Alaska (Paulson and Smith, 1974). Sequential density profiles measured at one lead are shown in Figure 1 (from Morison et al., 1992).
The lead was 33 km, 5｡?rue from Barrow. The sequence of CTD profiles was obtained 50m from the edge of the open lead over a period of over 12 hours.
Several features are apparent. The first is penetrative convection. During the period 2204 to 0039 hours, masses of low density (hence low salinity) water penetrated to as deep as 25 m, even though the pycnocline was at a depth from 10 to 15 m. The intrusions often caused unstable stratification, but were short lived. The profiles bear a strong resemblance to the observations of penetrative convection by Deardorff et al. (1969). The profiles suggest dense plumes from the surface were entraining less dense water from the mixed layer and penetrating the pycnocline. Typical perturbations in the profiles are 0.05 σt units (0.06 psu), which suggests the perturbations in salinity due to lead convection were of this order. Because of the shallow depth of the mixed layer, the salt rejected at the lead surface was confined to a shallow region and the resultant salinity disturbance was relatively large. Also, the lead was not moving, so the salt rejected at the surface was not spread out horizontally. More recent measurements made during the 1992 Lead Experiment (LeadEx) (Morison and McPhee, 1998) and theoretical studies (Smith and Morison, 1998) suggest the convection may be concentrate near the lead edges. This is further reason for penetration of the pycnocline near the measurement site.
Another important feature of Figure 1 is the mixed layer at the bottom of the profiles. The water depth at the site was about 115 m. The deepest 10 to 20 m of the water column was 0.2σt units greater in density than the bulk of the water column. As reported by Smith (1974), the water temperatures throughout the water column at ALEX were very close (within 5 millidegrees) to the freezing point. Thus, the high salinity layer had a surface freezing origin. Therefore, Smith (1974) and Morison et al. (1992) suggest this bottom boundary layer is the outflow of convection from the large open shore lead a few miles south of the measurement site.
There are a number of key questions that must be answered to determine the effect of a shore lead or polynya on the shelf and basin. The key driving forces are the rate of ice formation (and resulting salt flux) and the amount of mechanical mixing by wind and ice motion. These will determine the form of the saline convection. Weak mechanical mixing and strong salt flux may
