Two Scales of Mixed Rossby-gravity waves in the lower stratosphere

Two scales of mixed Rossby-gravity (MRG) waves in the lower stratosphere are identified through space time spectral, cross spectral and Principal Component analysis of filtered dynamical fields from radiosonde and reanalysis data. The classical Yanai waves are readily identified, and have periods of around 4 days and zonal wavenumber four meridional wind and streamfunction structures centered on the equator, with antisymmetric zonal wind and geopotential as expected. However a faster mode, with a period of around three days is also identified, with a zonal wavenumber three structure. This latter mode is actually much more coherent in cross spectra between antisymmetric zonal wind, geopotential and temperature at 10 degrees north and south. This scale matches the equatorial Rossby radius expected of a disturbance with an equivalent depth of about 120 meters, as identified in space-time spectra of various dynamical quantities obtained from lower stratospheric radiosonde and reanalyses. Strong modulation of this MRG activity by the QBO is also seen for both modes. Evidence that the MRG modes are excited by tropical convection is presented, whereby coherent OLR anomalies over the central Pacific peak several days before the stratospheric MRG activity. The convection itself appears to be associated with the central Pacific standing mode in OLR that we have recently identified by, which is itself identified as a convectively coupled MRG-type disturbance in the troposphere.