Direct determination of the air‐sea CO2 gas transfer velocity in Arctic sea ice regions

John Prytherch,Marc C Geibel,Christoph Humborg,Leif G Anderson,Dominic J Salisbury,Michael Tjernström,Brett F Thornton,Ian M Brooks,Patrick M Crill

doi:10.1002/2017gl073593

Abstract

AbstractThe Arctic Ocean is an important sink for atmospheric CO2. The impact of decreasing sea ice extent and expanding marginal ice zones on Arctic air‐sea CO2 exchange depends on the rate of gas transfer in the presence of sea ice. Sea ice acts to limit air‐sea gas exchange by reducing contact between air and water but is also hypothesized to enhance gas transfer rates across surrounding open‐water surfaces through physical processes such as increased surface‐ocean turbulence from ice‐water shear and ice‐edge form drag. Here we present the first direct determination of the CO2 air‐sea gas transfer velocity in a wide range of Arctic sea ice conditions. We show that the gas transfer velocity increases near linearly with decreasing sea ice concentration. We also show that previous modeling approaches overestimate gas transfer rates in sea ice regions.

Highlights

We show that the gas transfer velocity increases near linearly with decreasing sea ice concentration
For the Arctic Clouds in Summer Experiment (ACSE) measurements, these seasons are defined as day of year 186 (DOY) 186 to 239.5 and DOY 239.5 to 275, respectively [Sotiropoulou et al, 2016], with CO2 flux
We report direct eddy covariance (EC) determinations of CO2 keff in open water, marginal sea ice, and dense pack ice regions of the Arctic

Summary

Introduction

Primary production acts to draw down CO2. These processes combine for an estimated net air-sea flux of À66 to À199 Tg C yrÀ1 into the Arctic Ocean [Bates and Mathis, 2009], 4% to 12% of the net global oceanic CO2 uptake [Takahashi et al, 2009]. Sea ice is known to be permeable to gases [Miller et al, 2011] and may act both as a sink for CO2 during melt [Delille et al, 2014] and a source of CO2 during freezeup conditions [Delille et al, 2007]. When melt ponds form on the sea ice, they may act as an additional sink due to the initially low CO2 concentration in the meltwater [Geilfus et al, 2015]

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Discussion

Conclusion