Australian weather conditions explained
Weather underlies how we live and Australia farmers can be heavily impacted by its uncertainty.
Recent press headlines refer to a return to El Nino conditions in eastern Australia which is an expectation of below average rainfall. Not good if you have a winter crop that requires finishing rain, spring lambs or calves coming on, or have made farm plans based on receiving ‘average’ rainfall.
Weather forecasting and terminology can seem a dark art and herein we will seek to demystify the language and how different weather influences impact the Australian climate.
Weather patterns are driven by the Earth’s atmosphere which determines how wind and air moves around the world. Since surface air pressure is a measure of the weight of the atmosphere above any location, when weather reports talk of a developing low pressure system it means a region where there is somewhat less atmosphere (or air) overlying it. This low pressure anomaly allows clouds and rain to be produced and in extreme cases gives rise to tropical storms and cyclones. On the other hand, high pressure systems result in cooler, denser, drier air, which we experience as clear skies and calm weather.
This map from the Bureau of Meteorology provides an overview of typical weather conditions that impact Australia.
Moving around the map anticlockwise from the east coast we will look at each weather event and the weather conditions they tend to bring:
‘The east coast lows’ – Intense low pressure and hard hitting weather systems in autumn and winter in eastern Australia accompanied by heavy rainfall, high winds and wild seas.
‘The easterly trough’ – A blessing that brings rainfall to inland eastern Australia generally in summer. Formed by the topography of the Great Dividing Range and the intense inland heat of the day which draws the trough towards the coast causing showers and thunderstorms.
‘ENSO or El Nino Southern Oscillation’ – The movement between above average rainfall conditions in eastern Australia of La Nina to the lower than average rainfall conditions of El Nino. These conditions are measured by Southern Oscillation Index (‘SOI’) being monthly fluctuations in the air pressure difference between Tahiti and Darwin. Sustained negative SOI often indicates El Nino episodes with warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of Pacific trade winds and a reduction in rainfall over eastern and northern Australia. Positive SOI is associated with stronger Pacific trade winds, warmer sea temperatures to the north of Australia, increased cloudiness over northern Australia, Papua New Guinea and Indonesia and a higher probability of increased rainfall in eastern and northern Australia.
‘Trade winds’ – The east to south-easterly winds blowing across the southern hemisphere tropics. In the formative years of this great country it was a way to get back to Europe in a hurry. They bring rainfall in the summer months to the tropical areas of Australia’s east coast with an average rainfall per annum at its peak of 7,708 mm between Cairns and Innisfail Queensland. They are stronger in the winter months and cause drier conditions.
‘The Madden Julian Oscillation’ – This involves the period either side of the New Year with greater rainfall in northern Australia that influences the monsoonal conditions. This generally develops in association with tropical cyclones in 30 to 60 day cycles.
‘Upper level trough’ – A low pressure trough formed in the upper atmosphere bringing greater rainfall anywhere in Australia at any time of the year.
‘Tropical depressions’ – Mid strength low pressure systems that bring heavy rainfall to northern Australia between October and April and include thunderstorms, strong winds, heavy rain and flooding.
‘The Australian monsoon’ – This is the seasonal reversal of winds over northern Australia with the prevailing breeze being east/south east and with the heating up of the Australian continent this shifts to north west winds. A low pressure system forms drawing in the monsoon trough of moist air from the surrounding oceans. Monsoon is derived from the Arabic word ‘mausaum’ meaning season. The reversal of winds usually occurs in December around Darwin and a build-up of clouds can be seen.
‘Tropical cyclones’ – These are low pressure systems lasting between three and seven days formed over the warm tropical waters off northern Australia. They produce high rainfall, dangerous seas and gale force winds of 63km/h or greater, gusting to 90 km/h, which can extend hundreds of kilometres from the storm centre. Sustained winds of 118 km/h, gusting above 165 km/h, are categorised as severe tropical cyclones. The Australian cyclone season runs from November to April and in El Nino years they are less frequent. When they hit land they are accompanied by a destructive storm surge 60 to 80 kms wide that may be two to five metres above the ocean’s tide height.
‘Indian ocean dipole’ – The sea surface temperatures of the Indian Ocean appear to have an impact on our rainfall. Warmer temperatures coincide with higher rainfall whilst cooler sea temperatures coincide with lower rainfall.
‘West coast trough’ – During the warmer months this trough affects temperature, winds and thunderstorm development on the west coast of Australia. It is a zone of low pressure on the boundary of cooler Indian Ocean winds and warmer easterly winds off the Australian land mass. East of the trough can produce high temperatures and then thunderstorms whilst west of the trough will enjoy milder conditions and sea breezes.
‘Northwest cloud bands’ – These bring widespread and often heavy rain to north-western, central and south eastern Australia and are formed with warm, moist tropical air from the Indian Ocean between March and October
‘The sub-tropical ridge’ – Brings dry conditions to large parts of Australia and runs across a belt of high pressure that encircles the globe. Its position has a large impact on our weather; in the summer months it suppresses cold changes in southern Australia and moves north in winter allowing colder south-westerly winds and showers to occurs across southern parts of Australia.
‘Frontal systems’ – Bring rain to southern Australia and with higher intensity leading to higher rainfall. A cold front is formed when warm air is forced upwards and has the greatest impact during winter. A warm front is the opposite.
‘Southern annular mode’ – Also known as the Antarctic oscillation and can result in greater rainfall in southern Australia. This involves the north south movement of westerly winds. A southerly movement leaves high pressure and low rainfall whilst a northerly movement results in more storm systems and low pressure over southern Australia.
‘Cut off lows’ – Low pressure systems that have broken away from the main low pressure system and result in enhanced rainfall in southern Australia. Most common in autumn and winter with sustained heavy rainfall, strong winds and high seas.
‘Blocking highs’ – Strong high pressure and stationary systems that block the west to east movement of weather systems across southern Australia. On approaching these blocking highs the weather fronts slow down, weaken and slip to the south and can occur any time of the year. Blocking highs are associated with greater probabilities of fog and the dreaded frosts.
El Nino was touted as returning to trouble Australian farmers but this is now being forecast as a weaker spring event with warmer waters in the Indian Ocean (and a more favourable Indian Ocean Dipole) leading to a more favourable rainfall outlook.
For many years I hung on the messages of the weather forecasters in the hope of finishing rains for a winter crop but as a wise old mate told me once “it’ll rain when it rains!” In the absence of absolute certainty of future rainfall (timing and quantity) it is important to consider a range of rainfall scenarios when considering future production plans, working capital requirements and financial returns.
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