Title: Modelling the descent of nitric oxide during the elevated stratopause event of January 2013,
Abstract: Using simulations with a whole-atmosphere chemistry-climate model nudged by
meteorological analyses, global satellite observations of nitrogen oxide (NO)
and water vapour by the Sub-Millimetre Radiometer instrument (SMR), of
temperature by the Microwave Limb Sounder (MLS), as well as local radar
observations, this study examines the recent major stratospheric sudden warming
accompanied by an elevated stratopause event (ESE) that occurred in January
2013. We examine dynamical processes during the ESE, including the role of
planetary wave, gravity wave and tidal forcing on the initiation of the descent
in the mesosphere-lower thermosphere (MLT) and its continuation throughout the
mesosphere and stratosphere, as well as the impact of model eddy diffusion. We
analyse the transport of NO and find the model underestimates the large descent
of NO compared to SMR observations. We demonstrate that the discrepancy arises
abruptly in the MLT region at a time when the resolved wave forcing and the
planetary wave activity increase, just before the elevated stratopause reforms.
The discrepancy persists despite doubling the model eddy diffusion. While the
simulations reproduce an enhancement of the semi-diurnal tide following the
onset of the 2013 SSW, corroborating new meteor radar observations at high
northern latitudes over Trondheim (63.4$^{\circ}$N), the modelled tidal
contribution to the forcing of the mean meridional circulation and to the
descent is a small portion of the resolved wave forcing, and lags it by about
ten days.