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