Subseasonal variability in a two-level atmospheric general circulation model Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9019s488k

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  • The dynamical processes which maintain atmospheric disturbances in regions of strong wintertime variability of the northern hemisphere are examined using data from a GCM simulation. Time series of the dependent variables and diabatic heating components from 10 Northern Hemisphere winters simulated by the Oregon State University two-level GCM are used. Variance and covariance analyses are performed to determine the geographical distribution of the intensities and transport properties of high-frequency (periods between 2.5 and 10 days) and low-frequency (periods between 10 days and a season) eddies. These are compared with existing observations and the discrepancies are discussed in terms of their dynamical consistency with the time-mean circulation. The energetics of high-frequency and low-frequency eddies are studied. It is found that the behavior of high-frequency eddies is consistent with baroclinic instability theory. Low-frequency eddies appear to be maintained mainly by a high-latitude baroclinic energy cycle. Energy conversions characteristic of barotropic processes are also significant at jet-stream-latitudes. The process of wave-energy dispersion is found to be an important factor governing the geographical distribution of low-frequency activity at middle latitudes. The nature of the systems causing low-frequency variability over the North Pacific Ocean is examined by applying complex EOF analysis to the time series of geopotential height anomalies. The first eigenmode of this analysis describes a wave of planetary scale extending from northeastern Asia to the Gulf of Mexico across the North Pacific basin. While the phase of this wave retrogrades along the continental borders of the ocean basin, energy propagates in the opposite direction and penetrates as far as the central North Atlantic. The dynamical characteristics of this disturbance are examined by complex covariance analysis between the first mode's principal component and the dependent-variable fields. It is found that the disturbance grows mainly through baroclinic processes with some contribution from barotropic processes. On the basis of these results it is proposed that the observed differences between the high- and low-frequency disturbances result from their being generated in different geographical regions where sphericity and the properties of the stationary flow cause baroclinic growth of structurally different modes.
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