Sensitivity of the Global Ocean Carbon Sink to the Ocean Skin in a Climate Model

Abstract

The ocean skin is composed of thin interfacial microlayers of temperature and mass of less than 1 mm where heat and chemical exchanges are controlled by molecular diffusion. It is characterized by a cooling of ∼−0.2 K and an increase in salinity of ∼0.1 g/kg (absolute salinity) relative to the water below. A surface observation‐based air‐sea CO2 flux estimate considering the variation of the CO2 concentration in these microlayers has been shown to lead to an increase in the global ocean sink of the anthropogenic CO2 by +0.4 PgC yr−1 (15% of the global sink). This study analyzes this effect in more details using a 15‐year (2000–2014) simulation from an Earth System Model (ESM) that incorporates a physical representation of the ocean surface layers (diurnal warm layer and rain lenses) and microlayers. Results show that considering the microlayers increases the simulated global ocean carbon sink by +0.26 to +0.37 PgC yr−1 depending on assumptions on the chemical equilibrium. This is indeed about 15% of the global sink (2.04 PgC yr−1) simulated by the ESM. However, enabling the ocean skin adjustment to feedback on ocean carbon concentrations reduces this increase to only +0.13 (±0.09) PgC y−1. Coupled models underestimate the ocean carbon sink by ∼5% if the ocean skin effect is not included.

An Assessment of CO2 Storage and Sea‐Air Fluxes for the Atlantic Ocean and Mediterranean Sea Between 1985 and 2018

“…The uncertainty estimates provided here are mainly based on differences between the different products. Uncertainties and biases in gas transfer velocities, and impacts of near‐ surface pCO 2 gradients (Bellenger et al., 2023; Dong et al., 2022) are not taken into account but are estimated to increase the uncertainty in the pCO 2 products by 3‐fold on global scales (see Table 3, DeVries et al., 2023).…”

“…Further improvements in characterization of these gradients will improve the quantification of CO 2 fluxes (Dong et al, 2022). Of note is that the effect of gas transfer and near-surface gradients will be less in GOBMs than pCO 2 products because of the inherent feedback between fluxes and concentration gradients in GOBMs (Bellenger et al, 2023).…”

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

“…Within the GCB, all the data-product based assessments, except for the UoEx-Watson, do not include the vertical temperature gradients (Friedlingstein et al, 2023) and therefore have a single solubility term. Although the evidence is growing for the inclusion of these temperature gradients (Bellenger et al, 2023;Dong et al, 2022;Shutler et al, 2020;Watson et al, 2020;Woolf et al, 2016), the use of a single or two solubilities does not have a large effect on the uncertainty (i.e 0.10 Pg C yr -1 for two solubilities, or 0.08 Pg C yr -1 for one solubility assuming the solubilities are independent and uncorrelated).…”

Mediterranean Sea between

“…Uncertainty estimates provided here are mainly based on differences between the different products. Uncertainties and biases in gas transfer velocities, and impacts of near-surface pCO2 gradients (Dong et al, 2022;Bellenger et al, 2023) are not taken into account but are estimated to increase the uncertainty in the pCO2 products by 3-fold on global scales (See Table 3 , DeVries et al, 2023).…”

“…Further improvements in characterization of these gradients will improve the quantification of CO2 fluxes (Dong et al 2022). Of note is that the effect of gas transfer and near-surface gradients will be less in GOBMs than pCO2 products because of the inherent feedback between fluxes and concentration gradients in GOBMs (Bellenger et al, 2023).…”