Commentary: Energetic particle forcing of the Northern Hemisphere winter stratosphere: comparison to solar irradiance forcing

The latest solar cycle minimum sometime around 2009 showed unusually low solar activity and suggested the possibility of a grand solar minimum in the near future [1]. This event caused much attention to be focused on studies regarding solar influence on the Earth's climate. The recent review by Gray et al. [2] classified solar forcing on the Earth's climate to be of four types: galactic cosmic rays, total solar irradiance (TSI), solar ultraviolet radiation (UV), and energetic particle precipitations (EPP). Although EPP has not attracted much attention compared with TSI and UV in the past, several recent studies indicate that EPP could have a significant impact on the Earth's climate, comparable with that of TSI and UV [3–5]. Seppälä and Clilverd ([6]; hereafter SC14) divided the past 54 boreal winters into solar maximum, solar minimum, and high energetic particle forcing (EPF) years based on the F10.7 radio flux and the Ap index. SC14 produced composite figures for each group using the latest meteorological reanalysis data. SC14 identified a downward-propagating zonal wind anomaly during high EPF years and attributed it to the top-down mechanism common to the downward-propagating zonal wind anomaly induced by UV variations [7]. Although the stimulating results of SC14 are attracting much attention, I would like to point out that there is a significant error in these results. Figure 2 in SC14 shows composites of monthly-and zonal-mean zonal wind anomalies for October through February for high EPF, solar minimum, and solar maximum conditions. Figure 1 of this manuscript was created using the same method (i.e., a composite of monthly-and zonal-mean zonal wind anomalies in each month for each condition) and the same blended ERA-40 [9]) reanalysis data. It has been found that these two figures (i.e., Figure 2 of SC14 and Figure 1 of this manuscript) are the same in January and February, but clearly different from October through December. However, the figures from October through December become the same when those data are shifted back by 1 year as shown in Figure S1 in the Supplementary Material. This indicates that the analysis in SC14 was applied not to five consecutive months (i. This error has a significant impact on the results presented in SC14. In particular, no significant difference between the high EPF and the solar maximum conditions is observed in November, which indicates that the difference of circulation anomalies between the high EPF and the solar …