Year-to-year Variability in Arctic Minimum Sea Ice Extent and its Preconditions in Observations and the CESM Large Ensemble Simulations.

Arctic sea ice extent (SIE) achieves its minimum in September each year and this value has been observed to decline steeply over the satellite era of the past three decades. Yet large year-to-year fluctuations are also present in the September SIE and the mechanisms for this variability are still not clear. Here we address this issue by examining the preconditions in meteorological fields in the previous spring and summer from observations and a large ensemble of historical climate model simulations. The focus of this study is on the impact of anomalous moisture transport into the Arctic and the associated surface energy fluxes on the September SIE. We find that the below-normal September SIE is associated with enhanced moisture transport into the Arctic in spring, which induces downward thermal radiation at the surface. However, in summer, the anomalous moisture transport over the Arctic is divergent due to an anticyclonic atmospheric flow pattern and the ice albedo feedback plays a leading role in sea ice loss.