A. INTRODUCTION 



The expanding program of ice observation and forecasting at the 

 Hydrographic Office has emphasized the desirability of long-range ice 

 forecasting. There are many phases of the ice program for which long 

 range forecasts giving advance estimates of ice conditions of as much 

 as 150 days are required. This paper deals with long-range forecasting 

 of ice thickness in open seas whose salinity and density remain rela- 

 tively constant, such as Baffin Bay and the Labrador Sea. 



An operational method of forecasting for a long period must be 

 easy to use, require a minimum number of involved calculations, and 

 yield forecasts of reasonable accuracy. Most formulas previously devel- 

 oped have been complicated expressions which are not suited for opera- 

 tional use. In developing the following method, the goal has been to 

 perfect a technique from which quantitative results can be derived with 

 a minimum of computation and observation. 



B. THE ICE POTENTIAL OF A WATER COLUMN 



The severity of an ice season depends, among other things, upon the 

 amount of thermal energy stored in the water mass and upon the rate at 

 which convective mixing takes place. It is necessary to select a model 

 of convective mixing which will explain the variation in the thermohaline 

 conditions caused by heat removal and ice accretion. The method proposed 

 by Defant (1949) and by Zubov (1938) of computing the ice potential and 

 potential heat loss has been adopted as a basis for the present forecast- 

 ing technique. 



Before a forecast can be made, an analysis of the properties that 

 inhibit ice formation and growth must be perforated. Oceanographic meas- 

 urements within the area in question must be secured in order to obtain 

 information about salinity and thermal energy stored within the water 

 mass. Unnecessary computation can be avoided by making the measurements 

 at a time after the heat flux reverses, when thermal energy is being 

 continually removed from the water. Otherwise the amount of heat which 

 was added to the water column prior to the reversal of the heat flux 

 would have to be computed before the method of Defant and Zubov could 

 be utilized. 



Establishing the ice potential of a given water column requires 

 only the temperature and salinity at N levels within the column. The 

 column is divided into (N-l)jLayers and the mean temperature (T n ), 

 salinity (S n ), and density (cr n ), are computed for each layer. The 

 mixing model, as defined by the method of Defant and Zubov is constructed 

 by finding the rrvjan temperature and salinity of the surface and second 

 layers, i.e., 



hlh = § i,2 , hJ-h = T lf2 m 



