Dynamics of westerly jets and tropopause height.

摘要

The structure of westerly jets and their internal variability is investigated with a primitive equation model on the sphere. The model is forced with an idealized radiative equilibrium temperature profile which consists of a global-scale base profile and additional tropical heating and high-latitude cooling. The tropical heating controls the intensity of the subtropical jet, whereas the high-latitude cooling modulates the meridional width of the baroclinic zone.; A large number of dry model runs are carried out by varying the values of the tropical heating and high-latitude cooling parameters. This parameter study shows that, in a regime with weak tropical heating and strong high-latitude cooling, the polar-front jet separates from the subtropical jet, forming a double-jet state. In contrast, if the tropical heating rate is greater than a certain value, a strong single jet emerges. These results indicate that the presence of one or two jets is dependent upon the relative values of both the tropical heating and the baroclinic zone width. It is also found that the boundary between the single- and double-jet regimes is often sharp. This nonlinear behavior is compared with linear theory and differences are discussed with the aid of spectral analysis.; Keeping an application to the real atmosphere in mind, a set of moist model runs is also examined in which the moisture content is systematically varied. For a relatively small moisture content, the circulation prefers a double-jet state. However, for a moisture content greater than a certain value, the two jets collapse into a single jet. This threshold response is explained with the aid of the aforementioned dry model results. Based on the results of the dry and moist model calculations, we further discuss various physical interpretations of circulation responses to global warming presented in the literature.; The structure of the westerly jet has a significant influence on its internal variability, which is quantified by determining the first two leading empirical orthogonal functions of the zonal-mean zonal wind (EOF1 and EOF2)-. In the single-jet regime, characterized by strong tropical heating and weak high-latitude cooling, EOF1 represents a latitudinal meandering of the jet, the so-called zonal index. In contrast, in a double-jet regime of weak tropical heating and strong high-latitude cooling, EOF1 and EOF2 combine to represent coherent poleward propagation of zonal-mean flow anomalies. (Abstract shortened by UMI.)