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The atmospheric chemistry general circulation model ECHAM5/MESSy is used tosimulate polar surface air temperature effects of geomagnetic activityvariations. A transient model simulation was performed for the years1960–2004 and is shown to develop polar surface air temperature patternsthat depend on geomagnetic activity strength, similar to previous studies. Inorder to eliminate influencing factors such as sea surface temperatures (SST)or UV variations, two nine-year long simulations were carried out, withstrong and weak geomagnetic activity, respectively, while all other boundaryconditions were held to year 2000 levels. Statistically significanttemperature effects that were observed in previous reanalysis and modelresults are also obtained from this set of simulations, suggesting that suchpatterns are indeed related to geomagnetic activity. In the model, stronggeomagnetic activity and the associated NO<sub>x</sub> (= NO + NO<sub>2</sub>)enhancements lead to polar stratospheric ozone loss. Compared with the simulation with weakgeomagnetic activity, the ozone loss causes a decrease in ozone radiativecooling and thus a temperature increase in the polar winter mesosphere.Similar to previous studies, a cooling is found below the stratopause, whichother authors have attributed to a decrease in the mean meridionalcirculation. In the polar stratosphere this leads to a more stable vortex. Astrong (weak) Northern Hemisphere vortex is known to be associated with apositive (negative) Northern Annular Mode (NAM) index; our simulationsexhibit a positive NAM index for strong geomagnetic activity, and a negativeNAM for weak geomagnetic activity. Such NAM anomalies have been shown topropagate to the surface, and this is also seen in the model simulations. NAManomalies are known to lead to specific surface temperature anomalies: apositive NAM is associated with warmer than average northern Eurasia andcolder than average eastern North Atlantic. This is also the case in oursimulation. Our simulations suggest a link between geomagnetic activity,ozone loss, stratospheric cooling, the NAM, and surface temperaturevariability. Further work is required to identify the precise cause and effect of the coupling between these regions

atmospheric chemistry and physics

Geomagnetic activity related NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; enhancements and polar surface air temperature variability in a chemistry climate model: modulation of the NAM index