What is the controlling factor of trends in upward shortwave radiation over the Arctic?

The upward shortwave (SW) radiative flux at the top of the atmosphere (TOA) has been decreasing over the Arctic. However, there are a few quantitative assessments of surface and atmospheric factors contributing to TOA SW radiative flux. In this study, we investigated the relative contributions of several parameters to the trend of TOA SW radiative flux over the Arctic region from 2001 to 2019, using satellite observation data, reanalysis data, and radiative transfer model.

To assess the relative contribution of several factors (i.e., cloud, aerosol, water vapor, sea ice), we calculated the difference between two types of TOA SW radiative flux. The former was that all input variables are monthly data. The latter was that one variable was monthly climatological data obtained from averaging monthly data throughout the analysis period and the others were monthly data. TOA SW radiative flux was computed by using the SBDART model. Input data were cloud optical thickness, aerosol optical thickness, total column water vapor, total column ozone, and surface albedo. The surface albedo was estimated from the sea ice concentration.

Over the Arctic, there are the large decreasing trends over the Beaufort, Kara, Laptev Seas, and increasing trends over the Barents Sea and the northern Scandinavian Peninsula in June. The contribution analysis over the Beaufort, Kara, Laptev Seas shows that the decreasing trend is largely explained by decreasing the sea ice concentration. On the other hand, the increasing trend over the Barents Sea is explained mainly by increasing cloud optical thickness. These results indicate that the sea ice and cloud variations are important factors for TOA SW radiative flux variations over the Arctic.