Abstract
Abstract. The atmospheric chemistry general circulation model ECHAM5/MESSy is used to simulate polar surface air temperature effects of geomagnetic activity variations. A transient model simulation was performed for the years 1960–2004 and is shown to develop polar surface air temperature patterns that depend on geomagnetic activity strength, similar to previous studies. In order to eliminate influencing factors such as sea surface temperatures (SST) or UV variations, two nine-year long simulations were carried out, with strong and weak geomagnetic activity, respectively, while all other boundary conditions were held to year 2000 levels. Statistically significant temperature effects that were observed in previous reanalysis and model results are also obtained from this set of simulations, suggesting that such patterns are indeed related to geomagnetic activity. In the model, strong geomagnetic activity and the associated NOx (= NO + NO2) enhancements lead to polar stratospheric ozone loss. Compared with the simulation with weak geomagnetic activity, the ozone loss causes a decrease in ozone radiative cooling and thus a temperature increase in the polar winter mesosphere. Similar to previous studies, a cooling is found below the stratopause, which other authors have attributed to a decrease in the mean meridional circulation. In the polar stratosphere this leads to a more stable vortex. A strong (weak) Northern Hemisphere vortex is known to be associated with a positive (negative) Northern Annular Mode (NAM) index; our simulations exhibit a positive NAM index for strong geomagnetic activity, and a negative NAM for weak geomagnetic activity. Such NAM anomalies have been shown to propagate to the surface, and this is also seen in the model simulations. NAM anomalies are known to lead to specific surface temperature anomalies: a positive NAM is associated with warmer than average northern Eurasia and colder than average eastern North Atlantic. This is also the case in our simulation. Our simulations suggest a link between geomagnetic activity, ozone loss, stratospheric cooling, the NAM, and surface temperature variability. Further work is required to identify the precise cause and effect of the coupling between these regions.
Highlights
Solar variability manifests itself in several different ways
The surface air temperature effect found by Seppalaet al. (2009) triggered the present study, and an analogous analysis was carried out in that the dataset was separated according to the yearly average wintertime Ap index, used as proxy for the overall geomagnetic activity level
Seppalaet al. (2009) used reanalysis data to show that winter surface air temperatures divided up in years of low and high geomagnetic activity show significant temperature anomalies similar to Northern Annular Mode (NAM) patterns found at high latitudes
Summary
Solar variability manifests itself in several different ways. Amongst them is the variability of the solar wind, a stream of mainly electrons and protons that are able to escape the sun’s upper atmosphere. In sun-earth connection studies EPPNOx has not been regarded as important as variations in ultraviolet irradiance, which can exceed 50 % at some wavelengths Such variations have been shown to lead to stratospheric ozone changes and induce temperature variations over the 11 year solar cycle A number of publications have addressed the possible connections of changes in polar climate and solar or geomagnetic activity, but they generally do not consider EPP-NOx. For example, Boberg and Lundstedt (2002) have suggested a link between the electric field strength of the solar wind and the phase of the North Atlantic Oscillation (NAO). Lu et al (2008) investigated EPP-NOx influences on springtime polar stratospheric dynamics using the ERA40 data set Their results suggested that changes observed in stratospheric winds and temperatures were unlikely to be caused in situ in the stratosphere by EPP-NOx but were rather due to an indirect dynamical link, e.g. wave activity. Southern Hemisphere (SH) results are more difficult to interpret and are only briefly discussed here, warranting further studies
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