Two Scales of Mixed Rossby-gravity and Kelvin Waves in the lower Stratosphere

Abstract:

Two scales of mixed Rossby-gravity (MRG) and Kelvin 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. A standard covariance matrix EOF analysis of filtered wind along the equator at 50 hPa is used to isolate MRG and Kelvin waves, using ERA Interim analyses for the entire period 1979-2012. This approach uses 2-6 day meridional wind for MRG waves and 10-25 day eastward zonal wind for Kelvin waves as a basis for the two independent EOF analyses, respectively. Raw wind, geopotential height and CLAUS brightness temperature are then projected onto the EOF PCs at all levels from 1000 to 1 hPa to obtain the structure and evolution of the waves. In all cases EOF pairs are obtained corresponding to propagating modes. The classical Yanai MRG 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. Kelvin waves are similarly identified using equatorial zonal wind filtered for 10-25 day eastward travelling fluctuations. Two scales are evident here: a wave one and also a wave two disturbance, each with periods of around 14 days. Evidence that the Kelvin and 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.