Australia moves to El Niño alert and the drought is likely to continue


Skie Tobin, Australian Bureau of Meteorology; Catherine Ganter, Australian Bureau of Meteorology, and Robyn Duell, Australian Bureau of Meteorology

The chances of an El Niño developing late in 2018 have increased and this week the Bureau moved to El Niño ALERT. This means that model outlooks and observations indicate there is approximately a 70% chance that El Niño will develop in the coming months. Current patterns in the Pacific are similar to the early stages of past El Niño, with warm water shifting east towards South America.

We’re also seeing indications a positive Indian Ocean Dipole (IOD) has likely started, in which warmer waters near Africa drag moisture away from Australia. El Niño and positive IOD events typically mean below-average spring rainfall in central and southern Australia, and a drier start to the wet season in Queensland and the Northern Territory.




Read more:
Dipole: the ‘Indian Niño’ that has brought devastating drought to East Africa


The development of either would favour continued dry weather, and increase the likelihood that widespread drought relief will be delayed until 2019. Higher than average temperatures, heatwaves, and more severe bushfire weather are also more likely during El Niño and positive IOD events.

A dry year so far

September 2018 was a very dry month, adding to low rainfall seen across many parts of Australia so far this year. September 2018 was not only the driest September in 119 years of record for Australia, but it was also the second-driest for any month of the year (behind only April 1902).

Rainfall for the year to date has been exceptionally low over the mainland southeast, with much of the region experiencing totals in the lowest 10% of records for January–September. Many locations in eastern New South Wales, eastern Victoria, and southeast Queensland have received about 400 mm less rainfall than they usually would have by this time of the year.

Rainfall deciles for January to September 2018.
Bureau of Meterology

Much of southern Australia has experienced a persistent rainfall decline spanning several decades, which is adding to drought stress by drying the landscape.

Southwest Western Australia has experienced significantly lower cool season (April to October) rainfall since the mid-1970s, compared to observations since 1900, while for the southeast the drop has been more recent, emerging in the mid-1990s. These rainfall declines have been linked to circulation changes in the southern hemisphere influenced by the increase in greenhouse gases.

These rainfall changes have also been accompanied by much larger reductions in streamflow, particularly in the southwest of Australia where high flows have become much less frequent.

April to October rainfall anomalies (mm) for southwestern (left) and southeastern (right) Australia, showing the decline in totals with respect to the 1961 to 1990 average. The main feature of the decline is significantly fewer wet years, meaning recovery from the dry years is patchy.
Bureau of Meteorology

And it’s also been unusually warm

Low rainfall has also been accompanied by very high daytime temperatures so far this year. Of course, Australian temperatures are warming in line with global trends, but in individual years variations which are likely to be largely natural (such as droughts) may add to or subtract from the broader trends.




Read more:
Is Australia’s current drought caused by climate change? It’s complicated


Historically, droughts have often brought hot conditions, and this has been borne out in 2018. Maximum temperatures for January to September were the warmest on record for the Murray–Darling Basin and New South Wales, with neighbouring regions also much warmer than average.

These extremely warm days, combined with extremely low rainfall, have caused an intense drying of the Australian landscape in 2018, resulting in an early start to the bushfire season in New South Wales and Victoria, where damaging fire were observed as early as late winter.

So how might the year end?

Like all Australians, the Bureau hopes farmers and those suffering through drought get the rainfall they need, but unfortunately, the outlook indicates dry conditions are likely to continue for some time.

Large parts of southern and eastern Australia are likely to see a drier than average end to the year, though odds favouring drier than average conditions tend to moderate as we head towards summer. Most of the country is likely to see a dry October, though local heavy falls can occur against a backdrop of broadly suppressed rainfall.

Chance of exceeding median rainfall between October to December 2018.
Bureau of Meteorology

While some parts of New South Wales and southeastern Queensland have received very welcome rainfall in the first days of October, rainfall has been below average over much of over eastern Australia for so long (since early 2017) that this rainfall event hasn’t been enough to break the drought.




Read more:
Recent Australian droughts may be the worst in 800 years


Looking at temperature, outlooks show a very high chance of warmer than average days and nights through to the end of 2018. Considering the year so far has already been very warm, this means 2018 has the potential to rank as another significant warm year. Seven of Australia’s ten warmest years have occurred since 2005, with just one cooler than average year in the last decade (2011), highlighting how warmer than average temperatures now dominate Australia’s climate.The Conversation

Change of exceeding median maximum temperature between October to December 2018.
Bureau of Meteorology

Skie Tobin, Climatologist, Australian Bureau of Meteorology; Catherine Ganter, Senior Climatologist, Australian Bureau of Meteorology, and Robyn Duell, Senior Climatologist, Australian Bureau of Meteorology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Odds keep rising for a big El Niño in 2015


Jaci Brown, CSIRO; Andrew B. Watkins, Australian Bureau of Meteorology, and Madeleine Cahill, CSIRO

El Niño has arrived, it’s getting stronger, and it’s not about to go away soon. And already there are rumblings that this could be a big one. El Niño in Australia means warmer temperatures, and sometimes, but not always, drier conditions.

In 2014, some climatologists thought a big El Niño might have been on the cards. Ultimately, after some vigorous early warming in the Pacific, conditions only touched on El Niño thresholds. This year, with an event already established, climatologists are suggesting the odds are rising of an El Niño rivalling the record events of 1982 and 1997.

So what’s all the fuss about, and how are conditions different from last year?

Tropical Pacific Ocean temperatures are still rising

El Niño events are identified by equatorial Pacific Ocean temperatures. At the ocean surface, an El Niño is when these are sustained at about 0.8°C warmer than average. As we speak, temperature anomalies are exceeding twice that value.

In fact, we have just experienced twelve consecutive weeks with temperatures more than 1°C above average in all five of the key El Niño monitoring areas. The record was previously held by the 1997 El Niño, when this widespread warming lasted eight consecutive weeks.

But no two El Nino’s are exactly the same. Despite this warming in the tropical Pacific Ocean, in the Indian Ocean temperatures are far warmer than they were in 1997 (or 1982), which may mean different impacts for Australia. But more about that later.

Sea surface temperature differences from normal, June 1997

Sea surface temperature differences from normal June 2015

Sea levels are dropping north of Australia

When sea surface temperatures in the central equatorial Pacific get warm enough, the atmospheric circulation shifts and the usually strong trade winds reduce, sometimes even reversing.

The direct consequence of the changing wind pattern is that the sea level in the western equatorial Pacific is no longer “piled up” by the trade winds. Low sea levels north of New Guinea (shown boxed) are strongly correlated with Nino3.4, which is the index that relates best to Australian climate.

Sea level difference in height from the 1993-2000 normal for July 2015. Exceptionally low sea level to the north of Australia as expected with El Niño events.

Changes to sea level anomalies (relative to the 1993-2000 average) in the western equatorial Pacific (pink box in the previous image). This year is shaping up to be a lot like the El Niño of 1997. Also note the clear fall in western Pacific sea level in the El Niño events of 1994/95, 2002/03, 2006/07, and 2009/10.

At the peak (December) of the 1997 El Niño, the sea level in the western Pacific dropped nearly 30 cm. It is only August and already the sea level is nearly 25 cm below normal to the north of Australia.

Likewise, in the eastern Pacific, sea levels have risen by similar amounts as the weakened trades allow water to shift east. This half-metre difference in the normal sea level between the east and west is a classic strong El Niño signature.

A drop in sea level often means less water flows past Indonesia and down Australia’s west coast — weakening the Leeuwin Current and reducing the likelihood of coral bleaching in Western Australia.

Computer models are predicting a strong event

Climate forecast centres around the world are keenly monitoring the development of this year’s El Niño. Why? Because for some time, all the top dynamical (i.e., physics based) climate models have agreed that there is more warming of the tropical Pacific Ocean to come.

The current (late July) average forecast is for continued warming peaking at a Niño 3.4 value of +2.7°C by December. Such a value would put 2015 alongside the big El Niño events in 1982 (+2.8°C) and 1997 (+2.7°C).

The atmosphere is kicking the El Niño along

Reinforcement by the atmosphere is an essential part of El Niño development – as you can see in our Understanding ENSO video.

Last year the ocean began generating an El Niño but the atmosphere wouldn’t come to the party. This year the atmosphere is clearly responding.

Two exceptionally large westerly wind events have already occurred in the western equatorial Pacific this year, giving this El Niño a significant boost. Another wind event is forecast for August to kick the system along even further and add to the strength of this El Niño.

What does this mean for Australia?

Of the 26 El Niño events since 1900, 17 have brought widespread drought to Australia. In the big El Niño of 1982, drought devastated the eastern half of Australia and drove the devastating Ash Wednesday bushfires.

In contrast, the even stronger El Niño of 1997–98 brought more localised drought, with key rains in May and September meaning winter crops did reasonably well in most areas. Other years, such as 2002 and 1996, when weaker El Niño’s occurred, the drought was more severe.

For Australia, it’s not the size of El Niño that matters, it’s how it interacts with other rainfall drivers – such as sea surface temperatures around the continent and in the Indian and Southern Oceans, as well as random ‘weather noise’ – that governs the eventual rainfall over the continent.

Rainfall deciles for the strong El Niño of 1982
bom.gov.au

Rainfall deciles for the strong El Niño of 1997
bom.gov.au

Rainfall deciles for the weak to moderate El Niño of 2002
bom.gov.au

What can we expect?

A significant El Niño event is currently underway, and there’s a chance it could rival the big events of 1982 and 1997. While this may increase the chance of drought and higher temperatures in eastern Australia, many other factors influence potential impacts.

Typical El Niño impacts for Australia
bom.gov.au

We are already seeing that in the August–October Bureau of Meteorology seasonal outlooks, with the warmest June ocean temperatures on record in the southern Indian Ocean keeping the strengthening El Niño at bay by putting more moisture into the mid-levels of the atmosphere and changing weather patterns.

So what’s the final 2015 El Niño prediction?

The 2015 El Niño is already significant, and a big El Niño certainly remains a possibility. Widespread strong impacts haven’t (yet) raised their head for Australia and indeed, such as in 1997, may never do.

But managing El Niño is all about managing risk. The southern spring is the time when dry weather, frosts and heatwaves can hurt farmers and many others the most. And that’s when El Niño events, which raise the odds of these impacts, like to bite hardest.


The authors will be one hand for an Author Q&A between 12:30pm and 1:30 pm on Tuesday, August 11. Post your questions in the comments section below.

The Conversation

Jaci Brown is Senior Research Scientist at CSIRO.
Andrew B. Watkins is Manager of Climate Prediction Services at Australian Bureau of Meteorology.
Madeleine Cahill is Oceanographer at CSIRO.

This article was originally published on The Conversation. Read the original article.