Australia is expected to suffer a moderate-to-strong El Nino in the second half of 2015, potentially exacerbating prolonged drought in the eastern states.

The Bureau of Meteorology on Tuesday officially declared the existence of an El Nino, the precursor to many of Australia’s worst droughts.

El Nino is characterised by a warming of sea surface temperatures in the central and eastern Pacific Ocean and can cause below average rainfall in Australia in winter and spring, particularly in the east.

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David Jones, of the bureau’s climate information services branch, says El Nino does not always mean drought in Australia but since 1900 it has been a strong indicator of drier times ahead.

“We know that 17 of the last 26 El Ninos have seen reasonably extensive drought conditions over Australia,” he said.

“So while below-average rainfall is not guaranteed, there is a strong weighting and assistance towards below-average rainfall and increased risk of drought, which obviously isn’t good for those people already in drought.”

Vast areas of inland Queensland and NSW, western Victoria and southern South Australia are already affected by drought, with rainfall below 10 per cent of average for more than two years in some areas.

Worse, Dr Jones said global forecast models suggest this El Nino, the first observed since March 2010, is expected to peak in spring and early summer and to have a significant impact.

“Certainly the models aren’t predicting a weak event. They are predicting a moderate-to-strong El Nino event. So this is a proper El Nino event, this is not a weak one or a near miss as we saw last year,” he said.

Climatologists have debated the possible existence of an El Nino in the Pacific for the past eight months.

Sea surface temperatures have been at or near El Nino levels since spring last year, save for a brief dip after Christmas.

Just four weeks ago, the bureau raised to 70 per cent or more the chances an El Nino would form by spring 2015.

“We had a near miss in 2014-2015. Last year we saw some indices, such as the sea surface temperatures at times exceed El Nino thresholds … but we didn’t see them all coming together at the same time or we didn’t see it sustained,” Dr Jones said.

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

-AAP