Changes in cloudiness cause a changing seasonality in the Baltic Sea region

In the Baltic Sea region, a significant decrease and subsequent increase in solar radiation have been detected during the past half-century. But the rise in shortwave irradiance is not seen for all seasons; significant changes appear in the seasonality of the cumulative sum of daily shortwave irradiance and the sea surface temperature of the Baltic Sea. Kahru et al. (2016) show that the accumulated surface incoming shortwave (SIS) energy has decreased in winter and increases during the spring and summer. The cumulative thresholds of surface incoming shortwave irradiance up to 1000 W/m² are reached later in the season, but higher thresholds are reached earlier. The shift from later towards earlier cumulative thresholds occurs in spring, around March 15.

Changes in shortwave irradiance are associated with atmospheric transparency and cloudiness parameters like cloud fraction and albedo. The more substantial factor here is cloudiness, and therefore, we concentrate on reasons for changes in cloud properties. One of the most important reasons here is the synoptic-scale atmospheric circulation. The satellite-based cloud climate data record CLARA-A2 has been used to analyse regional time series and trends in the Baltic Sea region, from 1982-2019. The investigated cloud parameters were total fractional cloud cover (CFC) and SIS.

In March the interannual variability in CFC is high. The Increasing trend in incoming shortwave radiation could be explained by the decrease in CFC. The decrease in CFC is due to a smaller number of overcast days, that vary in the same rhythm with “cloudy” circulation patterns. This shows, that the shift in seasons that is connected to the earlier accumulated sums of SIS is at least partly explained by the changes in synoptic-scale atmospheric circulation.