Surface Cloud Warming Increases as Late Fall Arctic Sea Ice Cover Decreases.

During the Arctic night, clouds regulate surface energy budgets through longwave warming alone. During fall, any increase in low‐level clouds will increase surface cloud warming and could potentially delay sea ice formation. While an increase in clouds due to fall sea ice loss has been observed, quantifying the surface warming is observationally challenging. Here, we use a new observational data set of surface cloud warming at instantaneous 330 m × 90 m spatial resolution. By instantaneously co‐locating surface cloud warming and sea ice observations in regions where sea ice varies, we find October large surface cloud warming values (>80 W m−2) are much more frequent (∼+50%) over open water than over sea ice. Notably, in November large surface cloud warming values (>80 W m−2) occur more frequently (∼+200%) over open water than over sea ice. These results suggest more surface warming caused by low‐level opaque clouds in the future as open water persists later into the fall. Plain Language Summary: Over the past 40 years, Arctic sea ice cover has decreased in all months of the year, but especially in late summer and early fall. Through their impact on energy budgets, clouds have the potential to increase or decrease sea ice decline. More low‐level clouds over open water than over sea ice during non‐summer seasons have already been observed. But quantifying their radiative effect remains challenging. Therefore, this study seeks to answer the following question: By how much late fall Arctic clouds can change surface warming in response to sea ice loss? Using cloud surface warming data at high spatio‐temporal resolution, we found that large surface cloud warming values, higher than 80 W m−2, occurs much more frequently over open water than over sea ice during October and November months. This suggests that Arctic clouds favor sea ice loss by delaying sea ice recovery. As the Arctic continues to warm up due to human activities, cloud surface warming will delay sea ice freeze‐up later into the fall and may amplify Arctic sea ice loss. Key Points: During October, large surface cloud warming with values higher than 80 W m−2 occurs ∼+50% more often over open water than over sea iceCompared to October, November large surface cloud warming (>80 W m−2) occurs even more frequently (∼+200%) over open water than over sea iceMore frequent large surface warming caused by low‐level opaque clouds occurs as open water persists later into the fall [ABSTRACT FROM AUTHOR]