Using eddy covariance to measure the dependence of air–sea CO<sub>2</sub> exchange rate on friction velocity

Abstract

Abstract. Parameterisation of the air-sea gas transfer velocity of CO 2 and other trace gases under open-ocean conditions has been a focus of air-sea interaction research and is required for accurately determining ocean carbon uptake. Ships are the most widely used platform for air-sea flux measurements but the quality of the data can be compromised by airflow distortion and sensor cross-sensitivity effects. Recent improvements in the understanding of these effects have led to enhanced corrections to the shipboard eddy covariance (EC) measurements.Here, we present a revised analysis of eddy covariance measurements of air-sea CO 2 and momentum fluxes from the Southern Ocean Surface Ocean Aerosol Production (SOAP) study. We show that it is possible to significantly reduce the scatter in the EC data and achieve consistency between measurements taken on station and with the ship underway. The gas transfer velocities from the EC measurements correlate better with the EC friction velocity (u * ) than with mean wind speeds derived from shipboard measurements corrected with an airflow distortion model. For the observed range of wind speeds (u 10 N = 3-23 m s −1 ), the transfer velocities can be parameterised with a linear fit to u * . The SOAP data are compared to previous gas transfer parameterisations using u 10 N computed from the EC friction velocity with the drag coefficient from the Coupled Ocean-Atmosphere Response Experiment (COARE) model version 3.5. The SOAP results are consistent with previous gas transfer studies, but at high wind speeds they do not support the sharp increase in gas transfer associated with bubblemediated transfer predicted by physically based models.

Response to Reviewers' comments

“…The SOAP study was carried out on the RV Tangaroa during February and March 2012 in the biologically productive frontal waters of the Chatham Rise (44 • S, 174-181 • E), east of New Zealand, where nutrient-rich sub-Antarctic water meets warmer subtropical water (Law et al, 2017). The voyage strategy was to survey regional surface biogeochemical distributions during the night and focus sampling during the day on the areas showing highest DMS concentrations, chlorophyll a and CO 2 drawdown.…”

“…The SOAP experiment (Law et al, 2017) examined the role of surface ocean biogeochemistry in influencing marine boundary layer aerosol, with a strategy of targeting phytoplankton blooms east of New Zealand, which were potentially significant source regions for aerosol precursors. These blooms represented a significant source of DMS (Bell et al, 2015) and provided a valuable opportunity for validation of micrometeorological techniques and parameterizations.…”

Gradient flux measurements of sea–air DMS transfer during the Surface Ocean Aerosol Production (SOAP) experiment

“…(2): the bulk flux (corrected for airflow distortion) from the nearby RV Tangaroa, or the directly measured, motion-corrected u * from EC measurements on board Tangaroa. The gradient fluxes estimated using the EC u * (Landwehr et al, 2018) are shown in Fig. 5a in red, overlain on the EC continuous 10 min data, for 20 days of the SOAP voyage.…”

“…This slope is very close to that from a direct regression between u * from bulk fluxes and u * from EC. This suggests that the CFD modelling has overcompensated for the airflow acceleration, resulting in a u * that is too low, as examined in detail by Landwehr et al (2018). It is likely that small-scale features of the Tangaroa -such as the foremast and railings, which were not included in the modelmay have a significant effect on the airflow-corrected wind speeds.…”

“…In the absence of EC measurements on the ship the alternative is to use a bulk parameterization of u * based on measured wind speed; however, this may be subject to significant airflow distortion, which numerical CFD airflow correction cannot always fully resolve ( Fig. 6; Landwehr et al, 2018), resulting in GF estimates that do not compare as favourably with the EC fluxes (Fig. 6, this paper).…”