GEOPHYSICAL RESEARCH LETTERS

Transcription

1 GEOPHYSICAL RESEARCH LETTERS Supporting Information for: The Seasonal Cycle of Ocean-Atmosphere CO 2 Flux in Ryder Bay, West Antarctic Peninsula Oliver J. Legge 1,2, Dorothee C.E. Bakker 1, Martin T. Johnson 1,3, Michael P. Meredith 2, Hugh J. Venables 2, Peter J. Brown 4 and Gareth, A. Lee 1 Corresponding author: Oliver Legge, Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, Norfolk, NR4 7TJ, UK. (o.legge@uea.ac.uk) 1 Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, Norfolk, NR4 7TJ, UK 2 British Antarctic Survey,High Cross, Madingley Road, Cambridge CB3 0ET, UK 3 Centre for Environment Fisheries and Aquaculture Science, Lowestoft, NR33 0HT, UK 4 National Oceanography Centre, University of Southampton Waterfront

2 X - 2 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY Contents of this file 1. Carbonate system calculations 2. Water vapour pressure calculation 3. Atmospheric f CO 2 calculation 4. Calculation of the solubility of CO 2 in seawater 5. Seawater density calculation 6. Calculation of the CO 2 concentration difference between water and air 7. Schmidt number calculation 8. Calculation of the gas transfer velocity using two different methods 9. Calculation of the ocean-atmosphere CO 2 flux 10. Plot of total alkalinity at the RaTS site during the study period (figure S1) 11. Plot of mixed layer depth at the RaTS site during the study period (figure S2) Introduction This supporting information details the equations used to calculate ocean-atmosphere CO 2 flux from measured parameters. We also present total alkalinity and mixed layer depth data from the study period. Equations used to calculate ocean-atmosphere CO 2 flux Campus, European Way, Southampton, SO14 3ZH, UK

3 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY X - 3 The fugacity of CO 2 in water (f CO 2 w,) was calculated from dissolved inorganic carbon (µmol kg 1 ), total alkalinity (µmol kg 1 ), silicate (µmol kg 1 ), phosphate (µmol kg 1 ), seawater temperature ( C), salinity and in situ pressure (dbar) using the CO2SYS program [Van Heuven et al., 2011] and the dissociation constants of [Goyet and Poisson, 1989]. The CO2SYS matlab function is available from calc MATLAB v1.1 Water vapour pressure (ph 2 O, atm) was calculated from seawater temperature (T, K) and salinity (S) following Weiss and Price [1980] ph 2 O = exp( T T log S) (1) 100 f CO 2 in moist air (f CO 2 a, µatm) was calculated from seawater temperature (T, K), the atmospheric mole fraction of CO 2 (xco 2 a, µmol/mol), atmospheric pressure (P, atm) and water vapour pressure (ph 2 O, atm) using the gas constant, R (8.314 J mol 1 K 1 ) and coefficients (B, cm 3 mol 1 and D) from Weiss [1974]. B = T T T 3 (2) D = T (3)

4 X - 4 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY P (B + 2 D) fco 2 a = xco 2 a (P ph 2 O) exp R T (4) The solubility of CO 2 in seawater (s, mol kg 1 atm 1 ) was calculated from seawater temperature (T, K) and salinity (S) following Weiss [1974], taken from Dickson et al. [2007]. s = exp( T ) log T S ( T ( T 100 )2 )) (5) Seawater density (ρ, kg L 1 ) was calculated from seawater temperature (T, C), salinity (S) and atmospheric pressure (P, atm) using the TEOS-10 thermodynamic equation of seawater (available at ρ = gsw rho t exact(s, T, P ) 1000 (6) Pressure was multiplied by to convert from atm to db. Density was divided by 1000 to convert from kg m 3 to kg L 1.

5 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY X - 5 The CO 2 concentration difference between water and air ( C, µmol kg 1 ) was calculated from the fugacity of CO 2 in water (f CO 2 w, µatm), the fugacity of CO 2 in air (f CO 2 a, µatm) and the solubility of CO 2 in seawater (s, mol kg 1 atm 1 ) C = s (fco 2 w fco 2 a) (7) This concentration difference was converted from µmol kg 1 to volumetric units (µmol L 1 ) by multiplying by seawater density (ρ, kgl 1 ) The Schmidt number (Sc) was calculated from water temperature (T, C) following Wanninkhof [1992]. Sc = T T T 3 (8) The gas transfer velocity (k) was calculated using two separate methods. Firstly k (cm hr 1 ) was calculated from wind speed (u, m s 1 ) and the Schmidt number (Sc) following Wanninkhof et al. [2013]. k w13 = ( Sc 660 ) 0.5 u 2 (9) The gas transfer velocity calculated above, following Wanninkhof et al. [2013] was scaled linearly to the fraction of open water (wf ) k w13 lin = k w13 wf (10)

6 X - 6 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY Secondly, k (m d 1 ) was calculated from ice speed (m s 1 ), wind speed (m s 1 ), sea ice concentration (%), surface water temperature (T, C), air temperature (T, C) and (optionally) relative humidity (%), mixed layer depth (m) and salinity using a parameter model for effective gas transfer velocity in sea ice covered waters [Loose et al., 2014]. Ice speed was not known so a fixed value of 0.01 m s 1 was used throughout. Mixed layer depth was defined as the depth at which the density difference relative to the surface is 0.05 kg m 3 [Venables et al., 2013]. The model gives separate effective gas transfer velocities from ice and wave effects. These were summed to give the total effective gas transfer velocity and multiplied by to convert from m d 1 to cm hr 1. The Schmidt number correction was then applied. k L11 = (keff ice + keff wave) ( sc 660 ) 0.5 (11) Ocean-atmosphere CO 2 flux (F, mol m 2 yr 1 ) was calculated from the gas transfer velocity (k w13 or k L11, cm hr 1 ) and the CO 2 concentration difference between water and air ( C,µmol L 1 ) F = ( k C 1000) (12) 100

7 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY X - 7 k was divided by 100 to convert from cm hr 1 to m hr 1. C was multiplied by 1000 to convert from µmol L 1 to µmol m 3. Flux was multiplied by to convert from µmol m 2 hr 1 to mol m 2 yr 1. References Dickson, A. G., C. Sabine, and J. R. Christian (2007), Guide to best practices for ocean CO 2 measurements, PICES Special Publication, 3, 191 pp. Goyet, C., and A. Poisson (1989), New determination of carbonic acid dissociation constants in seawater as a function of temperature and salinity, Deep-Sea Research, 36 (11), Loose, B., W. R. McGillis, D. Perovich, C. J. Zappa, and P. Schlosser (2014), A parameter model of gas exchange for the seasonal sea ice zone, Ocean Science, 10, 1 12, doi: /os Van Heuven, S., D. Pierrot, J. Rae, E. Lewis, and D. Wallace (2011), MATLAB Program Developed for CO 2 System Calculations, ORNL/CDIAC-105b, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S.Department of Energy, Oak Ridge, Tennessee., doi: /cdiac/otg.co2sys. Venables, H. J., A. Clarke, and M. P. Meredith (2013), Wintertime controls on summer stratification and productivity at the western Antarctic Peninsula, Limnology and Oceanography, 58 (3), , doi: /lo

8 X - 8 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY Wanninkhof, R. (1992), Relationship between wind speed and gas exchange over the ocean, Journal of Geophysical Research, 97 (C5), 7373, doi: /92jc Wanninkhof, R., G. H. Park, T. Takahashi, C. Sweeney, R. Feely, Y. Nojiri, N. Gruber, S. C. Doney, G. a. McKinley, a. Lenton, C. Le Quéré, C. Heinze, J. Schwinger, H. Graven, and S. Khatiwala (2013), Global ocean carbon uptake: magnitude, variability and trends, Biogeosciences, 10 (3), , doi: /bg Weiss, R., and B. Price (1980), Nitrous oxide solubility in water and seawater, Marine Chemistry, 8 (4), , doi: / (80) Weiss, R. F. (1974), Carbon dioxide in water and seawater: the solubility of a non-ideal gas, Marine Chemistry, 2,

9 LEGGE ET AL.: CO 2 FLUX IN RYDER BAY X Figure S1. The seasonal cycle of total alkalinity from December 2010 until February 2014 at 15m depth at RaTS. Blue points indicate data from site 1, red from site 2. Error bars are uncertainty (2SD) based on measurement precision mixed layer depth (m) J FM AM J J A S O N D J FM AM J J A S O N D J FM AM J J A S O N D J F Figure S2. Mixed layer depth at RaTS site 1 from December 2010 until February Mixed layer depth was defined as the depth at which the density difference relative to the surface is 0.05 kg m 3. The blue line represents the carbon sampling depth of 15m