Friday, September 04, 2015

El Niño brings dryer weather to Australia

Some Climatology

Usually, the northern and eastern regions of Australia have a wet spring and summer. While October marks the beginning of the wet season, the rain doesn't just start falling on 1 October. Rather, it begins gradually with the occasional afternoon shower, and then 'builds up' to more frequent rains as the season progresses.

Although the risk is substantially higher during the first months of the year (January-April), the area might be affected by tropical cyclones from about November. There are on average 7.7 days per season when a cyclone exists in the Northern Region.

El Niño

Australia's weather is influenced by many climate drivers. El Niño and La Niña have perhaps the strongest influence on year-to-year climate variability in Australia. They are a part of a natural cycle known as the El Niño–Southern Oscillation (ENSO) and are associated with a sustained period (many months) of warming (El Niño) or cooling (La Niña) in the central and eastern tropical Pacific.

The El Niño cycle is driven by changes in the winds around the equator. North-East trade winds and South-East trade winds meet in the InterTropical Convergence Zone (ITCZ). These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry.

During El Niño, the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. On average, there are fewer tropical cyclones in the Australian region during El Niño years. This is particularly true around Queensland, where cyclones are half as likely to cross the coast during El Niño years compared to neutral years. This means a decreased likelihood of major damage and flooding related to strong winds, high seas and heavy rains associated with tropical cyclones.

During La Niña, the trade winds are stronger than usual. Warm ocean water is pushed harder to the west and the cold upwelling water along the South American coast is penetrating further towards the central part of the Pacific. Indonesia and Australia now get excessive rainfall, while Ecuador and Peru have to deal with extremely dry weather;

Current situation and outlook

There's currently a strong El Niño and it tends to persist.
The American Climate Predictions Center states that "There is a greater than 90% chance that El Niño will continue trough Northern Hemisphere winter 2015-16, and around an 85% chance it will last into early spring 2016."

I guess this means that we can hope for nice weather at the start of the World Solar Race!

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