Analysing and Modelling CO 2 fluxes across the air-sea boundary Anthony Bloom Project Supervisors: Ian Brooks, Conny Schwierz

pCO 2 sea pCO 2 air ΔpCO2 > 0ΔpCO2 < 0 Analyzing and Modelling CO2 fluxes across the air-sea boundary CO 2 fluxes: Air-Sea CO 2 Flux driven by a CO 2 partial pressure difference, ΔpCO2: CO 2 gas transfer velocity: dependent on friction velocity, white-capping and bubble mediated gas transfer. Bulk Flux Relationship: F CO2 = k CO2 s ΔpCO 2

CO 2 transfer velocity parameterisations: Non-linear relationship with wind speed No direct physical relationship with wind speed Considerable variation between parameterisations Significant divergence of parameterisations at wind speeds > 10ms -1 Wind speed CO 2 gas transfer parameterisations used as sole constraints in GCMs. Few open ocean measurements Analyzing and Modelling CO2 fluxes across the air-sea boundary

Cruise D317 North Atlantic: A large global CO 2 sink. High wind speeds are expected in springtime mid-latitude weather systems. Analyzing and Modelling CO2 fluxes across the air-sea boundary

Eddy Covariance Method Eddy Covariance Fluxes: F = ρ a · (w’ · c’) Analyzing and Modelling CO2 fluxes across the air-sea boundary Required Time Series include: Vertical Wind Speed (w) Atmospheric CO 2 concentration (c) Horizontal Wind Speed Air and Sea Surface Temperatures Atmospheric H 2 O concentration Air and Sea pCO 2 Platform motion

RRS Discovery Suitable ‘moving platform’ for eddy covariance flux measurements with a modest wind flow distortion. Instrument PackageMeasurementsSampling Interval (Freq)Group LiCOR 7500CO 2 /H 2 O concentrations,0.05s – (20Hz)Leeds Motion PackAcceleration, pitch, roll, heading.0.05s – (20Hz)Leeds/NOC SURFMETWind Speed and Direction, Rel Hum, Air Temperature, Sea Surface Temperature, Sea Water Salinity, Atmospheric Pressure. 30 s – (3 x Hz)Leeds PML air-sea pCO 2 instrumentation Air/Sea pCO 2 58 min – (3x10 -4 Hz)CAXIS/PML SURFMET LICOR & MOTIONPACK Analyzing and Modelling CO2 fluxes across the air-sea boundary

Motion Correction APPARENT VERTICALTRUE VERTICAL Analyzing and Modelling CO2 fluxes across the air-sea boundary

Ogive Functions BA Ogive Function = running integral from the highest to lowest frequencies of the co-spectral density. OR: Flux contributions at different frequencies. Analyzing and Modelling CO2 fluxes across the air-sea boundary

Well-behaved ogive functions chosen in order to eliminate: Erroneous CO 2 concentration effects Wave motion contamination effects

Main Filtering Criteria: Wind speed direction constrained Confined maximum correlation between w and c Well-behaved ogive functions Stability - unstable profiles only CO 2 concentration time series (<50mmol m -3 )

CO 2 – H 2 O concentration relationship: Taylor et al. (2007) correction 1. Removal of third order polynomial 2. Derivation of CO 2 flux from corrected data dc = c* dq q* 3. Addition of ‘derived gradient’ (Iterative method converges in 3 steps)

CO 2 Transfer Velocities (1)

CO 2 Transfer Velocities (2)

CO 2 Transfer Velocities (3)

Air-Sea CO 2 Flux Model (1) Mock spatial fields constructed for: Wind Speed Sea Surface Temperature ΔpCO 2 (Easily replaceable by real spatial fields)

Air-Sea CO2 Flux Model (2) Model Runs: McGillis et al., 2001 (M01) and Wanninkhof, 1992 (W92) parameterisations employed 36h model run at 6h time steps Daily/Yearly CO 2 Fluxes Derived Resulting CO 2 fluxes: W92 yearly fluxes are 25% greater than M01 fluxes

Conclusions Successful employment of the Eddy Covariance Method in the open Ocean. Extensive Measurements at high wind speeds are required to better constrain k at high windspeeds. Further research into the relationship between k and other sea state parameters is essential.

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