La Niña will give us a wet summer. That’s great weather for mozzies



Geoff Whalan/Flickr, CC BY-NC-ND

Cameron Webb, University of Sydney

The return of the La Niña weather pattern will see a wetter spring and summer in many parts of Australia.

We know mosquitoes need water to complete their life cycle. So does this mean Australia can expect a bumper mozzie season? How about a rise in mosquito-borne disease?

While we’ve seen more mosquitoes during past La Niña events, and we may well see more mosquitoes this year, this doesn’t necessarily mean we’ll see more related disease.

This depends on a range of other factors, including local wildlife, essential to the life cycle of disease-transmitting mosquitoes.

What is La Niña?

La Niña is a phase of the El Niño-Southern Oscillation, a pattern of ocean and atmospheric circulations over the Pacific Ocean.

While El Niño is generally associated with hot and dry conditions, La Niña is the opposite. La Niña brings slightly cooler but wetter conditions to many parts of Australia. During this phase, northern and eastern Australia are particularly likely to have a wetter spring and summer.

Australia’s most recent significant La Niña events were in 2010-11 and 2011-12.




Read more:
Explainer: El Niño and La Niña


Why is wet weather important for mosquitoes?

Mosquitoes lay their eggs on or around stagnant or still water. This could be water in ponds, backyard plant containers, clogged gutters, floodplains or wetlands. Mosquito larvae (or “wrigglers”) hatch and spend the next week or so in the water before emerging as adults and buzzing off to look for blood.

If the water dries up, they die. But the more rain we get, the more opportunities for mosquitoes to multiply.

Mosquito biting a person's hand
Mosquito populations often increase after wet weather.
Cameron Webb/Author provided

Mosquitoes are more than just a nuisance. When they bite, they can transmit viruses or bacteria into our blood to make us sick.

While Australia is free of major outbreaks of internationally significant diseases such as dengue or malaria, every year mosquitoes still cause debilitating diseases.

These include transmission of Ross River virus, Barmah Forest virus and the potentially fatal Murray Valley encephalitis virus.




Read more:
Explainer: what is Murray Valley encephalitis virus?


What happens when we get more rain?

We’ve know for a long time floods provide plenty of water to boost the abundance of mosquitoes. With more mosquitoes about, there is a higher risk of mosquito-borne disease.

The amount of rainfall each summer is also a key predictor for seasonal outbreaks of mosquito-borne disease, especially Ross River virus.




Read more:
Explainer: what is Ross River virus and how is it treated?


Inland regions of Queensland, New South Wales and Victoria, especially within the Murray Darling Basin, are particularly prone to “boom and bust” cycles of mosquitoes and mosquito-borne disease.

In these regions, the El Niño-Southern Oscillation is thought to play an important role in driving the risks of mosquito-borne disease.

The hot and dry conditions of El Niño aren’t typically ideal for mosquitoes.

But historically, major outbreaks of mosquito-borne disease have been associated with extensive inland flooding. This flooding is typically associated with prevailing La Niña conditions.

For instance, outbreaks of Murray Valley encephalitis in the 1950s and 1970s had significant impacts on human health and occurred at a time of moderate-to-strong La Niña events.




Read more:
Our new model shows Australia can expect 11 tropical cyclones this season


Over the past decade, when La Niña has brought above average rainfall and flooding, there have also been outbreaks of mosquito-borne disease.

These have included:

  • Victoria’s record breaking epidemic of Ross River virus in 2016-17 after extensive inland flooding

  • southeast Queensland’s outbreak of Ross River virus in 2014-15, partly attributed to an increase in mosquitoes associated with freshwater habitats after seasonal rainfall

  • eastern Australia’s major outbreaks of mosquito-borne disease associated with extensive flooding during two record breaking La Niñas between 2010 and 2012. These included Murray Valley encaphalitis and mosquito-borne illness in horses caused by the closely related West Nile virus (Kunjin strain).

We can’t say for certain there will be more disease

History and our understanding of mosquito biology means that with the prospect of more rain, we should expect more mosquitoes. But even when there are floods, predicting outbreaks of mosquito-borne disease isn’t always simple.

This is because of the role wildlife plays in the transmission cycles of Ross River virus and Murray Valley encephalitis virus.




Read more:
After the floods come the mosquitoes – but the disease risk is more difficult to predict


In these cases, mosquitoes don’t hatch out of the floodwaters carrying viruses, ready to bite humans. These mosquitoes first have to bite wildlife, which is where they pick up the virus. Then, they bite humans.

So how local animals, such as kangaroos, wallabies and water birds, respond to rainfall and flooding will play a role in determining the risk of mosquito-borne disease. In some cases, flooding of inland wetlands can see an explosion in local water bird populations.

How can we reduce the risks?

There isn’t much we can do to change the weather but we can take steps to reduce the impacts of mosquitoes.

Wearing insect repellent when outdoors will help reduce your chance of mosquito bites. But it’s also important to tip out, cover up, or throw away any water-holding containers in our backyard, at least once a week.

Local authorities in many parts of Australia also undertake surveillance of mosquitoes and mosquito-borne pathogens. This provides an early warning of the risk of mosquito-borne disease.




Read more:
The worst year for mosquitoes ever? Here’s how we find out


The Conversation


Cameron Webb, Clinical Associate Professor and Principal Hospital Scientist, University of Sydney

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

Meet El Niño’s cranky uncle that could send global warming into hyperdrive


Ben Henley, University of Melbourne; Andrew King, University of Melbourne; Chris Folland, Met Office Hadley Centre; David Karoly, University of Melbourne; Jaci Brown, CSIRO, and Mandy Freund, University of Melbourne

You’ve probably heard about El Niño, the climate system that brings dry and often hotter weather to Australia over summer.

You might also know that climate change is likely to intensify drought conditions, which is one of the reasons climate scientists keep talking about the desperate need to reduce greenhouse gas emissions, and the damaging consequences if we don’t.

El Niño is driven by changes in the Pacific Ocean, and shifts around with its opposite, La Niña, every 2-7 years, in a cycle known as the El Niño Southern Oscillation or ENSO.

But that’s only part of the story. There’s another important piece of nature’s puzzle in the Pacific Ocean that isn’t often discussed.

It’s called the Interdecadal Pacific Oscillation, or IPO, a name coined by a study which examined how Australia’s rainfall, temperature, river flow and crop yields changed over decades.

Since El Niño means “the boy” in Spanish, and La Niña “the girl”, we could call the warm phase of the IPO “El Tío” (the uncle) and the negative phase “La Tía” (the auntie).

These erratic relatives are hard to predict. El Tío and La Tía phases have been compared to a stumbling drunk. And honestly, can anyone predict what a drunk uncle will say at a family gathering?

What is El Tío?

Like ENSO, the IPO is related to the movement of warm water around the Pacific Ocean. Begrudgingly, it shifts its enormous backside around the great Pacific bathtub every 10-30 years, much longer than the 2-7 years of ENSO.

The IPO’s pattern is similar to ENSO, which has led climate scientists to think that the two are strongly linked. But the IPO operates on much longer timescales.

We don’t yet have conclusive knowledge of whether the IPO is a specific climate mechanism, and there is a strong school of thought which proposes that it is a combination of several different mechanisms in the ocean and the atmosphere.

Despite these mysteries, we know that the IPO had an influence on the global warming “hiatus” – the apparent slowdown in global temperature increases over the early 2000s.

Global temperatures are on the up, but the IPO affects the rate of warming.
Author provided, data from NOAA, adapted from England et al. (2014) Nat. Clim. Change

Temperamental relatives

When it comes to global temperatures we know that our greenhouse gas emissions since the industrial revolution are the primary driver of the strong warming of the planet. But how do El Tío and La Tía affect our weather and climate from year to year and decade to decade?

Superimposed on top of the familiar long-term rise in global temperatures are some natural bumps in the road. When you’re hiking up a massive mountain, there are a few dips and hills along the way.

Several recent studies have shown that the IPO phases, El Tío and La Tía, have a temporary warming and cooling influence on the planet.

Rainfall around the world is also affected by El Tío and La Tía, including impacts such as floods and drought in the United States, China, Australia and New Zealand.

In the negative phase of the IPO (La Tía) the surface temperatures of the Pacific Ocean are cooler than usual near the equator and warmer than usual away from the equator.

Since about the year 2000, some of the excess heat trapped by greenhouse gases has been getting buried in the deep Pacific Ocean, leading to a slowdown in global warming over about the last 15 years. It appears as though we have a kind auntie, La Tía perhaps, who has been cushioning the blow of global warming. For the time being, anyway.

The flip side of our kind auntie is our bad-tempered uncle, El Tío. He is partly responsible for periods of accelerated warming, like the period from the late 1970s to the late 1990s.

The IPO has been in its “kind auntie” phase for well over a decade now. But the IPO could be about to flip over to El Tío. If that happens, it is not good news for global temperatures – they will accelerate upwards.

Models getting better

One of the challenges to climate science is to understand how the next decade, and the next couple of decades, will unravel. The people who look after our water and our environment want to know things like how fast our planet will warm in the next 10 years, and whether we will have major droughts and floods.

To do this we can use computer models of Earth’s climate. In our recently published paper in Environmental Research Letters, we evaluated how well a large number of models from around the world simulate the IPO. We found that the models do surprisingly well on some points, but don’t quite simulate the same degree of slow movement (the stubborn behaviour) of El Tío and La Tía that we observe in the real world.

But some climate models are better at simulating El Tío and La Tía. This is useful because it points the way to better models that could be used to understand the next few decades of El Tío, La Tía and climate change.

However, more work needs to be done to predict the next shift in the IPO and climate change. This is the topic of a new set of experiments that are going to be part the next round of climate model comparisons.

With further model development and new observations of the deep ocean available since 2005, scientists will be able to more easily answer some of these important questions.

Whatever the case, cranky old El Tío is waiting just around the corner. His big stick is poised, ready to give us a massive hiding: a swift rise in global temperatures over the coming decades.

And like a big smack, that would be no laughing matter.

The Conversation

Ben Henley, Research Fellow in Climate and Water Resources, University of Melbourne; Andrew King, Climate Extremes Research Fellow, University of Melbourne; Chris Folland, Science Fellow, Met Office Hadley Centre; David Karoly, Professor of Atmospheric Science, University of Melbourne; Jaci Brown, Senior Research Scientist, CSIRO, and Mandy Freund, PhD student, University of Melbourne

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

2015-16 is shaping up to deliver a rollercoaster from strong El Niño to La Niña


Wenju Cai, CSIRO; Agus Santoso, UNSW Australia, and Guojian Wang, CSIRO

The anticipation is growing that this year’s newly formed El Niño will turn out to be very big. All climate models surveyed by the Australian Bureau of Meteorology are currently predicting a strong event later this year.

We’ve been here before – last year, in fact, when fears of a 2014 “super El Niño” proved anticlimactic. But it’s not over yet. The El Niño – Spanish for “the little boy”, which refers to a particular pattern of ocean and atmospheric temperatures across the Pacific – has resumed its growth this year and this time it is not showing any signs of slowing down.

It’s easy to see why this little boy gets so much attention. First, we are talking about a climate phenomenon that brings drought, rains, floods, heatwaves and other extreme weather events to many parts of the world.

Second, it is almost 20 years since the previous extreme El Niño. The 1997-98 event was the biggest in modern records and its worldwide catastrophic impacts earned it the infamous description of “the climate event of the 20th century”. A comparable but slightly weaker El Niño occurred in the summer of 1982-83, which was marked by severe drought in eastern Australia and the tragic Ash Wednesday bushfires.

Third, the latest climate model projections – reviewed by us in a study published today in Nature Climate Change – have shown that Earth will probably experience more super El Niños as the global climate warms. The projections also suggest that the extreme version of La Niña – the sister and “opposite” of El Niño – will also increase in frequency, as will the positive phase of the siblings’
“cousin”, a related phenomenon called the Indian Ocean Dipole. This also includes more successive occurrences of the trio.

A family gathering

This atypical “family gathering” has happened before. A positive Indian Ocean Dipole occurred in the southern spring of 1997, before the El Niño peaked the following summer. A La Niña then followed in the summer of 1998-99. For western Pacific rim countries, the overall result was drier-than-normal conditions in 1997, followed by unusually wet conditions in 1998. A similar series of events also occurred in 1982-83.

To understand this and to see how global warming spurs such events, we need to understand the physics of El Niño, taking the most recent unfolding events as a start.

The 2014-2016 chain of events would be interesting in its own right. While the failed 2014 super El Niño left many experts scrabbling for an explanation, its warming remnants in the central Pacific have now transformed into the official 2015 El Niño.

It is not common that two El Niño events would occur consecutively. The heat accumulated in the equatorial Pacific Ocean that fuels an El Niño is usually discharged, and some of this heat goes into the atmosphere, where it helps to accelerate warming in global surface temperature.

The discharge is proportional to the intensity of an El Niño. So the stronger the El Niño, the stronger the discharge. Also, the stronger the El Niño, the more dramatic the weakening of the Walker Circulation, with slackened trade winds and equatorial currents. This weakening Walker Circulation extends into the Indian Ocean which tends to induce a positive Indian Ocean Dipole during the southern spring.

Following the peak of an El Niño in the southern summer, the equatorial Pacific Ocean is depleted of heat and needs to be recharged. Winds associated with La Niña are effective in this recharge, and so an El Niño tends to be followed immediately by a La Niña.

Clearly the 2014 El Niño conditions were not strong enough for this to happen. Another similar exception occurred during 1986-1988. The 1986-87 El Niño was weak, the Pacific Ocean heat was not completely depleted, allowing for another somewhat stronger El Niño in 1987-88. These two events are considered weak to moderate.

The impact was mild and confined to northeastern Australia, in part because there were no concurrent positive Indian Ocean Dipole events which also act to channel El Niño’s impact to southern Australia.

However, after the two consecutive events, the equatorial Pacific was finally depleted of heat. The subsurface ocean was colder, facilitating surface cooling in the central Pacific through a suite of atmosphere-ocean positive feedback processes, leading to the extreme La Niña of 1988-89.

What is in store?

In terms of intensity and the growth rate up to July, the 2015 El Niño is second only to corresponding time of the 1997 event, and looks set to outpower the 1982 event. However, the eventual intensity of the 2015 El Niño is still hard to predict. What seems more certain is a La Niña in 2016.

For Australia, the extent and strength of the impact of the 2015 El Niño to a large extent depends on whether there is a concurrent positive Indian Ocean Dipole. In 2014, there was no positive Indian Ocean Dipole. To date, most models are predicting a positive dipole this year, raising the prospect of a strong El Niño preceded by a positive Indian Ocean Dipole and followed by a La Niña event – exactly as occurred in 1982-84 and 1997-99.

The pattern of Pacific Ocean temperatures during the last strong El Nino event in 1997.
NOAA

For Australia, the impacts of this sequence could be significant, as attested by the devastating Ash Wednesday bushfire in 1983 over southern Australia and the floods that hit the country’s northeast in early 1984.

This swing between opposite extremes from one year to the next could have globally damaging consequences too. On the far side of the Pacific, California may get a break from its a prolonged drought, although this hopefully won’t be in the form of intense storms and flooding.

Climate change bringing extremes

We cannot be sure if climate change plays a role in an individual event, and climate models are certainly not perfect. Observations need to be sustained to gather and compare robust statistics. But due to recent research we can now say that stronger El Ninos, La Ninas, and positive Indian Ocean Dipoles are all to be expected on a warming planet.

Climate models project an overall weakening of the Walker Circulation over the 21st century, underpinned by faster warming in the eastern equatorial Pacific (which is favourable for extreme El Niños, and in turn conducive to extreme La Niña). There will also be faster warming in the western than the eastern Indian Ocean, which would tend to promote positive Indian Ocean Dipole events. As a consequence, the sequence of an El Niño preceded by a positive Indian Ocean Dipole and followed by a La Niña event is projected to occur more frequently.

These sequences of events are likely to affect a vast swathe of the planet, extending from Africa, right through to South Asia and Australasia, and all the way across to the coastlines of the eastern Pacific.

The Conversation

Wenju Cai is Principal Research Scientist, Wealth from Oceans Flagship at CSIRO; Agus Santoso is Senior Research Associate at UNSW Australia, and Guojian Wang is Postdoctoral fellow at CSIRO

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