Modelling the residual mean meridional circulation at different stages of stratospheric warming events

Abstract

Abstract. Ensemble simulation of the general atmospheric circulation of the middle and upper atmosphere up to the lower thermosphere is performed using the 3-D nonlinear mechanistic numerical model MUAM. Residual mean meridional circulation (RMC) in terms of the Transformed Eulerian Mean is calculated for the boreal winter and changes in its vertical and meridional velocity components during different phases of simulated composite stratospheric warming (SW) events are studied. The simulation results show general decrease in RMC velocity components up to 30 % during and after SW in the mesosphere and lower thermosphere of the Northern Hemisphere. There are also increases in the downward and northward velocities at altitudes 50–70 km at the northern high latitudes. Associated changes in adiabatic heating/cooling rates can contribute to heating the stratosphere and cooling the mesosphere during the composite SW. The changes in the transport of conservative species (like ozone) during SWs are estimated. Weakening of ozone fluxes at the middle latitudes of the Northern Hemisphere may reach 30 % during SWs and 30–40 % after the events at the altitudes of stratospheric maximum of ozone concentration. Such statistically confident simulations of RMC reactions on SWs at altitudes up to the lower thermosphere are performed for the first time. The study of the residual meridional circulation is useful for effective analysis of wave impacts on the mean flow and for diagnostics of the transport of atmospheric gas species in the atmosphere.

Dynamical Impacts of Stratospheric QBO on the Global Circulation up to the Lower Thermosphere

“…A detailed description of the processes taken into account in the current version of the MUAM and the scheme of a numerical experiment can be found in Koval et al. (2021).…”

“…Nowadays, there is a program that combines interactive models to compare their ability to reproduce the QBO (Butchart et al., 2018). As it was stated above, the MUAM is not able to reproduce self‐consistently stratospheric QBO (unlike major SSW events; Koval et al., 2021), hence the simulations require specifying background and initial hydrodynamic fields for years with different QBO phases.…”

“…For better visibility, the vertical component is multiplied by 200. The meridional and vertical components of the residual mean meridional circulation in terms of the transformed Eulerian mean (TEM) are obtained from the ensembles of hydrodynamic fields simulated with the MUAM using standard formulas (e.g., Koval et al., 2021): $$$\bar{v}\,\ast =\bar{v}-\frac{1}{\partial \bar{\theta }/\partial z}\left(-\frac{\bar{{v}^{\prime }{\theta }^{\prime }}}{H}+\frac{\partial \bar{{v}^{\prime }{\theta }^{\prime }}}{\partial z}-\frac{\bar{{v}^{\prime }{\theta }^{\prime }}}{\sfrac{\partial \bar{\theta }}{\partial z}}\frac{{\partial }^{2}\bar{\theta }}{\partial {z}^{2}}\right),$$$ $$$\bar{w}\,\ast =\bar{w}+\frac{1}{a\,cos\,\varphi }\frac{1}{\sfrac{\partial \bar{\theta }}{\partial z}}\left(-sin\,\varphi \bar{{v}^{\prime }{\theta }^{\prime }}+cos\,\varphi \left(\frac{\partial \bar{{v}^{\prime }{\theta }^{\prime }}}{\partial \varphi }-\frac{\bar{{v}^{\prime }{\theta }^{\prime }}}{\partial \bar{\theta }/\partial z}\frac{{\partial }^{2}\bar{\theta }}{\partial z\partial \...$$…”