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

Abstract

Abstract. Ensemble simulation of the atmospheric general circulation at altitudes up to the lower thermosphere is performed using the 3-D nonlinear mechanistic numerical model MUAM. The residual mean meridional circulation (RMC), which is the superposition of the mean Eulerian and wave-induced eddy components, is calculated for the boreal winter. Changes in the vertical and meridional RMC velocity components are analysed at different stages of a simulated composite sudden stratospheric warming (SSW) event averaged over 19 model runs. The simulation results show a general decrease in RMC velocity components up to 30 % during and after SSW in the mesosphere and lower thermosphere of the Northern Hemisphere. There are also increases in the downward and northward velocities at altitudes of 20–50 km at the northern polar latitudes during SSW. Associated vertical transport and adiabatic heating can contribute to warming the stratosphere and downward shifting of the stratopause during the composite SSW. The residual mean and eddy mass fluxes are calculated for different SSW stages. It is shown that before the SSW, planetary wave activity creates wave-induced eddy circulation cells in the northern upper stratosphere, which are directed upwards at middle latitudes, northward at high latitudes and downwards near the North Pole. These cells increase heat transport and adiabatic heating in the polar region. During SSW, the region of upward eddy vertical velocity is shifted to high latitudes, but the velocity is still downward near the North Pole. After SSW, upward eddy-induced fluxes span the entire polar region, producing upward transport and adiabatic cooling of the stratosphere and providing the return of the stratopause to higher altitudes. The obtained statistically significant results on the evolution of RMC and eddy circulation at different SSW stages at altitudes up to the lower thermosphere can be useful for a better understanding the mechanisms of planetary wave impacts on the mean flow and for the diagnostics of the transport of conservative tracers in the atmosphere.

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

“…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): $$$\bar{v}\,\ast =\bar{v}-\frac{1}{\partial \bar{\theta }/\partial z}\left(-\frac{\bar{{v}^{\prime }{\theta }^{\prime }}}{H}+\frac{\partial \bar{{v}^{\prime }{\theta }^{\prime }}}{\partial z}-\frac{\bar{{v}^{\prime }{\theta }^{\prime }}}{\sfrac{\partial \bar{\theta }}{\partial z}}\frac{{\partial }^{2}\bar{\theta }}{\partial {z}^{2}}\right),$$$ $$$\bar{w}\,\ast =\bar{w}+\frac{1}{a\,cos\,\varphi }\frac{1}{\sfrac{\partial \bar{\theta }}{\partial z}}\left(-sin\,\varphi \bar{{v}^{\prime }{\theta }^{\prime }}+cos\,\varphi \left(\frac{\partial \bar{{v}^{\prime }{\theta }^{\prime }}}{\partial \varphi }-\frac{\bar{{v}^{\prime }{\theta }^{\prime }}}{\partial \bar{\theta }/\partial z}\frac{{\partial }^{2}\bar{\theta }}{\partial z\partial \...$$…”

“…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).…”

“…Starting from the 330th model day, seasonal changes in the zenith angle of the Sun are launched, and days 330-390 correspond to January-February. 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).…”

Numerical simulation of stratospheric QBO impact on the planetary waves up to the thermosphere

“…The MUAM includes a parameterization of thermal and dynamical effects produced by mountain waves 32 and a spectrum of non-orographic gravity waves 33 . Numerous analyses have been carried out to validate MUAM, which have shown that it correctly reproduces the structure of the atmospheric circulation, PWs, and tides (e.g., 8,9,21,34,35 ), sudden stratospheric warmings 36 , as well as the resonance properties of the atmosphere (normal atmospheric modes) 20 .…”