Observations of the Polar Frontal Zone 



To study further the variability of the polar frontal zone, 

 XBT sections were taken from vessels crossing the Antarctic 

 Circumpolar Current south of New Zealand, Australia, and 

 South America during the austral summers of 1976-77 and 

 1977-78. Figure 27 shows the tracks of vessels making these 

 observations in the region of the Drake Passage. Figure 28 

 shows results from one of these sections. Frontal boundaries 

 between different water masses are evident in XBT temperature 

 sections. At the southern end of the passage close to the South 

 Shetland Islands, a subsurface isothermal layer of cold water, 

 called Antarctic Continental Water, can be seen. This layer is 

 bordered on the north by a strong subsurface (>150m) tem- 

 perature gradient, the Continental Water Boundary (CWB). 

 North of the CWB in the upper 200 m is the cold Antarctic 

 Surface Water (ASW). Formed during the winter, this water 

 mass is characterized in summer by a subsurface temperature 

 minimum. At the northern terminus of the temperature mini- 

 mum, a strong temperature gradient is found — this temperature 

 gradient is called the polar front (an operational definition of 

 the polar front adopted by ISOS workers is the northern edge 



of the 2°C isotherm). North of the polar front is the Antarctic 

 polar frontal zone. The northern boundary of the Antarctic 

 polar frontal zone is the subantarctic front. The thermal ex- 

 pression of the subantarctic front is a subsurface temperature 

 gradient between 2° and 5°C. 



The series of sections collected during 12 crossings of the 

 Drake Passage in the austral summer of 1976-77 indicated the 

 development of cold features within the Antarctic polar frontal 

 zone. These cold features are most likely expressions of eddies 

 or meanders formed at the polar front. They appear to widen 

 the Antarctic polar frontal zone by intensifying the subantarctic 

 front and moving it to the north. The majority of the tempera- 

 ture sections contain such cold features, whose signature could 

 be identified as an inflection in average profiles of 450 m heat 

 content and sea-surface temperature. 



Observations made by American and Soviet scientists on the 

 RV Professor Zubov in the polar frontal zone south of 

 Australia along 132°E between January 24 and March 1, 1977, 

 provided a more detailed view of the structure of the polar 

 frontal zone. A strong cyclonic eddy, containing ASW, from 

 south of the polar front, was found to the south of and com- 

 bined with a meander of the subantarctic front. Current meters 



60W 



50W 



• ■•■•• « . 



• •••»• ■ a . 



v ••! "-• 



■ ■ • •• ■ _ 



• ■■VS. • 



■ •■..<: '. 



■ • • ■ 



• o 



*A 



• A 



A A. 



±A 



55S 



. • ' <^v 



Figure 27. — Position of XBT observations in the Drake Passage 1976-77. 



41 



