Mesoscale frontal dynamics: shaping the environment of primary production in the Antarctic Circumpolar Current

The frontal regions of the Antarctic Circumpolar Current (ACC) differ from other parts of the ACC due to higher phytoplankton concentrations and primary production rates. We hypothesise that the enhancement of primary production results from the mesoscale frontal dynamics, in particular the cross-front circulation related to baroclinic instability. The hypothesis is corroborated by data collected in austral summer 1995/1996 at the Antarctic Polar Front between 1degreesW and 12degreesE during quasi-synoptic surveys with a measuring system combining towed and vessel-mounted instruments. Further confirmation is obtained from moored current meters and an earlier section worked across the front in austral winter 1992. The quasi-synoptic surveys cover a meander structure of the front and a cold cyclonic eddy located to its south. The highest chlorophyll concentrations, correlating with enhanced primary production rates, are found in a band of mesoscale patches aligned with the front and in a tongue extending southward from the front along the leading edge of a meander ridge. The increased chlorophyll concentrations at the meander edge are explained by confluence of surface water, which carries with. it the phytoplankton grown in favourable light conditions. While mesoscale surface confluence structures the synoptic chlorophyll distribution pattern, the mean enhancement of primary production at the front can be attributed to the influence of cross-front circulation on stratification. Ageostrophic cross-front circulation related to mesoscale upwelling and downwelling was identified at a site of eddy/front interaction. Consistent with the principle of potential vorticity conservation, upwelling was found to occur on the anticyclonic, equatorward side of the jet and downwelling on the cyclonic, poleward side in the frontogenetic situation. The associated cross-front circulation is characterised by poleward motion of light water at the surface and a reversed flow of dense water at greater depth; thus it contributes to stratification and thereby to a more favourable photic environment for the phytoplankton growing in the shallower mixed layer. While the cross-front circulation varies on horizontal scales of < 10 to > 100 km and time scales of days to weeks, it is constrained to sites of available potential energy, i.e. fronts marked by sloping isopycnals. With mixed-layer shallowing by cross-front circulation, we suggest a mechanism to explain the enhancement of primary production by mesoscale frontal dynamics in regions and during periods of light limitation. This complements the understanding of the importance of frontal dynamics for primary production, hitherto focussed on mesoscale upwelling of nutrients in the oligotrophic regime. (C) 2002 Elsevier Science Ltd. All rights reserved.