Mesoscale physical and bio-optical structure of the Antarctic Polar Front near 170 degrees W during austral spring

摘要

As part of the U.S. Joint Global Ocean Flux Study Southern Ocean program, high-resolution surveys of the Antarctic Polar Front near 170 degreesW were conducted during October-November 1997 with a towed undulating system equipped with conductivity-temperature-depth and bio-optical sensors. Transects along 170 degreesW and two successive mapping surveys revealed zonal bands with sharp meridional gradients in east-west velocity. The Polar Front (PF) was characterized by a sea surface temperature drop from 1.6 degrees to -1.6 degreesC between 60.35 degrees and 61.10 degreesS, with eastward velocities of 0.4-0.5 m s(-1) in the core of the PF jet. Deep mixed layers (> 200 m) were found within and north of the PF, but mixed layers shoaled to 100-125 m south of the PF to the edge of loose ice at 62.3 degreesS. Highest mixed layer chlorophyll concentrations (0.35 mg m(-3)) in late October along 170 degreesW were to the south of the PF and associated with cold, fresh water. A large meander of the PF was observed with an alongfront wavelength of 175 km, a cross-front peak-to-peak amplitude of 100 km, and an eastward phase propagation of 0.05-0.08 m s(-1), all of which are consistent with its formation via hydrodynamic instability of the PF jet. Highest-phytoplankton biomass was located just poleward of the center of the PF jet. A high-chlorophyll (up to 1.1 mg m(-3)) 50 by 50 km region was found downstream of the cyclonic bend associated with the meander. A survey 7.5 days later revealed growth of this high biomass region so that chlorophyll was in excess of 0.8 mg m(-3) over an 80 km cross front by (at least) 80 km alongfront region. High biomass was observed to grow in place with respect to the meander rather than being displaced far downstream as would be expected from advection. This pattern is consistent with meander-driven upwelling of nutrients and/or trace metals, which in turn stimulates phytoplankton growth. Detailed cross sections of the PF reveal narrow 10-20 km wide bands or filaments of phytoplankton biomass that have temperature/salinity properties distinct from surrounding water and are coherent for at least 120 km alongfront. [References: 45]