Arctic Cloud and Atmospheric Regime Interactions: An Annual Cycle Perspective

Arctic clouds exhibit a robust annual cycle with maximum cloudiness in fall and a minimum in winter. These variations affect energy flows in the Arctic with a large influence on the surface radiative fluxes and therefore surface temperature, sea ice, and atmospheric variability. Contemporary climate models struggle to reproduce the observed Arctic cloud amount annual cycle and significantly disagree with each other. Climate models tend to fall in to one of two well-defined groups producing either a maximum cloud amount in winter or a maximum cloud amount in summer. The goal of this analysis is to quantify the cloud influencing factors (e.g., lower tropospheric stability, 500 hPa vertical velocity, relative humidity, surface turbulent fluxes, and sea ice) responsible for placing a model into one of these two groups and assess the impacts on Arctic atmospheric variability. First, we find that the total cloud amount annual cycle is primarily caused by differences in low, not high, clouds with the largest differences between the surface and 950 hPa. Secondly, the discrepancies between the two model groups exhibit little spatial variability and are consistent between land and ocean and are only weakly influenced by sea ice concentration. Stratifying cloud amount by a range of cloud influencing factors, we find that model groups disagree most under strong lower tropospheric stability, weak to moderate mid-tropospheric subsidence, and cold near-surface air temperatures. The cloud amount dependence on cloud influencing factors explains most of the inter-group differences in cloud amount whereas differences in the mean atmospheric conditions and frequency of occurrence of atmospheric conditions explain little. The cloud amount dependence on cloud influencing factors represents cloud parameterization, therefore, the results show that cloud parameterization differences are responsible for the discrepancies in the Arctic cloud amount annual cycle and that differences due to atmospheric dynamics and thermodynamics can be ruled out. However, we find evidence that the cloud parameterization influences Arctic atmospheric variability and dynamic/thermodynamic state selection.