Comparing air-sea fluxes of carbon dioxide estimated using the inertial dissipation method with fluxes measured using the eddy-correlation method

Introduction

In the global carbon cycle the total exchange at the sea surface is not very accurately described. This is explained by lack of understanding of the processes controlling air-sea exchange. For near coastal regions this lack of understanding has a greater impact due to larger variability of the parameters controlling the transfer. The exchange of CO2 between the ocean and the atmosphere is controlled by the air-sea difference in partial pressure of CO2 at the surface and of the efficiency of the transfer processes (by the transfer velocity). 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. The largest resistance to the CO2 transfer is found in the molecular diffusion through laminar layers in the water and in the air. Due to significant scatter in measurements it is difficult to decide the most accurate formulation. To determine the air-sea exchange of CO2 we thus need to be able to describe the biological, chemical, and physical properties of the ocean as well as the meteorological conditions in the atmosphere.

Measurements

In 2006 a Danish scientific expedition, Galathea 3, sailed around the world collecting a large number of meteorological, oceanographic, chemical and biological data. It was a one year cruise covering most of the world's oceans. The concentration of CO2 was continuously measured in the air as well as in the water. The vertical flux of CO2 was estimated using the eddy correlation method, where the vertical wind was measured with a Sonic anemometer and CO2 fluctuations with a Licor on each side of the ship. Since the ship moves, a correction for the ship movement needs to be applied when calculating the fluxes with the eddy correlation method. The correction of the movement is performed using 6 accelerometers placed on the ship.

Results

The part of the Galathea data that has been used is data measured from Greenland to Cape Town. The route covers upward fluxes as well as downward fluxes. The accelerometers are used to correct the fluctuation of the vertical wind. The correction works satisfactory during some periods giving relatively accurate fluxes.

The CO2 flux is also calculated using two other methods, the inertial dissipation method and the co-spectral method (Larsen, 1996, 2001 and Sörensen, 2002). Comparing the three methods shows periods where the fluxes are reliable and the accuracy of the methods can be estimated. The data is then used to study the transfer velocity in different regions during various conditions.