Seasonality of Interannual Inter-hemispheric Oscillations over the Past Five Decades

Air mass is inter-hemispherically redistributed, leading to an interesting phenomenonknown as the Inter-Hemispheric Oscillation (IHO). In the present article, the seasonality of theinterannual IHO has been examined by employing monthly mean reanalyses from NCEP/NCAR, EAR40, andJRA25 for the period of 1958--2006. It is found that the IHO indices as calculated from differentreanalyses are generally consistent with each other. A distinct seesaw structure in all four seasonsbetween the northern and southern hemispheres is observed as the IHO signature in both the surfaceair pressure anomalies (SAPAs) and the leading EOF component of the anomalous zonal mean quantities.When the SAPAs are positive (negative) in the northern hemisphere, they are negative (positive) inthe southern hemisphere. Large values of SAPAs are usually observed in mid- and high-latitude areasin all but the solstice seasons. In boreal summer and winter, relatively stronger perturbations ofIHO-related SAPA are found in the Asian monsoon region, which shows a large difference from thestatus in boreal spring and fall. This suggests that seasonal mean monsoon activity is globally linkedvia air mass redistribution globally on interannual timescales, showing a very interesting linkagebetween monsoons and the IHO in the global domain. In all seasons, large values of SAPA always existover the Antarctic and the surrounding regions, implying a close relation with Antarctic oscillations.