A NUMERICAL INVESTIGATION OF MESOSCALE PHENOMENA IN THE SUMMER MARGINAL ICE ZONE USING A SIMPLE ICE-OCEAN COUPLED MODEL
Yasushi Fukamachi*
Hokkaido University, Sapporo, Japan
1. INTRODUCTION
In the marginal ice zone (MIZ), mesoscale ice features, such as meanders, tongues, and eddies have been observed from remote sensing data. Using NOAA AVHRR imagery Yamanouchi and Seko (1992) and Fukamachi et al. (1998) identified ice eddies and tongues during summer in the region of East Queen Maud Land in Antarctica. Figure 1 is an AVHRR visible-band image showing those characteristic sea-ice distribution. Furthermore, Fukamachi et al. (1998) revealed that the growth of these features is not correlated with the wind field and patterns of these features are associated with those of cold water offshore. In this study, a simple ice-ocean coupled model, which includes both dynamic and thermodynamic processes for ice and ocean, is used to investigate these mesoscale phenomena.
2. THE ICE-OCEAN COUPLED MODEL
The coupled ice-ocean model is a fully nonlinear system. The ice model is essentially the same as that of Kantha and Mellor (1989). In this model, both lateral and bottom melting processes are considered. The ocean model is essentially the same as thermodynamic, reduced-gravity model of Mc-Creary et al. (1989), except that a salinity equation is included. In this model, the change in the upper-layer density is caused by surface heat flux from atmosphere, heat and salinity fluxes due to ice melting. It is assumed that ice prevents surface heat flux from reaching ocean. Figure 2 shows the vertical section of this model schematically.
The model basin has zonal and meridional dimensions of 50 km and its resolution is 1 km. At the eastern and western boundaries, cyclic conditions are applied, and at the northern and southern boundaries, closed conditions are applied. The model parameters are based on those observed in the Southern Ocean during summer. The initial ice concentration is increased from 0 to 80 % within l0 km. In this MIZ, surface heat flux, which is uniform in time and space, is applied. In order to isolate the effects of thermodynamic forcing, wind forcing is not included.
3. MODEL RESULTS
When the model is forced by the surface heat, salinity gradients are generated near the ice edge. There are ice-edge currents associated with these salinity gradients. These currents start to meander as they grow stronger. A wavelength of meanders is about 14 km. The meandering currents exchange cold and fresh water in the ice-covered region and warm and saline water in the ice-free region, and advect ice into the ice-free region to form ice tongues. In Figure 3, these features are seen in solution at day 35.
In order to determine types of instabilities that cause the formation of meanders in the nonlinear model, a linear instability analysis is performed. The linearized model consists of an ocean model only because ice is passively advected by the upper-layer currents in the nonlinear solution. The unstable wave found in the linearized model is caused by the baroclinic instability associated with horizontal density gradients. The structure of this wave is similar to that of the perturbation fields in the nonlinear solution. This suggests that the same baroclinic instability generates the meanders in the nonlinear model as well.
4. REFERENCES
Fukamachi, Y., K.I. Ohshima, and T. Ishikawa, 1998: Antarctic Res. Ser., 74, 317-324.
Kantha, L.H., and G.L. Mellor, 1989: J, Geophys. Res., 94, l0,921-935.
McCreary J,P., Y. Fukamachi, and P.K. Kundu, 1991: J. Geophys. Res., 96, 2,515-534,
Yamanouchi, T., and K, Seko, 1992, National Institute of Polar Rssearch, Tokyo, 91pp.
*Corresponding author address. Yasushi Fukamachi, Hokkaido Univ., Inst. of Low Temperature Science, Sapporo, 060-0819 Japan; e-mail yasuf｡?oya.lowtem.hokudai.ac.jp
