V. ANTARCTIC CONVERGENCE 



A. General 



The Antarctic Convergence is considered as the zone where the cold 

 and more dense surface •water of the Antarctic region sinks below the 

 warmer and less dense surface water to the north. This zone is usually 

 marked by a sharp north- south decrease in the surface water temperature 

 of 1* to 3*C (2° to 6*F)* The mean surface temperature associated with 

 this drop is about 2*C (361P) during January through March; this gradi- 

 ent (north to south temperature decrease) is also generally found at 

 moderate depths. The mean temperature of the Convergence surface 

 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, recognizable by its minimum salinity. 

 It is emphasized that the main water circulation in the Convergence 

 area is west to east, and the north-south movements are vectors of small 

 magnitude* 



Four temperature profiles are presented from data collected by a 

 900-foot BT across the Convergence. BT's were taken half -hourly except 

 when prevented by rough seas. One other Convergence crossing is pre- 

 sented with temperature, salinity, and density profiles. These data 

 were obtained by Nansen casts. 



B. Bathythermograph Sections 



1. Figure 12 presents the vertical distribution of temperatures for 

 two crossings of the Antarctic Convergence taken by ATKA south of New 

 Zealand, towards McMurdo Sound. The first is the result of observations 

 made in January i960. The position, during this early summer crossing, 

 was between 62°30'S. and 63 °S. This section provides a good example of 

 the major characteristics of the Convergence. The rapid surface tempera- 

 ture change, 37° to 33 °F in about fifteen miles, and the nearly vertical 

 isotherms in this zone are clearly indicated. To the north, the 

 Subantarctic Water is shown with an isothermal layer to about 300 feet; 

 to the south, the Antarctic Surface Water, with an isothermal layer to 

 about 150 feet. 



The second crossing by ATKA was made during March i960, between 

 6l°^7*S and 62*30 *S. The Convergence in this section is not so 

 readily apparent by a rapid surface change; however, below the surface, 

 the typical structure is noted. During this period, late Bummer, the 

 surface layer of the Antarctic Surface Water is considerably deeper. 



2. Figure 13 is a crossing of the Convergence made by GLACIER during 

 transit from New Zealand to Thurston Peninsula in February i960. The 

 vessel crossed the Convergence at an angle, at approximately 60* to 6l*S. 

 An interesting feature of this section is the warmer surface layer of the 

 Antarctic Surface Water than noted in the previous cross sections of this 

 report. 



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