Short timescale variations of ƒCO2 in a North Atlantic warm‐core eddy: Results from the Gas‐Ex 98 carbon interface ocean atmosphere (CARIOCA) buoy data

Abstract

During a Lagrangian deliberate tracer study in the North Atlantic, the 1998 Gas Exchange Experiment, hourly measurements of wind speed, sea surface temperature, ƒCO2, and fluorescence were made from two carbon interface ocean atmosphere (CARIOCA) drifting buoys in a warm‐core eddy near 46°N and 21.5°W over a period of approximately 20 days. Shipboard measurements of ƒCO2 near the buoys were used to verify the buoy operation, calibrate the buoy measurements, and assess the performance of the ƒCO2 sensor. The strong air‐sea ƒCO2 gradient in the eddy and intense atmospheric forcing during the experiment provided ideal conditions for demonstrating the potential of autonomous drift buoy measurements for studies of surface ocean biogeochemistry, where changes of ƒCO2 were rapid and large. During the experiment a storm occurred with wind speeds reaching as high as 16–17 m s−1, leading to a sharp decrease in sea surface temperature and an increase in ƒCO2 of ∼30 μatm. The magnitude of this sudden change in ƒCO2 is equal to approximately half of the annual range of ƒCO2 in this area. The air‐sea flux estimate for the ∼20 day experiment using the Wanninkhof [l992] gas transfer velocity formulation was −0.012 mol m−2 d−1 and using the Liss and Merlivat [1986] formulation was −0.007 mol m−2 d−1. The storm event, lasting 3–4 days, accounted for ∼38% of the flux over this period. Approximately 16 hours after the onset of the storm, there was an increase in surface fluorescence coincident with the initial increase in ƒCO2. Nitrate measurements made from the ship in the eddy show a sharp peak in surface concentrations ∼24 hours after the increase in winds and ∼6–8 hours after the increase in surface fluorescence. After the upwelling of the NO3 the fluorescence increases more sharply while the ƒCO2 decreases, consistent with biological productivity. The surface fluorescence measurements remain higher than prestorm conditions for ∼5 days after the NO3 has disappeared.