Weekly Tropical Climate Note

Record rainfall across northern Australia

Unseasonable rainfall has affected much of the Northern Territory and Queensland during the last week.  A broad swathe of Australia’s tropical region, extending from the Northern Territory’s Top End to inland southern Queensland, received between 50 and 100 mm. Many sites observed record September daily rainfall totals and many more are on track to have their highest September rainfall on record. Several sites in the central Top End had daily falls of more than 100 mm. Typically this part of northern Australia sees rainfall totals of 5 to 20 mm during September.

The unusually heavy rainfall was driven by a combination of weather and climate factors. A series of deep, low-pressure troughs have tapped into the enhanced moisture provided by the very warm waters off Australia’s northwest. This is occurring as a strong negative Indian Ocean Dipole (IOD) combines with a La Niña–like pattern in the Pacific Ocean. Both of these climate patterns tend to push warmer-than-normal water into the Timor and Arafura seas, increasing the likelihood of above-average rainfall for northern Australia in the September to November period. Climate models suggest the negative IOD will weaken later in the southern hemisphere spring. See the Climate Outlook for more information on what to expect in the coming months.

While some La Niña–like patterns are present in the Pacific Ocean, and the Southern Oscillation Index has recently exceeded La Niña thresholds, the El Niño–Southern Oscillation (ENSO) state remains neutral (neither La Niña nor El Niño).  Nearly all climate models surveyed by the Bureau indicate a neutral ENSO state will persist for the coming months; however, La Niña-like effects can still occur even if ocean thresholds are not exceeded. During La Niña, northern Australia typically experiences above-average wet season rainfall, with the first rains of the season typically arriving earlier than normal.

See the Bureau’s ENSO Wrap-Up for official El Niño, La Niña and IOD information.

Madden–Julian Oscillation strengthens

In the last week, the Madden–Julian Oscillation (MJO) strengthened as it moved over the eastern Indian Ocean and into the Maritime Continent. Typically, when the MJO is over the Maritime Continent at this time of the year, there is an increase in cloudiness and rainfall over South-East Asia; however, there is no significant impact on rainfall variability over Australia.

Nearly all international climate models suggest the recent strengthening of the MJO will be a short-lived event. The MJO is expected to become weak and indiscernible in the coming week.

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