Graduate Thesis Or Dissertation
 

Quasi-stationary atmospheric responses to large-scale forcing

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  • A time-dependent, spectral, barotropic model and a similar two-layer primitive equation model are developed to investigate the planetary-scale wave responses to various types of large-scale forcing: vorticity and heat sources, and sea surface temperature (SST) anomalies. Both models are linearized about the zonal mean states of January climatology. The characteristics of forced Rossby waves are extensively studied based on both the barotropic model experiments and the theory of Rossby wave propagation on the sphere (Hoskins and Karoly, 1981). In particular, both model and theoretical results show that the responses are dominated by ultra-long wave components (zonal wavenumber m = 1, 2, and 3), and that the large responses occur for vorticity sources located at the subtropics and at the high latitudes near 45°N. The model experiments for the wavenumber-dependent sources located at various latitudes show that the ultra-long waves behave like a north-south seesaw between the high and middle latitudes (m = 1) or between the high latitudes and subtropics (m = 2). The north-south seesaw of zonal wavenumber 1 component is in good agreement with that observed by Gambo and Kudo (1983). The responses of long waves (m > 3) are, however, localized in the source regions with relatively small amplitudes. The characteristics of baroclinic responses to prescribed heat sources located at various latitudes are also examined. Over the source latitudes baroclinic responses are dominant; however, the remote responses have a barotropic structure. The north-south seesaws appearing in the barotropic model are also observed in the baroclinic model. A series of baroclinic model experiments, in which surface heat fluxes and internal heating are computed in terms of the model variables, are also conducted to investigate the linear effect of sea-surface temperature (SST) anomalies on the atmospheric circulation. The experiments for prescribed SST anomalies, taken equal to twice those of Rasmusson and Carpenter (1982), simulate many aspects of the associated observed atmospheric anomalies, and suggest, therefore, that a large part of the atmosphere's responses occur via linear dynamics. It is also suggested that the rather weak responses in the North Pacific are due to the lack of a zonally varying basic state. In the case where the SST anomalies are located in the middle latitudes, the responses are about five times smaller than for the tropical SST anomalies. This result is also fairly consistent with the GCM experimental results by Chervin et al. (1976). Subsequent experiments, using climatological January SSTs in the tropics, suggest that the tropical Pacific SST can be an important factor in maintaining the climatological standing waves, at least over the western half of the Northern Hemisphere.
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