About the Australian desert(s)

Looking at Australia in my World Atlas, I discover the Tanami Desert, Great Sandy Desert, Little Sandy Desert, Gibson Desert, Simpson Desert, Sturt Stony Desert and the Great Victoria Desert. I hope I'm not forgetting one. Apparently, most of Australia is dry and hostile for fauna and flora. But why? What makes this part of the world so dry? Why do we find so many deserts here?

It has all to do with the global circulation of air around our planet. And this circulation is driven by the differential heating by the sun.We all know that the Earth is receiving solar energy much more efficiently around the equator than around the North Pole and the South Pole. Therefore, the air around the equator is warmer and less dense than the air at the Poles, and it begins to rise. At the North Pole and the South Pole, the air sinks.
In the Tropopause, the temperature is nearly constant, and in the Stratosphere, it is getting warmer with height. This temperature inversion acts as some kind of a ceiling, through which the air around the equator can no longer rise. It has to move aside, partly to the north and partly to the south.
At the poles, the sinking air meets the surface of the Earth and there also, it has to move aside. It flows back to the equator, over the surface of the planet. That way, a circulation has started up between the equator and the poles.

But, this model is far to simple. In reality, the air also sinks around 30 ° latitude, in the northern hemisphere as well as in the southern hemisphere. When this sinking air hits the surface, it will move partly to the equator (so closing a circulation known as the "Hadley cell") and partly to the pole. Roughly between 50 and 60 degrees latitude, this poleward moving air meets the air that was moving from the pole in the direction of the equator. In that zone, the air will rise again. This zone, where these two different air masses meet, is known as the Polar Front. Depressions and perturbations are born here.

Back to the Hadley cell. Under the sinking air around 30 ° latitude, we find the subtropical highs. The most important subtropical high for Europe is the Azores' high, which brings dry and mainly sunny weather, when it is moving over the continent. Because of the sinking air in a subtropical high, clouds and precipitation have great difficulty to form. Only when the high is temporarily moving away somewhat, some rain can be observed, but mostly, the amounts of rain are very limited.

No wonder that almost all deserts of the planet are situated at or close to 30 ° latitude.

 

Australia is roughly situated between 10 and 40 degrees south. A big part of it is mostly covered by a subtropical high. Therefore there are so many deserts in Australia.

Darwin's second sea-breeze

The passage of a sea-breeze front is a regular occurence in Darwin, Northern Territory, Australia. But, on some days, something like a “second sea-breeze” is observed during the evening, a phenomenon that has puzzled local forecasters for ages. In 2008, a study conducted by Gerald Thomsen and Roger Smith at the Meteorological Institute of the University of Munich, Germany, brought the solution.

Some sea-breeze theory

A sea-breeze is a local circulation, induced by differences in surface temperature between the land and the sea. While the sea surface temperature doesn't change much throughout a sunny day, the surface temperature of the land often increases very rapidly. As a result the air above the land also warms up and becomes less dense than the air above the sea. (1) This difference in density then produces a difference in air pressure at a certain altitude (2), with a higher pressure above land and a lower pressure above the sea. A pressure difference is at the origin of an air flow (= wind) from the higher pressure to the lower pressure. (3)

As a result of this wind at a certain altitude, more air particles are being advected towards the sea and at the surface, the pressure rises, while at land, the surface pressure decreases, simply because the air is escaping aloft. (4)

Again, this difference in air pressure at the surface produces a wind, blowing from the sea to the land. (5) This is the sea-breeze.

When the sea-breeze hits the coast, the temperature may drop by several degrees and typically the wind shifts and increases. At the same time, the humidity increases, as a more moist air mass is advected by the sea-breeze.

At Darwin though

Such a circulation typically starts up around noon or in the early afternoon, but at Darwin, a similar sudden jump of the wind, increasing wind speed and increasing humidity often have been observed in the evening.

The researchers from the University of Munich found at that this second sea-breeze isn't a sea-breeze at all.

From the early morning on sunny days on, a band of dry inland air, lying over the “Top End”, is being advected northwestwards towards the Tiwi Islands by the prevailing easterly to southeasterly winds. This dry airmass subsequently is moving southwestwards to Darwin with the sea-breeze. When it passes in the late evening, it is finally replaced by moist maritime air. At this moment, a jump in the wind direction and an increasing wind speed is mostly observed.

But, although it looks like a sea-breeze, it's certainly not the same thermally driven phenomenon.

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!