Impacts of the Asia-Australia dipole and ENSO on climate variability in the Asia-Australia region

Abstract

An empirical orthogonal function (EOF) analysis of upper troposphere thickness between 200 and 300 hPa reveals a north–south dipole over the Asian–Australian (A–A) monsoon region in boreal winter (December–February, DJF). The dipole centres are located above the exit of the polar jet in the north and over the equatorial ‘warm pool’ in the south. The structure is defined as the Asia–Australia dipole (AAD) in this study. Its intensity reflects the temperature gradient and interaction of the circulation between the Southern and Northern Hemispheres, as well as illustrating the relations between the polar front jet and the ‘warm pool’ climate. The results show that the AAD strongly affects the climate in the A–A region after excluding the El Nino Southern Oscillation (ENSO) signal in the upper troposphere. The precipitation in the western Eurasian continent and the surface temperature in most of the A–A region are closely linked through the AAD, while the ENSO signal in the upper troposphere influences the climate at lower latitudes. Furthermore, the AAD index (AADI) signal can be found in most 5th Phase of the Coupled Model Intercomparison Project (CMIP5) model results. Thus, both observational analysis and modelling results demonstrate that the AAD is an important structure in the upper troposphere, largely reflecting differences in the upper troposphere between the Northern and Southern Hemispheres.