Modeling and Tracer Studies of the Arctic Shelves
Douglas G. Martinson
Lamont-Doherty Earth Observatory Palisades, NY 10964
Tel: 1-914-365-8830
Fax: 1-914-365-8736
dgm@ldeo.columbia. edu
R. Newton, P. Schlosser, D. Martinson, W. Maslowski, A. Semtner
Shelf-derived plumes of cold, salty water are critical in the large-scale thermodynnanics of the Arctic ocean. They feed the halocline through horizontal detrainment along isopycnals, and thereby maintain the halocline which segregates the warm, salty Atlantic layer from the surface waters. If the halocline were to deteriorate, there is good reason to believe that the Arctic ice cover would be threatened through the combined action of vertical mixing of heat from the Atlantic Layer and positive feedbacks due to the difference in albedo between ice and water. To the extent that the plumes break past the surface and intermediate waters, they can be driven to great depth, where, on long timescales, they set the properties of abysal waters. Horizontal and temporal transients in these properties are critical evidence about the history of large-scale patterns of convection and abysal flow. In addition, recent measurements show that the horizontal density structure of the Arctic surface waters, below the ice cover, are strongly influenced by plumes of shelf-water which have been freshened by river runoff or fresh inflow from the Bering Strait. By setting up horizontal density fronts, the shelf waters have a strong influence over the strength and structure of the baroclinic component of the Arctic circulation. Unfortunately, direct measurements of plume sizes, volumes and densities are very incomplete. Even the points of separation of surface "streams" of fresh or dense water from the shelf are poorly documented.
The Arctic shelf break is an area of steep and complicated topography, strong along-shelf currents and significant tidal mixing. These complex dynamics make the explicit modeling of shelf-plumes very difficult. However, recent advances in tracer measurements for the Arctic are making it possible to track their influence far from the source regions. Oxygen 18/16 ratios can distinguish between sea-ice (with a delta-O-18 value very close to the ocean value) and precipitation, most of which enters the Arctic as river runoff (with a value of approximately-21). In addition, chemical tracers can further discriminate between fresh waters from the Canadian rivers (using barium), the Bering Strait (using silica anddisolved nutrients) and perhaps even between the Russian rivers (using industrial or agricultural pollutants).
We are engaged in a program of closely integrated tracer field studies and model experiments. Measurements of oxygen-isotope, tritium, helium, chloroflorocarbons (freons) and noble gases in the Arctic have been used, for example, to: (1) identify the source of freshwater pooling in the surface of the Beaufort Gyre (river runoff); (2) quantify changes in openwater convection in the Greenland-Iceland-Norwegian seas(3) trace shelf-plumes of Weddell Sea deep-water, and (4) measure water-mass gradients in the abysal Arctic basins. In the model runs we add several freshwater tracers to a coupled ice-ocean general circulation model based on the a free-surface version of the Bryan/Cox/Semtner/Chervin codes. The model uses Y.Zhang's implementation of Hibler's plastic-viscous theology.
The atmospheric forcing is taken from the ECMWF re-analysis of its operational forecasts. The model grid offers unprecedented resolution for a basin scale model: 18-km horizontal resolution and 30 vertical levels. For each river, long term measurements of flow and temperature are used to establish average monthly flow-rates and heat inputs. These are interpolated to the model timestep, so that a source for fleshwater and heat is create at the boundary.
