Modulation of Stratospheric Kelvin Wave Activity from the Troposphere

The structure and time variability of stratospheric Kelvin waves is isolated through an EOF analysis of space-time filtered zonal wind. The frequency spectrum of equatorial zonal wind at 50 hPa reveals peaks at periods of around 14 days associated with two scales of Kelvin waves, as is well-known. The leading mode has a zonal wavenumber one structure and the second mode is dominated by zonal wave two, although the latter structure propagates at roughly half the phase speed of the first, yielding nearly the same period. Apart from the well-documented variability associated with the stratospheric quasi-biennial oscillation, there is large interannual, seasonal, and intraseasonal variability in the activity of these modes. The leading structure displays a strong association with convectively-coupled Kelvin wave activity in the troposphere, especially during northern winter. Both modes are also apparently modulated by the El Nino-Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO). In the case of ENSO, changes in the basic state and perhaps the location of convective forcing lead to distinct changes in the vertical propagation characteristics and consequently result in longitudinal variations of Kelvin activity within the stratosphere. Some of this variability is likely related to the static stability and zonal wind properties near the tropical tropopause layer (TTL).