Wintertime Cooling of the Arctic TOA by Low-Level Clouds

Globally, clouds are known to warm the climate system in the thermal infrared because they typically emit thermal radiation to space at effective temperatures lower than the combined cloud-free atmosphere and surface. However, here we show that & SIM;40% of low-level clouds over sea ice tend to cool the Arctic system at TOA and contribute to a radiative cooling of the Arctic winter climate by -2.3 Wm-2, or a & SIM;16% reduction over the infrared warming effect of all clouds during winter. Based on satellite observations, low-level clouds residing in surface-based temperature inversions emit more longwave radiation to space than would occur in cloudless skies. While these clouds are known to significantly warm the surface, they cool the Arctic climate system overall. Our results imply that accurately representing the cloud radiative effects unique to the Arctic could help to constrain the regional energy budget. The Arctic has become emblematic of climate change, with rapid warming that is at least twice as fast as the rest of the planet. However, major uncertainties in our confidence to understand and predict Arctic climate persist, particularly regarding the radiative effects of clouds. Here we use satellite data to quantify the radiative effects of Arctic low-level clouds, and find approximately 40% of low-level clouds over sea ice tend to radiatively cool the Arctic climate system (Earth's surface and the atmosphere) in winter, rather than warm the climate system as is typical for most clouds in the thermal infrared regime. This cooling effect is governed by the widespread surface temperature inversions (layers in which temperature increases with altitude), which cap these low-level clouds and allow more longwave radiation to escape from the Earth to space compared to clear skies. This finding reveals a fundamental, but overlooked, characteristic of cloud radiative effects in the wintertime Arctic and establishes a new perspective for understanding Arctic climate change. A full range of radiative effects for Arctic wintertime low clouds over sea ice is investigatedAbout 40% of low clouds over sea ice tends to cool the Arctic at the top of the atmosphere in the polar nightThese low clouds with a cooling effect at the top of the atmosphere reside within frequent surface-based temperature inversions