . - ". ~. . . '. . . ~. . . .~. . . -. . . - ~ / / ' ' ,": . . . . .
.z, ..............
I
/
.'.','-';:::.
,', . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
'-:--~-~ . . . . .
,m
I
• . , ~ . . . . . . . . . . . . . . . . . . . . .
. . . . . .
"~',." ; ............
-
~:
,...::
: ,',,
• ._.~
),,:..,
.....
~,--~.,
..
._"..,,~
'
I.~..A
~'~,k'.-a~---./
"~_i
.,,,
.
,,...,.-~,/.,,
~,_.,~_- ..~ ~
" : <
~
~ ."/'.,L'".
%.. -10
I'-
%
,~J ~ . , . ' .
; z_-:7-". ,
, ~ 'E
" 130
I
I
I
140
150
160
•
I 170
~
I
180 190 Longitude (W)
.t/~ , % "/__.
K'<-
:"
I
I
I
I
200
210
220
230
Fig. 5 Monthly mean currents on August 15 at 5000 m depth from the POCM Model (Semtner and Chervin, 1992), as used by the Lagrangian Dispersion Model (Dewey and Stegen, 1995a, b). Note the prominant north-eastward abyssal flow in the northwest Pacific off Kamchatka.
361
Marine Pollution Bulletin Concentration
(units)
50°
% %
40
155
160
165
170
175
180
Longitude (E) Fig. 6 Vertically integrated concentrations (relative to the assumed release concentration) of a tracer released at 5000 m depth at 50°N and 160°E, computed from the Lagrangian Dispersion Model after 10 years (Dewey and Stegen, 1995a,b). Lateral dispersion of the tracer has presumablynot stopped (the model run was stopped after a 10-year model run), but has significantly slowed due to constraints imposed on the abyssal circulation by the bottom topographic features bounding the north-east Pacific Basin(see Fig. 2).
second model having high diffusivity and a low amount of advection.
Conclusions The conceptual and numerical model results, field results and trajectory model output presented above suggest four possible flow pathways from the dumpsites: • advection northward along the Kamchatka Trench, through the Kamchatka Strait (4420 m) and into the Bering Sea; • advection northward along the Kamchatka Trench, or advection by the abyssal currents in the north-west Pacific Basin, to the Aleutian Trench, then eastward along the southern edge of the Aleutian Trench to the Gulf of Alaska; • eastward advection in the north-west Pacific Basin through gaps in the Emperor Seamount chain (sill depth of about 5000 m) into the north-east Pacific Basin, then northward to the Aleutian Trench and continuing on to the Gulf of Alaska; and, finally, 362
• slow upwelling to the mid-depth circulation and then flow at mid-depth towards Alaska with potential upper layer outcropping. The first three pathways would transport radionuclides into the abyssal north-east Pacific Ocean or Bering Sea bordering Alaska. Upwelling of this deep water would then be required to transport radionuclides from the abyss into the biologically active upper ocean. Additionally, upwelling would be required for the second and third pathways before passage of dissolved materials through the Aleutians into the Bering Sea. While this occurs according to the theory summarized above, which holds that such upwelling may in fact be strongest in the north-east Pacific Ocean, available observational and modelling results are inadequate to predict an upwelling rate. The above pathways involve generally slow advection speeds (1~, cm s - I in the boundary currents and 13 cm s-~ in the interior flows), and significant mixing may occur along the pathway. The actual spreading rates may, however, be considerably less than implied by these numbers. Doney and Jenkins (1994) researched
Volume 35/Numbers 7-12/July-December 1997 Log(Concentration). D e p t h = 4 0 0 0 m 80 70!
60
,-, 40 Z gl
,~
30 ¸ 2O
10 0
-10 -20
250
200
150
100
Longitude (W)
-9
- 8
-7
-6
-5
-4
Logl0 Tracer Concentration Fig. 7 Concentration of a tracer after 10 years injected at 48.75°N and 163.75°E at 4000 m depth with a local bottom depth of 4785 m. Concentrations are shown for level 14, with the model interfaces for this depth level at 3500 and 4500 m (Bacastow and Maier-Reimer, 1990). The tracer has not become isolated in the strict sense of the word. However, after first spreading eastward it has not advected significantly outward from the north-east Pacific Basin which contains the oldest water in the Pacific Ocean (see also Fig. 4).
the spreading rate o f tritium and 3He in the deep western boundary currents in the N o r t h Atlantic and found the spreading rates of the tracers to be slower than the measured currents. Directly measured currents in the area ranged from 4-10 cm s - t , whereas tracer spreading rates were measured to be 1-2 cm s - 1. Doney and Jenkins (1994) hypothesized that the current has recirculating cells that allow the higher current speeds and lower spreading rates. Whatever the mechanism, if it acts similarly in the abyssal currents of the Pacific, then the transport rates o f the radionuclides might be slower by a factor o f 2 to 4 than suggested above. At the high-end advection rate o f 4 cm s -~, about 5 years would be required for materials to reach the north-east Pacific Ocean from the dumpsites, while at the low-end rate o f 1 cm s - t, decreased still farther by a factor of 5
to allow for Doney and Jenkins (1994) results, about 100 years would be required. Our information on regional scale abyssal advective processes is at present inadequate to provide a significantly better estimate of this time scale. The numerical simulations show 4-5 order of magnitude decreases in particle concentrations along the pathway from their initial locations at the dumpsites to their final abyssal locations, yielding dilutions of 104105 .
The authors wish to thank Bert Semtner for use of the POCM data and Ernst Maier-Reimer for providing their 15 level circulation field. This work was supported by the ONce of Naval Research under contract # N00014-93-C-0222with Science Applications International Corporation. 363
Marine Pollution Bulletin Bacastow, R. and Maier-Reimer, E. (1990) Ocean-circulation model of the carbon cycle. Climate Dynamics 4, 95-125. Bennett, A. F. (1987) A Lagrangian analysis of turbulent diffusion. Reviews in Geophysics 24, 799--822. Dewey, R. K., Mourn, J. N., Paulson, C. A., Caldwell, D. R. and Pierce, S. D. (1991) Structure and dynamics of a coastal filament. Journal of Geophysical Research 96, 14885-14907. Dewey, R. K. and Stegen, G. R. (1995a) A global ocean 3d plume dispersion model. Proceedingsof the lOth Conferenceon Engineering Mechanics, Boulder, CO, May 22, Vol. 1, pp. 207-210. Dewey, R. K. and Stegen, G. R. (1995b) A 3-dimensional ocean plume model using stochastic dispersion. Proceedings for Comptational Fluid Dynamics, Banff, AL, June 25-27, Vol 1. Doney, S. C. and Jenkins, W. J. (1994) Ventilation of the deep western boundary current and abyssal western North Atlantic: estimates from tritium and He 3 distributions. Journal of Physical Oceanography 24, 638--659. Kawase, M. (1993) Effects of a concave bottom geometry on the upwelling-driven circulation in an abyssal ocean basin. Journal of Physical Oceanography 23, 400-405. Knauss, J. A. (1962) On some aspects of the deep circulation of the Pacific. Journal of Geophysical Research 67, 3943--3954. Mantyla, A. W. and Reid, J. L. (1983) Abyssal characteristics of the world oceans. Deep-Sea Research 30, 805-833. Maier-Reimber, E. (1993) Geochemical cycles in an ocean general circulation model: preindnstrial tracer distributions. Global Biochemical Cycles 7, 645--677. Nan'niti, T. and Akamatsu, H. (1966) Deep current observations in the Pacific Ocean near the Japan Trench. Journal of the Oceanography Society of Japan 22, 154-160. Pedlosky, J. (1992) The Baroelinic Structure of the Abyssal Circulation. Journal of Physical Oceanography 22, 652-659. Pedlosky, J. and Chapman, D. C. (1993) Baroclinic structure of the
364
abyssal circulation and the role of meridional topography. Journal of Physical Oceanography 23, 979-991. Reed, R. K. (1970) On the anomalous deep water south of the Aleutian Islands. Journal of Marine Research 28, 371-372. Reed, R. K., Khen, G. V., Stabeno, P. J. and Verkhunov, A. V. (1993) Water properties and flow over the deep Bering Sea basin, summer 1991. Deep-Sea Research 140, 2325--2334. Sawford, B. L. (1984) The basis for, and some limitations of, the Langevin Equation in atmospheric relative dispersion modelling. Atmospheric Environment 18, 2405-2411. Sayles, M. A., Aagaard, K. and L. K. Coachman (1979) Oceanographic Atlas of the Bering Sea Basin. University of Washington Press. Semtner, A. J. Jr and Chervin, R. M. (1992) Ocean general circulation from a global eddy-resolving model. Journal of Geophysical Research 97, 5493-5550. Stommel, H. (1958) The Abyssal Circulation. Deep-Sea Research 5, 580-582. Stommel, H. and Arons, A. B. (1960a) The Abyssal Circulation of the World Ocean I. Deep-Sea Research 6, 140-154. Stommel, H. and Arons, A. B. (1960b) The Abyssal Circulation of the World Ocean II. Deep-Sea Research 6, 217-233. TaUey, L. D., Joyce, T. M. and DeSzoeke, R. A. (1991) Transpacific sections at 47°N and 152°W: distribution of properties. Deep-Sea Research 38(Suppl.), 63-82. Warren, B. A. (1976) Structure of deep western boundary currents. Deep-Sea Research 23, 129-142. Warren, B. A. and Owens, W. B. (1988) Deep currents in the central subarctic Pacific Ocean. Journal of Physical Oceanography 18, 529551. Wooster, W. S. and Volkmann, G. H. (1960) Indications of deep Pacific circulation from the distribution of properties at five kilometers. Journal of Geophysical Research 65, 1239-1249.