Climate: Wet spring ahead for the south

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Southern Australia may experience above-average rainfall during late winter-spring. Southern Australia may experience above-average rainfall during late winter-spring. Southern Australia may experience above-average rainfall during late winter-spring.
When a negative IOD event and La Nina coincide, winter–spring rainfall has been above average over large parts of Australia. When a negative IOD event and La Nina coincide, winter–spring rainfall has been above average over large parts of Australia. When a negative IOD event and La Nina coincide, winter–spring rainfall has been above average over large parts of Australia.
A negative IOD event brings above-average rainfall, in line with the impacts typically expected in La Nina. A negative IOD event brings above-average rainfall, in line with the impacts typically expected in La Nina. A negative IOD event brings above-average rainfall, in line with the impacts typically expected in La Nina.

Farmers in Southern Australia can expect above-average rainfall and cooler-than-average daytime temperatures, according to the Bureau of Meteorology (BOM).


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A negative Indian Ocean Dipole (IOD) pattern in the Indian Ocean has prompted BOM to also predict warmer day and night-time temperatures in northern Australia.

Current weekly IOD index values are the lowest they have been in the last 15 years. Cimate models predict the negative IOD pattern will persist and develop through the southern winter and spring.

Sea surface temperatures in the tropical Pacific Ocean have continued to cool in recent weeks and the tropical Pacific Ocean is in a neutral El Nino–Southern Oscillation (ENSO) state.

But a large volume of cooler than normal water below the ocean surface suggests La Nina remains possible later this year. This means the likelihood of La Nina forming later in 2016 is around 50 per cent.

Typically during La Nina, winter-spring rainfall is above average over northern, central and eastern Australia. If La Nina does develop, climate models suggest it is unlikely to reach levels seen in the most recent event of 2010–12, which was one of the strongest La Nina events on record.

Indian Ocean sea surface temperatures impact rainfall and temperature patterns over all of Australia. Warmer than average sea surface temperatures can provide more moisture for frontal systems and lows crossing Australia.

 

Indian Ocean Dipole explained

Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole or IOD.

The IOD is one of the key drivers of Australia's climate and can have a significant impact on agriculture. This is because events generally coincide with the winter crop growing season.

The IOD has three phases: neutral, positive and negative. Events usually start around May or June, peak between August and October and then rapidly decay when the monsoon arrives in the southern hemisphere around the end of spring.

A neutral IOD phase occurs when water from the Pacific flows between the islands of Indonesia, keeping seas to Australia's northwest warm. Air rises above this area and falls over the western half of the Indian Ocean basin, blowing westerly winds along the equator. Netural IODs have little influence on Australia's climate.

 

Positive IOD phase

Westerly winds weaken along the equator allowing warm water to shift towards Africa. Changes in the winds also allow cool water to rise up from the deep ocean in the east. This sets up a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west.

Generally this means there is less moisture than normal in the atmosphere to the northwest of Australia. This changes the path of weather systems coming from Australia's west, often resulting in less rainfall and higher than normal temperatures over parts of Australia during winter and spring.

 

Negative IOD phase

Westerly winds intensify along the equator, allowing warmer waters to concentrate near Australia. This sets up a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west.

A negative IOD typically results in above-average winter-spring rainfall over parts of southern Australia as the warmer waters off northwest Australia provide more available moisture to weather systems crossing the country.

 

Wetting influence expands

Some negative IOD events occur at the same time as La Nina. However, negative IOD events rarely coincide with El Nino. The relationship between La Nina and the IOD is complicated, with the level of dependence between the two being an area of active research.

Since 1960, when reliable records of the IOD began there have been nine negative IOD and nine positive IOD events.


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