result in a sharply defined form of free convection with relatively little entrainment. In such a case highly saline water may drain off the shelf in a shallow bottom boundary layer or along the deepest channels on the shelf. If mechanical mixing is strong and the shelf is wide and shallow we might expect to see a forced convection regime with more entrainment. This may result in formation of a shelf break front and a modification of the shelf circulation.
2. Future Study of Nearshore Lead Convection
Experimental work is needed to determine the form of saline convection. It is critical that, for a variety of shore lead or polynya conditions, the rate of ice formation, salt flux, and mechanical mixing be related to the spatial distribution of the convection and the amount of mixing on the shelf. The consequent shelf circulation patterns should also be determined. Given the harsh ice conditions in the coastal environment, the measurements needed to answer these questions have been difficult. However recent advances in instrumentation now make such observations feasible.
Figure 2 shows a possible experimental arrangement to study winter shore lead or polynya convection. A combination of dedicated process studies and longer term monitoring would be used. New methods of observing convection with Autonomous Underwater Vehicles (AUV) and other instruments offer the opportunity to study and parameterize the effect of nearshore convection sources. Morison and McPhee (1998) discuss observations of pack-ice lead convection with an AUV called the Autonomous Conductivity Temperature Vehicle (ACTV) during the 1992 Lead Experiment (LeadEx). They have developed techniques to estimate turbulent heat and salt flux as well as the spatial distribution of water properties. The mobility of the AUV provides a 3-dimensional view of the convection. Development of a more sophisticated Autonomous Micro-conductivity and Temperature Vehicle (AMTV) is underway. This should expand our operational capabilities further. Acoustic Doppler Current Profilers (ADCP) measure velocity from either the sea bottom or underside of the ice, and they measure Ice velocity as well. Work during the ongoing Surface Heat Balance of the Arctic (SHEBA) program demonstrates the ability to make long term turbulent flux measurements with moored sensors (McPhee, personal communication).
In the experiment AUVs would be used as at LeadEx along with fixed turbulence sensors to measure the vertical flux of salt and heat. They would map the spatial variability of the convective plume in terms of mean water properties and turbulent fluxes. ADCPs and fixed temperature and salinity sensors would measure vertical structure and horizontal fluxes over long periods.
3. References
Deardorff, J.W., G. E. Willis, and D. K. Lilly, 1969, Laboratory investigation of non-steady penetrative convection, J Fluid, Mech., 35, part 1,7-31.
Morison, J.H., M.G. McPhee, T. Curtin and C.A. Paulson, 1992: The oceanography of winter leads. J, Geophys. Res., Vol. 97, No, C7, pp. 11,199.11,218,
Morison, J. H., M. G. McPhee, 1998, Lead convection measured with an autonomous underwater vehicle, J. Geophys, Res., (in press).
Paulson, C. A., and J. D. Smith, 1974, The AIDJEX Lead Experiment, in AIDJEX Bulletin, 23, 1-8, Univ. of Wash., Seattle. (Available as PB-230378, from Nat. Tech. Inf. Serv,, Springfield, Va.)
Smith, J. D., Oceanographic investigations during the AIDJEX Lead Experiment, in AIDJEX Bulletin, 27,125-133, Univ. of Wash., Seattle, 1974. (Available as PB-238574 from Nat. Tech. Inf. Serv., Springfield, Virginia)
Smith, D.C., IV and J.H, Morison, 1998: Nonhydrostatic haline convection underneath leads in seab ice. J. Geophys. Res., (in press).
