Observing gas transfer between ocean and atmosphere from space

 Short wind waves in the order of centimetres can be observed by satellite altimeters; their relation with gas transfer velocity through the sea surface is used to develop gas transfer algorithms for the world’s oceans.

 

Air-sea gas exchange has been studied for decades because of its role in predictions of climate change, the global carbon cycle and ocean acidification. The oceans absorb more carbon dioxide (CO₂) than they release, but there are major differences in net carbon uptake calculations. Gas transfer velocity, K, is a key variable and the commonly used K parameterizations based on empirical relations with 10 m wind speed (U₁₀) are not good enough on a global scale. An alternative is linking K with short wind waves (Fig. 1) as this relation is more direct than with wind and is measurable from space.

Figure 1

Satellite altimeters can measure the mean square slope of short wind waves, <s²>, and K increases with <s²>.  Two approaches using implicit relationships between <s²> and K have been reported. (1) Linear relations determined from wind-wave tank experiments¹ have been applied^{2, 3}. The same linear relationships are not likely to hold in the open ocean, however. (2) K has been obtained in the field from extrapolations of heat transfer velocity measurements^4,5. Although this method produces reasonable results it needs to be confirmed with in situ K measurements of gas. Our study is the first to correlate K measured at sea and the satellite altimeter signal directly.

An altimeter emits a radar wave and measures backscatter. Space base altimeters were developed to measure sea surface height but nadir looking altimeters can also measure <s²>.  As the wave steepness increases, and hence <s²>, the backscatter in altimeter view decreases. Altimeters on board satellites (ERS-1, ERS-2, TOPEX/POSEIDON, GEOSAT, JASON-1, JASON-2 and ENVISAT) have been measuring the Ku-band backscatter coefficient, σ_Ku, from the ocean surface for 20 years. A homogeneous and calibrated data set is available from the IFREMER site ftp://ftp.ifremer.fr/ifremer/cersat/products/swath/altimeters/waves/data/. For normal incidence <s²> should be proportional to 1/σ_Ku. We therefore correlated K measured during eight cruises around the world with concurring 1/σ_Ku from IFREMER’s data base.

The gas in question was dimethyl sulfide (DMS). K was scaled to a gas with a Schmidt number, Sc, of 660 and signified direct gas transfer through the unbroken surface. K best related to altimeter σ_Ku following K = C + (A/σ_Ku)², while K (cm/hr) was  roughly double U₁₀ (m/s)⁶. We can apply our calibrations for DMS to direct gas transfer of any other gas if we know the diffusivity (expressed by Sc) and solubility of that gas ⁶. For a gas less soluble than DMS, such as CO_2, we also need to add a small term to account for bubble mediated gas transfer^3,7. This term can be derived using the fraction of whitecapping, W,⁷ estimated from other Earth observation, EO, data (model or satellite)⁸. 

 

Disappointingly the K algorithm for DMS based on σ_Ku did not perform better than the traditional algorithm based on U₁₀ (in situ and altimeter). A likely explanation for this is that in the open ocean longer swell waves are usually present (Fig. 2) affecting the altimeter back scatter and muddying the signal of the short wind waves. A way to attenuate the contribution of longer swell waves is to subtract the back scattering signal of a second, lower frequency wave^2,4,5,9.

Figure 2



The Ku band signal frequency (wavelength) is 13.6 GHz  (2.1 cm). Recently IFREMER added C-band data (5.3 GHz / 5.5 cm) of the JASON-1 and -2 (Fig. 3) altimeters to the database. Using these data we explored two band algorithms and found a best fit for a relation of the form K = C + A(1/σ_Ku - B/ σ_C). We found some evidence that for small separation errors between altimeter overpasses and K sample stations (dx < 0.5° and dt < 2 hr) the dual-frequency algorithm reduced the uncertainty in the K estimation by ~0.5 cm/hr compared to both the single-band and wind speed (in situ and altimeter) parameterizations. This is an interesting discovery that will be investigated further.

Figure 3



It is described above how satellite altimeters can monitor air-sea gas transfer velocity, K, over the oceans and how our calibration for DMS can be applied to CO₂ using diffusivity and solubility of CO₂ and whitecap coverage (derived from other EO products). It is then possible to produce maps of CO₂ flux distributions from the product of K for CO₂ and air-sea CO₂ concentration differences.  It is expected that better K parameterizations and EO data will result in improved calculations of the total oceanic CO₂ budget. 

REFERENCES

1.    Bock, E. J., T. Hara, N. M. Frew, and W. R. McGillis (1999), Relationship between air-sea gas transfer and short wind waves, J. Geophys. Res., 104, 25,821-25,831.

2.    Glover, D. M., N. M. Frew, S. J. McCue, and E. J. Bock (2002), A multi-year time series of global gas transfer velocity from the TOPEX dual frequency, normalized backscatter algorithm, in Gas Transfer at Water Surfaces, Geophysical Monograph, vol. 127 edn., edited by M. Donelan, W. M. Drennan, E. Saltzman, and R. Wanninkhof, pp. 325-331, American Geophysical Union, Washington, DC.

3.    Fangohr, S. and D. K. Woolf (2007), Application of new parameterization of gas transfer velocity and their impact on regional and global marine CO₂ budgets, J. Mar. Syst., 66, 195-203.

4.    Frew, N. M. et al. (2004), Air-sea gas transfer: Its dependence on wind stress, small-scale roughness, and surface films, J. Geophys. Res., 109.

5.    Frew, N. M., D. M. Glover, E. J. Bock, and S. J. McCue (2007), A new approach to global air-sea gas transfer velocity fields using dual-frequency altimeter backscatter, J. Geophys. R., VOL. 112.

6.    Goddijn-Murphy, L. M., D. K. Woolf, and C. A. Marandino (2012), Space-based retrievals of air-sea gas transfer velocities using altimeters: Calibration for dimethyl sulfide, J. Geophys. Res., 117. doi:10.1029/2011JC007535

7.    Woolf, D. K. (2005), Parametrization of gas transfer velocities and sea-state-dependent wave breaking, Tellus, 57-B, 87-94.

8.    Goddijn-Murphy, L., D. K. Woolf, and A. H. Callaghan (2011), Parameterizations and algorithms for oceanic whitecap coverage, J. Phys. Oceanogr., 41(4), 742-756, doi: 10.1175/2010JPO4533.1.

9.    Chapron, B., K. Katsaros, T. Elfouhaily, and D. Vandemark (1995), A note on relationships between sea surface roughness and altimeter backscatter, in Air-Water Gas Transfer, 3rd International Symposium on Air-Water Gas Transfer, edited by B. Jähne and E. C. Monahan, pp. 869-878, AEON, Germany.