Uncertainty of the global oceanic CO<sub>2</sub> uptake induced by wind forcing: quantification and spatial analysis

Abstract

Abstract. The calculation of the air-water CO 2 exchange (FCO 2 ) in the ocean not only depends on the gradient in CO 2 partial pressure at the air-water interface but also on the parameterization of the gas exchange transfer velocity (k) and the choice of wind product. Here, we present regional and global-scale quantifications of the uncertainty in FCO 2 induced by several widely used k-formulations and 4 wind speed data products (CCMP, ERA, NCEP1 and NCEP2). The analysis is 10 performed at a 1° x 1° resolution using the sea surface pCO 2 climatology generated by Landschützer et al. (2015) that the range of global FCO 2 , calculated with these k-relationships, diverge by 12 % when using CCMP, ERA or NCEP1.Due to differences in the regional wind patterns, regional discrepancies in FCO 2 are more pronounced than global. These global/regional differences significantly increase when using NCEP2 or other k-formulations which include earlier relationships (i.e. Wanninkhof et al., 2009) as well as numerous local/regional parameterizations derived experimentally. To minimize uncertainties associated with the choice of wind product it is possible to recalculate the 20 coefficient c globally (hereafter called c * ) for a given wind product and its spatio-temporal resolution, in order to match the last evaluation of the global k value. We thus performed these recalculations for each wind product at the resolution and time period of our study but the resulting global FCO 2 estimates still diverge by 10 %. These results also reveal that the Equatorial Pacific, the North Atlantic and the Southern Ocean are the regions in which the choice of wind product will most strongly affect the estimation of the FCO 2 , even when using c * . 25

A comprehensive analysis of air-sea CO2 flux uncertainties constructed from surface ocean data products

“…Recently the evaluation of wind speed products has indicated that a 0.09 Pg C yr -1 Roobaert et al, 2018) uncertainty stems from the wind speed uncertainty. We have shown a slightly Manuscript submitted to Global Biogeochemical Cycles larger but still consistent value of 0.14 Pg C yr -1 (Table 2) using a different methodology.…”

Impact of Predictor Variables on Estimates of Global Sea-Air CO2 Fluxes Using an Extra Trees Machine Learning Approach

“…The effect of different wind speed products and parameterizations have been detailed in Roobaert et al (2018), including discussion of the rationale for normalizing the wind products and gas transfer-wind speed dependencies. Two different parameterizations are compared here that differ in their assumptions of environmental forcing as detailed in Wanninkhof et al (2009).…”

“…Different gas transfer velocity formulations and wind speed products can impact the global flux estimates with past studies indicating that this is a primary source of uncertainty in global flux estimates (Woolf et al, 2019). The impact of time averaging and the effect of different wind fields has been investigated (Wanninkhof et al, 2002;Roobaert et al, 2018;and Gregor et al, 2019) but conical quadratic wind speed relationships to parameterize gas transfer are used in most flux estimates, including those in RECCAP2 (DeVries et al, 2023) and the GCB (Friedlingstein et al, 2022). A common procedure is to normalized the coefficient in the relationship (Eqn.…”

“…Inconsistencies in modeled surface areas, wind speed products and the method of calculation of fluxes contribute to differences. To account for these differences area normalization and ensembles (or multi-product averages) are increasingly common in intercomparison studies and improve consistency (Fay et al, 2021;Roobaert et al, 2018). A summary of the annual global sea-air CO 2 fluxes for different ML approaches used in the Global Carbon Budget (Friedlingstein et al, 2022) is provided in Figure 1 that show correspondence over time between the observation-based methods at the global scale.…”