The low frequency variability in the surface air pressure (SAP) field is investigated using the NCEP/NCAR 40-year reanalysis data. The EOF1 (EOF2), which accounts for 36% (21%) of total variance of the zonal mean anomalous SAP, corresponds to the air mass oscillation in the middle and high latitudes of the Southern (Northern) Hemisphere. While the total air mass is conserved globally, EOF3 (14.5%) represents the air mass redistribution between the two hemispheres on time-scales ranging from several months to a decade. This is referred to as the interhemispheric oscillation (IHO). An index is defined to describe the IHO variability. The EOF3 explains 65% of the total variance of the IHO index. Moreover 14.5% of the total variance of the zonal mean SAP anomalies represents the variability of the IHO. ¿ 2001 American Geophysical Union