Mesosphere-to-stratosphere descent of odd nitrogen in February-March 2009 after sudden stratospheric warming

We use the 3-D FinROSE chemistry transport model (CTM) and Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) observations to study connections between atmospheric dynamics and middle atmospheric NOx (NOx = NO+NO2) distribution. Two cases are considered in the northern polar regions: (1) descent of mesospheric NOx in February-March 2009 after a major sudden stratospheric warming (SSW) and, for comparison, (2) early 2007 when no NOx descent occurred. The model uses the European Centre for Medium-Range-Weather Forecasts (ECMWF) operational data for winds and temperature, and we force NOx at the model upper altitude boundary (80 km) with ACE-FTS observations. We then compare the model results with ACE-FTS observations at lower altitudes. For the periods studied, geomagnetic indices are low, which indicates absence of local NOx production by particle precipitation. This gives us a good opportunity to study effects of atmospheric transport on polar NOx. The model results show no NOx descent in 2007, in agreement with ACE-FTS. In contrast, a large amount of NOx descends in February-March 2009 from the upper to lower mesosphere at latitudes larger than 60 degrees N, i.e. inside the polar vortex. Both observations and model results suggest NOx increases of 150-200 ppb (i.e. by factor of 50) at 65 km due to the descent. However, the model underestimates the amount of NOx around 55 km by 40-60 ppb. According to the model results, chemical loss of NOx is insignificant during the descent period, i.e. polar NOx is mainly controlled by dynamics. The descent is terminated and the polar NOx amounts return to pre-descent levels in mid-March, when the polar vortex breaks. The break-up prevents the descending NOx from reaching the upper stratosphere, where it could participate in catalytic ozone destruction. Both ACE-FTS observations and FinROSE show a decrease of ozone of 20-30% at 30-50 km from mid-February to mid-March. In the model, these ozone changes are not related to the descent but are due to solar activation of halogen and NOx chemistry.