Cyclones and El Nino

(pic: cyclone in Townsville, 1903)

When it is a dry season in Western Australia it can be a wet one in Queensland and vice versa. The Southern Oscillation is a see-sawing of atmospheric pressures between the South Pacific and Indian oceans. When the atmospheric pressure is abnormally high over the Indian Ocean it is usually abnormally low over the South Pacific. Pressure systems manifest as opposing trends on opposite sides of the continent.

Active monsoon seasons and the increase in frequency of tropical cyclones occur together. As they diminish they are followed by a period of droughts in the east of Australia. The process is governed by the moon and its influence on undersea currents which are forced to alternate on a 4.5 year cycle on average but may vary between 2-7 years. It is climate change caused by moon, not man.

Undersea currents in the Pacific normally go east to west from Peru to Australia. The currents transfer to winds above the sea which used to be called the Trade Winds. As a result of the westward sea flow, after about 2-3 years the sea level gets higher near Australia by nearly a metre and a point is reached when it must flow the other way to restabilise, just as water pushed to one end of a bathtub will return to establish equilibrium. It is then that the currents slow and with them diminish the strength of the above surface wind systems. This return phase is shorter, between 1-1.5 years.

When the westerly trade winds weaken, a mass of warm water forms off north-eastern Australia and is pushed eastward to South America. This mound of water can be 20 cm high and cover an area larger than Australia. The warmth of this water compared with surrounding water means it can be detected by radar from satellites. The phenomenon is known as the El Nino effect which can bring heavy rain to South America. Because winds are weakened at this time they can cause a delay in the arrival of the north-western monsoon and a late start to the wet season in Australia.

The Southern Oscillation Index is traditionally measured by the difference in sea level atmospheric pressure between Tahiti and Darwin. When the index is positive it is the signal that the trade winds are still blowing strongly westward across the Pacific towards Australia. They feed moisture into and converge with the monsoons of Asia and Australia. This is the La Nina or Trade Wind side of the cycle and is the predominating situation. When the index is negative, the trade winds are weak and reverse, depriving the monsoons of their energy source. The winds blow the other way because undersea currents are reversed.

This becomes the El Nino, and is a time of lessened cyclones and monsoonal activity.  It is why they call El Nino "the brake on cyclones". Rainfall is below average and tropical cyclones are fewer, and later, widespread drought occurs over eastern and northern Australia. These dry conditions usually commence early in the year and last for about 12 months. The droughts of 1990-91, that affected the whole Pacific region, and 1969-70 and 1979-83 that affected eastern Australia were associated with El Nino systems.

Cyclones develop over the sea within about 6° of the equator from November to April, when abundant moisture is available from the evaporation of warm water. It is only near the equator that the surface of the water can get hot enough (26-28C)  from the heat of the sun to enable widespread evaporation that is needed for a cyclone to become established. Cyclones very rarely cross the equator, and in this hemisphere the cyclones that affect Queensland develop to the east in the Coral Sea, SolomonSea and South Pacific Ocean and to the north (Gulf of Carpentaria and Arafura Sea).

From there they can vary in direction. Many approach the eastern coast of northern Queensland. They curve back near the coast and pass southward, frequently as far as Brisbane, and then move off the coast to the east and the south-east and into the Pacific. Others curve back some distance east of the coast, near the Great Barrier Reef, and pass south and then east without causing much damage to Australia but may resurge over the PacificIslands. These will occur in the coming season, together with cyclones that form west of northern WA and travel south.

Still others, especially those that strike the northern part of Cape York, pass inland, crossing into Queensland’s interior before passing back out to sea. Some cyclones cross across the top towards WA, while others cross the land from the west to Queensland and regenerate when they reach the Pacific. While most lose force shortly after crossing land and become rain depressions, some strengthen if they reach the coast again.

To begin with, cyclones were named after annoying politicians. Naming was formalised by international agreement in the 1964-65 cyclone season when female names were selected, as it was considered (by mainly male meteorologists) that females, like weather systems are unpredictable and temperamental. Feminists protested and from 1975 the prepared list was extended to include male names. Male and female names are now used alternately. Cyclones are named by the region in which they form.

At the moment we are in neutral SOI phase, and an active monsoon season with cyclonic activity is anticipated over the coming 2014-15 summer months on both sides of northern Australia. The season will start in WA in mid December 2014 and end when monsoonal activity dissipates around the end of March 2015. Most cyclones will stay in the north of Australia or east of Queensland, and will not venture far into the interior, but cyclonic remnants will go on to affect NZ. EL Nino will be a feature of 2015 from autumn onwards for Australia, and may continue into 2016.