Gas Transfer Velocity from QuikSCAT/SeaWinds Backscatter

A major goal of NASA's Earth Science Enterprise is to assess the net global flux of CO₂, a climatologically important, radiatively active gas, into the ocean (Asrar and Dozier, 1994). Flux estimates based on air-sea exchange models are an important diagnostic tool and must be consistent with the long-term sequestering of CO₂ predicted by various carbon cycle models, both general circulation models (GCM) and isotopic estimates (¹⁴C). In certain regions of the ocean, exchange across the air-sea interface may be the rate-limiting step, as in high latitudes where episodic deep advection processes occur on time scales that are shorter than ocean-atmosphere equilibration times (Broecker and Peng, 1982). These regions apparently account for much of the sensitivity of carbon flux models to variations in the gas transfer coefficient (Sarmiento et al., 1992; Johnson, 1995).

Gas exchange flux is calculated from Eqn. (1) which contains two factors: a concentration gradient across the air-water interface and the exchange coefficient or transfer velocity representing a parameterization of a combination of important near surface exchange mechanisms.

(1)

Here F is the flux of mass (mol cm^-2 hr^-1), k is the transfer velocity (cm hr^-1) and Δ C is the concentration difference (mol cm^-3). Transfer velocity fields predicted by various parameterizations based on wind speed (Liss and Merlivat, 1986; Wanninkhof, 1992; Tans et al., 1990; Erikson, 1993) lead to widely varying estimates of zonal and global net CO₂ fluxes (Etcheto and Merlivat, 1988; Boutin and Etcheto, 1995). These are not sufficiently constrained to validate GCM models (Sarmiento et al., 1992; Keeling et al., 1989; Stocker et al., 1994), which suggest a global uptake of 2±0.8 GtC/yr (in the mid-1980's), or to shed light on the apparent ``missing sink'' for anthropogenic CO₂ (~1.6 GtC/yr). The uncertainty is a significant fraction of the total annual 3.5 GtC uptake by non-atmospheric sinks (Johnson, 1995). Thus, the Intergovernmental Panel on Climate Change (IPCC, 1996) has identified uncertainty in the gas exchange coefficient as a significant limitation in assessing the role of the ocean in absorbing anthropogenic CO₂ and has called for increased study of its global spatial and temporal variations in order to help close the global carbon budget.

Current parameterizations of k are based on wind speed at 10 m height, U₁₀ (Liss and Merlivat, 1986; Wanninkhof, 1992; Wanninkhof and McGillis, 1999; Nightingale et al., 2000). Such parameterizations would be extremely useful in estimating CO₂ fluxes both seasonally and spatially, since global wind fields can be estimated from space-based scatterometers (Chelton et al., 1990). Recent studies, however, have shown that k is not a unique function of wind speed (Frew et al., 1995; Frew, 1997; Hara et al., 1995, Bock et al., 1999). Evidence for this is seen in the considerable scatter in both laboratory and field data when correlated with either U₁₀ or friction velocity u_* (Wanninkhof, 1992). Other factors have a strong influence on k, most notably wave fetch (Wanninkhof, 1992), boundary layer stability (Erickson, 1993), and the presence of surface-active organic matter, which affects the small-scale wave field and surface turbulence (Frew et al., 1990; 1995; Bock et al., 1995; Frew, 1997). These factors are expressed largely as modulations of surface roughness (and hence k).

We have avoided complications introduced by the U₁₀ model by developing an alternative method for predicting k using the TOPEX dual-frequency normalized altimeter backscatter (Frew et al., 2005; Glover et al., 2002). The modulating factors cited above are assimilated using a direct measure of surface roughness, the mean square surface slope, which has been shown to be directly related to the transfer velocity (Jähne et al., 1987; Hara et al., 1995; Bock et al., 1999). The transfer velocity is derived from the inverse relationship between σ^o and the mean square slope () of the wave field from which it is reflected (Jackson et al., 1992). Winds derived from altimeters, radiometers and scatterometers are all empirical estimates based on surface roughness scaled to obtain wind speed. Nonetheless, the relation between U₁₀ and k cannot be specified precisely. The use of altimeter data instead of other sensors such as SSM/I or the NSCAT replacement (SeaWinds), which would provide better spatio-temporal coverage was attractive because of the existence of a rationale (theoretical model and empirical data) that allowed us to relate roughness to gas exchange rate. This report details the status of our project to extend our altimeter-based algorithm to a scatterometer-based algorithm by using altimeter results to bootstrap calibrate the scatterometer k-σ^o function.

We believe a radar backscatter-gas transfer velocity relationship represents a significant improvement over using wind speed to predict transfer velocity. While the initial development of the gas exchange algorithm has taken place using altimetry data (as members of the Jason-1 SWT), a relationship between scatterometer backscatter and transfer velocity has greatly accelerate further development of both algorithms and ultimately leads to a better data product in terms of both spatial and temporal coverage. Our contribution brings together scatterometer and altimeter remote sensing with in situ data from field programs. The combination of transfer velocity fields with pCO₂ fields derived from general circulation models (Gent et al., 1998) or field programs (Takahashi et al., 1997) allows us to identify the major CO₂ source and sink regions in the world's oceans and provides an unprecedented continuous record (using the combination of TOPEX/Poseidon, extended TOPEX/Poseidon, Jason-1, ALT, QuikSCAT and ADEOS-2 SeaWinds missions) of estimated CO₂ flux on monthly, annual, and decadal time scales.