SYMPOSIUM ON MARINE BIOGEOGRAPHICAL 

 PROVINCES IN THE PACIFIC 



SURFACE BOUNDARIES IN THE SOUTHERN OCEAN 

 By G. E. R. Deacon, Admiralty Research Laboratory, England 



[^4 bstract] 



More information about the Antarctic Convergence, the boundary 

 l^etween x\ntarctic and subantarctic waters at the surface, has been 

 ■obtained by the detailed analysis of thermograph records obtained by 

 the vessels of the " Discovery " Investigations (Mackintosh, 1946). The 

 mean position is little different from that pubhshed in 1937, and the new 

 figures strengthen the conclusion (Deacon, 1937) that the position of the 

 surface boundary is closely related to the deep and bottom currents 

 which are subject to httle variation. Secondary fluctuations can be 

 attributed to local causes, but it is found that half the reported positions 

 fall within 25 miles of the mean position, and the extreme displacement 

 may not exceed 100 'miles. The average difference of temperature is 

 2° c, from 4° c. to 6° c. in summer, and 1° C. to 3° c. in winter, and if it 

 is less than 2° c. observations of surface temperature alone may not be 

 sufficient to show exactly where the boundary is crossed. Many of the 

 biologists who have examined the " Discovery " collections have remarked 

 on the significance of the convergence as a boundary for species of 

 plankton, fish, and non-abyssal bottom-living animals. Its position is 

 also related to the distribution and stratification of the bottom sediments. 

 R. C. Murphy has given a hst of fifteen birds typical of the Antarctic 

 zone, twenty-nine typical of the subantarctic zone, and eleven common 

 to both. 



The subtropical convergence between subantarctic and subtropical 

 water is usually marked by a difference of 4 or 5° c, from approximately 

 12° c. to 16° c. in summer and 8° c. to 12° c. in winter, and the increase 

 in temperature is accompanied by a sharp increase in salinity. It has 

 been found to Hmit the distribution of some species of plankton, fish, 

 and bottom-Hving animals. The simplest explanation of the sharp 

 difference in physical properties is that it marks the convergence of 

 water movements with northward and southward components. The 

 boundary is sharpest where a current convergence is most likely, as in 

 the regions south of the Brazil current, Agulhas current, and East 

 Austrahan current, and it appears to be less sharp in the eastern half 

 of each ocean, where the current in middle latitudes has a more northerly 

 trend. The position of the subtropical convergence appears to be subject 

 to considerable variation, especially in the eastern half of each ocean, and 

 Defant (1938) has suggested that it would be better to describe it as a 

 convergence region rather than a fine. This should prove a useful 

 suggestion, though it is likely to prove that the convergence region is 

 one over which a sharp boundary fluctuates, rather than one of gradual 

 change. 



Stephenson (1947), deahng with littoral regions, thinks the term 

 " subtropical " misleading when it is applied to water which may be as 

 cold as 12° c. in winter, and prefers the names "warm temperate" 

 and " cold temperate " to subtropical and sub- Antarctic. 



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