Isolating the Surface Type Influence on Arctic Low-Clouds

Interactions between sea ice and clouds represent a mechanism through which sea ice influences climate. We composite cloud properties for ice-free, marginal ice zone (MIZ), and ice-covered surfaces during MIZ crossing events to analyze the cloud property differences between surface types. Restricting the analysis to MIZ crossing events enables the isolation of the sea ice effect on clouds from meteorological factors. We find larger cloud fraction (CF) and total water concentration below similar to 1.5 km over ice-free relative to ice-covered surfaces during non-summer months. During summer, the results suggest larger CF and total water concentration over ice-free surfaces, however differences do not exceed observational uncertainty. Cloud property differences are linked to atmospheric thermodynamic profile differences, namely ice-free surfaces are warmer, moister, less stable, and have more positive surface turbulent fluxes than ice-covered surfaces. Ice-free minus ice-covered cloud property differences scale with surface temperature differences and are only found in the presence of a surface temperature difference. Our results suggest a 0.02 CF and 0.005 g m(-3) total water concentration increase (similar to 5%) at the level of maximum CF between 2000 and 2021 due to the observed Arctic sea ice decline in fall, corresponding to similar to 2 W m(-2) increase in the net surface radiative flux. Our results support a positive sea ice-cloud radiative feedback in fall and winter and a negative sea ice-cloud radiative feedback in spring. We propose an updated conceptual model where the average surface type influence on cloud properties is mediated by surface temperature differences between ice-free and ice-covered surfaces. Plain Language Summary The response of clouds to sea ice decline is a key uncertainty in projections of future Arctic climate change. This study analyzes the cloud properties across the Arctic sea ice edge to quantify the impact of a change in the surface characteristics from sea ice to ice-free ocean on cloud properties. The results suggest that clouds respond to a transition of the surface from sea ice to ice-free ocean by increasing their area coverage and their total water amount. This results supports an amplifying feedback in fall and winter where increases in cloudiness occur as a result of decreases in sea ice that warm the surface and promote less sea ice growth. Lastly, our results indicate that differences in surface temperature between sea ice and ice-free ocean surface is the root cause of the cloud differences between surface types. This represents an update to our thinking about the factors that influence the cloud response to sea ice loss.