Final Report Summary - AIRSEA (Air-sea fluxes of climatically relevant gases in the marine atmospheric boundary layer)
Introduction
Mounting evidence indicates that climate change is one of the greatest environmental challenges today, with the potential to significantly alter how we live. Climate change is largely attributable to human activity where greenhouse gases (GHGs) are released into the atmosphere through the burning of fossil fuel for the production of electricity and the provision of transportation, as well as agricultural activities. It is well recognised that the atmosphere-ocean flux of trace gases is important for the cycling of climatically relevant gases. Direct measurements of fluxes have presented a significant challenge to scientists in the past several decades. This led to the development of parameterisations for the gas exchange rate or transfer velocity, with which fluxes could be calculated with atmospheric and aqueous concentrations. These parameterisations for the transfer velocity are largely based on wind speed using empirically-determined coefficients, and are often used to estimate global air-sea gas fluxes. However, it is well recognised that wind speed alone is not adequate to describe air-sea exchange, and in this report we describe the research that we have conducted which takes a broader view of air-sea exchange by studying a variety of processes in unison which are known to affect the air-sea fluxes of GHGs.
For example, turbulence in the upper ocean, induced by wind shear, breaking waves, and convection, periodically erodes the hierarchy of molecular sublayers immediately below the air-sea interface. The viscous, thermal, haline, and diffusive sublayers control the air-sea fluxes of momentum, heat, water, and gas (Lorke and Peeters 2006). The greater the amount of turbulence, the more enhanced are the fluxes between the ocean and atmosphere. The nature of turbulence is such that energy gets transferred from larger scales to smaller until they are dissipated by viscosity, and the kinetic energy is transferred into heat (Thorpe 2004). Therefore it is critical to quantify the turbulence at the ocean surface, which is a controlling process for air-sea exchange (Zappa et al. 2007), although there has been very few measurements in this region of the ocean.
The overall objectives of this project were to provide funding to establish a research programme for air-sea exchange. The primary scientific focus of the project was to study the atmosphere and ocean as a coupled system in order to determine the processes that control the air-sea fluxes of GHGs, specifically carbon dioxide (CO2). This required making measurements of ocean turbulence below the air-sea interface, as well as measuring the fluxes of CO2 above the air-sea interface. Efforts were spent during the initial period of this project to develop methods for measurement and analysis, and during the second period of the project we had the opportunity to participate in a field experiment in the North Atlantic in June - July 2011 in collaboration with scientists from the United States of America (USA). This allowed us to make these challenging measurements under open ocean conditions, providing us with a unique dataset with which to study several of the key processes at the ocean surface.
Main results
The culmination of the research on this project was the participation in a field experiment in June - July 2011 in the North Atlantic. This was the first time that our research group was in a position to make measurements both above and below the air-sea interface, and where the turbulence data with the profiler was situated within the footprint of the flux measurements. The track of the ship is descrived the R/V Knorr, which left from Woods Hole on the East Coast of the USA on 25 June 2011, and returned to Woods Hole on 18 July 2011. There was a total of 23 days of scientific research under open ocean conditions (see http://bloomcruise.blogspot.ie/(si apre in una nuova finestra) online for further details). We made measurements of upper ocean microstructure using our air-sea interaction profiler (ASIP), eddy covariance flux measurements of heat and gas, and surface wave measurements using our ultrasonic ranging device. These instruments are described below.
- Upper ocean microstructure
ASIP is an autonomous vertically profiling instrument designed to profile from below so as to provide undisturbed measurements all the way to the surface. It is equipped with high-resolution sensors for the measurement of temperature, salinity, light, oxygen, and turbulence. There are three thrusters which submerge it to a programmed depth (maximum 100 metres), whereupon it ascends through the water column towards the surface under its own buoyancy, recording data at 1 000 Hz, generating 192 kbytes per second which is stored with an single board computer. Once the surface is reached, ASIP acquires a GPS fix and then transmits this message using the iridium short burst data service to the scientists on board the research vessel from which it was deployed. ASIP is then placed in a low-power mode for a specified period, whereupon it repeats the profile cycle. ASIP is controlled with a miniature embedded computer system, and it can receive commands which allow real-time update of mission parameters. Power is provided with rechargeable lithium-ion batteries which typically allow 100 profiles during a single deployment.
- Eddy covariance fluxes
In combination with the upper ocean turbulence measurements, we conducted direct flux measurements using the eddy covariance method, which allows the direct measurement of the gas fluxes across the air-sea interface. The instrumentation consists of a sonic anemometer which can measure the small-scale fluctuations of vertical velocity, and a gas analyser which can determine the concentration of CO2 in the lower atmosphere. These instruments were mounted on a mast at the bow of the ship in order to minimise the effects of flow distortion.
- Ocean surface waves
The system employed to provide a record of ocean surface waves consist of an ultrasonic altimeter and an inertial motion unit. The altimeter is mounted at the bow of the ship (the wave sensor is suspended from the bow and attached to the end of a steel pole) and continuously records the elevation of the surface as it changes with the movement of the waves and the ship. The data from the IMU are used to remove the ship motion, providing only accurate measurements of the surface elevation, from which one-dimensional wave spectra can be derived. We have tested and compared the data from this device to a waverider, and to models, and the results show excellent agreement.
Lorke and Peeters (2006) have suggested that if we can predict turbulence at the ocean surface, we can parameterise this quantity and therefore provide better estimates of air-sea gas exchange. As we had the data available from our measurements, we were able to determine how easy it was to predict this. During one of the ASIP deployments the wind rapidly decreased followed by a rapid increase. This is a result of a low-pressure system passing over our area as can be seen from the pressure record and the rainfall on the leading edge. During this deployment a total of 54 profiles were made from a depth of 50 meters to the surface with each profile separated by approximately 20 minutes. The time-depth plots for the OBL as measured by ASIP is also shown and the time series for the turbulent Langmuir number are also shown. The stratification of the water column, also known as the buoyancy frequency or Brunt-Vaisala frequency, is shown with the depth of the OBL following the peak which is expected as there is often an abrupt jump in density at the base of the OBL. What is interesting is that the dissipation rate responds nearly instantly to the wind stress and is within an order of magnitude of that predicted by the law-of-wall scaling.
The mean dissipation profiles for three distinct periods during the deployment are shown. Each measured profile is an hourly mean with the 95 % confidence intervals are compared with law-of-wall scaling, the model of Terray et al. (1996) and the model incorporating Stokes drift for the wind and swell and their sum. There is no simple and consistent method to separate the wind and swell components from a one-dimensional (1D) spectrum (Wang and Hwang, 2001; Hwang et al., 2012) so the separation frequencies were estimated for each spectrum individually which looked reasonable. The wave number for each spectral section was calculated from the mean frequency calculated using the second statistical moment (Webb and Fox-Kemper, 2011) and converting this to a wavenumber using the dispersion relation for deep-water gravity waves. There is a surprising agreement between the observed dissipation profiles and the wave scaling of Huang and Qiao (2010), especially at depths approaching and deeper than $D$. This is especially true where there was a clearer separation between the wind and swell in the wave spectrum.
These results have been published in Sutherland et al. (2012).
Conclusions and impacts
The scientific objectives if this project were to study the processes governing air-sea exchange so that they could be better modelled. Compared to wind speed, dissipation in the ocean is a much more challenging to measure as it requires sophisticated instrumentation and an ability to provide data close to the air-sea interface for the application of air-sea transfer parameterisation. According to Lorke and Peeters (2006), there is a need for a new generation of experiments which combine turbulence data with estimates of gas transfer velocities. We have provided such a dataset and have shown that the turbulence at the ocean surface can be modelled. The next stage in this ongoing research is to incorporate these results into a parameterisation for air-sea exchange.
The objective of the IRG project was to provide funds to allow the award recipient to establish himself as a scientist at a European Institute after having spent several years outside of the European Union (EU). This project was hugely successful and the grant recipient is now a staff member at the host institute with a research group consisting of 6 Doctor of Philosophy (PhD) students and 2 post-doctoral researchers.
Climate change is currently a major challenge facing society, and the growth in GHG emissions makes it more critical to understand the role of the ocean in the uptake of these gases.
References Cited
Huang, C. J. and F. Qiao (2010). Wave-turbulence interaction and its induced mixing in the upper ocean. J. Geophys. Res. 115. C04026, doi: 10.1029/2009JC005853.
Hwang, P. A., F. J. Ocampo-Torres, and H. Garcia-Nava (2012). Wind sea and swell separation of 1d wave spectrum by a spectrum integration method. J. Atmos. Oceanic Technol. 29, 116-128.
Lorke, A. and F. Peeters, 2006: Toward a unified scaling relation for interfacial fluxes. J. Phys. Oceanogr., 36 (5), 955-961.
Sutherland, G., K. H. Christensen, and B. Ward (2012). Wave-turbulence scaling in the ocean mixed layer. Ocean Sci. Discuss. 9, 3761-3793.
Terray, E. A., M. A. Donelan, Y. C. Agrawal, W. M. Drennan, K. K. Kahma, A. J. Williams III, P. A. Hwang, and S. A. Kitaigorodskii (1996). Estimates of kinetic energy dissipation under breaking waves. J. Phys. Oceanogr. 26, 792-807.
Thorpe, S. A., 2004: Recent developments in the study of ocean turbulence. Annual Review of Earth and Planetary Sciences, 32, 91-109.
Wang, D. W. and P. A. Hwang (2001). An operational method for separating wind sea and swell from ocean wave spectra. J. Atmos. Oceanic Technol. 18 (12), 20522062.
Webb, A. and B. Fox-Kemper (2011, January). Wave spectral moments and Stokes drift estimation. Ocean Modelling 40 (3-4), 273-288.
Zappa, C. J., W. R. McGillis, P. A. Raymond, J. B. Edson, E. J. Hintsa, H. J. Zemmelink, J. W. H. Dacey, and D. T. Ho, 2007: Environmental turbulent mixing controls on air water gas exchange in marine and aquatic systems. Geophys. Res. Lett., 34, L10601, doi:10.1029/2006GL028790.
Mounting evidence indicates that climate change is one of the greatest environmental challenges today, with the potential to significantly alter how we live. Climate change is largely attributable to human activity where greenhouse gases (GHGs) are released into the atmosphere through the burning of fossil fuel for the production of electricity and the provision of transportation, as well as agricultural activities. It is well recognised that the atmosphere-ocean flux of trace gases is important for the cycling of climatically relevant gases. Direct measurements of fluxes have presented a significant challenge to scientists in the past several decades. This led to the development of parameterisations for the gas exchange rate or transfer velocity, with which fluxes could be calculated with atmospheric and aqueous concentrations. These parameterisations for the transfer velocity are largely based on wind speed using empirically-determined coefficients, and are often used to estimate global air-sea gas fluxes. However, it is well recognised that wind speed alone is not adequate to describe air-sea exchange, and in this report we describe the research that we have conducted which takes a broader view of air-sea exchange by studying a variety of processes in unison which are known to affect the air-sea fluxes of GHGs.
For example, turbulence in the upper ocean, induced by wind shear, breaking waves, and convection, periodically erodes the hierarchy of molecular sublayers immediately below the air-sea interface. The viscous, thermal, haline, and diffusive sublayers control the air-sea fluxes of momentum, heat, water, and gas (Lorke and Peeters 2006). The greater the amount of turbulence, the more enhanced are the fluxes between the ocean and atmosphere. The nature of turbulence is such that energy gets transferred from larger scales to smaller until they are dissipated by viscosity, and the kinetic energy is transferred into heat (Thorpe 2004). Therefore it is critical to quantify the turbulence at the ocean surface, which is a controlling process for air-sea exchange (Zappa et al. 2007), although there has been very few measurements in this region of the ocean.
The overall objectives of this project were to provide funding to establish a research programme for air-sea exchange. The primary scientific focus of the project was to study the atmosphere and ocean as a coupled system in order to determine the processes that control the air-sea fluxes of GHGs, specifically carbon dioxide (CO2). This required making measurements of ocean turbulence below the air-sea interface, as well as measuring the fluxes of CO2 above the air-sea interface. Efforts were spent during the initial period of this project to develop methods for measurement and analysis, and during the second period of the project we had the opportunity to participate in a field experiment in the North Atlantic in June - July 2011 in collaboration with scientists from the United States of America (USA). This allowed us to make these challenging measurements under open ocean conditions, providing us with a unique dataset with which to study several of the key processes at the ocean surface.
Main results
The culmination of the research on this project was the participation in a field experiment in June - July 2011 in the North Atlantic. This was the first time that our research group was in a position to make measurements both above and below the air-sea interface, and where the turbulence data with the profiler was situated within the footprint of the flux measurements. The track of the ship is descrived the R/V Knorr, which left from Woods Hole on the East Coast of the USA on 25 June 2011, and returned to Woods Hole on 18 July 2011. There was a total of 23 days of scientific research under open ocean conditions (see http://bloomcruise.blogspot.ie/(si apre in una nuova finestra) online for further details). We made measurements of upper ocean microstructure using our air-sea interaction profiler (ASIP), eddy covariance flux measurements of heat and gas, and surface wave measurements using our ultrasonic ranging device. These instruments are described below.
- Upper ocean microstructure
ASIP is an autonomous vertically profiling instrument designed to profile from below so as to provide undisturbed measurements all the way to the surface. It is equipped with high-resolution sensors for the measurement of temperature, salinity, light, oxygen, and turbulence. There are three thrusters which submerge it to a programmed depth (maximum 100 metres), whereupon it ascends through the water column towards the surface under its own buoyancy, recording data at 1 000 Hz, generating 192 kbytes per second which is stored with an single board computer. Once the surface is reached, ASIP acquires a GPS fix and then transmits this message using the iridium short burst data service to the scientists on board the research vessel from which it was deployed. ASIP is then placed in a low-power mode for a specified period, whereupon it repeats the profile cycle. ASIP is controlled with a miniature embedded computer system, and it can receive commands which allow real-time update of mission parameters. Power is provided with rechargeable lithium-ion batteries which typically allow 100 profiles during a single deployment.
- Eddy covariance fluxes
In combination with the upper ocean turbulence measurements, we conducted direct flux measurements using the eddy covariance method, which allows the direct measurement of the gas fluxes across the air-sea interface. The instrumentation consists of a sonic anemometer which can measure the small-scale fluctuations of vertical velocity, and a gas analyser which can determine the concentration of CO2 in the lower atmosphere. These instruments were mounted on a mast at the bow of the ship in order to minimise the effects of flow distortion.
- Ocean surface waves
The system employed to provide a record of ocean surface waves consist of an ultrasonic altimeter and an inertial motion unit. The altimeter is mounted at the bow of the ship (the wave sensor is suspended from the bow and attached to the end of a steel pole) and continuously records the elevation of the surface as it changes with the movement of the waves and the ship. The data from the IMU are used to remove the ship motion, providing only accurate measurements of the surface elevation, from which one-dimensional wave spectra can be derived. We have tested and compared the data from this device to a waverider, and to models, and the results show excellent agreement.
Lorke and Peeters (2006) have suggested that if we can predict turbulence at the ocean surface, we can parameterise this quantity and therefore provide better estimates of air-sea gas exchange. As we had the data available from our measurements, we were able to determine how easy it was to predict this. During one of the ASIP deployments the wind rapidly decreased followed by a rapid increase. This is a result of a low-pressure system passing over our area as can be seen from the pressure record and the rainfall on the leading edge. During this deployment a total of 54 profiles were made from a depth of 50 meters to the surface with each profile separated by approximately 20 minutes. The time-depth plots for the OBL as measured by ASIP is also shown and the time series for the turbulent Langmuir number are also shown. The stratification of the water column, also known as the buoyancy frequency or Brunt-Vaisala frequency, is shown with the depth of the OBL following the peak which is expected as there is often an abrupt jump in density at the base of the OBL. What is interesting is that the dissipation rate responds nearly instantly to the wind stress and is within an order of magnitude of that predicted by the law-of-wall scaling.
The mean dissipation profiles for three distinct periods during the deployment are shown. Each measured profile is an hourly mean with the 95 % confidence intervals are compared with law-of-wall scaling, the model of Terray et al. (1996) and the model incorporating Stokes drift for the wind and swell and their sum. There is no simple and consistent method to separate the wind and swell components from a one-dimensional (1D) spectrum (Wang and Hwang, 2001; Hwang et al., 2012) so the separation frequencies were estimated for each spectrum individually which looked reasonable. The wave number for each spectral section was calculated from the mean frequency calculated using the second statistical moment (Webb and Fox-Kemper, 2011) and converting this to a wavenumber using the dispersion relation for deep-water gravity waves. There is a surprising agreement between the observed dissipation profiles and the wave scaling of Huang and Qiao (2010), especially at depths approaching and deeper than $D$. This is especially true where there was a clearer separation between the wind and swell in the wave spectrum.
These results have been published in Sutherland et al. (2012).
Conclusions and impacts
The scientific objectives if this project were to study the processes governing air-sea exchange so that they could be better modelled. Compared to wind speed, dissipation in the ocean is a much more challenging to measure as it requires sophisticated instrumentation and an ability to provide data close to the air-sea interface for the application of air-sea transfer parameterisation. According to Lorke and Peeters (2006), there is a need for a new generation of experiments which combine turbulence data with estimates of gas transfer velocities. We have provided such a dataset and have shown that the turbulence at the ocean surface can be modelled. The next stage in this ongoing research is to incorporate these results into a parameterisation for air-sea exchange.
The objective of the IRG project was to provide funds to allow the award recipient to establish himself as a scientist at a European Institute after having spent several years outside of the European Union (EU). This project was hugely successful and the grant recipient is now a staff member at the host institute with a research group consisting of 6 Doctor of Philosophy (PhD) students and 2 post-doctoral researchers.
Climate change is currently a major challenge facing society, and the growth in GHG emissions makes it more critical to understand the role of the ocean in the uptake of these gases.
References Cited
Huang, C. J. and F. Qiao (2010). Wave-turbulence interaction and its induced mixing in the upper ocean. J. Geophys. Res. 115. C04026, doi: 10.1029/2009JC005853.
Hwang, P. A., F. J. Ocampo-Torres, and H. Garcia-Nava (2012). Wind sea and swell separation of 1d wave spectrum by a spectrum integration method. J. Atmos. Oceanic Technol. 29, 116-128.
Lorke, A. and F. Peeters, 2006: Toward a unified scaling relation for interfacial fluxes. J. Phys. Oceanogr., 36 (5), 955-961.
Sutherland, G., K. H. Christensen, and B. Ward (2012). Wave-turbulence scaling in the ocean mixed layer. Ocean Sci. Discuss. 9, 3761-3793.
Terray, E. A., M. A. Donelan, Y. C. Agrawal, W. M. Drennan, K. K. Kahma, A. J. Williams III, P. A. Hwang, and S. A. Kitaigorodskii (1996). Estimates of kinetic energy dissipation under breaking waves. J. Phys. Oceanogr. 26, 792-807.
Thorpe, S. A., 2004: Recent developments in the study of ocean turbulence. Annual Review of Earth and Planetary Sciences, 32, 91-109.
Wang, D. W. and P. A. Hwang (2001). An operational method for separating wind sea and swell from ocean wave spectra. J. Atmos. Oceanic Technol. 18 (12), 20522062.
Webb, A. and B. Fox-Kemper (2011, January). Wave spectral moments and Stokes drift estimation. Ocean Modelling 40 (3-4), 273-288.
Zappa, C. J., W. R. McGillis, P. A. Raymond, J. B. Edson, E. J. Hintsa, H. J. Zemmelink, J. W. H. Dacey, and D. T. Ho, 2007: Environmental turbulent mixing controls on air water gas exchange in marine and aquatic systems. Geophys. Res. Lett., 34, L10601, doi:10.1029/2006GL028790.
