JOURNAL OFGEOPHYSICAL RESEARCH, VOL. 90, NO. D3, PAGES 5676-5686, JUNE 20, 1985 SEA ICE: MULTIYEAR CYCLES AND WHITE ICE Tamara Shapiro Ledley Departmentof SpacePhysics and Astronomy,Rice University, Houston, Texas Abstract. An interesting feature of previous sea ice modeling studies [Semtner, 1976; Wash- ington et al., 1976] was the multiyear cycles in sea ice thickness despite repeating year to year forcing. It is shownhere that a number of factors combine to produce these cycles. The most important of these is the insulating prop- erties of the snow layer; however, the timing of the refreezingof new ice at the endof an ihe free summer season, and the vertical ocean flux play a role in determining the length and char- acter of these cycles. It is also shown that the inclusion of the formation of white ice in the model greatly reduces its ability to produce multiyear cycles. 1 ß Introduction One of the interesting features of the sea ice studies done by Semtner [1976] and Washing- ton et al. [1976] with simple thermodynamic models of sea ice was the multiyear thickness cycles. These were produced in cases where open ocean was predicted during the summer season once every few years despite an unchanging year to year forcing. Semtner suggested that these cycles were the result of the differing thermal conductivities of snow and ice. The scenario of these cycles was that following the appearance of open ocean in a given summer, the refreezing of ocean did not occur until most of the heavy snowfall season was over. As a result, the newly formed ice was less insulated from the cold winter temperatures; thus it grew quickly to a relatively large thickness which required several years with usual winter snow cover to eliminate. He then • went on to speculate that this process may be important in explaining multiyear anomalies in the observed ice cover. Washington et al. [1976], using a one-layer version of the Semtner model to examine sea ice variations over the Arctic Ocean and around Antarctica, also obtained multiyear cycles ß However, this phenomenon was not examined fur- ther due to the expense of running the model to equilibrium. Results presented in this article will show that the insulating effect of snow on the sur- face of the sea ice is important in producing these multiyear cycles given the physics includ- ed in the model; however, when the formation of white ice is included, these cycles almost dis- appear. White ice is the ice that forms at the snow-ice interface when the snow layer becomes thick enough to depress the ice below the water level. Water then infiltrates the snow by Copyright 1985 by the American Geophysical Union. Paper number 5D0192. 0148-0227/85/005D-0192505.00 coming through the ice at leads and generally freezes there, forming white ice. The inclusion of white ice restricts the thickness that the snow layer can attain and thus its ability to insulate the ice layer from the cold 'temperatures of the winter season. This process is relatively rare to observe because in most regions the snow layer never becomes very thick; however, it does occur occasionally over ice on lakes [Adams and Roger- son, !968] and off the coast of Labrador [Weeks and Lee, 1958] ß Also, when modeling long-term climate change and extreme glacial climates, situations involving very thick snow layers may occur. For these reasons the formation of white ice •shoutd be included in the models. 2 ß The Model The sea ice model used here is a modification of that used by Semtner [1976]. It is a three- layer thermodynamic model which includes conduc- tion within the ice and snow, penetration of solar radiation into ice layers, and surface energy balances. This model is coupled to a very simple mixed layer ocean model which allows the initiation of new ice on open ocean. The temperatures and accretion and ablation of the snow and ice are governed by one-dimen- sional heat equations, surface energy balances, and a snowfall parameterization. The numerical formulation of the one-dimensional heat equation as applied to the layers is given in Appendix B of Ledley [1985]. The major terms included in the surface energy balance calculations are the latent heat flux, sensible heat flux, absorbed short wave radiation, long wave radiation from the atmosphere, long wave radiation from the surface, conductive fluxes from within the ice, and vertical oceanic heat flux. The formula- tions of these terms are summarized in Table 1. The snowfall rates are determined from present precipitation rates; and the saturation vapor pressure and the fraction of precipitation that will fall as snow, both of which are com- puted as a function of air temperature (see Table 2). For a more complete description of the parameterizations used in the surface energy balances and the snowfall parameterization, see Ledley [1983, 1985]. The transition from ice-free to ice-covered ocean is made by applying a heat budget to a 70- m mixed layer ocean unt$1 it again reaches the freezing point, at which time new ice forms. These equations are solved numerically using a semi-implicit numerical scheme for the time derivative which allows time steps on the order of 1 week. • A previous study [Ledley, 1985] has shown that the sea ice model can be sensitive to time step size; however, this sensitivity has mainly been associated with the inclusion of a lead 5676