to WHOI Arctic Group Projects
the Formation and Offshore Transport of Dense Shelf Water
from High-Latitude Coastal Polynyas
Senior Scientist, WHOI
Associate Scientist, WHOI
The research described here was originally supported
by the Arctic
System Science (ARCSS) program at NSF,
and more recently by the Western
Arctic Shelf-Basin Interactions (SBI) project, which is co-sponsored
by NSF through ARCSS and by the High-Latitude
Dynamics Program at ONR. The
fundamental goal of SBI is to understand the physical and biogeochemical
processes that link the arctic shelves, slope and deep basins within
the context of global change. The focus of SBI is the region including
Bering Strait and the Chukchi and Beaufort Shelves (Figure
1). Our focus has been to understand the physical processes that
modify the shelf water and exchange shelf water with deep basin water.
Directly beneath the ice cover of the deep Arctic basins is a 50-100
m layer of relatively fresh water that is near the freezing temperature.
This surface layer is separated from the underlying warmer, saltier
Atlantic water by the cold halocline layer, in which the temperature
is near freezing while the salinity increases rapidly with depth.
The cold halocline water is not a simple mixture of the surface layer
and the Atlantic water. Aagaard et al. (1981) suggested that the cold
halocline layer must be maintained by the lateral injection of cold,
salty water; the most likely source being from the broad, shallow
Arctic continental shelves where cooling rapidly reduces the temperature
to near freezing and salt rejection accompanying ice formation can
increase the salinity (and, hence, density) of the shelf waters.
The Chukchi Shelf is a good candidate for such dense-water formation
because (i) it is very shallow (40-50 m deep), (ii) there is a large
seasonal variation in ice cover (i.e. seasonal ice formation), (iii)
there are persistent wind-driven coastal polynyas in which ice can form
at about ten times the seasonal rate, and (iv) there is little
freshwater runoff to reduce the salinity of the shelf water.
Figure 1: Map of the region
of interest for the Western Arctic Shelf-Basin Interactions (SBI)
project. Arrows indicate the directions of known mean flows through
Bering Strait and across the Chukchi Shelf (click here for more details). [Larger image]
The basic scenario is as follows. A mean transport of about 0.8
Sverdrups (1 Sv = 10^6 m^3/s) of relatively fresh and warm Pacific
water flows through Bering Strait onto the shallow Chukchi Shelf (Figure
1). This flow splits into several branches as it crosses the shelf
before possibly entering the Canada Basin. (Click here
for details of this circulation.) During it's transit, it may be modified
considerably by atmospheric cooling and ice formation processes. Our
initial goal was to understand the ocean response to salt rejection
during ice formation in a coastal polynya:
Our approach has been to use numerical models to address these and
other related questions.
- What ocean circulation is induced by a coastal polynya?
- How dense can the water beneath a coastal polynya become?
- How does the dense water move across the shelf and into the deep
- How much dense water can coastal polynyas produce?
2. Modeling Philosophy
Numerical modeling studies can be grouped into two broad categories:
(1) full-physics simulations, and (2) process studies. Full-physics
simulations attempt to use a comprehensive model with complete physics,
the most realistic geometry, forcing and boundary conditions either
to reproduce existing observations or to predict future behavior.
In our view, it is often difficult to isolate and understand individual
processes that are embedded among many processes in such models. Frequently,
the "numerical data" must be analyzed in the same manner as real ocean
observations, making the interpretation unclear.
Process studies attempt to gain an understanding of some basic underlying
physics by studying one or more processes in specific circumstances.
Generally, the model geometry, forcing and/or boundary conditions are
simplified in order to isolate the processes of interest, often involving
parameterizations of effects for which a full model would be overly
complicating. For example, we often study the ocean response to wind
forcing by applying a stress to the ocean surface, in order to avoid
using a full atmospheric model. Likewise, we can study the ocean response
to ice formation in a coastal polynya by imposing a surface density
flux to represent salt rejection, perhaps based on satellite images
of ice distribution and estimates of heat flux,
without a full ice-dynamics model. The results of process studies are
universal, so they are useful for building intuition and are easily
applied to new situations.
We have adopted the process study approach in our research. The
choices of model, model domain, coastline and bathymetry depend on
the particular scenario being considered, and we simplify the forcings,
boundary conditions and/or geometry where appropriate in order to
reduce the complexity of the dynamical system while maintaining enough
dynamics to understand the important processes. We are ultimately
interested in applying the results to observable situations, so we
add more realism to the scenarios as we progress, trying not to compromise
our likelihood of understanding the results.
3. Idealized, static polynyas
We began with the simplest coastal polynya scenario we could imagine
-- a static polynya of specified size and shape with an imposed salt
flux at the surface to mimic the primary effect of salt rejection
accompanying ice formation (Figure
2). The coastline is assumed straight and the bottom slopes gently
away from the coast. We ignore the details of wind forcing and ice
dynamics, and assume a constant ice formation rate (i.e. constant
surface buoyancy flux).
The ocean starts from rest. The water beneath the polynya becomes denser
than the surrounding water, creating a density front along the edge
of the polynya. This front adjust toward geostrophy, developing currents
along the front -- clockwise at the surface and counter-clockwise at
the bottom (Figure 2). The
front is unstable, so any perturbations of the frontal currents (e.g.
where the front intersects the coastal boundary) grow and form eddies
that break away from the front and carry dense water with them. This
is the mechanism by which the dense water is carried across the shelf.
It does not flow as a smooth gravity current, but rather is carried
by small-scale (15-30 km diameter), intense eddies with swirl velocities
of 20-30 cm/s. The eddies move slowly offshore as counter-rotating vortex
pairs with translation speeds of 1-2 cm/s (Figure
3). More details can be found in Gawarkiewicz and Chapman (1995).
With continued buoyancy forcing, the eddies eventually carry the
dense water offshore as fast as the surface flux can increase the
density, so a quasi-equilibrium is reached in which the density increase
beneath the polynya is fairly constant (Figure
4). A simple theory, based on the approach of Visbeck, Marshall
and Jones (1996), allows us to estimate the time it takes to reach
this quasi-equilibrium and the maximum density increase produced by
the polynya, as a simple algebraic function of polynya size and shape,
water depth, and buoyancy forcing (Chapman and Gawarkiewicz, 1997).
These results have helped in the understanding of some recent laboratory
experiments on convection in a two-layer fluid (Narimousa, 1996).
Surface cooling sometimes causes the upper layer fluid to penetrate
through the interface into the lower layer, while sometimes it does
not. Our simple theoretical estimates of the quasi-equilibrium density
increase explain this behavior (Chapman, 1997). If the parameters
are such that the increased density in the upper layer at equilibrium
is larger than the density difference across the interface, then convection
will penetrate into the lower layer. If the equilibrium density increase
is less than the density difference across the interface, the dense
water remains lighter than the lower layer water, and hence it remains
in the upper layer.
An important aspect of our specified polynya is the thin ice region
surrounding the polynya over which the ice formation rate (i.e. surface
buoyancy flux) decays from its maximum in the polynya to zero outside
the polynya, where the water is totally ice covered. If this region
is large, eddy formation can be delayed, and a much larger density
increase can be produced within the polynya before the quasi-equilibrium
is reached. Thus, the width of the forcing decay region appears explicitly
in the theoretical estimates described above. The relation of these
estimates to those without the forcing decay region, and the transition
between the two cases, has been explored in Chapman (1998).
We have examined some effects of bottom topography and ambient alongshelf
currents on the formation and offshore transport of dense water from
coastal polynyas. First, we included a single submarine canyon that
intersected the original idealized, static polynya (Chapman and Gawarkiewicz,
1995). The basic formation of dense water eddies was qualitatively
identical to the smooth topography cases. However, eddies that happen
to move into the canyon quickly cascade down the steep canyon walls
and are channeled offshore as an intermittent gravity current that
rides along the right wall of the canyon into deeper water (Figure
5). The gravity current tends to "pulse" as each eddy passes.
The current is slow and strongly stratified, so little mixing with
ambient fluid takes place, in marked contrast to gravity currents
associated with large overflows, e.g. the Mediterranean Sea. The model
features are consistent with recent observations of dense water currents
in Barrow Canyon (Weingartner et al., 1998).
An ambient alongshelf current carries water out of the polynya during
ice formation, thereby reducing the density increase for a given surface
buoyancy flux. Furthermore, the distribution of water with increased
density is altered substantially. Surprisingly, an alongshelf current
can inhibit the tendency of dense water eddies to enter submarine
canyons because the alongshelf current itself tends to follow isobaths
and flow around the canyon, carrying the dense water eddies with it
(Figure 6). This suggests the
combined importance of ambient shelf currents and bottom topography
in determining the pathways of dense water transport across continental
shelves. Details are found in Chapman (2000).
5: Plan views of the density anomaly at the bottom at four
different times for an idealized, static polynya with a constant
surface density flux (as in Figure
3), but with a submarine canyon intersecting the polynya
(indicated by the black depth contours). As eddies move close
to the canyon, they slide down the steep canyon walls and flow
along the canyon axis as a slow and intermittent gravity current.
The colorbar shows the density increase in kg/m^3. [Larger image]
6: Plan views of the density increase at the bottom at four
different times for an idealized, static polynya, but with an alongshelf
current from left to right of 10 cm/s imposed at the left boundary
of the model. A constant surface density flux is imposed within
the white half-ellipse adjacent to the coastline. The colorbar indicates
the density increase in kg/m^3. The dashed contours are isobaths,
indicating a submarine canyon crossing the shelf at x=125 km. Note
that the dense water does NOT flow down the canyon, despite moving
directly toward the canyon edge. Instead, the alongshelf current
flows around the canyon head, almost parallel to the isobaths, carrying
the dense water eddies with it. [Larger image]
The estimates for the quasi-equilibrium density increase and time
to reach this equilibrium depend on a proportionality coefficient
that represents the efficiency with which eddies transport dense water
across the front at the polynya edge. This has traditionally been
treated as an unknown constant that is estimated by fitting numerical,
laboratory and observed responses. In an attempt to understand this
coefficient from dynamical principles, Spall and Chapman (1998) developed
a theoretical model in which cross-frontal heat (or density) flux
is accomplished by heton pairs. They showed that the unknown coefficient
can be estimated reasonably well as the ratio of the cross-frontal
translation velocity of the heton pairs to the along-frontal current
4. Time-dependent polynyas
The static polynya studies assume that the polynya remains open
with a fixed shape and that the ice formation rate is constant. Of
course, real polynyas open and close on time scales of days to weeks,
with their shapes varying tremendously. In fact, polynyas may not
remain open long enough for the quasi-equilibrium to be reached. To
address these issues, an investigation of the ocean response to time-dependent
polynyas has been made (Chapman, 1999).
The opening and closing of the polynya was modeled using the Pease
(1987) model in which polynya width is determined by a balance between
the surface wind stress blowing the newly formed ice offshore (opening
the polynya) and the new ice formation (closing the polynya). Using
idealized atmospheric variables (air tempreature and wind speed),
as well as estimates based on observations, it was shown that the
ocean basically integrates the effects of short-duration polynyas
and behaves in much the same manner as the idealized, static polynyas.
The density within the polynya increases with each successive polynya
until eddies form and begin to carry the dense water away from the
polynya region. The density increase is then limited, despite additional
polynya events throughout the winter. The time-dependent response
is nearly the same as if the polynya were static and forced with the
seasonally averaged surface buoyancy flux, suggesting that the simple
algebraic estimates of density increase may be useful even for variable
polynyas on longer time scales (Figure
5. Shelf-edge processes
The effect of dense water transport across an idealized continental
shelf has important implications for exchange between the shallow continental
shelves and the deep ocean basins within the Arctic Ocean. Gawarkiewicz
(2000) found that dense water eddies are capable of inducing flows which
cross the shelfbreak. Using ambient (linear) stratification which is
typical of the Chukchi Shelf, and forcing typical of coastal polynyas,
the dense water was injected over the continental slope at a depth of
100 m, roughly the depth of the shelfbreak (Figure
8). A theoretical estimate of this depth of maximal transport of
dense water was developed, which depended on both the strength of the
surface buoyancy forcing in the coastal polynya as well as the strength
of the ambient stratification.
A key result of this work is that fluxes from the shelf to the slope
are highly variable in both time and space (Figure
9), depending on the detailed trajectory and structure of individual
eddies as they reach the shelfbreak. We anticipate that future observations
will show similar complex behavior in terms of cross-shelfbreak fluxes,
with large variations on time scales of days and spatial scales of order
8 : Along-slope distribution of a passive tracer seaward of
the shelfbreak after 90 days of a model run. The maximum concentrations
are at a depth of 90 to 120 meters, which is similar to the depth
of the shelfbreak. The tracer was initialized next to the coast,
within an idealized coastal polynya. [Larger image]
Figure 9: Time series
of (negative) buoyancy fluxes across the shelfbreak (y=50 km), and
slope (y=62 and 73 km) from dense eddies crossing the shelfbreak.
Note the large day to day variability at the shelfbreak as individual
eddies cross. [Larger image]
6. Interannual variability
Several estimates have been made of the total amount of dense water
capable of feeding the cold halocline layer that could be produced
on Arctic continental shelves by coastal polynya processes (e.g. Cavalieri
and Martin, 1994; Winsor and Bjork, 2000). These results are based
on estimates of salt rejected during ice formation, which is then
available to increase the density of the shelf water. However, the
studies have not included a dynamical ocean model to determine the
actual density increase from the polynyas. Therefore, Winsor and Chapman
(2001) have used the polynya model of Winsor and Bjork (2000) to force
an idealized ocean model to estimate the dense water production on
the Chukchi Shelf for the winters of 1978-1998.
Results show that the density increase during typical winters is substantially
less than expected from previous estimates. Only in a few of the years
is the density increase enough (when added to the density of the Bering
Strait inflow) to produce cold halocline water. There is considerable
interannual variability in both the volume of dense water produced and
the maximum density increase (Figure
10). These results are consistent with the intermittency of dense
water found on the Chukchi Shelf and suggest that other sources of cold
halocline water are probably important.
Aagaard, K., L. K. Coachman, and E. Carmack, 1981. On the halocline
of the Arctic Ocean. Deep-Sea Res., 28A, 529-545.
Cavalieri, D.J. and S. Martin, 1994. The contributions of Alaskan,
Siberian, and Canadian coastal polynyas to the cold halocline
layer of the Arctic Ocean, J. Geophys. Res., 99, 18343-18362.
Chapman, D.C., 1997. A note on isolated convection in a rotating,
two-layer fluid, J. Fluid Mech., 348, 319-325.
Chapman, D.C., 1998. Setting the scales of the ocean response to
isolated convection, J. Phys. Oceanogr., 28, 606-620.
Chapman, D.C., 1999. Dense water formation beneath a time-dependent
coastal polynya, J. Phys. Oceanogr., 29, 807-820.
Chapman, D.C., 2000. The influence of an alongshelf current on the
formation and offshore transport of dense water from a coastal polynya,
J. Geophys. Res., 105, 24007-24019.
Chapman, D.C. and G. Gawarkiewicz, 1995. Offshore transport of dense
shelf water in the presence of a submarine canyon, J.
Geophys. Res., 100, 13373-13387.
Chapman, D.C. and G. Gawarkiewicz, 1997. Shallow convection and buoyancy
equilibration in an idealized coastal polynya, J. Phys. Oceanogr.,
Gawarkiewicz, G. and D.C. Chapman, 1995. A numerical study of dense
water formation and transport on a shallow, sloping continental shelf.
J. Geophys. Res., 100, 4489-4507.
Gawarkiewicz, G., 2000. Effects of ambient stratification and shelfbreak
topography on offshore transport of dense water on continental shelves, J. Geophys. Res.,
Narimousa, S., 1996. Penetrative turbulent convection into a rotating
two-layer fluid, J. Fluid Mech., 321, 299-313.
Pease, C. H., 1987. The size of wind-driven coastal polynyas,
J. Geophys. Res., 92, 7049-7059.
Spall, M.A. and D.C. Chapman, 1998. On the efficiency of baroclinic
eddy heat transport across narrow fronts, J. Phys. Oceanogr.,
Visbeck, M., J. Marshall and H. Jones, 1996. Dynamics of isolated
convective regions in the ocean, J. Phys. Oceanogr., 26, 1721-1734.
Weingartner, T.J., D.J. Cavalieri, K. Aagaard and Y. Sasaki, 1998.
Circulation, dense water formation, and outflow on the northeast Chukchi
Shelf. J. Geophys. Res., 103, 7647-7661.
Winsor, P. and G. Bjork, 2000. Polynya activity in the Arctic Ocean
from 1958 to 1997, J. Geophys. Res., 105, 8789-8803.
Winsor, P. and D.C. Chapman, 2001. Distribution and interannual variability
of dense water production from coastal polynyas on the Chukchi Shelf,
J. Geophys. Res., submitted.
to WHOI Arctic Group Projects