pite the small tidal ranpe, there is much horizontal transport of 

 water by the tides entering Amundsen Gulf from the Beaufort Sea, 

 The bottom topograph;/ shows a sill located between Cape Bathurst 

 and Thesiger Bay, averaging 200 to 2!jO meters in depth, and over 

 which a large quantity of water is transported by tides, creating a 

 large-scale tidal rip. The effect of this tidal phenomenon is to 

 decrease the ice potential by causing uptrelling over the sill be- 

 tween the Beavfort Sea and the Amundsen Gulf basia; therefore, the 

 area of low ice potential is formed. 



The above theory seems reasonable uhtil it is considered that the 

 actual thickness of the surface layer in'/olved in the ice potential 

 (about 20 to 25 meters) is only about a tenth of the depth involved 

 in the horizontal tidal transport. In addition, the location of the 

 low ice potential is northwest of the sill. There is evidently some 

 other unknown source of unstable water which causes the low ice po- 

 tential in this area. 



3. Depth of Convection in 1951 



As explained previously, a depth of convection is determined when- 

 ever a fixed total heat loss is used. A chart shovdng these convective 

 depths for a total heat loss of 20 kg.cal./cm.^ is given for 19^1 (fig. 

 3) and may be used in conjunction with figure 1. Figure 3 shoi-/s less 

 regularity than figure 1. There are several centers of shallow and 

 deep convection evident on the chart. In general, the depth of con- 

 vection induced by the fixed heat loss is greatest to the north and 

 least toward the south. Convective depths of over $0 meters are found 

 north of 76°N, whereas depths of less than 10 meters are found in 

 Mackenzie Bay. Since a shallow depth of corr/ection is caused by strati- 

 fication, the area affected by the runoff of the Mackenzie River contains 

 the most stratified surface water of any area on the chart. In some 

 cases, high ice potential values are associated with deep convection, 

 while in other cases, the opposite association is found. For example, 

 in the area of high ice potential north of Barter Island the convec- 

 tive depths are less than 20 meters, although the same amount of po- 

 tential ice thickness (200 cm.) accompanies convection reaching a depth 

 of 1)5 meters near 76°N 170°V/. Also, , the area of low potential ice 

 thickness at the entrance to Amundsen Gulf is not evident on the flat 

 isoline field in figure 3o 



Because of the nature of the available data, it was not possible 

 to present a mean chart of cbnvective depth for the period 1950 to 1951io 

 Inasmuch as the mean chart for ice potential (fig. 2) resembles the chart 

 for 1951 (fig. 1) in most respects, it is felt that a mean chart of con- 

 vection would nearly duplicate the main features shown, on figiore 3» 



li. -Stability Index for 1951 



By combining the data shown in figures 1 and 3, an index of stability 

 related to ice forrnation is obtairied by finding the average potential ice 

 thickness per unit of depth pf convection. Since potential ice thickness 

 is given in centiwiters nni corwecfcive depth in meters, the ice potential 



^6- 



