V. ANTARCTIC CONVERGENCE 



A. General 



The exact definition of Antarctic Convergence is controversial 

 and considered beyond the scope of this report. However, as the con- 

 vergence is one of the most interesting oceanographic features in the 

 Antarctic area and forms one of the best criteria for delineating the 

 Antarctic region from the Subantarctic Region, it vdll be discussed 

 briefly. 



For the purpose of this report, the Antarctic Convergence will be 

 considered as the zone where the cold and slightly denser surface water 

 of the Antarctic region sinks below the warmer and slightly lighter 

 surface water to the north. This zone is usually marked by a sharp 

 north-south decrease in the surface water tenperature ran^ng from 2° to 

 6°F, The mean surface temperature associated with this drop is about 

 liO^ during the southern summer; this gradient (north to south drop) 

 generally is also found at moderate depths. The mean tenperature of the 

 convergence sxirface gradient decreases as winter approaches. At greater 

 depths sinking water mixes with adjacent water and eventually spreads 

 to the north as the Antarctic Intermediate Water Mass, always recognizable 

 by its minimum salinity. 



The entire area south of the convergence is typified in summer by a 

 mixed surface layer as deep as 150 feet. Below this, tenperatures de- 

 crease strongly to a minimum of about 30°F at depths from 1$0 to 500 feet. 

 At greater depths the temperature increases to as much as 36°F reflecting 

 the presence of the deep warm water mass. Over continental shelf areas 

 the water is generally isothermal below the immediate surface layer. It 

 is emphasized, however, that the main water circulation in the convergence 

 area is west to east, and the north-south movements described above are 

 vectors of small magnitude. 



The Antarctic Convergence is not always a clear phenomenon and may 

 not be readily apparent. The most reliable indication of the position 

 of the convergence is given by the first indication, when traveling south- 

 ward, of a distinct subsurface temperature minimum at moderate depths. 



Frequently the surface water characteristics near the convergence 

 will fluctuate or show some inregularity. These may result from eddies, 

 tongues, presence of associated divergences, seasonal or meteorological 

 effects, or other causes, 



B. Atlantic Ocean Section 



The temperature structure of the Convergence zone extrapolated from 

 BT»s taken by the STATEN ISLAND during its southward transit in December 



23 



