Interhemispheric Oscillations of Air Mass
Zhaoyong Guan and Toshio Yamagata
Frontier Research System for Global Change, Institute for Global Change Research, Tokyo
Abstract.
The surface air pressure (SAP) field obtained from NCEP/NCAR 40-year reanalysis is used to investigate the low frequency variabilities in the atmosphere. It is found that there exist interesting oscillations of air mass between the northern and southern hemisphere, which is referred to as the interhemispheric oscillation (IHO). An index of this air mass oscillation is defined to explore both the time evolution and the spatial pattern of IHO. The air mass is redistributed between both hemispheres on time-scales ranging from several months to decades.
The EOF analysis is performed for the zonal mean area-weighted dtrended anomalous SAP to reveal the IHO patterns. The first EOF that accounts for 36.4% of total variance relates to the oscillations in the Southern Hemisphere whereas EOF2 that explains 21.1% is the principal mode of the oscillations in the Northern Hemisphere. In the first 2 principal modes, the residual of the stronger oscillations of air mass in the one hemisphere is balanced by the weaker fluctuations of air mass in the other hemisphere, which accounts for 12.3% of total variance of IHO index. The third principal mode shows a seesaw pattern between two hemispheres. It accounts for 65.1% of the total variance of IHO index and for 14.5% of total variance of zonal mean SAP anomalies. The secular tendency of SAP has also been discussed.
