The Structure and Propagation of Intraseasonal Oscillations Appearing in a GFDL General Circulation Model

Abstract

The three-dimensional structure and temporal evolution of quasi-periodic, planetary-scale tropospheric oscillations simulated by a 15-wavenurnber 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 traveling features with spatial scales of zonal wavenumbers 1 and 2, and with temporal scales of 25–40 and 15–20 days, respectively. The flow pattern associated with the oscillations of both spatial scales is characterized by circulation cells oriented along the equatorial zonal plane, with the zonal wind and geopotential height fluctuations near the sea level being negatively correlated with the corresponding fluctuations in the upper troposphere. The movement of these zonal circulation cells along the equatorial bell is accompanied by systematic migration of the global-scale horizontal divergence field, and by dipole-like precipitation structures within the Indonesian/Pacific sector. The preferred sites for such oscillatory behavior exhibit a notable seasonal dependence, with the most active zonal circulation cells being located in the summer hemisphere. 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. The principal characteristics of the model-generated phenomena analyzed in this study are compared with corresponding results reported in the observational literature. Although the period of the simulated wavenumber-1 phenomena is somewhat shorter than the corresponding observed values, it is demonstrated that the spatial structure, propagation characteristics and seasonal dependence of the model features are consistent with observations. The model findings are also interpreted in terms of current theoretical understanding of tropical and extratropical motions.