The structure and propagation of intraseasonal oscillations appearing in a GFDL general circulation model.

The three-dimensional structure and temporal evolution of quasi-periodic, planetary-scale tropospheric oscillations simulated by a 15-wavenumber GCM are investigated by applying cross- spectral, eigenvector, composite and temporal correlation techniques to 12 years of model output. Evident from this diagnostic study is the presence in the model tropics of well-defined eastward travelling features with spatial scales of zonal wavenumbers 1 and 2, and with temporal scales of 25-40 and 15-20 days, respectively. During the northern summer, the 25-40 day oscillations coincide with the occurrence of northward moving, zonally elongated rainbands over the monsoon region of South Asia. During the northern winter the 25-40 day phenomena in the tropics are linked to well-organized extratropical wave trains spanning the Eurasian and Pacific/North American sectors. -from Authors