The FuGas 2.3 Framework for Atmosphere–Ocean Coupling: Comparing Algorithms for the Estimation of Solubilities and Gas Fluxes

Abstract

Accurate estimates of the atmosphere-ocean fluxes of greenhouse gases and dimethyl sulphide (DMS) have great importance in climate change models. A significant part of these fluxes occur at the coastal ocean which, although much smaller than the open ocean, have more heterogeneous conditions. Hence, Earth System Modelling (ESM) requires representing the oceans at finer resolutions which, in turn, requires better descriptions of the chemical, physical and biological processes. The standard formulations for the solubilities and gas transfer velocities across air-water surfaces are 36 and 24 years old, and new alternatives have emerged. We have developed a framework combining the related geophysical processes and choosing from alternative formulations with different degrees of complexity. The framework was tested with fine resolution data from the European coastal ocean. Although the benchmark and alternative solubility formulations generally agreed well, their minor divergences yielded differences of up to 5.8% for CH 4 dissolved at the ocean surface. The transfer velocities differ strongly (often more than 100%), a consequence of the benchmark empirical wind-based formulation disregarding significant factors that were included in the alternatives. We conclude that ESM requires more comprehensive simulations of atmosphere-ocean interactions, and that further calibration and validation is needed for the formulations to be able to reproduce it. We propose this framework as a basis to update with formulations for processes specific to the air-water boundary, such as the presence of surfactants, rain, the hydration reaction or biological activity.

The FuGas 2.5 Updated for the Effects of Surface Turbulence on the Transfer Velocity of Gases at the Atmosphere–Ocean Interface

“…The FuGas user may opt for one simple parameterization, or its comprehensive iterative estimation involving the wind log-linear profile: first, a preliminary u * is estimated, and then z 0 and u * must be solved for convergence. In both COARE [63,64] and previous FuGas algorithms [74][75][76], convergence was generally attained after three iterations. Although the new FuGas schemes could be substantially more complex, convergence was still attained within three iterations, at most, and often with only one or two.…”

“…∆ppm is the difference between atmospheric and oceanic CO 2 measured concentrations. The parameter k Hcp is the Henry's constant, providing the CO 2 solubility in its c water /ρ air form (mol•L −1 •atm −1 ), and estimated from the measured surface water temperature and salinity [73][74][75][76]. Simultaneously, we measured the atmospheric and oceanic variables related with turbulence and commonly used in formulations estimating gas transfer velocities.…”

“…The FuGas framework [74][75][76] was developed for the estimation of the air-water fluxes of virtually any gas in the atmosphere, currently having 110 alternative or complementary formulations. The details about its application are provided in its software tutorial (see Section 5: Code and Data Availability).…”

“…The wind is most often low or moderate over the world's oceans [18,24,32,33], and particularly in tropical regions [34], resulting in large gas exchange between atmosphere and ocean [18,24,33]. Hence, by Johnson [73], we developed the FuGas (Flux of Gases) numerical scheme [74][75][76] with over 150 available formulations, some competing, others complementary, allowing the users to tune their model from a wide range of driving factors and degrees of complexity. This work reports on a FuGas update (version 2.5) with formulations that, either directly or indirectly, account for the effects of the wind, wave field and air-flow type on the gas transfer velocity.…”

Local Variability of CO2 Partial Pressure in a Mid-Latitude Mesotidal Estuarine System (Tagus Estuary, Portugal)

“…Inherent differences in various measurement techniques is another source of uncertainty, leading to the emergence of new alternatives for standard formulations for the solubility and gas transfer velocities across air-water surfaces (e.g., [51][52][53][54]). However, such formulations require more comprehensive simulations of atmosphere-ocean interactions, and further calibration and validation.…”