scholarly journals Natural variability in air–sea gas transfer efficiency of CO2

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Mingxi Yang ◽  
Timothy J. Smyth ◽  
Vassilis Kitidis ◽  
Ian J. Brown ◽  
Charel Wohl ◽  
...  
Keyword(s):  
Wind Speed ◽  
Transfer Efficiency ◽  
Gas Transfer ◽  
Transfer Velocity ◽  
Concentration Difference ◽  
Near Surface ◽  
Wind Speeds ◽  
Naturally Occurring ◽  
Surface Active ◽  
The Mean

AbstractThe flux of CO2 between the atmosphere and the ocean is often estimated as the air–sea gas concentration difference multiplied by the gas transfer velocity (K660). The first order driver for K660 over the ocean is wind through its influence on near surface hydrodynamics. However, field observations have shown substantial variability in the wind speed dependencies of K660. In this study we measured K660 with the eddy covariance technique during a ~ 11,000 km long Southern Ocean transect. In parallel, we made a novel measurement of the gas transfer efficiency (GTE) based on partial equilibration of CO2 using a Segmented Flow Coil Equilibrator system. GTE varied by 20% during the transect, was distinct in different water masses, and related to K660. At a moderate wind speed of 7 m s−1, K660 associated with high GTE exceeded K660 with low GTE by 30% in the mean. The sensitivity of K660 towards GTE was stronger at lower wind speeds and weaker at higher wind speeds. Naturally-occurring organics in seawater, some of which are surface active, may be the cause of the variability in GTE and in K660. Neglecting these variations could result in biases in the computed air–sea CO2 fluxes.

Suppression of air-sea CO2 transfer by surfactants – direct evidence from the Southern Ocean

2021 ◽  
Author(s):  
Mingxi Yang ◽  
Timothy Smyth ◽  
Vassilis Kitidis ◽  
Ian Brown ◽  
Charel Wohl ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Direct Evidence ◽  
Gas Transfer ◽  
Climate Projections ◽  
Transfer Velocity ◽  
Wind Speeds ◽  
Naturally Occurring ◽  
Mean Wind Speed ◽  
Surface Active ◽  
Global Mean

<p>Uncertainty in the CO<sub>2</sub> gas transfer velocity (K<sub>660</sub>) severely limits the accuracy of air-sea CO<sub>2</sub> flux calculations and hence hinders our ability to produce realistic climate projections.  Recent field observations have suggested substantial variability in K<sub>660</sub>, especially at low and high wind speeds.  Laboratory experiments have shown that naturally occurring surface active organic materials, or surfactants, can suppress gas transfer.  Here we provide direct open ocean evidence of gas transfer suppression due to surfactants from a ~11,000 km long research expedition by making measurements of the gas transfer efficiency (GTE) along with direct observation of K<sub>660</sub>.  GTE varied by 20% during the Southern Ocean transect and was distinct in different watermasses.  Furthermore GTE correlated with and can explain about 9% of the scatter in K<sub>660</sub>, suggesting that surfactants exert a measurable influence on air-sea CO<sub>2</sub> flux.  Relative gas transfer suppression due to surfactants was ~30% at a global mean wind speed of 7 m s<sup>-1</sup> and was more important at lower wind speeds.  Neglecting surfactant suppression may result in substantial spatial and temporal biases in the computed air-sea CO<sub>2</sub> fluxes.</p>

scholarly journals DMS gas transfer coefficients from algal blooms in the Southern Ocean

2014 ◽  
Vol 14 (21) ◽  
pp. 28453-28482
Author(s):  
T. G. Bell ◽  
W. De Bruyn ◽  
C. A. Marandino ◽  
S. D. Miller ◽  
C. S. Law ◽  
...  
Keyword(s):  
Wind Speed ◽  
Southern Ocean ◽  
Eddy Covariance ◽  
Algal Blooms ◽  
Biologically Active ◽  
Gas Transfer ◽  
Transfer Coefficients ◽  
Transfer Velocity ◽  
Gas Transfer Velocity ◽  
Wind Speeds

Abstract. Air/sea dimethylsulfide (DMS) fluxes and bulk air/sea gradients were measured over the Southern Ocean in February/March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (>15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s−1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A~flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data shows no obvious modification of the gas transfer-wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.

Detection and Characterization of Microscale Breaking Waves

2003 ◽  
Author(s):  
M. H. Kamran Siddiqui ◽  
Mark R. Loewen
Keyword(s):  
Wind Speed ◽  
Water Surface ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Skin Layer ◽  
Wave Crest ◽  
Gas Transfer ◽  
Near Surface ◽  
Wind Speeds ◽  
The Waves

Microscale breaking waves are short wind-generated waves that break without air entrainment. At low to moderate wind speeds microscale breaking waves play an important role in enhancing air-water heat and gas transfer. We report on a series of experiments conducted in a wind-wave flume at Harris Hydraulics Laboratory (University of Washington, Seattle) designed to investigate the importance of microscale breaking waves in generating near-surface turbulence and in enhancing air-sea gas and heat transfer rates. Non-invasive experiments were performed at wind speeds ranging from 4.5 m/s to 11 m/s and at a fetch of 5.5 m. The skin-layer or water surface temperature was measured using an infrared (IR) imager and digital particle image velocimetry (DPIV) was used to obtain simultaneous measurements of the two-dimensional velocities immediately below the water surface. Analysis of the simultaneous DPIV and infrared datasets revealed that microscale breaking waves generate strong vortices in their crests that disrupt the cool skin layer at the water surface and create thermal wakes that are visible in the infrared images. While non-breaking waves do not generate strong vortices and hence do not disrupt the skin layer. We developed a scheme based on the magnitude of vorticity in the wave crest to identify microscale breaking waves. The results show that at a wind speed of 4.5 m/s, 11% of the waves broke. The percentage of breaking waves increased with wind speed and at a wind speed of 11 m/s, 91% of the waves were microscale breaking waves. Comparison of different geometric and flow properties of microscale breaking and non-breaking waves revealed that microscale breaking waves are steeper, larger in amplitude and generate more turbulent kinetic energy compared to non-breaking waves.

Characteristics of the wind drift layer and microscale breaking waves

2007 ◽  
Vol 573 ◽  
pp. 417-456 ◽  
Author(s):  
M. H. KAMRAN SIDDIQUI ◽  
MARK R. LOEWEN
Keyword(s):  
Wind Speed ◽  
Wave Breaking ◽  
Breaking Waves ◽  
Mean Velocity ◽  
Wind Drift ◽  
Near Surface ◽  
Wind Speeds ◽  
The Mean ◽  
Rate Of Dissipation ◽  
Drift Layer

An experimental study, investigating the mean flow and turbulence in the wind drift layer formed beneath short wind waves was conducted. The degree to which these flows resemble the flows that occur in boundary layers adjacent to solid walls (i.e. wall-layers) was examined. Simultaneous DPIV (digital particle image velocimetry) and infrared imagery were used to investigate these near-surface flows at a fetch of 5.5 m and wind speeds from 4.5 to 11 m s−1. These conditions produced short steep waves with dominant wavelengths from 6 cm to 18 cm. The mean velocity profiles in the wind drift layer were found to be logarithmic and the flow was hydrodynamically smooth at all wind speeds. The rate of dissipation of turbulent kinetic energy was determined to be significantly greater in magnitude than would occur in a comparable wall-layer. Microscale breaking waves were detected using the DPIV data and the characteristics of breaking and non-breaking waves were compared. The percentage of microscale breaking waves increased abruptly from 11% to 80% as the wind speed increased from 4.5 to 7.4 m s− and then gradually increased to 90% as the wind speed increased to 11 m s−. At a depth of 1 mm, the rate of dissipation was 1.7 to 3.2 times greater beneath microscale breaking waves compared to non-breaking waves. In the crest–trough region beneath microscale breaking waves, 40% to 50% of the dissipation was associated with wave breaking. These results demonstrated that the enhanced near-surface turbulence in the wind drift layer was the result of microscale wave breaking. It was determined that the rate of dissipation of turbulent kinetic energy due to wave breaking is a function of depth, friction velocity, wave height and phase speed as proposed by Terray et al. (1996). Vertical profiles of the rate of dissipation showed that beneath microscale breaking waves there were two distinct layers. Immediately beneath the surface, the dissipation decayed as ζ−0.7 and below this in the second layer it decayed as ζ−2. The enhanced turbulence associated with microscale wave breaking was found to extend to a depth of approximately one significant wave height. The only similarity between the flows in these wind drift layers and wall-layers is that in both cases the mean velocity profiles are logarithmic. The fact that microscale breaking waves were responsible for 40%–50% of the near-surface turbulence supports the premise that microscale breaking waves play a significant role in enhancing the transfer of gas and heat across the air–sea interface.

scholarly journals Variability of air-sea gas transfer velocities in the Baltic Sea

2018 ◽  
Author(s):  
Leila Nagel ◽  
Kerstin E. Krall ◽  
Bernd Jähne
Keyword(s):  
Heat Transfer ◽  
Wind Speed ◽  
Baltic Sea ◽  
Active Material ◽  
Gas Transfer ◽  
The Baltic Sea ◽  
Transfer Velocity ◽  
Wind Speeds ◽  
The Baltic ◽  
Surface Active Material

Abstract. Heat transfer velocities measured during three different campaigns in the Baltic Sea using the Active Controlled Flux Technique (ACFT) with wind speeds ranging from 5.3 to 14.8 m s−1 are presented. Careful scaling of the heat transfer velocities to gas transfer velocities using Schmidt number exponents measured in a laboratory study allows to compare the measured transfer velocities to existing gas transfer velocity parameterizations, which use wind speed as the controlling parameter. The measured data and other field data clearly show that some gas transfer velocities are much lower than the empirical wind speed parametrizations. This indicates that the dependencies of the transfer velocity on the fetch, i.e., the history of the wind and the age of the wind wave field, and the effects of surface active material need to be taken into account.

scholarly journals Uncertainties in wind speed dependent CO<sub>2</sub> transfer velocities due to airflow distortion at anemometer sites on ships

2010 ◽  
Vol 10 (11) ◽  
pp. 5123-5133 ◽  
Author(s):  
F. Griessbaum ◽  
B. I. Moat ◽  
Y. Narita ◽  
M. J. Yelland ◽  
O. Klemm ◽  
...  
Keyword(s):  
Wind Speed ◽  
Three Dimensional ◽  
Co2 Flux ◽  
Co2 Concentration ◽  
Gas Transfer ◽  
Co2 Uptake ◽  
Flow Distortion ◽  
Simulation Code ◽  
Transfer Velocity ◽  
Concentration Difference

Abstract. Data from platforms, research vessels and merchant ships are used to estimate ocean CO2 uptake via parameterisations of the gas transfer velocity (k) and measurements of the difference between the partial pressures of CO2 in the ocean (pCO2 sw) and atmosphere (pCO2 atm) and of wind speed. Gas transfer velocities estimated using wind speed dependent parameterisations may be in error due to air flow distortion by the ship's hull and superstructure introducing biases into the measured wind speed. The effect of airflow distortion on estimates of the transfer velocity was examined by modelling the airflow around the three-dimensional geometries of the research vessels Hakuho Maru and Mirai, using the Large Eddy Simulation code GERRIS. For airflows within ±45° of the bow the maximum bias was +16%. For wind speed of 10 m s−1 to 15 m s−1, a +16% bias in wind speed would cause an overestimate in the calculated value of k of 30% to 50%, depending on which k parameterisation is used. This is due to the propagation of errors when using quadratic or cubic parameterisations. Recommendations for suitable anemometer locations on research vessels are given. The errors in transfer velocity may be much larger for typical merchant ships, as the anemometers are generally not as well-exposed as those on research vessels. Flow distortion may also introduce biases in the wind speed dependent k parameterisations themselves, since these are obtained by relating measurements of the CO2 flux to measurements of the wind speed and the CO2 concentration difference. To investigate this, flow distortion effects were estimated for three different platforms from which wind speed dependent parameterisations are published. The estimates ranged from −4% to +14% and showed that flow distortion may have a significant impact on wind speed dependent parameterisations. However, the wind biases are not large enough to explain the differences at high wind speeds in parameterisations which are based on eddy covariance and deliberate tracer methods.

scholarly journals Dimethylsulfide gas transfer coefficients from algal blooms in the Southern Ocean

2015 ◽  
Vol 15 (4) ◽  
pp. 1783-1794 ◽  
Author(s):  
T. G. Bell ◽  
W. De Bruyn ◽  
C. A. Marandino ◽  
S. D. Miller ◽  
C. S. Law ◽  
...  
Keyword(s):  
Wind Speed ◽  
Southern Ocean ◽  
Eddy Covariance ◽  
Algal Blooms ◽  
Biologically Active ◽  
Gas Transfer ◽  
Transfer Coefficients ◽  
Transfer Velocity ◽  
Gas Transfer Velocity ◽  
Wind Speeds

Abstract. Air–sea dimethylsulfide (DMS) fluxes and bulk air–sea gradients were measured over the Southern Ocean in February–March 2012 during the Surface Ocean Aerosol Production (SOAP) study. The cruise encountered three distinct phytoplankton bloom regions, consisting of two blooms with moderate DMS levels, and a high biomass, dinoflagellate-dominated bloom with high seawater DMS levels (> 15 nM). Gas transfer coefficients were considerably scattered at wind speeds above 5 m s−1. Bin averaging the data resulted in a linear relationship between wind speed and mean gas transfer velocity consistent with that previously observed. However, the wind-speed-binned gas transfer data distribution at all wind speeds is positively skewed. The flux and seawater DMS distributions were also positively skewed, which suggests that eddy covariance-derived gas transfer velocities are consistently influenced by additional, log-normal noise. A flux footprint analysis was conducted during a transect into the prevailing wind and through elevated DMS levels in the dinoflagellate bloom. Accounting for the temporal/spatial separation between flux and seawater concentration significantly reduces the scatter in computed transfer velocity. The SOAP gas transfer velocity data show no obvious modification of the gas transfer–wind speed relationship by biological activity or waves. This study highlights the challenges associated with eddy covariance gas transfer measurements in biologically active and heterogeneous bloom environments.

scholarly journals Measurements of air–sea gas transfer velocities in the Baltic Sea

Ocean Science ◽  
2019 ◽  
Vol 15 (2) ◽  
pp. 235-247 ◽  
Author(s):  
Leila Nagel ◽  
Kerstin E. Krall ◽  
Bernd Jähne
Keyword(s):  
Heat Transfer ◽  
Wind Speed ◽  
Baltic Sea ◽  
Active Material ◽  
Gas Transfer ◽  
The Baltic Sea ◽  
Transfer Velocity ◽  
Wind Speeds ◽  
The Baltic ◽  
Surface Active Material

Abstract. Heat transfer velocities measured during three different campaigns in the Baltic Sea using the active controlled flux technique (ACFT) with wind speeds ranging from 5.3 to 14.8 m s−1 are presented. Careful scaling of the heat transfer velocities to gas transfer velocities using Schmidt number exponents measured in a laboratory study allows us to compare the measured transfer velocities to existing gas transfer velocity parameterizations, which use wind speed as the controlling parameter. The measured data and other field data clearly show that some gas transfer velocities are much lower than those based on the empirical wind speed parameterizations. This indicates that the dependencies of the transfer velocity on the fetch, i. e., the history of the wind and the age of the wind-wave field, and the effects of surface-active material need to be taken into account.

scholarly journals Uncertainties in wind speed dependent CO<sub>2</sub> transfer velocities due to airflow distortion at anemometer sites on ships

2009 ◽  
Vol 9 (5) ◽  
pp. 18839-18865
Author(s):  
F. Griessbaum ◽  
B. I. Moat ◽  
Y. Narita ◽  
M. J. Yelland ◽  
O. Klemm ◽  
...  
Keyword(s):  
Wind Speed ◽  
Three Dimensional ◽  
Co2 Flux ◽  
Co2 Concentration ◽  
Gas Transfer ◽  
Co2 Uptake ◽  
Flow Distortion ◽  
Simulation Code ◽  
Transfer Velocity ◽  
Concentration Difference

Abstract. Data from research vessels and merchant ships are used to estimate ocean CO2 uptake via parameterizations of the gas transfer velocity (k) and measurements of the difference between the concentration of CO2 in the ocean (pCO2sw) and atmosphere (pCO2atm) and of wind speed. Gas transfer velocities estimated using wind speed dependent parameterisations may be in error due to air flow distortion by the ship's hull and superstructure introducing biases into the measured wind speed. The effect of airflow distortion on estimates of the transfer velocity was examined by modelling the airflow around the three-dimensional geometries of the research vessels Hakuho Maru and Mirai, using the Large Eddy Simulation code GERRIS. For airflows within ±45° of the bow the maximum bias was +16%. For wind speed of 10 m s−1 to 15 m s−1, a +16% bias in wind speed would cause an overestimate in the calculated value of k of 30% to 50%, depending on which k parameterisation is used. This is due to the propagation of errors when using quadratic or cubic parameterizations. Recommendations for suitable anemometer locations on research vessels are given. The errors in transfer velocity may be much larger for typical merchant ships, as the anemometers are generally not as well-exposed as those on research vessels. Flow distortion may also introduce biases in the wind speed dependent k parameterizations themselves, since these are obtained by relating measurements of the CO2 flux to measurements of the wind speed and the CO2 concentration difference. To investigate this, flow distortion effects were estimated for three different platforms from which wind speed dependent parameterizations are published. The estimates ranged from –4% to +14% and showed that flow distortion may have a significant impact on wind speed dependent parameterizations. However, the wind biases are not large enough to explain the differences at high wind speeds in parameterizations which are based on eddy covariance and deliberate tracer methods.

scholarly journals Impact of Nonzero Intercept Gas Transfer Velocity Parameterizations on Global and Regional Ocean–Atmosphere CO2 Fluxes

Geosciences ◽  
2019 ◽  
Vol 9 (5) ◽  
pp. 230
Author(s):  
Mariana Ribas-Ribas ◽  
Gianna Battaglia ◽  
Matthew P. Humphreys ◽  
Oliver Wurl
Keyword(s):  
Wind Speed ◽  
Ocean Model ◽  
Global Ocean ◽  
Gas Transfer ◽  
Co2 Fluxes ◽  
Average Wind Speed ◽  
Transfer Velocity ◽  
Gas Transfer Velocity ◽  
Wind Speeds

Carbon dioxide (CO2) fluxes between the ocean and atmosphere (FCO2) are commonly computed from differences between their partial pressures of CO2 (ΔpCO2) and the gas transfer velocity (k). Commonly used wind-based parameterizations for k imply a zero intercept, although in situ field data below 4 m s−1 are scarce. Considering a global average wind speed over the ocean of 6.6 m s−1, a nonzero intercept might have a significant impact on global FCO2. Here, we present a database of 245 in situ measurements of k obtained with the floating chamber technique (Sniffle), 190 of which have wind speeds lower than 4 m s−1. A quadratic parameterization with wind speed and a nonzero intercept resulted in the best fit for k. We further tested FCO2 calculated with a different parameterization with a complementary pCO2 observation-based product. Furthermore, we ran a simulation in a well-tested ocean model of intermediate complexity to test the implications of different gas transfer velocity parameterizations for the natural carbon cycle. The global ocean observation-based analysis suggests that ignoring a nonzero intercept results in an ocean-sink increase of 0.73 Gt C yr−1. This corresponds to a 28% higher uptake of CO2 compared with the flux calculated from a parameterization with a nonzero intercept. The differences in FCO2 were higher in the case of low wind conditions and large ΔpCO2 between the ocean and atmosphere. Such conditions occur frequently in the Tropics.

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