Improved key process in representing Arctic warming (D3.5)

• Main Authors: Davy, Richard, Gao, Yongqi, Gastineau, Guillaume, Tian, Tian, Zhang, Ying
• Format: Text
• Language: English
• Published: Zenodo 2020
• Subjects: Arctic; Sea ice
• Online Access: https://dx.doi.org/10.5281/zenodo.3559469; https://zenodo.org/record/3559469

In the work presented in this deliverable the Blue-Action teams focused on improving the representation of some of the most important physical processes which contribute to Arctic warming within the climate models used by the consortium. The two processes we addressed were the effect on the atmospheric state due to the presence of leads in the sea ice cover and turbulence under strongly stable thermal stratification. The work done: We first analysed the results of previously performed large eddy simulations which resolved the turbulence over leads to determine the effect leads have on sensible heat flux from open water. Because of the effect of three-dimensional structures in the turbulent mixing above leads, the heat flux coming from leads can be amplified compared to the fluxes one would get from open water under the same air-sea temperature difference. The amplification effect strongly depends on the width of the lead, with the largest effect occurring for leads of widths around 1.4 km. We assessed the functional sensitivity of this amplification effect to key parameters used in the turbulence-resolving model, including the length scale for the convective boundary layer, which characterizes the background stability in the atmosphere. We combined this relation between the amplification effect of heat fluxes as a function of lead width with observed distributions of lead widths. These were taken from the peer-reviewed literature. Together, this gives us a scheme to describe how the presence of leads affects the surface sensible heat flux, and this depends upon the concentration of sea ice and the background atmospheric stability. We implemented this scheme in four climate models (NorESM, EC-Earth3, IPSL/ LMDZ6A and CAS-ESM/ IAP4) and tested the scale of the effect using multiple single-model ensemble simulations of historical climate. The key findings: The presence of leads in sea ice dramatically alters the surface energy balance in the Arctic. There is a large seasonal cycle to the effect of the presence of leads, because the flux from the leads depends strongly on the background stability in the atmosphere. In the winter when the atmosphere is often strongly stably stratified, the leads greatly amplify the surface sensible heat flux coming from open water. In the summer there is the opposite effect and the generally weaker atmospheric stability reduces the flux coming from leads. The net effect is to increase near-surface temperatures over sea ice in winter, with little or no change in the summer. Therefore this scheme may be used to address the long-standing winter cold bias over ice in many contemporary global climate models, as shown in CMIP6. Linked Zenodo records: Guidelines for improving the representation of surface heat flux in the Arctic https://zenodo.org/record/4728073 : The Blue-Action project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No 727852.