Eddy-induced cross-slope exchange maintaining summer high productivity of the Bering Sea shelf break

摘要

Eddy-related cross-slope exchange along the Bering Sea shelf break was investigated using a hydrographic observations data set and a numerical model. Results of observations in summer of 2001 showed a shelf break front that formed at a shelf break near an anticyclonic eddy, high nitrate-nitrite concentrations in the subsurface layer, and high chlorophyll a (Chl-a) concentrations (≥6 mg m~-3) in the surface layer. A hydrographic observation in summer of 2002 exhibited relatively high Chl-a concentrations at the surface around the anticyclonic eddy. Tracer experiments revealed two types of cross-slope exchange. Under isopycnals, nutrient-rich water in the basin is transported to the shelf and there is about a 64.53% increase in integrated nitrate-nitrite on-shelf flux (50 m depth ～bottom), when mesoscale eddies are formed and propagated along the shelf break. At the surface, high Chl-a waters in the shelf are advected to the deep basin area by eddy transport and propagation. These indicate that (1) mesoscale eddies supply nutrients and sustain primary productivity at the shelf break, and (2) eddies expand the high Chl-a area to the basin, then to the highly productive area, so that the Green Belt is maintained.