The role of the annual cycles for the air–sea exchange of CO2

The annually integrated air–sea flux of CO2 is governed by two quantities: the basic state of the ocean (e.g., the properties of the winter mixed layer) and the signal from the annual cycles. In this study, I focus on the role of the annual cycles in mixing, sea-surface temperature and biological production. By integrating these cycles, it is shown that the annually integrated flux of CO2 can be written as F=α(CTW−CEq), where F is the mean annual air–sea flux of CO2, α is an equilibrium rate constant (m/year), CTW is the total dissolved inorganic carbon concentration in the winter mixed layers and CEq is the dynamic equilibrium concentration for CTW. In the formula, α and CEq capture the influence of the annual cycles as well as the spatial variations in, for example, the alkalinity. I analyze data from the Ocean Weather Stations (OWS) in the North Atlantic and some results from the MIT biogeochemical ocean model to find adequate descriptions of α and CEq. According to these model/data estimates, the influence of the annual cycles in sea-surface temperature and biological production on the dynamic equilibrium concentration is on the order of −5 to −20 and 0–30 μmol/kg, respectively. These numbers are not negligible showing that the annual cycles must be considered when analyzing the oceanic carbon cycle. The spatial distribution of the quantities CTW, α, and CEq are estimated for the North Atlantic using data from the TTO/NAS expedition and a combination of model and data analyses. The air–sea flux is calculated according to the above formula, and the seawards flux is estimated to be 0.4 Gt C/year for the area north of 30°N.