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

Abstract

Abstract The annually integrated air–sea flux of CO 2 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 CO 2 can be written as F = α ( C T W − C Eq ), where F is the mean annual air–sea flux of CO 2 , α is an equilibrium rate constant (m/year), C T W is the total dissolved inorganic carbon concentration in the winter mixed layers and C Eq is the dynamic equilibrium concentration for C T W . In the formula, α and C Eq 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 C Eq . 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 C T W , α , and C Eq 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.