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

The annually integrated air-sea flux of CO, is governed by two quantities: the basic state of the ocean (e.g., the properties of the winter mixed laye r) and the signal from the annual cycles. In this study, I focus on the rol e of the annual cycles in mixing, sea-surface temperature and biological pr oduction. By integrating these cycles, it is shown that the annually integr ated flux of CO2 can be written as F = alpha (C-T(W) C-Eq), where F is the mean annual air-sea flux of CO2, alpha is an equilibrium rate constant (m/y ear), C-T(W) is the total dissolved inorganic carbon concentration in the w inter mixed layers and C-Eq is the dynamic equilibrium concentration for C- T(W). In the formula, alpha and C-Eq capture the influence of the annual cy cles as well as the spatial variations in, for example, the alkalinity. I a nalyze data from the Ocean Weather Stations (OWS) in the North Atlantic and some results from the MIT biogeochemical ocean model to find adequate desc riptions of alpha and C-Eq. According to these model/data estimates, the in fluence of the annual cycles in sea-surface temperature and biological prod uction on the dynamic equilibrium concentration is on the order of - 5 to - 20 and 0-30 mu mol/kg, respectively. These numbers are not negligible show ing that the annual cycles must be considered when analyzing the oceanic ca rbon cycle. The spatial distribution of the quantities C-T(W), alpha, and C -Eq are estimated for the North Atlantic using data from the TTO/NAS expedi tion and a combination of model and data analyses. The air-sea flux is calc ulated according to the above formula, and the seawards Bur is estimated to be 0.4 Gt C/year for the area north of 30 degreesN. (C) 2000 Elsevier Scie nce B.V. All rights reserved.