A Subtropical Oceanic Ring of Magnitude (STORM) in the Eastern North Atlantic: physical, chemical and biological properties

Abstract

A Subtropical Oceanic Ring of Magnitude (STORM) that budded from the Azores Current was studied in the Eastern North Atlantic Ocean in April 1999. The cyclonic eddy, called Leticia, was centred at ~32.4°N–28.7°W and extended across more than 200 km in an east–west direction. Leticia had been previously detected by means of TOPEX/POSEIDON altimetry data and further intensively investigated using expendable bathythermograph and conductivity–temperature–depth stations. The altimeter surface signature of the eddy revealed a ~20-cm sea-level depression, an upward displacement of the 16°C isotherm of ~125 m was linked to the eddy centre and Geostrophic transport of the amount of water swirling cyclonically was 7 Sv. The temperature, nitrate, and oxygen anomalies on the 0=26.5 isopycnal surface inside Leticia were ?0.4°C, ?1 mol l?1, and +0.3 ml l?1, respectively, with respect to surrounding waters. Shallowing (>50 m) of the deep chlorophyll maximum (>0.3 mg m?3) was observed at the eddy centre associated with vertical displacements of the isotherms (>100 m) within the photic layer. Vertical excursions of the isopycnals and the deep chlorophyll maximum of more than 30 m were observed over temporal scales of a few (3–6) h. Integrated nitrate concentration over the photic layer was ~4-fold higher inside Leticia than outside, however, primary production rates were only slightly higher. Eddy diffusive fluxes across the nitracline, located ~30 m below the deep chlorophyll maximum, ranged from 0.02 to 0.10 mmol NO3 m?2 d?1, and explained <25% of the nitrate required to sustain the estimated new production (0.24–0.48 mmol NO3 m?2 d?1).