A monsoon refers to a seasonal reversal of wind direction. In Northern Australia, south easterly trade winds characterise the Dry Season, which lasts April through November, and the wet or monsoon season is characterised by a north westerly monsoonal flow. The south east winds are moderately cool and very dry because they originate from the centre of the desert. They push the monsoon system north into South East Asia and the Indian subcontinent. Heat from the western Pacific proceeds into the Indian Ocean, which reduces sea surface temperatures in the north of Australia, thereby reducing evaporation of water and making the air drier, which reduces rainfall just south of the equator for a few months.
From late September / early October, above the Himalayas and Nepalese plateau, high air pressure forms and remains until the following March. Cold dry air descends blowing out to the south. There is a relationship between amounts of snow in the Himalayas and amounts of rain reaching NW Australia. By late October the flow from the Pacific to the Indian Ocean then to the north west of Australia, slows, stops, and may even reverse, as the Indian Ocean counter current flows west to east into the Timor gap. This leads to a large raft of relatively still water to the immediate north west of Australia. Under the tropical sun the body of water begins to heat. As sea surface temperature rise, evaporation increases and moist air from this region will eventually fall and develop into the Australian monsoon.
With the change of seasons, as the sun’s path swings to the south from late October to March, the Pilbara to Alice Springs in the red centre heats up and the vast area becomes extremely hot. It takes several months for this heat over the landmass to reach its peak heat in December. This level is then sustained until late January or early February. The very hot air close to the land surface rises, leaving a region of lowered air pressure (as the air that was there has risen), and a heat low is formed. Heat lows only occur over large landmasses. Air flows in to fill this area of lower pressure and helps to pull the monsoon south over the Australian continent.
The ensuing wet season lasts for about six months, between November and March. It is hotter than the dry season, with temperatures between 30C and 50C. This heat is added to by the high humidity caused by large amounts of water in the air. There is often a lot of rain, which frequently causes flooding. During the wet season there are usually two or three major monsoon events, from when the monsoon trough moves south over the land mass of north Western Australia.
The trough may remain over the land for periods of one day to several weeks. It usually overlies the continent for about seven days, during which time strong north westerly winds blow and rain falls almost constantly. The later phases in February and March are often longer and more intense. According to aboriginal custom, in the Northern Territory in southeast Arnhemland, November and December always brought first rains, and then January to March constituted the brunt of the wet season.
Predicting monsoons is possible by examining the Southern Oscillation El Nino cycle. Increasing and decreasing solar wind pressure expands and contracts the atmosphere like a bellows. This creates a push-pull effect that causes weaker tropical winds to stall or reverse and the Jet Stream to switch from low to high amplitude. Acting in accord, the moon swings closer and further away from earth on average every 4 years.
The position of the moon can indicate how severe a season will be. In years when the moon is closer to earth the monsoons will often be more intense. The moon is typically averagely closer to earth during La Nina years. Strong La Ninas can signal wetter seasons. During El Nino the moon is often further away, causing a weakening of currents and a slowing down of air flows. It is typically when monsoonal activity and cyclones may reduce.
In 2011 the moon was averagely closer to earth. It was the year that since 1900 it was averagely the 4th closest to earth. We know how destructive 2011 was, with the Japanese tsunami, Cyclone Yasi and Christchurch earthquakes. By 2013 the moon was further away but 2015 sees the moon drift slightly back closer. 2013 and 2014 have been neutral years, when monsoons have been late and cyclones fairly weak.
2015 is expected to be a weak El Nino year, and the moon will be the 26th closest since 1900. Tropical Cyclones should number about seven in 2015 (with 1-2 over WA) but mostly they will quickly lose their strength and dissipate to the east. The monsoon season should avoid December and extend from January to March, with most activity over January and February. It should be completely finished by mid April.
The first TC develops in the Solomons in the first week of January but drifts to the southeast just as a second TC develops in the western Gulf. Then in mid Jan another starts in the northeast Coral Sea which affects central and southern QLD coasts before moving southeast to New Zealand. In the last week of February a fourth TC forms east-northeast of Cairns, followed at the end of February by a fifth TC forming in the north Coral Sea and again heading south-southeast towards New Zealand. In the second week of March, the sixth TC develops in the east-northeast region north of Townsville. Then, from the fourth week of March to first week of April, the last TC of the season develops in the Coral Sea and moves towards the QLD east coast.
Ken Ring of www.predictweather.com is author of the Weather Almanac for NZ for 2015