Air–sea dimethylsulfide (DMS) gas transfer in the North Atlantic: evidence for limited interfacial gas exchange at high wind speed

T G Bell,E S Saltzman,K H Christensen,W De Bruyn,S D Miller,B Ward

doi:10.5194/acp-13-11073-2013

Abstract

Abstract. Shipboard measurements of eddy covariance dimethylsulfide (DMS) air–sea fluxes and seawater concentration were carried out in the North Atlantic bloom region in June/July 2011. Gas transfer coefficients (k660) show a linear dependence on mean horizontal wind speed at wind speeds up to 11 m s−1. At higher wind speeds the relationship between k660 and wind speed weakens. At high winds, measured DMS fluxes were lower than predicted based on the linear relationship between wind speed and interfacial stress extrapolated from low to intermediate wind speeds. In contrast, the transfer coefficient for sensible heat did not exhibit this effect. The apparent suppression of air–sea gas flux at higher wind speeds appears to be related to sea state, as determined from shipboard wave measurements. These observations are consistent with the idea that long waves suppress near-surface water-side turbulence, and decrease interfacial gas transfer. This effect may be more easily observed for DMS than for less soluble gases, such as CO2, because the air–sea exchange of DMS is controlled by interfacial rather than bubble-mediated gas transfer under high wind speed conditions.

Highlights

Gas exchange between the ocean and atmosphere is a major term in the global budgets of many compounds with biogeochemical and climatic importance
A better understanding of gas transfer rates and their controlling factors is needed in order to predict how air–sea gas fluxes will vary in the future in response to changing climate and anthropogenic emissions
The air–sea flux of gas is proportional to the concentration difference across the interface ( C) and a gas transfer coefficient, K, expressed in water-side units: Kw (Liss and Slater, 1974): FDMS = Kw · C

Summary

Introduction

Gas exchange between the ocean and atmosphere is a major term in the global budgets of many compounds with biogeochemical and climatic importance. Parameterization of air–sea gas transfer is one of the major uncertainties in global biogeochemical models A better understanding of gas transfer rates and their controlling factors is needed in order to predict how air–sea gas fluxes will vary in the future in response to changing climate and anthropogenic emissions. The air–sea flux of gas is proportional to the concentration difference across the interface ( C) and a gas transfer coefficient, K, expressed in water-side units: Kw (Liss and Slater, 1974): FDMS = Kw · C. Whitecaps/bubble production, wind–wave interactions and surface films all play a role in determining the rate of gas transfer (Wanninkhof et al, 2009)

Methods

Results

Discussion

Conclusion