In a previous paper, Tomas and Webster documented large scale, quasi-stationary oscillations about the equator with a period of about 4 days. The oscillations were first observed in the space-time spectra of the meridional component of the divergent wind from the European Center for Medium Range Weather Forecast (ECMWF) reanalyses. This quantity was used to produce indices that provided the basis for the construction of composite averaged data fields. A detailed analysis of the composite fields revealed that many properties of the oscillation were consistent with mixed Rossby gravity/inertia gravity modes from shallow water theory. The signature of the modes was found in composite fields of the outgoing longwave radiation (OLR), which are essentially independent of the reanalyses.

We sought to determine the origin of these modes in the reanalyses and to this end, we analyzed the time dependent energetics. We were able to come to the following conclusions:

1) The modes are of tropical origin and are not forced by the mid-latitudes. 2) The forecast model, which provides the first guess field for the reanalyses, appears to simulate the growth and decay of the modes fairly well. 3) Convective heating, as parameterized by the forecast model, is an important source of energy for the modes. However, the degree to which convective heating forces the modes appears to be underestimated. 4) The incorporation of data into the analyses is an important energy source for the modes and makes up for shortcomings in the first guess simulation. 5) The modes are wave-CISK like in that the low level convergence owing to the circulation leads to convection and the heating owing to convection is an energy source for the circulation. 6) The conversion of energy from latent heating to the circulation is extremely inefficient. 7) In some cases, the latent heat energy source is immediately converted into disturbance kinetic energy. In other cases, it also is converted into disturbance potential energy. 8) Depending upon the phase of mode relative to the phase of the heating, the upper or lower branch of the mode is preferentially forced.