Unusual nitrate distribution in an anticyclonic eddy during summer stratification.
Abstract
Typical open ocean eddies are usually recognized by vertical deformations in a density surface at the base of the euphotic zone. Cyclonic eddies can lift nutrients into the surface layer, which may be rapidly consumed by biota. On the contrary, anticyclonic eddies carry a deepening on density surface that eventually brings down nutrient depleted water, out of the euphotic zone. Aside these classical definitions, rare anticyclonic features known as “mode water eddies” have been reported to enhance a depression in the main thermocline while lifting the seasonal one, isolating intermediate water in the middle and leading to anomalous nutrient distribution in surface waters. In 2001, the POMME program (Programme Ocean Multidisciplinaire Meso Echelle) conducted three cruises from February to October in the Eastern North Atlantic (16° - 22°W, 38° - 45°N) in the vicinity of a mode water subduction area. Displacement of the σ ≈ 26.9 isopycne was intense near 41° - 42°N, characterizing the permanent mixed layer depth discontinuity zone (MLDZ). North of the MLDZ, winter convection controlled nutrient concentrations. Average mixing values oscillating between 100 and 75 m from winter to spring and allowed surface nitrate concentrations of over 5 to 7 µM until 44.5°N. In the southern area, no relationship was detected between convective mixing and integrated nitrates over the mixed layer. The main mesoscales features observed during the winter and spring cruises had no influence on nutrient distribution. Only one anticyclonic eddy (named A1) located permanently near 43°N, reflected some specific patterns. Changes in the inner structure of the eddy were observed in spring, when it evolved from a warm core to a cold core structure and finally behaved as a “mode water eddy” during summer stratification. A thick lens of water (σ ≈ 26.9-27) lifted the seasonal thermocline and depressed the main one. Consequently, surface nitrate concentrations greatly increased in the core of A1 (1µM at 25 m depth) offering favorable conditions for phytoplankton growth during oligotrophic summer conditions.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA......810F