Coherent Wave–Zonal Mean Flow Interactions in the Troposphere

Abstract

Spatial structure and temporal evolution of synoptic time scale variations of the tropospheric zonal mean flow are studied by extensive cross-correlation analyses, and the degree to which observations agree with two-dimensional, adiabatic theory is determined. Observational data from seven years of operational daily global analyses are studied, along with data from the NCAR general circulation model. Observed zonal mean zonal wind and temperature tendencies are compared with analyzed adiabatic forcing terms. Although significant correlations are found throughout the extratropics, there are significant equation residuals in both operational analyses and model data. Model momentum residuals result from calculational inaccuracies (interpolation to pressure surfaces and spectral aliasing of nonlinear terms) and biases introduced by once daily sampling; diabatic terms are also important for the daily thermodynamic balance. Coherent wave-zonal mean flow interactions are revealed via cross-correlation analyses, including fluctuations in zonal mean temperature, three-dimensional winds, and quadratic wave quantities. Equatorward propagating wavelike patterns in the meridional plane are observed for both zonal wind and temperature tendencies. These patterns result from midlatitude baroclinic-wave life cycles, and the signatures associated with wave growth and decay are revealed with novel detail. New observed features of baroclinic wave life cycles shown here include coherent fluctuations of the extratropical mean meridional circulation (Ferrel cells), and equatorward propagation of midlatitude wave activity (Rossby-wave radiation) as far as the equator. An individual case study is presented to show the variability associated with a particular event.