Fan (2007) recently documented the zonal asymmetry of the Antarctic oscillation (AAO) in the austral winter. In this research, the zonal asymmetry of the northern annular mode, or the Arctic oscillation (AO), in the interannual variability is studied for the boreal winter. It is shown that there is zonal asymmetry of the AO as well, similar to the case of the Antarctic oscillation (AAO). However, the zonal asymmetry of the AO is considerably weaker than that of the AAO. This is far beyond the speculation, since the zonal asymmetry of the geography is larger in the Northern Hemisphere than the Southern Hemisphere. The Western and Eastern Hemispheres portions of the AO are correlated at 0.54 for 1959― 1998, comparing with 0.23 for the case of the AAO. The authors also discussed the physical reason for this inter-hemispheric difference, and partly attributed it to the El Nio and Southern Oscillation (ENSO) cycle which may be represented by the SO index. It is indicated that the SO associated sea-level pressure (SLP) patterns are more zonal symmetric in the high latitudes of the Northern Hemisphere than the Southern Hemisphere.
Relationships between the North Pacific Oscillation (NPO) and the typhoon as well as hurricane fre-quencies are documented. The correlation between NPO index in June-July-August-September and the annual typhoon number in the western North Pacific is 0.37 for the period of 1949―1998. The NPO is correlated with the annual hurricane number in the tropical Atlantic at -0.28 for the same period. The variability of NPO is found to be concurrent with the changes of the magnitude of vertical zonal wind shear, sea-level pressure patterns, as well as the sea surface temperature, which are physically asso- ciated with the typhoons and hurricanes genesis. The NPO associated atmospheric circulation vari- ability is analyzed to explain how NPO is linked with variability of the tropical atmospheric circulation in the western Pacific and the tropical Atlantic, via the atmospheric teleconnection.
