Transition of Zonal Asymmetry of the Arctic Oscillation and the Antarctic Oscillation at the End of 1970s

In this study, the interdecadal changes in the zonal symmetry of both Arctic Oscillation (AO) and Antarctic Oscillation (AAO) were analyzed. To describe the zonal asymmetry, a local index of AO and AAO was defined using the normalized sea level pressure (SLP) differences between 40o and 65o (latitudes) in both hemispheres. The zonal covariability of local AO and AAO can well represent the zonal symmetry of AO and AAO. Results show that the zonal asymmetry of both AO and AAO significantly changed in the late 1970s. AO was less asymmetric in the zonal direction in the boreal winter season during the latter period, while in the boreal summer it became more asymmetric after 1979. The zonal symmetry of AAO in both austral summer and winter has also significantly decreased since the late 1970s. These changes may imply interdecadal transition in the atmospheric circulation at middle and high latitudes, which is of vital importance to understanding climate variability and predictability across the globe, including the African--Asian--Australian monsoon regions.