



What the hell happened to autumn, and what do long dry spells bode?

Sunday Explainer

■ Unseasonably warm weather and hardly any rain - are we likely to get a warmer winter after this positively balmy autumn?

There is a degree of inertia in the weather, mostly to do with soil moisture. Dry soils can result in warmer temperatures through less evaporative cooling.

Even the return of normal rainfall can take time to have an impact on temperatures as soils soak up the moisture. Ocean temperatures, though, have a greater influence on Australia's seasonal weather conditions than the antecedent conditions over the continent itself.

Weather models generate their forecasts mostly from ocean temperatures. Last week's seasonal outlook from the weather bureau indicated all of Victoria had at least a 75 per cent chance of a warmer than usual winter, while the odds are about 60 per cent for it to be drier than normal.

■ How can we be sure current conditions are predictors of what's to come rather than random?

The Bureau of Meteorology invests millions of dollars in supercomputers, and will soon tender for a more advanced system. Ocean conditions are the basis of seasonal outlooks. The models identify persistent trends, but also changes that will alter forecasts.

Longer term, the trends from climate change are clear. The World Meteorological Organisation last week noted that the warming effect from greenhouse gases has increased by 34 per cent since 1990. Warmer than average temperatures are now much more likely than abnormally cool ones.

■ What does this mean for the snow season?

Weatherzone says we shouldn't expect a ''proper'' cold front for about three weeks as strong high-pressure systems push cool conditions further south than usual. Warm conditions also don't help snow-making, which works best at about minus 2 degrees.

A late start to a snow season isn't that uncommon, and it only needs a couple of decent dumps - ideally just before and during the winter school holidays to help bring a flood of snow bunnies to the mountains.

Still, alpine regions are on a long-term drying and warming trend, which is not only bad news for ski resorts, it may also reduce snow-melt waters for hydropower and irrigation. Threatened alpine species will also be at even greater risk.

■ And what about the spring and summer beyond - and the bushfire season?

Soil moisture levels may have the biggest impact on the bushfire season. Fuel loads are obviously more combustible after a dry spell.

Risks vary though. Rain followed by dry conditions favours grass fires. Persistently dry conditions and drought favour forest fires.

Victoria has lately had some decent rainfall, so it is really a case of looking at the soil moisture closer to the start of the fire season. For NSW, that means looking towards the end of winter, and for Victoria, a little later in the year.

■ Are these unseasonal conditions - more akin to spring than autumn - possible evidence of a shift in the timing of the seasons?

Australia has warmed in all seasons and in all states and territories. Averaged across the country, it is about 0.9 degrees warmer than it was in 1910. Since the frequency of warm weather has increased and cool spells are less so, extended warm sequences are more common.

Species previously found in warmer climates to the north are turning up much further south.

One season that is changing is the fire season, which is extending further into spring and autumn.

■ Why can't meteorologists tell for sure if we are entering a La Nina (wet) or El Nino (dry) cycle?

Each year around autumn, the key El Nino Southern Oscillation (ENSO) goes into reset mode, making it harder to tell whether conditions will tilt towards El Nino or La Nina, particularly if the previous setting was neutral. At present, the bureau is rating the chance of an El Nino at more than 70 per cent. Climate experts say it's more of a gradient than a switch being flipped, so if we are trending towards an El Nino - as we are now - Australia will likely start to see those types of conditions.

■ Could another El Nino mean we're heading back into prolonged drought and, if so, what warning signs should we look for?

El Nino is very often associated with drought in Australia. Meteorological centres monitor central Pacific sea-surface temperatures and atmospheric pressure across the ocean.

The Bureau of Meteorology has an ENSO Tracker system, which is currently producing an ''alert'' reading.

■ What would it take for a La Nina to last years, such that Australia could enjoy a sustained period of lush green environments, especially in the southern part of the continent?

La Nina events have sometimes come in sequence in Australia, such as from 2010 to 2012 and during the early 1970s. The nature of the recharge phase (La Nina) and the discharge one (El Nino) makes multiple La Ninas more likely than El Ninos.

This is simply an expression of the ENSO cycle. Adding to the complexity is the tendency for a decadal ''signal'' in the ENSO variability, with some decades having more La Nina events and some more El Ninos. This variability is sometimes called the Pacific decadal oscillation, or the interdecadal Pacific oscillation.

Scientists are currently unable to predict the future phase of the interdecadal oscillation.

With assistance from the Bureau of Meteorology.

Featured advertisers