Linear wavetrains in models of the stratosphere

Abstract

AbstractExperiments with a primitive equation model of the stratosphere and mesosphere are described which investigate the effect on the stratosphere of the growth of localized, stationary forcing in the troposphere. Two basic states are used which are representative of stratospheric flow in early and late winter in the southern hemisphere. Simple mechanistic models are then introduced to examine the wave phenomena which occur during the first 10 days of the experiments when the dynamics are known to be linear.The localized forcing grows to a peak amplitude of 600 gpm and half‐width of 15° of latitude, values typical of the mid‐latitude upper troposphere. Stratospheric travelling waves are excited with phase velocities which are sensitive to the zonal mean winds and to the speed of the switch‐on of the forcing, and wave activity spreads quickly from the localized forcing with a typical vertical group velocity of 10 km d−1. Strong horizontal curvature in the basic states can profoundly affect the vertical and horizontal structures of the wavetrains: mid‐latitude regions of negative northward potential vorticity gradient in the stratosphere (as seen in the southern hemisphere in late winter) lead to the confinement and amplification of waves in high latitudes. Three‐dimensional diagnostics of wave activity propagation reveal wave reflections missed by zonally‐averaged EP flux diagnostics.Significant wave reflection and trapping occur on the flow from late winter, eventually producing a steady linear wavetrain with an intense local centre of circulation in the upper stratosphere, the maximum anomaly being ∼1000gpm at 1mb. It is argued that such wavetrains are unrealistic because nonlinear ageostrophic winds drive the dynamics away from any linear steady state.