The Stratospheric Circulation 669 



the out-flow from the Labrador Sea is subject to large variations dependent on a 

 number of diff"erent phenomena occurring at the sea surface of the polar regions (see 

 also KiiLERiCH, 1939). 



Table 156 also contains a heat budget for the Labrador Sea. The heat gain amounts 

 to 1-7 X 10^ kg cal/sec. If the mean temperature of the waters which sink below 

 1500 m is taken as 3-2°C then the heat flux with the outflow mentioned above will be 

 about 6-1 X lO^kgcal. This then gives for the Labrador Sea a heat deficit of 

 4-4 X 10^ kg cal. It is not improbable that this heat deficit according to its magnitude 

 is totally compensated by the heat absorption of solar radiation in the water during the 

 summer. It can be calculated that of the total radiation from sun and atmosphere about 

 20 X 10^ kg cal reach the sea surface of the Labrador Sea. Of this then more than 

 40% (8 X 10^) is lost by reflection; the remaining 12 x 10^ kg cal goes to radiation, 

 evaporation and absorption. Since the radiation is probably not very eff"ective, about 

 two-thirds of this goes to evaporation and one third or about 4 X 10^ kg cal to 

 absorption. This quantity is of the same order of magnitude as the quantity given above, 

 but due to the uncertainty of the calculation this result should only be accepted with 

 reservations. 



3. The Processes which occur at the Antarctic Convergence Zone 



The causes for the formation of an Antarctic convergence within the broad oceanic 

 West Wind Drift of higher latitudes in the Southern Hemisphere were discussed on 

 p. 549. This discontinuity layer in the thermo-haline structure of the upper water 

 masses appears in the pressure field as a discontinuous step in the meridional slope of 

 the isobaric surfaces and the physical sea level (Fig. 253). This can also clearly be seen 

 in representations of the dynamic topography of the isobaric surfaces constructed by 

 Deacon (1937) according to the data obtained by the "Discovery" for the broad ring 

 of water surrounding the Antarctic continent. Figure 316 shows the dynamic topo- 

 graphy of the physical sea level (relative to that of the 3000-decibar level) for this 

 oceanic region. The downward slope of the pressure surfaces towards south at all meri- 

 dians is not uniform and a discontinuity extends all around the earth that makes the 

 meridional gradient much stronger in a belt coinciding with the Antarctic polar front. 

 This frontal zone is also shown to exist in the topographies of the isobaric surfaces 

 for larger depths; but corresponding to the much smaller gradient it is less strongly 

 developed in the deep sea. 



From the analysis of a series of vertical sections between Antarctica and South 

 America (partly in the Drake Strait) as well as at 30° W. in the South Atlantic between 

 36° and 50° S. based on the observations of the "Discovery" Sverdrup (1933a) has 

 deduced the vertical circulation in the Antarctic Convergence Zone. Since conditions 

 around the Antarctic continent are very uniform, the results should be typical for the 

 whole of the circumpolar region. The essential details can be seen in the temperature, 

 salinity and oxygen sections at 30° W. shown in Fig. 317. According to all such meri- 

 dional sections and also according to those for the other oceans the water masses of 

 the upper layers south of the Antarctic Convergence sink down along the boundary 

 layer. In the salinity distribution this is clearly shown by a tongue of weakly saline 

 water. At the polar front at first the water sinks immediately down to 400 m and then 

 spreads almost horizontally to the latitude of the subtropical convergence region where 



