Weekly Tropical Climate Note

Madden–Julian Oscillation approaches Australian region

A pulse of the Madden–Julian Oscillation (MJO) has moved into the eastern Indian Ocean and is approaching Maritime Continent longitudes, north of Australia. The general consensus amongst surveyed climate models is that this pulse of the MJO will weaken as it moves across the Maritime Continent, some time in the next fortnight. Independent of its strength, the MJO pulse may affect the regional environment by significantly increasing tropical moisture around northern Australia.

At this time of year, the MJO in this region can lead to above-average rainfall across parts of northern Australia. A pulse of the MJO in this region is also favourable for monsoon onset at Darwin. The typical date of monsoon onset at Darwin is mid-December in La Niña years, about 2 weeks earlier than in non-La Niña years.

Read more about the Madden–Julian Oscillation

Heatwave conditions across northern Australia

Well-above-average temperatures have been observed across northern Australia throughout November 2020. Temperatures averaged across Australia, north of 26° South (roughly parallel to the Northern Territory/South Australia border), indicate the mean daily daytime and overnight temperatures for the month to date are the highest ever recorded in the month of November. Many regional locations across northern Australia have seen extended periods of very high temperatures, fulfilling the criteria for a heatwave.

There are many environmental factors that contribute to heatwave conditions. During recent weeks, a notable weather factor appears to have been prolonged high pressure across northern Australia.

A climate driver such as the MJO could potentially lower the surface pressure across northern Australia in the coming fortnight, while at the same time increasing the likelihood of above-average cloudiness and rainfall. This would likely lead to significantly cooler temperatures, particularly if the MJO pulse contributes to the development of monsoonal conditions across northern Australia.

Read more about the Bureau's Heatwave Service for Australia

Above-average rainfall likely for northern Australia this wet season

Australian climate outlooks indicate a high chance (greater than 70%) of above-average rainfall across most of northern Australia in the December 2020 to March 2021 period. The rainfall outlooks are consistent with the ongoing La Niña, a seasonal climate driver typically associated with above-average rainfall across northern Australia during the wet season months of October to April.

Monsoon arrival (at Darwin) is normally earlier during La Niña, with an average date around mid-December, compared to the average date, a few days after Christmas.

The first tropical cyclone to form in the Australian region typically occurs about two weeks earlier than normal during La Niña years—based on data since 1970, the average date during La Niña years is mid-December. Tropical cyclone and tropical low frequency across northern Australia is often greater during La Niña, compared to non-La Niña years.

Recent climate models indicates the current La Niña is likely to persist until at least the end of February 2021. The Bureau's model, along with most of the surveyed international models, also predict the current La Niña is likely to briefly reach relatively strong levels.

Read more about the El Niño-Southern Oscillation (ENSO)