The Arctic is one of the cloudiest regions on the Earth. White carpet of clouds typically covers the Arctic during three quarters of year. In summertime, clouds reflect sunlight helping to keep the Arctic cool. In winter time, they capture and backscatter heat making the Arctic warmer than it could be under a clear sky. Figure 1. Historical variations since 1930 of the observed convective cloud fraction at the Wrangel Island station (redrawn from Chernokulsky and Esau, 2019). Temperature anomalies are taken from the CarbonBrief portal (https://www.carbonbrief.org/mapped-how-every-part-of-the-world-has-warme...). Satellite images (Terra/Aqua platform) are obtained from the NASA WorldView portal (https://worldview.earthdata.nasa.gov/). The stratiform cloud view (photo) is taken from http://vk.com/ostrovwrangelya.Today, more and more changes are observed in this white carpet as the warming of the Arctic advances. Sea ice loss opens millions of square kilometers of relatively warm water. Heat and moisture from the Arctic Ocean transform the white carpet making holes in it, as veil of stratiform clouds clumps more frequently into convective cloud cells. These and many other changes can be traced with a new integrated, quality controlled and systematically aggregated data set produced by the collaboration between the Norwegian Nansen Centre and the A.M. Obukhov Institute for Atmospheric Physics in Russia. For the first time, this data set cover the full extent of the historical cloud cover variability in the European and Asian Arctic. Now, this data set has been made freely available for all interested users. 

by Alexander Chernokulsky and Igor Esau

Climate scientists have recognized that clouds may be an unknown wildcard in the Earth’s climate system. Clouds have an impact on the surface heat budget, weather patterns and climate feedback mechanisms. Despite of this understanding, little was known about the Arctic clouds in previous cool and warm periods. Climate models are inaccurate in representation of the icy Arctic clouds. Satellite observations of clouds are covering only the most recent decades of global warming; and they have difficulties to distinct sea ice and low clouds. Nevertheless, we still can follow the transformations of the Arctic clouds observed from the cold beginning of the 20^th century through the early warm period of 1940s and subsequent recent cool period of 1960s to the unprecedented warming we witness today. As an example, Figure 1 illustrate the dramatic increase of convective clouds around Wrangel Island – the remote Arctic area where the sea ice loss has been the most pronounced in the 21^st century.

Analysis of our data set reveals that the Arctic cloudiness increases in the warm periods. However, increasing cloudiness might not counteract the amplified Arctic warming as one may expect. Firstly, increases in winter clouds have a clear warming effect as they capture the heat closer to the surface of the Earth. Secondly, clouds become clumpier leaving larger area of clear sky open to penetration of solar heating. Whether or not the changes in the Arctic clouds induce strong feedbacks in the regional climate is a scientific challenge to be addressed with improved and better calibrated climate models to which this data set could be a valuable reference.

Figure 2:: Figure 2. Seasonal strength of the links between several climate indices and the total cloud cover for the main three Eurasian parts of the Arctic. Dark red and blue colors signal stronger links. The black dots show the statistically significant links. The links are defied through the Mann-Kendall correlation coefficients.Our study of the cloud-to-climate connections has already found strong connections with the Atlantic weather types and climate indices. Figure 2 lists a number of climate indices, such as e.g. the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) – two index based on the surface level pressure difference between high- and low-latitudes, the meridional moisture transport over the Barents-Kara Seas (MT BK), and the local sea ice cover (SIC) in the Atlantic sector of the Arctic (BK) and its Siberian sector (LESC). Each colored square visualizes the strength of links (correlations) between the given index at the given station; the station names are listed below the panel. Dark red and blue squares show the strongest links. The NAO, AO, MT BK and other indices sensitive to the transport of heat and moisture to the high Arctic have the strongest direct effect on the cloud cover – additional heat and moisture increase cloudiness in all Eurasian Arctic regions reaching the most remote parts of the East Siberian and Chukchi coastal areas. Similarly, retreating sea ice also increases cloudiness in the region as the SIC BK and SIC LESC show with blue colors signaling anti-correlations. Although the strongest effects are local and more pronounced in the autumn season, clouds emerge over open water in other seasons as well.

Alexander Chernokulsky at the A.M. Obukhov Institute for Atmospheric Physics (Russia) and Igor Esau at the Nansen Center (Norway) have processed and published cloud observations from more than 100 meteorological stations in the Eurasian Arctic. Although most of the stations have made observations since 1930s, some of them began to report cloud cover and cloud forms since the end of 19^th century. Support from the Norwegian Research Council (#273463) have contributed to make the processed data set freely available at the Nansen Centre (https://www.nersc.no/project/gcloudl).

References

Chernokulsky, A. V., Esau, I., Bulygina, O. N., Davy, R., Mokhov, I. I., Outten, S., & Semenov, V. A. (2017). Climatology and Interannual Variability of Cloudiness in the Atlantic Arctic from Surface Observations since the Late Nineteenth Century. Journal of Climate, 30(6), 2103–2120. https://doi.org/10.1175/JCLI-D-16-0329.1

Chernokulsky, A., & Esau, I. (2019). Cloud cover and cloud types in the Eurasian Arctic in 1936–2012. International Journal of Climatology, (March), joc.6187. https://doi.org/10.1002/joc.6187