Trends in North Atlantic sea-surface fCO2 from 1990 to 2006

We examine observations from 1990 to 2006 from four voluntary observing ships and two time-series stations in the North Atlantic, fitting a sinusoidal annual cycle and linear year-on-year trend at all locations where there are sufficient data. Results show that in the subtropical regions, sea-surface fCO(2) has closely followed the increasing trend in atmospheric fCO(2). In contrast, farther north, sea-surface fCO(2) has increased faster than fCO(2) in the atmosphere. The resulting AfCO(2), driving air-sea flux of CO2, has therefore decreased in the North Atlantic, particularly at higher latitudes, as has the annual mean air-sea flux. Several underlying causes may have led to the observed changes in sea-surface fCO(2). Low-frequency modes, such as the North Atlantic Oscillation, lead to changes in the sea-surface temperature, in sea-surface circulation and in vertical mixing, affecting sea-surface fCO(2) through biogeochemical processes. A comparison with measurements covering a longer time period shows that the sea-surface fCO(2) rise has accelerated since 1990 in the northern North Atlantic. (C) 2008 Elsevier Ltd. All rights reserved.