LAGRANGIAN ASPECTS OF THE SURFACE CIRCULATION AND MIXING ON WIDE ARCTIC SHELVES
Andreas Munchow*
Institute of Marine and Coastal Sciences, Rutgers University New Brunswick, New Jersey, U.S.A.
1. ABSTRACT
The Arctic Ocean is presently ice covered not for the cold air temperatures but for its vertical salinity stratification. The fresh water that stratifies this ocean derives from the inflow of riverine waters which subsequently spread and mix over the generally broad and shallow continental shelves. Using recent observational data from in situ platforms, I here discuss kinematic aspects of the circulation, mixing, and exchange of buoyant shelf waters in the East Siberian and the Mackenzie Shelf Seas.
More specifically, I present direct velocity observations from surface drifters that indicate correlations, terms in the momentum balance, and dispersive characteristics. At mid-latitudes riverine discharges generally force coastally trapped along-shore coastal currents (Munchow and Garvine, 1993). Often these currents are in semi-geostrophic balance that constrains across-shelf exchange. In contrast, Arctic riverine discharges prior to the onset of winter freezing appear on the shelf as unsteady and diffusive flows susceptible to but not always correlated with wind stress forcing. Drifters cross isobaths easily and exhibit isotropic dispersive characteristics. Dispersion coefficients are about 2-3×107cm2s-1. These characteristics resemble those of open ocean drifters more than they resemble the generally anisotropic dispersive characteristics of drifters released into the coastal ocean.
2. INTRODUCTION
The Arctic Ocean constitutes a major source of fresh water for the global ocean. The fresh water enters the vast Arctic shelf seas in the form of riverine discharges.
Brackish riverine waters impacts circulation, mixing, stratification, and ice formation both on shelves and adjacent deep basins. Shelf processes support and maintain the strong vertical density stratification that allows ice to cover much of this ocean. The Arctic thermohaline circulation derives its forcing from shelf processes such as river forced plumes, tides, wind stress, and the interaction of these processes. The lateral flux of mass and buoyancy from rivers around the seasonally ice free rim of the Arctic Ocean affects both the dynamics on the adjacent continental shelves and the deep basins through thermohaline forcing. Furthermore, the Arctic thermohaline circulation extends into the North-Atlantic where it potentially impacts the connective gyres in the Greenland and Labrador Sea through the lateral injection of fresh water (Aagaard and Carmack, 1989).
The pre-conditioning of generally ice free shelf waters by shelf processes partly determines the amount and properties of brines that are formed during the freezing process (Melling and Moore, 1995). The brines are subsequently mixed on the shelf (Gawarkiewicz and Chapman, 1995) and, if they are sufficiently fresh, supply and ventilate the Arctic halocline (Aagaard et al., 1981). The halocline is also ventilated in canyons during strong wind forced upwelling such as observed by Munchow and Carmack (1997). This study, however, concentrates on the first step, i.e,, the pre-conditioning of shelf waters. I describe the circulation, mixing, and across-shelf exchange on the East Siberian and Mackenzie Shelf Seas before, during, and after the onset of winter freezing. Future studies need to investigate the dynamics of these flows that are forced by the interaction of a large lateral buoyancy flux and wind stress.
3. METHODOLOGY
During the ice free summer seasons of 1994 and 1995 15 and 39 drogued surface drifters were deployed into Canadian and Siberian shelf seas, respectively. Figure 1 shows these study areas on the same scale as two mid-latitude shelf areas (the Mid-Atlantic Bight and the North Sea).
The 1994 deployment used CODE-type drifters (Davis, 1985) designed and built at the Scripps institution of
* Corresponding author address. Andreas Munchow,
Institute of Marine and Coastal Sciences, Rutgers University, 71 Deadly Rd., New Brunswick, NJ 0890l-8521, U.S.A., andreas@ahab.rutgers.edu
http://marine.rutgers.edu/ac/munchow.html
