The influence of boreal winter extratropical North Pacific Oscillation on Australian spring rainfall

The North Pacific Oscillation (NPO) is a key atmospheric intrinsic mode. This study concerns mechanisms that the NPO influences Southern Hemisphere climate anomalies after several months. It is found that the boreal wintertime NPO has a significant negative connection with austral spring rainfall anomalies in Australia, particularly in the Northern Territory, Queensland, New South Wales and Victoria. A positive NPO phase tends to be followed by dry conditions, while a negative phase by wet conditions in austral spring over much of northern and eastern Australia. The physical mechanism by which the boreal winter NPO affects Australian rainfall arises from the NPO seasonal footprinting mechanism that generates significant tropical central and eastern Pacific sea surface temperature (SST) warming anomalies during austral spring. Then, the positive NPO-related SST warming anomalies over the tropical central-eastern Pacific further induce a weakened Walker circulation, with its western subsiding branch over Australia, suppressing convection and thereby reducing rainfall. Furthermore, the NPO plays an important role in contributing to the significant long-term trends of the Northern Territory and Queensland rainfalls. The decreasing amplitude of the NPO contributes much of the observed rainfall wetting trends in the Northern Territory (~50 %) and Queensland (~60 %) regions during 1951–2010. Noted that caution is recommended regarding the Australian spring rainfall trend results obtained in this study as the austral spring rainfall trends may be sensitive to the choice of the selected time period.