A large increase of the CO2 sink in the western tropical North Atlantic from 2002 to 2009

Determining a robust trend of surface ocean carbon dioxide (CO2) for a period shorter than a decade is challenging due to large seasonal variability and a sparsity of data. Here, we estimate the multiannual trend of surface CO2 in the region of 19 degrees N-20 degrees N, 65 degrees W-68 degrees W for the period of 2002-2009. We used an unprecedented number of high-quality underway data of the fugacity of CO2 in surface seawater (fCO(2SW)) collected from 137 cruises using an automated system onboard the cruise ship Explorer of the Seas. The growth rate of fCO(2SW) was estimated by two de-seasonalization approaches that showed similar and significantly lower values than the atmospheric increases, leading to a large increase in the CO2 sink. The seasonal difference in the trends was significant, with fCO(2SW) values in winter showing no increase, while summer fCO(2SW) values lagged only slightly with the atmosphere. We attribute the lack of an increase in winter fCO(2SW) values to sea surface temperature changes, which are closely correlated with the El Nino-Southern Oscillation cycle, and to changes in the mixed layer depth. The slower increase of fCO(2SW) is also related to decreases in salinity. The 8-year averaged annual net sea-air CO2 flux was -0.06 +/- 0.18 mol m(-2) yr(-1) compared to a climatology that shows a flux out of the ocean of +0.11 mol m(-2) yr(-1). The increasing flux differs from previous, mostly longer-term results for regional studies and time series stations in the North Atlantic, which suggests a decrease or no change in oceanic CO2 uptake.