A numerical simulation study of the influence of the low boundary condition on the Arctic Oscillation

The Arctic Oscillation (AO) or the Northern Hemisphere annular mode (NAM) is the leading mode of the wintertime variability of sea level pressure over the Northern Hemisphere. The studies on the AO are receiving more and more attention because of its important role in the world's weather and climate. In this paper, the influences of topography and land-sea contrast on the AO or the NAM were examined by use of the NCAR GCM CAM3.0, and the interaction between the wave-mean flow associated with the AO in the stratosphere and that in the troposphere were explored preliminarily. A comparison between the control simulation results and the NCEP/NCAR reanalysis data indicates that CAM3.0 is able to simulate to a great extent the horizontal annular structure and vertical barotropic structure of the AO. Two sensitive experiments were carried out. In the first experiment the topography was removed, and in the second experiment the globe was covered fully by water (an aqua-planet experiment). The first experiment shows that mountains can influence the intensity, range and position of the Pacific and the Atlantic action centers of the AO. The second experiment reveals that the two action centers would disappear and instead a ring structure was found surrounding the Arctic action center, should the globe be covered by water. Both experiments indicate that the existence of the AO is basically independent of the low boundary condition, i.e., the AO is probably the result of some internal processes in the atmosphere, however, the spatial structure of the AO does depend on it. The influences of quasi-stationary waves in the stratosphere and transient waves in the troposphere on the AO were also examined by the comparison between the control and sensitive experiments. The maximum zonal wind speed center in the stratosphere associated with the AO index disappears when the quasi-stationary waves forced by the topography and land-sea contrast are absent, indicating that the AO activities are related to the interaction between the quasi-stationary wave and the zonal flow in the stratosphere. The fact that the AO still exists under only the transient waves in the troposphere indicates that quasi-stationary waves in the stratosphere are not necessary for the existence of the AO. The relationship between the quasi-stationary/transient wave and the mean flow in the stratosphere/troposphere and the origin of the AO may be obtained through an E-P flux diagnosis in the future.