Explaining observed surface radiation trends 2001–2023 on Alaska's North Slope

Abstract. Changes in surface radiation are a key contributor to Arctic surface warming. However, the details of Arctic surface radiation change are not fully known due to a lack of multi-decadal observations. Here, we leverage 23 years (2001–2023) of ground-based observations at two neighboring coastal tundra sites on the North Slope of Alaska (NSA) to explain surface radiation change. US Department of Energy Atmospheric Radiation Measurement (ARM) and National Oceanic and Atmospheric Administration (NOAA) facilities in Utgiagvik, Alaska have documented warming (0.9 K/decade since 2001) and surface radiation change. Surface downwelling longwave radiation increases year-round. Surface downwelling and net shortwave radiation decrease during sunlit months (April–September). Decreasing net shortwave radiation is driven by changing cloud properties and increasing surface albedo over exposed tundra in mid-summer. Increasing downwelling longwave radiation during both sunlit and dark months (October–March) is driven by atmospheric warming, increasing water vapor, and changing cloud properties. Atmospheric warming and increasing water vapor explain about half the total observed increase in downwelling longwave radiation, while changing cloud properties explain the rest. Assessed cloud drivers (liquid water path, cloud cover, and low cloud fraction) explain increasing longwave radiation during sunlit months but are insufficient during dark months. Changes in assessed cloud drivers increased longwave radiation in all seasons except SON, when there was no change. These results reveal substantial surface radiative changes on the NSA, where cloud radiative effect changes amplify surface warming during dark months and dampen it during sunlit months.