62 DISCOVERY REPORTS 



northern side of the South Shetland Islands, with the result that at the junction heavy 

 overfalls are seen. When, however, north-easterly winds predominate, the south-west- 

 going stream on the northern side of the island is considerably strengthened, with the 

 result that this current overcomes the northerly set passing up the narrow straits, and 

 passes into the individual straits. When this happens the two contrary directed currents 

 meet inside the narrow straits and it is here that the heavy overfalls are now found. 



In this latter condition a prominent line of demarcation is to be seen between the two 

 currents, showing that on the western side of the narrow straits there is a north-going 

 set and on the eastern side there is a south-going set. During the time the R.R.S. 

 ' Discovery II ' was lying at anchor in Nelson Strait we observed this last condition very 

 well. A north-east wind commenced to blow and later the northerly set could be seen 

 to be gradually overcome by the now strengthened southerly current. 



SUMMARY 



i. Previous workers have suggested the probability that the Bransfield Strait consists 

 of an enclosed basin. This is confirmed and it is shown that two such basins exist, 

 bounded by islands and submarine ridges. The effect of the bathymetric features is such 

 that the strait is cut off from free circulation with the outside seas. As a result the 

 temperature below the surface layer is abnormally low, the horizontal circulation is 

 restricted very considerably at depths below those of the confining ridges and the 

 amount of warm deep water within the strait is thus considerably less than exists outside, 

 the lowest layer, consisting of Antarctic bottom water, being correspondingly increased. 

 Temperature-salinity diagrams for stations both inside and outside the strait are given 

 in illustration of the differences in composition of the water masses in this area. 



2. Vertical sections of temperature, salinity and density show in general an increase 

 of salinity and density and a decrease of temperature in the surface layer from the South 

 Shetland Islands towards Trinity Peninsula. The amount of warm deep water is greatest 

 at the south-west end of the strait and close to the South Shetland Islands, but decreases 

 towards Trinity Peninsula. In February 1929, on the line from King George Island, 

 thermal evidence of the presence of warm deep water was restricted to the station nearest 

 the island, whereas in November of the same year the vertical section of temperature 

 showed a wedge of this water reaching to at least within 24 miles of Trinity Peninsula. 



3. Apart from the effects of the bottom topography the chief features of the circula- 

 tion in the strait appear to be due to two main currents. At the south-western end water 

 from the Bellingshausen Sea enters the strait between Low, Smith and Snow Islands 

 and flows both north and south of Deception Island, finally appearing as a north-east 

 going set close to the southern shores of the South Shetland Islands. At the eastern end 

 of the strait a current of cold, more dense water, whose origin is the Weddell Sea, flows 

 round Joinville Island and across the strait, some of it spreading down the Trinity 

 Peninsula coast. In the available data there is one feature of the water which later forms 

 the north-east set close to the South Shetland Islands, which appears to be permanent. 

 The surface water after passing Deception Island makes a characteristic bend to the 



