E. Shelf Water 



Shelf Water, the densest water mass found in the antarctic region, 

 is formed on the continental shelf within the Ross Sea. Other similar 

 shelf areas in Antarctica also may produce this water type; however, no 

 oceanographic observations are available to substantiate this hypoth- 

 esis. Shelf Water is characterized by temperatures less than -1.80°C 

 (28.76°F), salinities greater than 34.75%o, and sigma-t values equal 

 to or greater than 28.00 (U.S. Naval Oceanographic Office, 1965). 



This water mass is believed to form during the austral winter season 

 when the formation of sea ice increases the salinity of the surface layer, 

 causing mixing to occur. Meteorological conditions at McMurdo station 

 are capable of forming a maximum new ice layer 190 centimeters (75 inches) 

 thick (U.S. Navy Weather Research, 1961, and Zubov, 1943). In the western 

 Ross Sea, this amount of ice formation causes mixing to the bottom with 

 a resultant salinity greater than 34.75%o. Because Shelf Water is formed 

 at freezing temperatures, unmodified Shelf Water would have a temperature 

 of -1.90°C (28.58°F). Near the bottom some temperatures were observed 

 to average 0.03°C below the freezing point, indicating, that at times, 

 Shelf Water may be formed under super-cooled conditions. 



A continuous influx of Antarctic Circumpolar Water during the winter 

 would hamper cooling effects, but its high salinity of 34,72%o may play 

 a role in the formation of Shelf Water. The large volumes of Shelf Water 

 formed each winter probably flow north and eastward from the Ross Sea as 

 a bottom current; Antarctic Circumpolar Water, flowing up over the conti- 

 nental shelf, would then replace the north and eastward tending Shelf 

 Water. 



Shelf Water appears to influence the dynamics of the Ross Sea to such 

 an extent as to be the controlling factor. Due to its density, it may be 

 effective in barring the entrance of Antarctic Circumpolar Water into 

 parts of the Ross Sea during the late winter and early spring. 



During the summer season, Shelf Water extends from the southwest to- 

 ward the north and east (Fig. 19). In 1963, Shelf Water was found at 

 173°W in the south central Ross Sea at stations 65 and 66. At both of 

 these stations, it was observed within 20 meters (66 feet) of the bottom. 

 Farther east, during 1964, no evidence of Shelf Water was observed; how- 

 ever, the deepest observations were 50 to 75 meters (164 to 246 feet) 

 from the bottom, perhaps missing the Shelf Water in that area. Year to 

 year variations undoubtedly occur in the Ross Sea. 



Only near the edge of the continental shelf and over Pennell Bank, 

 was Shelf Water not observed in 1963. It existed in all depressions, 

 in some cases close to the shelf edge, indicating that it is highly prob- 

 able that Shelf Water spills over the slope in winter. No stations were 

 taken over Pennell Bank east of 179°E north of 75°S; however, the shallow 

 nature of this bank should exclude Shelf Water during the austral summer. 



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