The Bureau of Meteorology has clarified its position on the La Nina pattern expected to affect northern Australia this season and it’s not good.
The tropical cyclone season runs between 1 November and 30 April and the bureau advises there’s a greater chance of cyclones and more so, rain.
There’ve been fewer tropical cyclones on average in recent years but the bureau noted that the last significant La Nina event in 2010-12 delivered Cyclone Yasi. This is an extract from its latest advice.
La Niña can cause changes to much more than just tropical cyclone numbers.
The biggest impact is to rainfall, with large parts of northern and eastern Australia typically having much more rain than normal.
This outlook increases the risk of widespread and prolonged flooding over large parts of eastern and northern Australia especially in Queensland, New South Wales and Victoria.”
The weather patterns associated with La Niña can lead to repeated flooding in the same location or very long duration flooding.
In Queensland in the 2010–11 season, some rivers and townships experienced several flood peaks that progressively increased in size over the course of the summer (Gympie, Roma, Dalby, Goondiwindi) and other locations remained at high flood levels for weeks (Rockhampton, St George, Theodore).
The 2020–21 severe weather season will be driven by very different climate settings than the past two seasons.
This year the BOM’s Tropical Cyclone Season Outlook model predicts a 66 percent chance of an above-average number of tropical cyclones in the Australian region.
Typically, between 9 and 11 tropical cyclones form each season in the Australian region, with around 4 crossing Australia’s coastline.
In fact, there has never been a season without at least one tropical cyclone crossing the coast.
The number of tropical cyclones can vary a lot between years, and this can be caused by several factors.
One of the main ones is the temperature of waters to the north of Australia, as warm ocean temperatures are the fuel for tropical cyclones.
One of the biggest drivers of change in ocean temperatures is the El Niño–Southern Oscillation, or ENSO.
During La Niña phases of ENSO, the warmest waters in the equatorial Pacific build up in the western Pacific and to the north of Australia. This region becomes the focus for more cloud, rainfall, and an increased chance of tropical cyclone formation.
The potential for intense rainfall also increases risks of flash flooding in urban areas and fast-responding streams.
Ex-tropical cyclones and tropical lows can also bring large amounts of rainfall inland. In 2010–11, moisture from ex-tropical cyclones Anthony and Yasi made it all the way to Victoria, contributing to flooding in the west of the state.