Structure and dynamics of the Barents Sea Polar Front near the Great Bank and associated plankton distribution in August 2007

Abstract

The Polar Front separating Atlantic Water (AW) and Arctic Water (ArW) is one of the most dominant mesoscale features in the Barents Sea and a highly productive region. Here, the results of sub-mesoscale variability in physical and biological fields associated with the Barents Sea Polar Front (BSPF) at the Great Bank are reported from a high-resolution survey in August 2007. A nearly synoptic data set was collected using a ship-mounted Acoustic Doppler Current Profiler (ADCP) and an undulating instrument platform, equipped with a fluorometer, a Laser Optical Plankton Counter (LOPC) and Conductivity-Temperature-Depth (CTD) sensors. AW was found beneath ArW, leading to a surface and a deep expression of the front, which were shifted by about 40 kilometers and characterized by strong salinity and temperature gradients. Thermohaline compensation eliminated the cross-frontal density gradient at depth. The most dominant flow feature was a southeastward barotropic along-frontal jet with two cores at the surface and the deep expressions of the front. Tidal currents were negligible (estimated using the Arctic Ocean Inverse Tidal Model), while other ageostrophic processes retarded the frontal circulation and stimulated cross-frontal exchange. The hydrographic small scale structure of the front was highly patchy, presumably caused by turbulent stirring. The Arctic side of the front had a strong pycnocline, and the plankton community was in a post-bloom condition there. A summer phytoplankton bloom was observed on the Atlantic side, where nutrient upwelling was probably facilitated through shoaling isopycnals and a weaker pycnocline. Zoo- and phytoplankton occurred in small-scale patches that were correlated with the hydrographic small-scale variability.