Seasonal sea-surface carbon dioxide in the Azores area

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The seasonal evolution of total inorganic carbon and CO 2 air–sea fluxes in the Eastern North Atlantic Subtropical Gyre (Azores area) was investigated by means of studying a data set from 10 cruises covering a seasonal cycle. Monthly CO 2 fugacity was modelled as a function of surface temperature and month for 1998. So, the seasonal cycle of CO 2 and its air–sea fluxes were obtained using monthly average surface data in the area. Over the year, the Azores area (2.25·10 12 m 2) acts as a weak net sink of CO 2 (0.38 mmol m −2 day −1). From December to May, the zone is a rather strong sink for CO 2 (10.3 mmol m −2 day −1), while between June and November, it behaves as a CO 2 source (9.9 mmol m −2 day −1), August presents the highest outgassing (3.88 mmol m −2 day −1). Moreover, a box budget was established to evaluate the relative contribution of the physical and biological processes affecting the seasonal CO 2 variability in the mixed layer of the Azores area. The most important contributor to the average mass balance of CO 2 was the mixing with the lower layer (7.8 mmol m −2 day −1) and biological activity (−8.9 mmol m −2 day −1). Conversely, air–sea exchange (0.17 mmol m −2 day −1) and advection (1.7 mmol m −2 day −1) contribute with a very small input. There is a strong coupling between biological activity, advection, and mixing in the mixed layer depth. The biological activity is supported by mixing and advection that provide CO 2 and nutrients to the mixed layer, so we combine the three processes in only one term (ΔC AMB) that represents the net biology production in the water column, and re-evaluated the CO 2 mass balance to discriminate the importance of the physical and biological contributions. The effect of temperature, wind, and net biological process contribute in 42%, 12%, and 46%, respectively, to the explained variance of total CO 2 mass balance in the upper layer.