J4.3
Three-dimensional structure and evolution of the barotropic and baroclinic components of the MJO

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Wednesday, 5 February 2014: 9:00 AM
Room C114 (The Georgia World Congress Center )
Ángel F. Adames, University of Washington, Seattle, WA; and J. M. Wallace

The two leading principal components of the unfiltered daily 150 hPa minus 850 hPa global velocity potential in the ERA Interim (1979-2011) data are used as time-varying indices of the Madden Julian Oscillation (MJO). Regression maps and cross sections based on these indices are used to document the structure and evolution of the zonal wind (u) and geopotential height (Z) anomalies in the MJO cycle. The MJO vertical structure is decomposed into baroclinic and barotropic modes by performing maximum covariance analysis on vertical profiles in the stack of regression maps for u and Z in the equatorial belt. The baroclinic mode is dominated by an equatorially-trapped Kelvin wave flanked by Rossby waves that propagate eastward in unison, circumnavigating the globe in a full MJO cycle. The weaker barotropic mode consists of equatorially-symmetric Rossby waves only. The primary centers of the Rossby gyres lie ∼28°N/S, in regions of strong upper level westerlies, while the Kelvin wave is confined to an equatorial channel in which absolute vorticity is very small. While the u anomalies in the Rossby waves are passing through the climatological-mean jet exit region in the central Pacific, the waves extract kinetic energy from the background flow, giving rise to strong Z anomalies that project upon the Pacific / North American (PNA) pattern. This occurs in the phase of the MJO cycle when enhanced / suppressed convection is passing over the Marine Continent. The climatological zonal mean flow is thus important in determining the structure of the MJO.