A Numerical Simulation Study Of The Influence Of The Lower Boundary Condition On The Arctic Oscillation

The Arctic Oscillation (AO) is the leading mode of wintertime variability of sea levelpressure over the Northern Hemisphere. It becomes a new research topic of the atmosphericcirculation since the late 20th century. The studies on the AO are receiving more and moreattentions because of the important roles played by it in the world's weather and climate. Themain issues on the AO studies could be reduced to three aspects: the origin of the AO, thecoupling between the stratosphere and troposphere associated with the AO and themechanism of its influences on regional climates. The addresses of the last two issues dependon that of the first one. It has been clear that the origin of the AO is closely related to theactivities of the transient and quasi stationary Rossby waves at middle and high latitudes.Lower boundary condition with topography and land sea contrast can producequasi stationary waves which in turn could force the AO. Also, the lower boundary conditioncan, via the basic climate state determined by it, affect the structure of the AO. Therefore, theresearch of the influence of the lower boundary condition on the AO is an important aspect ofmechanism research of theAO.This paper is focused on the roles of the lower boundary condition in the formations ofAO's structure and the wave mean interactions associated with the AO in the stratosphere andtroposphere, through which the origin of theAO is explored as well.Firstly, one control and two sensitive runs of the NCAR GCM CAM3.0 are conducted.In the first sensitive run the topography was removed, while in the second the globe wascovered fully by water, or aqua planet is assumed. The monthly mean results of theseexperiments were analyzed to determine the influence of the lower boundary condition on theannular structure and vertical barotropic structure. The comparison between the results of thecontrol run and the analysis of the NCEP data indicates that CAM3.0 can simulate in greatextent the annular structure and vertical barotropic structure of the AO. All the threeexperiments demonstrate the existence of the AO is basically independent of the lowerboundary condition, and the AO is the result of some internal processes in the atmosphere.However, the spatial structure of the AO do depend on it.The first sensitive experiment showsthat the mountains can influence the intensities, the ranges and the positions of the Pacific andthe Atlantic centers of action of the AO. While the second sensitive experiment with aqua planet shows that these two centers of action are replaced by an annular structuresurrounding the arctic center of action and the maximum center of zonal wind associated withtheAO in the stratosphere is absent.Secondly, in order to clarify how the lower boundary condition influences the AO viawave mean interaction, we find from the analysis of NCEP daily data the relation between thewave mean interaction and the origin of the AO in the stratosphere and troposphere,respectively. Empirical orthogonal function (EOF) analysis is emplyed to extract the E P fluxpatterns characterizing quasi stationary wave mean interaction in the stratosphere and thatcharacterizing the transient wave mean interaction in the troposphere. Then we find therelationships between certain E P flux pattern and the AO at different time scales. Theanalysis shows that both the quasi stationary and transient wave mean interactions areresponsible for the fluctuations of the AO. These interactions result in forcings imposed on theAO independently. Usually, anormaly eddy forcing leads the response of the AO about 1 or 2days. Furthermore, the result of the numerical control run indicates that the characters of theabove wave mean interaction and their relationship with the AO are described quite well bythe model.Finally, the influence of the lower boundary condition on the AO via the wave meaninteraction is analyzed with the daily output of the two sensitive experiments. We focus on thechanges of the relationship between the quasi stationary/transient wave mean interaction inthe stratosphere/troposphere and the origin of the AO. In the sensitive experiment withtopography removed, both the quasi stationary wave mean interaction in the stratosphere andthat of transient wave in the troposphere are responsible for the fluctuations of the AO.Whereas, in the sensitive experiment of aqua planet, this causal relation between thequasi stationary wave mean interaction and the AO disappeares, which is closely related tothe absence of the maximum center of zonal wind associated with the AO in the stratospherein this case.In this dissertation, the influence of the lower boundary condition on the AO via basicclimate state and wave mean interaction are identified. In addition, the detailed dynamics ofAO in terms of wave mean interaction is clarified. We get an even deeper insight into theorigin of the AO and the issue of the coupling phenomena between the stratosphere and thetroposphere. The main conclusions and the issues discussed in this dissertation may provide apotential reference for other researchers in this field.