Using eddy covariance to measure the dependence of air–sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; exchange rate on friction velocity

Sebastian Landwehr,Brian Ward,Eric S Saltzman,Thomas G Bell,Murray J Smith,Scott D Miller

doi:10.5194/acp-18-4297-2018

Abstract

Abstract. Parameterisation of the air–sea gas transfer velocity of CO2 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 CO2 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 (u10 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 u10 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 bubble-mediated transfer predicted by physically based models.

Highlights

Mass exchange across the air–sea interface is an important component of the Earth’s climate system
The improved correction has relatively little impact on transfer velocities measured while the ship was on station, giving results that are similar to those previously published by Landwehr et al (2014)
Due to the lower u10 N estimated by the airflow distortion (AFD)-corrected wind-speed measurements on Surface Ocean Aerosol Production (SOAP), the usage of those would lead to about 20 % and 10 % higher k660 values at u10 N = 10 m s−1 and u10 N = 20 m s−1, respectively

Summary

Introduction

Mass exchange across the air–sea interface is an important component of the Earth’s climate system. Understanding the processes that control the ocean–atmosphere exchange of CO2 is important in order to estimate global carbon fluxes and to assess the evolution and future impact of ocean uptake on Earth’s climate. The flux of CO2 across the air–sea interface can be written as FCO2 = pCO2 αCO2 kCO2 , (1). Where pCO2, αCO2 , and kCO2 are the partial pressure difference, the solubility, and the transfer velocity. Several different experimental approaches have been used to quantify air–sea gas exchange: (i) tracer studies utilising ambient gases (14CO2) Wanninkhof, 1992; Sweeney et al, 2007) which integrate the flux over timescales of years, (ii) deliberately introduced tracers (3He / SF6) Several different experimental approaches have been used to quantify air–sea gas exchange: (i) tracer studies utilising ambient gases (14CO2) (e.g. Wanninkhof, 1992; Sweeney et al, 2007) which integrate the flux over timescales of years, (ii) deliberately introduced tracers (3He / SF6) (e.g. Nightingale et al, 2000; Ho et al, 2006) which integrate the flux over

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Discussion

Conclusion