| Summary: | The dynamics of low-frequency fluctuations (periods between 10 days and a season) is investigated. While those fluctuations are known to be forced, at least in part, by the underlying surface, the emphasis in this thesis is placed on processes taking place within the atmosphere. The thermal interaction between the high-frequency (periods 2 to 10 days) and the low-frequency flow is first investigated. The temporal and spatial relationship between the heat flux convergence by the synoptic-scale eddies and the low-frequency temperature field is identified. It is shown that the low-frequency temperature fluctuations are negatively correlated with the heat flux convergence by the synoptic-scale eddies, implying the damping effect of high frequency eddy heat flux on the slowly varying temperature field. The damping effect is not homogeneous in space, however, and different temperature patterns have different damping rates. The low-frequency temperature patterns over the North American and Siberian inland areas, where the strongest low-frequency temperature variances are observed, are associated with weak damping from the high-frequency eddies. A stronger dissipation is found for the low-frequency temperature variations in the storm track regions. We then examine the El Nino-related interannual variations of the transient eddy activity and the associated multiscale interactions. A dataset from the U.S. National Meteorological Center (NMC) of 24 years is used. The purpose is to determine when the seasonal mean flow is less dependent on processes internal to the atmosphere and therefore more dependent on external forcing and thus more predictable. The results show that during El Nino winters the low-frequency eddy activity is reduced over the North Pacific and the high-frequency baroclinic waves are shifted south-eastward of their normal position in the Pacific. Over the North Pacific less kinetic energy is supplied to the low-frequency eddies both from the large-scale seasonal mean flow and from the synoptic-scale eddies. Thus the atmosphere in that region is more "stable" during El Nino winters, and its state depends more on the external forcings. Finally, the atmospheric predictability is studied explicitly. A large number of numerical experiments are performed to determine whether the forecast skill is dependent on the weather regimes. The predictions are made with a T21 three-level quasi-geostrophic model. The relationship between the forecast behaviour and the "interannual" variation of the Pacific/North American (PNA) anomaly is investigated. Comparison of the error growth for the forecasts made during the positive and negative PNA phases indicates that little differences of error growth can be realized before about a week. After that period the forecast error grows faster during the negative PNA phases. The forecast skill for the medium- and long-range predictions over the North Pacific, the North American and the North Atlantic regions is higher during the positive PNA phase than that during the negative PNA phase. A global signal of this relationship is also observed. The physical mechanism for the difference of error growth is discussed.
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