Frontal positions and mixed layer evolution in the Seasonal Ice Zone along 140°E in 2001/02

• Main Authors: Aoki,Shigeru, Rintoul,Stephen R., Hasumoto,Hiroshi, Kinoshita,Hideki
• Format: Report
• Language: English
• Published: Institute of Low Temperature Science, Hokkaido University/CSIRO Marine and Atmospheric Research and Antarctic Climate and Ecosystems Cooperative Research Centre/Oceanographic Research Institute, The University of Tokyo/Hydrographic and Oceanographic Department, Japan Coast Guard 2006
• Subjects: Seasonal Ice Zone; Antarctic Circumpolar Current; mixed layer; cyclonic eddies; Antarctic; Southern Ocean; The Antarctic; Antarc*; ice pack; Polar bioscience; Sea ice
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=6254; http://id.nii.ac.jp/1291/00006254/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=6254&item_no=1&attribute_id=18&file_no=1

We describe the circulation and seasonal development of the upper ocean in the Seasonal Ice Zone (SIZ) of the Southern Ocean along 140°E. The 140°E section was repeated four times between November 2001 and March 2002, spanning the period from early spring to autumn. The sea ice edge was located at 62°-63°S in November, and retreated to 65°S in January. The circulation in the region is dominated by several fronts: the southern branch of Polar Front (PF-S) was located between 60° and 61.5°S; the northern branch of Southern ACC front (sACCf-N) was located at 61.5°-63°S, and roughly corresponds with the winter sea ice edge; and the southern branch of sACCf, the southern boundary of the ACC, and the Antarctic Slope Front (ASF) were closely spaced and found between 64°S and 65°S. Vigorous cyclonic (clockwise) eddies were identified in the region between the sACCf-N and sACCf-S throughout the period. Changes in salinity made the dominant contribution to changes in density in the SIZ, while changes in temperature made the largest contribution to density changes in the AZ, north of the sACCf. The depth of the mixed layer generally shoaled to the south, in all seasons. The decrease in mixed layer depth occurred in a series of steps. Seasonal variability in the depth of the mixed layer was strongest in the AZ, where summer warming formed a strong seasonal thermocline above the relatively deep (100 m) Winter Water layer. In the SIZ, the mixed layer became warmer, fresher and lighter in summer but the depth of the mixed layer remained at about 50 m throughout the year. The freshest surface waters were observed in the SIZ in January, immediately following the melt and retreat of the sea ice pack. An increase in mixed layer salinity from January to March likely reflects the effect of mixing with saltier waters below the mixed layer. Mixed layer depths south of the ASF were highly variable, both within and between seasons, varying from a minimum of ～20 m in January to over 500 m in March.