Quantifying the impact of variable solar forcing on Sudden Stratospheric Warmings (SSWs)

Abstract

Sudden stratospheric warmings (SSWs) are characterised by the rise of polar stratospheric temperatures by several tens of kelvins. Here, we investigate the SSW of 2009 using the ECHAM/MESSy (EMAC) chemistry climate model. We study in particular how the SSWs are affected by variable solar forcing: EUV photo-ionisation that dominates the changes during high solar activity and geomagnetic storms. The warmings are preceded by a slowing then reversal of the westerly winds in the stratospheric polar vortex which then becomes easterly and it is also closely associated to polar vortex breakdown. 20 ensemble members are considered and different onset dates of the free running ensembles for the SSW event in January are tested to see the development of the polar vortex and its breakdown in the different ensemble members. Ionisation rates from the AISSTORM model are used in this case. And the results are compared with a geomagnetic storm (consisting of mostly electrons that are in the range of a few kilo-electron volts (keVs) to about 1 MeV) included on the day of the SSW, i.e., 25th of January. For the experiments considered here, the EUV photoionization was doubled and halved, and in both cases an increase in stratospheric temperature compared to the normal EUV was observed. Overall, effects of both EUV photoionization and particles on the temperature, wind fields, NOy and ozone in the middle atmosphere was observed. As ozone is one of the key species in radiative heating and cooling of the stratosphere, changes in its concentration can be linked to dynamical changes in the middle atmosphere.