A non-hydrostatic model of dense water formation in the partially ice-covered ocean
Naosuke Okada
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Tel: 81-11-706-2299
Fax: 81-11-726-6234
okada@ees.hokudai.ac.jp
S. Minobe, and M. Ikeda
In the ocean partially covered by sea ice, nearly uniform atmospheric cooling can induce much more intense cooling through an open water area, and hence, more rapid brine rejection from ice formation than the ice-covered portion. This nonuniform negative buoyancy flux produces a unique situation of convection: an area of convection is comparable with a convective plume.
A 3-dimensional ocean model is developed without a hydrostatic assumption. The model domain is a square in the plan view with the side lengths of 12.8km and a depth of 1000m. The spatial resolution is 100m in the horizontal directions and 50m in the vertical direction. The negative buoyancy flux is given to the initially homogeneous water for 6 days.
We carry out 3 experiments at which the negative buoyancy flux is given in different areas. These areas are
Case 1. Large single disk, the diameter of which is 4km.
Case 2. 16 disks distributed uniformly with diameters of lkm.
Case 3. 16 disks clustered near the center of the domain with diameters of lkm.
Here, the total forcing area is kept unchanged over the domain.
A density histogram is Produced at Day 6: i.e., an effective density anomaly ( =density deviation from the initial volume multiplied by its volume ) is plotted as a function of the density deviation. As the forcing area is clustered closely, the peak of the histogram shifts toward a larger density deviation.
Thus, the dense water produced by brine rejection varies depending on the sizes of lead and polynia. This information is extremely useful for parameterizing dense water formation under the ice cover in a numerical model with a large (a few tens of km or larger) grid size inevitably using a hydrostatic approximation. In order to simulate the ice-covered ocean, we should include a size distribution of lead and polynia in addition to grid-averaged ice concentration.
