Understanding the multi-scale sructures in the tropics

After Wheeler and Kiladis (1999), we use the two-dimensional (2D) power spectrum of outgoing longwave radiation (OLR) to study the tropical circulation. We also use power spectra of variables in the NCEP reanalysis and GCM output. Our objective is to understand all the features of these spectra – not only the spectral peaks associated with the MJO and equatorial waves but also the concentration of power at low frequencies and small wavenumbers. Using dry and moist idealized GCMs without topography, we compare the effect of forcing by mid-latitude disturbances with forcing by disturbances that originate in the tropics.

The MJO, the most striking peak in the OLR spectrum, is a planetary scale mode of intraseasonal variation in the tropical Indian and western Pacific Oceans. We test the hypothesis that the MJO is not a wave but a wave packet⎯the interference pattern produced by a narrow frequency band of mixed Rossby-gravity (MRG) waves. As such, the MJO would propagate with the MRG group velocity, which is eastward at ~ 5 m s-1. (Yang and Ingersoll 2011, JAS)

Through the 2D power spectrum of the mid-latitude zonal wind from both dry and moist GCMs, we can see spectral peaks in the wavenumber and frequency range that are consistent with the theoretically derived baroclinic wave dispersion relations. When the meridional temperature gradient is increased, the power of the spectral peaks increases. We can see the same spectral peaks in the 2D power spectrum of the tropical zonal wind. Moreover, the strength of the tropical power spectrum increases with the meridional temperature gradient. By comparing the 2D power spectrum of the dry GCM output with that of the NCEP reanalysis data, we find that the power spectrum of the dry GCM is significantly weaker than that of the NCEP. Comparing the 2D power spectrum of the moist GCM output with that of the NCEP, we find that the amplitudes of the power spectra are closer. Such comparisons indicate that the moisture is important to get the correct picture of the tropical meteorology. We increase the moisture in the tropics by increasing the sea surface temperature in the tropics, but keep the same meridional temperature gradient. Then we can separate the convective forcing in the tropics from forcing by the mid-latitude disturbances. The results show that the moisture favors some part of the spectrum, and reduces the power of the other part of the spectrum. Details and further results will be presented.