EFFECTS OF AMBIENT STRATIFICATION AND SHELFBREAK TOPOGRAPHY ON TRANSPORT OF DENSE WATER FROM ARCTIC SHELVES: IMPLICATIONS FOR SHELF-BASIN INTERACTIONS
Glen Gawarkiewicz*
Department of Physical Oceanography Woods Hole Oceanographic Institution
The ventilation and maintenance of the Arctic halocline has been a critical and long-standing problem in the study of the Arctic Ocean. The classic work of Aagaard et al, (1981) has identified brine production within Arctic coastal polynyas as an important physical process through which water of the proper salinity can be formed. Numerous tracer studies of the composition of the halocline have confirmed this basic insight. However, the physical process by which the dense water is carried from he coastal polynyas to the shelfbreak, and then into the Arctic basins, has remained problematical.
Recently, the possiblility of eddy fluxes driving the dense water offshore has received attention in both theoretical and idealized numerical modelling studies (McDonald, 1993; Gawarklewicz and Chapman, 1995). Chapman and Gawarkiewicz (1997) have developed a simple formula for the maximum density attainable within a coastal polynya, pe. This equilibrium density results a balance between the negative buoyancy input at the surface within the polynya and the lateral eddy fluxes at the periphery of the polynya. The equilibrium density is a function of the magnitude of the negative buoyancy forcing as well as the shape of the polynya as well as the forcing decay region which surrounds the polynya. The relation for the equilibrium density assumes that there is no ambient stratification present.
When ambient stratification is added, the equilibrium density is changed only by the density present at the bottom at the coastal boundary due to the initial stratification. This is given by the relation
p'e=pe + hn2p0/g (1)
where h is the depth of the water column at the coast N is the stratification which is initially imposed, and p is the equilibrium density for the case with no initial stratification. As in the case with no ambient stratification, eddies are self-advected directly off-shore, until reaching the shelfbreak.
At the shelfbreak, the fate of the dense water is dependent on the ratio of the adjusted equilibrium density, P'e, to the ambient density at the shelfbreak,
Psb = hsubN2P0/g (2)
where hsb is the depth at the shelfbreak. When this ratio is large, the dense water moves rapidly down the slope. The alongshelf scale of the dense water flowing down the continental slope is limited by the size of the eddies which transport the dense water offshore from the polynya region (typically 20-30 km for parameters representative of the western Arctic). When this ratio is small, there is little or no downslope buoyancy force to propel the dense fluid down the continental slope. Some of the dense fluids remains at the shelfbreak, forming a bottom-trapped current that moves in the direction of coastal-trapped wave propagation (with the coast on the right in the northern hemisphere).
In addition to the bottom-trapped current, dense water which is less dense than p'e is carried offshore over the continental slope at roughly the depth of the shelfbreak. This results in the continuity of the isopycnals between the middle of the shelf and the continental slope (Figure 1). This is suggestive that the dense water eddies have the potential of acting as a ventilation mechanism for the Arctic halocline. This section bears a striking resemblance to observations of the salinity field at the shelfbreak within the Beaufort Sea reported by Melling and Moore (1993).
Passive tracer distributions reveal more about the water which originally is located within the polynya. By initializing a passive tracer within the polynya (with a value of 1.0) and setting the value outside the polynya to zero, one can track the dense water offshore. The tracer distributions at the shelfbreak and over the upper slope are highest within the eddies which pass offshore (Figure 2). The typical alongshelf scale for the tracer maxima is 20-30 km, consistent with the
* Glen Gawarkiewicz
Mail Stop #21
Woods Hole Oceanographic Institution Woods Hole, MA 02543 USA
