What is an El Nino?

WHAT IS AN EL NINO?

* A warming of sea surface temperatures in the eastern and central Pacific Ocean that disrupts weather patterns across the Pacific.

* Can cause a corresponding cooling of the ocean in the western Pacific and around northern Australia

* Cooler seas near Australia, Papua New Guinea and Indonesia reduces convection - moist air at the surface that is heated, rises, cools and condenses into rain-bearing clouds.

* Convection migrates towards the eastern Pacific and can deliver increased rain to the west coast of North and South America

* El Nino can disrupt the trade winds that, in the southern tropical Pacific, blow moisture-laden air towards eastern Australian coast

WHAT ARE THE EFFECTS OF AN EL NINO IN AUSTRALIA?

LOWER RAINFALL

* El Nino increases chances of below-average rainfall through winter and spring in much of Australia, especially the north and east

* El Nino does not always mean drought but nine out of the 10 driest winter/spring periods happened in El Nino years

* Australia's severest droughts - 1982/83, 1994, 2002 and 2006 were all associated with El Nino

* Since 1900, El Ninos delivered winter/spring rainfall 28 per cent lower than the long-term average

TEMPERATURE EXTREMES

* Warmer-than-average weather, particularly in southern Australia and particularly in the second half of the year. Decreased cloud cover increases surface heating and assists to keep rainfall low

* Background warming of the atmosphere has made El Nino years warmer since the 1950s, the Bureau of Meteorology says

* In warmer months, El Nino can cause fewer slow-moving "blocking" high pressure systems, worsening heat extremes for cities such as Adelaide and Melbourne with an increase in extreme hot days and heatwaves further north

* Frost increases in El Nino years in cooler months in eastern Australia because clearer skies contain less daytime heat and lead to reduced minimum temperatures

* Between 15 and 30 per cent more frost days in El Nino years than average in northern Victoria and southern NSW, affecting agriculture

* Australia's record low temperature of -23C at Charlotte Pass, NSW, on June 29, 1994, occurred during a strong El Nino

INCREASED BUSHFIRE RISK

* El Nino droughts dry the bush and have led to disasters including the 1983 Ash wednesday bushfires that killed 71 people in Victoria and South Australia.

DISRUPTED TROPICAL WEATHER

* Fewer tropical cyclones, especially for Queensland

* Later onset of northern monsoon rains

* Below-average wet season rains early in the season, with average rain later in the season

REDUCED WINTER SNOWFALL

* The four lowest recorded peak snow depths in Australia's alpine country occurred in El Nino years, including severe drought years in 1982 and 2006