Enhanced transfer velocity of carbon dioxide during unstable stratification

1. Introduction

To understand the effects of anthropogenic emissions of CO2 it is crucial to understand all parts of the carbon cycle. The ocean is a sink of a large fraction of the anthropogenically produced CO2. Understanding gas exchange across the air-sea interface is an important component in global climate dynamics. There have been a large number of measurements of oceanic CO2 during the last decades but the quantification of the total oceanic uptake as well as the regional distribution is still uncertain.

In Takahashi et al. (2002) global estimates of the total CO2 uptake by the oceans are increased by 70% using two different formulations of the efficiency of the transfer.

The exchange of CO2 between the ocean and the atmosphere is controlled by the air-sea difference in partial pressure of CO2 (Δp CO2) at the surface and of the efficiency of the transfer processes. The partial pressure at the water surface is controlled by biological, chemical and physical processes in the ocean.

The efficiency of the transfer processes is determined by the resistance to the transfer in the atmosphere as well as in the ocean. The largest resistance to the CO2 transfer is found in the molecular diffusion through the laminar layer in the water. In most investigations the efficiency of the transfer is described by a transfer velocity. The transfer velocity is most commonly described by quadratic or cubical wind speed dependence. Most investigations of the relation between the transfer velocity and wind speed show a significant scatter, in part due to uncertainties in measurements, but also because there are other processes of importance. In Erickson (1993) it was shown that the atmospheric stability influences the transfer velocity between 20 and 50% and that this can be of very large importance for large air-sea temperature differences (for example for cold air outbreaks over warm water).

This effect is further investigated in the present study.

2. Measurements

With a unique measuring site in the Baltic Sea (the Östergarnsholm site, Smedman et al., 1999) extended data of direct measurements of the flux of CO2, using the eddy covariance method, the difference in partial pressure between the atmosphere and the ocean, as well as some parameters that probably influences the transfer (turbulence, heat fluxes and waves) gives the opportunity to gain significant new understanding of the processes controlling the transfer of CO2 (as well as other gases) at the air-sea interface. The station has been running since 1995, with measurement of the flux of CO2 since 2001 and measurements of the partial pressure of carbon dioxide at the ocean surface since 2005.

3. Results

The variability of the partial pressure of CO2 is surprisingly large both in the ocean and in the atmosphere. The atmospheric concentration varies between 360 and 410 ppm, the partial pressure in the ocean varies from 100 to 900 µatm. There is relatively good agreement between buoy data close to Östergarnsholm and ship data from the central parts of the Baltic Sea. There are, however, large differences during some situations.

The flux of CO2 is thus much more sensitive to footprint area than the fluxes of heat and humidity previously calculated for the same site (Rutgersson et al, 2001).

In a substantial part of the data with unstable atmospheric stratification the air-sea flux is significantly larger than the corresponding flux calculated using the difference in partial pressure and transfer velocity, this indicates the presence of processes strongly influences by atmospheric stratification of importance for air sea exchange. These results will be shown and possible explanations discussed.

References

Rutgersson, A., A. Smedman and A. Omstedt, Measured and simulated latent and sensible heat fluxes at two marine sites in the Baltic Sea. Bound.-Layer Meteor., 99, 53-84, 2001

Smedman, A., U. Högström, H. Bergström, A. Rutgersson, K. K. Kahma and H. Pettersson, A case-study of air-sea interaction during swell conditions, J. Geophys. Res., 104(C11), 25833-25851, 1999

Takahashi, T., and e. al. 2002. Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep Sea Research 49:1601-1622.