Observational constraints on Arctic Ocean clouds and radiative fluxes during the early 21st century

摘要

Arctic Ocean observations are combined to create a cloud and radiation climatology for the early 21st century (March 2000 to February 2011). Data sources include: active (CloudSat, CALIPSO) and passive (MODIS) satellite cloud observations, observed top-of-atmosphere (TOA) radiative fluxes (CERES-EBAF), observationally constrained radiative flux calculations (2B-FLXHR-LIDAR), and observationally constrained cloud forcing calculations (CERES-EBAF, 2B-FLXHR-LIDAR). Uncertainty in flux calculations is dominated by cloud uncertainty, not surface albedo uncertainty. The climatology exposes large geographic, seasonal, and interannual variability cloud forcing, but on average, Arctic Ocean clouds warm the surface (+10 W m~(-2), in 2B-FLXHR-LIDAR) and cool the TOA (-12 W m~(-2), in CERES-EBAF and 2B-FLXHR-LIDAR). Shortwave TOA cloud cooling and longwave TOA cloud warming are stronger in 2B-FLXHR-LIDAR than in CERES-EBAF, but these two differences compensate each other, yielding similar net TOA values. During the early 21st century, summer TOA albedo decreases are consistent with sea ice loss but are unrelated to summer cloud trends that are statistically insignificant. In contrast, both sea ice variability and cloud variability contribute to interannual variability in summer shortwave radiative fluxes. Summer 2007 had the largest persistent cloud, radiation, and sea ice anomalies in the climatology. During that summer, positive net shortwave radiation anomalies exceeded 20 W m~(-2) over much of the Arctic Ocean. This enhanced shortwave absorption resulted primarily from cloud reductions during early summer and sea ice loss during late summer. In summary, the observations show that while cloud variability influences absorbed shortwave radiation variability, there is no summer cloud trend affecting summer absorbed shortwave radiation. Key Points New Arctic Ocean cloud/radiation constraints are analyzed for the early 21st C Sea ice loss, not clouds, explain increased summer absorbed shortwave ('00-'10) Cloud and sea ice variability explain summer absorbed SW variability (esp. '07)