Abstract:
Mesoscale surveys to examine the relationship between the physical processes and biological response along the Antarctic Polar Front (PF) were conducted as part of the US JGOFS Southern Ocean Program during austral spring and summer 1997/98. Multiple crossings of the PF near 170° W, using a towed undulating instrument, provided high-resolution, 'quasi-synoptic' spatial profiles of the region. Increased chlorophyll levels were distributed in a band along the axis of the PF and extended southward. Cross-correlation analysis between the physical and biological properties was used to estimate the average spatial separation between features during austral spring and summer. Oceanographic sections revealed a meandering front in early November. The separation between the strongest horizontal temperature and chlorophyll gradients displayed spatial offsets of <10 km in the anticylonic portion of the PF while weaker gradients displayed offsets of 20+ km in the cyclonic portion of the PF. Changes in geopotential anomaly and temperature were used to estimate the width of the PF. The width of the PF was small, averaging 50 km in the anti-cyclonic portion of the meander, and increased to 90 km in the cyclonic portion. Mixed layer depths were deep and averaged 200 m. In austral summer, spatial gradients were weaker across the entire survey region and separations between temperature and chlorophyll were variable. The width of the PF averaged 120 km during the austral summer. Mixed layer depths shoaled from the 200 m observed in November to 60 m in January. The results suggest that spatial separation between the physical and biological properties across the front is associated with PF width and meandering. Upwelling, as a consequence of secondary circulation and/or frontal meanders, induces cross-front circulation which promotes stratification of surface waters, creating favorable light conditions suitable for phytoplankton growth and enhanced levels of chlorophyll along the PF.
