Surges, cross-equatorial flows and their links with the Australian summer monsoon circulation and rainfall

Links between wind surges from the South China Sea and Australian monsoon circulation features are analysed for twelve summers from 1980-81 to 1991-92 using European Centre for Medium Range Forecasts (ECMWF) data. Composite analyses and lagged correlation methods were used to investigate the link in circulation features between these two regions. During strong surge events, winds are stronger over the South China Sea and east Asia. Anticyclonic circulations occur over east China and to the southwest of Australia during surge events. Cross-equatorial flows occur over the Indonesian region five days after surge events over the South China Sea that later influence the circulation pattern over Indonesia and northern Australia. Composites of winds in relation to monsoon onset dates show the development of surges over the South China Sea five to ten days prior to the monsoon onset. It appears that the relationship between surges and monsoon activity is strong prior to and during the onset of the Australian monsoon, but is weak once the monsoon is well established in the Australian region. Other influencing factors, such as intraseasonal oscillations, tropical cyclones and the El Niño-Southern Oscillation (ENSO) phenomenon and local thunderstorm activity, make the above-mentioned link more complex. The strongest correlation between north-south winds over the South China Sea, the west coast of Australia, east-west winds over the Timor Sea and Coral Sea was found five to ten days before the monsoon onset. Power spectra of wind components over the Timor Sea and rainfall in northern Australia for all twelve years also show dominant peaks at five to ten days. These results suggest that the periodicity around a five to ten-day period could arise from synoptic-scale circulation patterns associated with surges over the South China Sea. The relationship between the South China Sea winds and northern Australian rainfall is negative, but weak. The relationship between Timor Sea winds and northern Australian rainfall is positive and strong. In general, the onset of the monsoon is associated with a steady increase in westerly winds and rainfall which follow stronger northerlies over the South China Sea. This relationship does not always exist. In some years heavy rainfall preceded strong winds and in others heavy rainfall spells were not observed, although there were strong relationships between wind components between the South China Sea and the Timor Sea.