Genesis of Intraseasonal Oscillations and Equatorial Waves

Abstract

The particular role of evaporation?wind feedback, and as well cumulus convection and dissipation, in the formation of the Madden?Julian 30?60-day intraseasonal oscillation (MJO) and equatorially trapped waves, including Kelvin, equatorial Rossby, mixed Rossby?gravity, and eastward inertio-gravity waves, has been studied using a global two-level primitive equation instability model. The evaporation has been specified through a bulk aerodynamic formula, and the convection through a generalized Kuo-type parameterization with a moist static stability that is positive everywhere so that wave-conditional instability of the second kind (CISK) is not possible. Both three-dimensional and zonally averaged basic states for January 1979 have been employed, and the e-folding times, periods, structures, and propagation characteristics of these waves have been analyzed and compared with their observed properties. The MJO modes appear to be particularly realistic when the three-dimensional basic state includes evaporation?wind feedback, which promotes the zonal wavenumber-1 component of the eastward-propagating velocity potential, and cumulus convection. The tropical velocity potential propagates eastward relatively quickly around the globe (≈13.6 m s?1, corresponding to a period of 34.4 days), while in the convective region between the Indian Ocean and the date line the divergence propagates more slowly (≈4.5 m s?1). The MJO has a largely first internal mode structure in the Tropics with a more equivalent barotropic structure of the streamfunction in the extratropics, including distinct Pacific?North American and Eurasian patterns. The MJO modes do not lie along any of the theoretical dispersion curves of equatorially trapped waves and have distinctly longer periods than the model convectively coupled Kelvin waves, even at zonal wavenumber 1, as in the observational study of M. Wheeler and G. N. Kiladis. The Kelvin mode streamfunction fields also do not exhibit the largely equivalent barotropic teleconnection patterns in the extratropics typical of the MJO. With cumulus convection and evaporation?wind feedback, the model equatorially trapped Kelvin, equatorial Rossby, mixed Rossby?gravity, and eastward inertio-gravity waves have periods, structures, and propagation characteristics comparable to those of corresponding convectively coupled observed waves.