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Weekly Tropical Climate Note

Issued 29 September 2015

Pre-wet-season rainfall over Australia's Top End

Northern Australia is beginning the transition into a wet-season-like weather pattern. Increased humidity over the northwestern Northern Territory resulted in some significant rainfall during the past week. Several locations around the Darwin region recorded weekly totals greater than 25 mm; some locations received more than the September average rainfall in a single rainfall event. Showers are forecast for the Northern Territory coast this week, coinciding with the official start of the northern wet season on 1 October.

Madden-Julian Oscillation weak but tropical cyclone activity continues

The Madden-Julian Oscillation (MJO) signal remains weak or indiscernible. There is disagreement between international climate models as to the forecast location of the MJO signal but most models predict a weak signal will persist over the next two weeks. If the MJO signal remains weak in the near-term, the dominant driver of tropical activity in the Australian region is expected to be the warm waters of the tropical Pacific Ocean, related to El Niño.

Despite a weak MJO signal, tropical cyclone activity persists across the Pacific region. Super typhoon Dujuan brought very strong wind gusts and high rainfall to Taiwan and the islands south of Japan in recent days. Both the weakening tropical storm Niala, over the central Pacific Ocean, and tropical storm Marty, in the eastern Pacific Ocean, are expected to have little impact on any land masses.

See the Bureau's MJO Monitoring for current MJO information.

El Niño in tropical Pacific Ocean and positive Indian Ocean Dipole

The 2015 El Niño continues to dominate the Pacific Ocean. The 30-day Southern Oscillation Index (SOI) to 27 September is −18.1 and the 90-day SOI is −17.9 which are strongly negative. The latest weekly NINO3.4 sea surface temperature anomaly is +2.0°C for the week ending 27 September.

The Indian Ocean Dipole (IOD) index has been at or above +0.4°C for the past 8 weeks, so the Indian Ocean meets the minimum criteria for what could be classified as a positive IOD event. When the IOD is in a positive phase, it typically reinforces the below-average rainfall patterns over eastern and northern Australia during El Niño. However, warmer-than-average sea surface temperatures across the whole Indian Ocean appear to have moderated the influence of the positive IOD and El Niño over Australia so far. Four out of five international models suggest this positive IOD event will persist through November.

See the Bureau of Meteorology's ENSO Wrap-Up for official El Niño information.

Next update expected by 6 October 2015 | Product Code IDCKGEW000

Further information

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