Calculating long‐term global air‐sea flux of carbon dioxide using scatterometer, passive microwave, and model reanalysis wind data

Abstract

[1] Global air-sea flux of carbon dioxide (CO 2 ) is calculated from wind data acquired by the satellite scatterometer QuikSCAT, the passive microwave radiometer AMSR-E, and the model reanalysis ERA-40 using four of the most commonly used wind speed dependent parameterizations of gas transfer velocity. Assuming QuikSCAT as reference, the results are compared to obtain an estimate of that relative uncertainty in the flux calculations which results solely from the origin of the input wind data. We illustrate the discrepancies between these data sets and quantify the uncertainty in the computed air-sea CO 2 flux that arises from data processing such as temporal and spatial averaging using AMSR-E as an example data set. The impact of temporal variability of wind speed is shown to be significantly greater than that of spatial variability. However, simple parameterizations of temporal variability are found to be sensor-specific and cannot be applied in a straightforward way to data sets with lower temporal resolutions from other sensors. We show a simple methodology to correct monthly mean data in such a way that seasonally and zonally varying parameterizations of temporal variability derived from QuikSCAT data can be applied to data from AMSR-E and ERA-40. This allows us to produce a global 44-year time series of gas transfer velocity and to present a more coherent estimate of air-sea transfer of carbon dioxide from the three most commonly available types of wind data.

Progress in satellite remote sensing for studying physical processes at the ocean surface and its borders with the atmosphere and sea ice

“…Hence studies will typically use a combination of satellite remote-sensing data, in situ and model derived data to derive air-sea gas fluxes at the global, basin and regional scales. Despite this limitation the improved quasi-global availability of EO data has facilitated recent studies of CO 2 air-sea exchange over a substantial area of the Arctic Seas (Else et al, 2008;Arrigo et al, 2010;Land et al, 2013), Atlantic Ocean (Kettle and Merchant, 2005;Kettle et al, 2009) and global oceans (Fangohr et al (2008) and figure 6a).…”

“…Most existing gas transfer parameterisations use wind speed and these are often applied at regional or global scales using remote-sensing retrievals of wind speed (Fangohr et al, 2008;Boutin et al, 2002). All retrievals of wind speed from passive and active microwave instruments rely on surface roughening by the wind.…”

“…These datasets are generally used to determine the gas transfer velocity, solubility and to correct in situ based climatology datasets to allow the determination of appropriate pCO 2w and pCO 2a values (Fangohr and Woolf, 2007;Fangohr et al, 2008;Land et al, 2013;Else et al, 2008). Currently it is not possible to derive all of the required variables from satellite observations.…”

A comparative assessment of monthly mean wind speed products over the global ocean

“…Parameterizations of gas transfer velocity of CO 2 commonly rely on wind speed (Wanninkhof, 1992;Nightingale et al, 2000;Equation (4)) and vary between square and cubic functions of wind speed so nonlinear effects are particularly important (Wanninkhof et al, 2002;Fangohr et al, 2008). In an attempt to parameterise those surface processes that do not scale with wind speed, some more recent theoretical parameterizations of k also include a dependence on remotely sensed mean square slope, friction velocity and whitecapping (Glover et al, 2002;Woolf, 2005;Fangohr and Woolf, 2007).…”

Transfer Across the Air-Sea Interface

“…Because of the quadratic or cubic dependency of k w with U, the accuracy of the wind speed fields used to build k w fields is a big issue and the merging of several satellite wind speeds requires a thorough crossvalidation (e.g. Fangohr et al 2008). Not only the average wind speed must be well known but also at GEOSECS inventories by [Broecker et al, 1985] New 14C constraint [Naegler and Levin, 2006] New 14C constraint [Sweeney et al, 2007] New 14C and 13C constraints [Krakauer et al, 2006] k660 (cm/hr) <kLM> <kW> <kN> <kH> k14C…”