Weekly Tropical Climate Note

Shift towards La Niña continues

The cooling trend in the tropical Pacific Ocean continued during the past week. Water temperatures in the central Pacific Ocean are edging closer to La Niña thresholds, while there is mounting evidence that the atmosphere is responding to the water temperature patterns. A mature La Niña is characterised by both the pattern of sea surface temperature across the equatorial Pacific Ocean, and changes in the atmospheric circulation overlying the region.

All surveyed international climate models indicate La Niña thresholds will be reached by October and persist until at least the end of 2020.

The Indian Ocean Dipole (IOD) has returned to neutral during the last fortnight, after 3 weeks in a row when the negative threshold had been exceeded. A majority of surveyed climate models continue to forecast a negative IOD developing during spring 2020.

La Niña and the negative IOD are both typically associated with above-average rainfall across Northern Territory and Queensland during spring. Rainfall is usually near, to slightly above, average across northern Western Australia during spring with either of these seasonal climate drivers. The influence of La Niña on northern Australian rainfall normally extends into early summer, whereas the influence of the IOD typically wanes significantly once spring concludes, as monsoonal winds migrate to the southern hemisphere.

La Niña and the negative IOD usually result in an earlier-than-normal monsoon onset date (at Darwin). These climate drivers are typically also associated with above-average wet season (October to April) rainfall totals across northern Australia. Tropical cyclone activity is typically influenced by both of these climate drivers, with La Niña having a more pronounced effect. During La Niña, tropical cyclone numbers across the Australian region are normally higher than in average years, and the first cyclone of the season normally forms earlier than usual.

Read more about the ENSO Outlook

Early northern rainfall onset likely for most of Australia's tropics

The final issue of the Bureau’s Northern Rainfall Onset (NRO) outlook for season 2020/21 was issued on 27 August. The outlook indicates that the first rainfall accumulation of 50mm or more is likely to arrive earlier than normal for much of tropical Australia. Early rainfall onset likelihood is highest across much of Queensland and the southern Northern Territory, while the western half of Western Australia is predicted to have roughly equal chances of an early or late rainfall onset.

Earlier-than-usual rainfall onset is linked to La Niña forming during spring, which would increase the likelihood of above-average rainfall across much of Australia. A negative Indian Ocean Dipole also increases the chance of above-average rains during the early wet season months. Expected warmer-than-average waters around northern Australia are another factor conducive to above-average rainfall across northern Australia.

From October, the NRO webpage tracks rainfall accumulations after 1 September across northern Australia. The NRO observations page also displays maps which show a location's rainfall onset status, and whether it was earlier or later than normal.

Read more about the northern rainfall onset