Speaker
Description
Whether there is association between solar activity and tropospheric circulation, is still an open research question. Various characteristics of circulation have been analyzed in this respect, including its composites and correlation / regression fields, teleconnections (such as the North Atlantic Oscillation), position and duration of blocking events, and frequency of circulation types. Although many studies suggest that associations between solar activity and circulation are significant particularly in the European-North Atlantic sector, there are many unknowns, such as the temporal stability of such associations and the degree to what they may be (or may be not) mistaken for a response to other forcings, most notably to sudden stratospheric warmings.
Here we present an analysis, which is based on Hess-Brezowsky (H-B) catalogue of circulation types. In the catalogue, every day is assigned to one of 29 circulation types according to the airflow direction, position of cyclones and anticyclones, (anti)cyclonicity of airflow, etc., over central Europe. Two versions of the H-B catalogue are utilized as a source of data on tropospheric circulation: original subjective (starting from 1881), and modified objective (starting from 1851). The description of atmospheric circulation in terms of the H-B catalogue has the advantages of covering a much longer time span than what full-input atmospheric reanalyses allow, and of being independent of surface-input atmospheric reanalyses, which – although also cover periods extending into the 19th century – do not incorporate potential solar effects coming from above via interactions with stratospheric ozone.
The analysis of frequencies of H-B circulation types conditioned by levels of solar activity (expressed as sunspot numbers) and/or phases of solar cycle shows a significant association between solar activity and tropospheric circulation over central Europe. Under low solar activity, westerly types tend to be less frequent, and northerly and easterly types more frequent. Anticyclonic situations occur more frequently under low solar activity, compensating for the reduced frequency of cyclonic types. However, the relationships between solar activity and frequencies of H-B types change over time. They are particularly significant since the 1940s and in the early part of the analyzed period shortly after the middle of the 19th century. These periods of significant associations coincide with high solar activity cycles. This suggests that it is the level of solar activity, rather than the phase of the solar cycle, that plays a crucial role in the relationship between solar activity and tropospheric circulation. That is, weak solar cycles do not allow the association with tropospheric circulation to develop. Taken from a different perspective, the weakening or even lack of the solar-to-circulation association, which is observed in the early 20th century, does not mean that what we observe as an association is in fact random (which some studies seem to suggest); instead, it is an impact of solar cycles being weak.