DEPTH OF THE ANTARCTIC SURFACE LAYER 41 



The curves in the upper half of Fig. 10 show the temperature of the water column 

 between the surface and 400 m. at intervals of 4 hours, and those in the lower half of the 

 figure the depth of the isotherms at the same intervals. Both sets of curves show that 

 the temperature change between the two layers is sharpest where the temperature is 

 — 0-5 to o-o° C, and the depth of this stratum affords an approximate measure of the 

 depth of the layer. The greatest depth was about 230 m. at 0600 hours on the second 

 day, and the least depth 180 m. 6 hours later; the range of the vertical movements is 

 50 m. The examination of these and similar data is so far only in a very preliminary 

 stage, and it cannot be proved that changes go on incessantly ; these observations were 

 made at the time of new moon, so that the possible effect of tidal influences on the 

 layer should be measured when it was likely to be most pronounced. The times of the 

 moon's upper and lower transits are indicated in the diagram, and it is approximately 

 at these times that the isotherms are nearest the surface. The undulations of the iso- 

 therms during the 24-hour period suggest that their depths are changed by a succession 

 of internal waves, and the period of the waves indicates that they are of tidal character. 

 At the time of the first and third quarters of the moon the waves will probably have a 

 smaller amplitude and the depth of the layer will not be subject to such large variations. 

 It is also not certain that there are such fluctuations in the depth of the layer at all points 

 in the Antarctic Zone. The observations described were made in deep water; the sound- 

 ing was 3820 m. and the nearest shallow water — in the neighbourhood of Bouvet Island 

 — 250 miles away. Until the question of these changes has been thoroughly investigated 

 any account of the depth of the layer must necessarily be only approximate. 



The nineteen sections in this report which show the conditions in each sector of the 

 Southern Ocean make it quite clear that the depth of the layer varies only within com- 

 paratively small limits. It is shallowest in the divergence region between the East Wind 

 Drift near the Antarctic Continent, and the West Wind Drift farther north. The ob- 

 servations made in this region show that in the Atlantic and Indian Oceans the depth of 

 the layer is only 60-80 m. ; in the Pacific Ocean, however, the depth increases from west 

 to east, and in the eastern half of the ocean it is about 150 m. 



From the divergence region the depth of the layer increases towards the north and 

 south ; it is deepest near the convergence on the one hand and near the continent on the 

 other. In each direction the slope is caused partly by the effect of the earth's rotation 

 on the current flowing at right angles to the slope, but the northward slope at least must 

 also depend on the north and south movements in the bottom and deep layers. 



On the western side of the Atlantic Ocean the depth of the Antarctic layer near the 

 convergence is 200-300 m. ; on the eastern side the depth is slightly less, only 1 50-200 m. 

 South-east of the Cape of Good Hope it is as much as 300 m. ; at St. 850 in section 8 

 (Plates XIII-XV), the cold stratum of the layer was found between 200 and 300 m., its 

 great depth being due to the presence of a large volume of highly saline cold water 

 which is probably derived from the Weddell Sea current. South of Australia the depth 

 of the layer near the convergence is 200-300 m., but it shallows towards the east, and 

 south of the Tasman Sea is only 150-200 m. On the western side of the Pacific Ocean 



