Numerical Simulation of Sudden Stratospheric Warmings

Abstract

A mechanistic, quasi-geostrophic, semi-spectral model with a self-consistent calculation of the mean zonal flow fields is used to numerically simulate sudden stratospheric warmings generated by a single zonal harmonic (m) planetary wave. The development of a warming depends critically on the two factors which govern the transmission of planetary waves to the upper stratosphere: 1) the strength of the westerly winds in the lower stratosphere and 2) the magnitude of wave damping in the same region. Major warmings can only develop when the prewarming lower stratospheric winds and tropospheric forcing are strong but insufficient to trap the wave at low altitudes. Damping controls the maximum amplitude that a warming can attain and the time constant for its growth rate. The growth rate of a m = 2 warming is accelerated during the westerly zonal wind phase of the quasi-biennial oscillation, but the maximum amplitude of the warming is independent of QBO phase. The evolution of m = 1 and m = 2 warmings are very different. An m = 1 warming is characterized by pronounced oscillation of wave amplitude and mean flow that result from resonantly trapped, westward propagating planetary waves moving in and out of phase with the tropospherically forced stationary planetary wave. This oscillation can reach sufficient amplitude to decelerate the zonal flow during a cycle to easterlies and create a critical level. Although formation of a mesopheric critical level is not required to initiate a warming, the development and propagation of critical levels in the middle atmosphere is central to the evolution of sudden warmings. During an m = 1 event the critical level forms initially in the polar region and advances equatorward in its development. But a m = 2 critical level first develops in the equatorial region and advances poleward. A m = 2 warming is also characterized by a sudden intensification after an initially slow growth in contrast to slowly developing m = 1 warmings. Both m = 1 and m = 2 warmings are accompanied by polar mesospheric cooling, low-latitude stratospheric cooling and equatorial mesospheric heating as a result of the induced secondary circulation in response to eddy heat transport to the polar stratosphere. Long-term integrations with a steady forced m = 1 wave show that the mean flow evolves to a steady, asymptotic state with net cooling in the polar mesosphere from planetary waves. But steady m = 2 forcing of greater than approximately 200 m at 300 mb leads to multiple generation of warmings which are similar to stratospheric vacillations.