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

Abstract

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 NO_x (NO_x = NO + NO₂) distribution. Two cases are considered in the northern polar regions: (1) descent of mesospheric NO_x in February–March 2009 after a major sudden stratospheric warming (SSW) and, for comparison, (2) early 2007 when no NO_x descent occurred. The model uses the European Centre for Medium-Range Weather Forecasts (ECMWF) operational data for winds and temperature, and we force NO_x 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 NO_x production by particle precipitation. This gives us a good opportunity to study effects of atmospheric transport on polar NO_x. The model results show no NO_x descent in 2007, in agreement with ACE-FTS. In contrast, a large amount of NO_x descends in February–March 2009 from the upper to lower mesosphere at latitudes larger than 60° N, i.e. inside the polar vortex. Both observations and model results suggest NO_x 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 NO_x around 55 km by 40–60 ppb. According to the model results, chemical loss of NO_x is insignificant during the descent period, i.e. polar NO_x is mainly controlled by dynamics. The descent is terminated and the polar NO_x amounts return to pre-descent levels in mid-March, when the polar vortex breaks. The break-up prevents the descending NO_x 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 NO_x chemistry.