Modeling influence of sea ice on gas exchanges between atmosphere and ocean in a global Earth System Model

Global Earth System Models (ESMs) seek to simulate physical, chemical and biological processes that are relevant for the evolution of global climate. One key feature of an ESM is the ability to simulate fluxes of greenhouse gases and aerosols between the atmosphere and ocean, keep track of the inventories in the respective model components and allow for feedback on the climate system. These fluxes are usually calculated based on bulk formulations derived from open water measurements, and are restricted by the sea ice fraction in regions covered by sea ice.

The air-sea gas exchange is determined by the difference in concentration across the air-sea interface, and a gas transfer velocity that is specific for the gas in question. Using CO₂ as example, the air-sea gas exchange is
          F_CO2 = (1 - βC_sea-ice ) ⋅ k_w(CO2) ⋅ ( [CO2]_sea - α[CO2]_air)   (1)
where C_sea-ice is the sea ice concentration, k_w(CO2) is the gas transfer velocity, and α is the Ostwald solubility coefficient. Traditional formulas use β = 1 (complete barrier), but in order to account for cracks and leads in the sea ice, Steiner et al. (2013) proposed a modified formula with β ∈ [0, 1], allowing the sea ice to act as a partial barrier (0 < β < 1) or allowing free exchange in sea ice covered regions (β = 0).

We implement the modified gas exchange formula (eq. 1) in the Norwegian Earth System Model NorESM2, for all model tracers exchanged over the air-sea interface (CO₂, O₂, N₂, N₂O, DMS). Experimenting with different β values, we find that small increases (β ∈ [0.01, 0.02]) may result in either increased or decreased gas fluxes in high latitude regions. This can be attributed to the internal variability of the sea ice area, in particular for the summer minimum, which responds to changes in greenhouse gases and aerosols in the atmosphere. For β ∈ [0.1, 0.2] we find an increase in CO₂ flux of 16% — 22% north of 68°N, and 5% — 8% south of 60°S. Observational datasets based on eddy covariance data for CO₂ in the atmospheric boundary layer will be used in
future work in order to determine a realistic range for β.

REFERENCES:

N. S. Steiner, W. G. Lee, and J. R. Christian. Enhanced gas fluxes in small sea ice leads and cracks: Effects on CO2 exchange and ocean acidification. Journal of Geophysical Research: Oceans, 118(3):1195–1205, 2013. doi: 10.1002/jgrc.20100.