Indo-Pacific bottlenose dolphins (Tursiops aduncus) are a regular sight in the waters around Australia, including the Bunbury area in Western Australia where they attract tourists.
The dolphin population here, about 180km south of Perth, has been studied quite intensively since 2007 by the Murdoch University Cetacean Unit. We know the dolphins here have seasonal patterns of abundance, with highs in summer/autumn (the breeding season) and lows in winter/spring.
But in winter 2009, the dolphin population fell by more than half.
This decrease in numbers in WA could be linked to an El Niño event that originated far away in the Pacific Ocean, we suggest in a paper published today in Global Change Biology. The findings could have implications for future sudden drops in dolphin numbers here and elsewhere.
A Pacific event
The El Niño Southern Oscillation (ENSO) results from an interaction between the atmosphere and the tropical Pacific Ocean. ENSO periodically fluctuates between three phases: La Niña, Neutral and El Niño.
During our study from 2007 to 2013, there were three La Niña events. There was one El Niño event in 2009, with the initial phase in winter being the strongest across Australia.
Coupled with El Niño, there was a weakening of the Leeuwin Current, the dominant ocean current off WA. There was also a decrease in sea surface temperature and above average rainfall.
ENSO is known to affect the strength of the south-ward flowing Leeuwin Current.
During La Niña, easterly trade winds pile warm water on the western side of the Pacific Ocean. This westerly flow of warm water across the top of Australia through the Indonesian Throughflow results in a stronger Leeuwin Current.
During El Niño, trade winds weaken or reverse and the pool of warm water in the Pacific Ocean gathers on the eastern side of the Pacific Ocean. This results in a weaker Indonesian Throughflow across the top of Australia and a weakening in strength of the Leeuwin Current.
The strength and variability of the Leeuwin Current coupled with ENSO affects species biology and ecology in WA waters. This includes the distribution of fish species, the transport of rock lobster larvae, the seasonal migration of whale sharks and even seabird breeding success.
The question we asked then was whether ENSO could affect dolphin abundance?
What happened during the El Niño?
These El Niño associated conditions may have affected the distribution of dolphin prey, resulting in the movement of dolphins out of the study area in search of adequate prey elsewhere.
This is similar to what happens for seabirds in WA. During an El Niño event with a weakened Leeuwin Current, the distribution of prey changes around seabird’s breeding colonies resulting in a lower abundance of important prey species, such as salmon.
In southwestern Australia, the amount of rainfall is strongly connected to sea surface temperature. When the water temperature in the Indian Ocean decreases, the region receives higher rainfall during winter.
High levels of rainfall contribute to terrestrial runoff and alters freshwater inputs into rivers and estuaries. The changes in salinity influences the distribution and abundance of dolphin prey.
This is particularly the case for the river, estuary, inlet and bay around Bunbury. Rapid changes in salinity during the onset of El Niño may have affected the abundance and distribution of fish species.
In 2009, there was also a peak in strandings of dead bottlenose dolphins in WA (between 1981-2010), but the cause of this remains unknown.
Of these strandings, in southwest Australia, there was a peak in June that coincided with the onset of the 2009 El Niño.
Specifically, in the Swan River, Perth, there were several dolphin deaths, with some resident dolphins that developed fatal skin lesions that were enhanced by the low-salinity waters.
What does all this mean?
Our study is the first to describe the effects of climate variability on a coastal, resident dolphin population.
We suggest that the decline in dolphin abundance during the El Niño event was temporary. The dolphins may have moved out of the study area due to changes in prey availability and/or potentially unfavourable water quality conditions in certain areas (such as the river and estuary).
Read more: Explainer: El Niño and La Niña
Long-term, time-series datasets are required to detect these biological responses to anomalous climate conditions. But few long-term datasets with data collected year-round for cetaceans (whales, dolphins and porpoises) are available because of logistical difficulties and financial costs.
Continued long-term monitoring of dolphin populations is important as climate models provide evidence for the doubling in frequency of extreme El Niño events (from one event every 20 years to one event every ten years) due to global warming.
With a projected global increase in frequency and intensity of extreme weather events (such as floods, cyclones), coastal dolphins may not only have to contend with increasing coastal human-related activities (vessel disturbance, entanglement in fishing gear, and coastal development), but also have to adapt to large-scale climatic changes.
Kate Sprogis, Research associate, Murdoch University; Fredrik Christiansen, Postdoctoral Research Fellow, Murdoch University; Lars Bejder, Professor, Cetacean Research Unit, Murdoch University, Murdoch University, and Moritz Wandres, Oceanographer PhD Student, University of Western Australia
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.
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.
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.
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.
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
For Australia’s climate, 2016 was a year of two halves. The year started with one of the strongest El Niño events on record in place in the Pacific Ocean, and the opening months of 2016 were generally hot and dry, especially in northern and eastern Australia.
From May onwards there was a dramatic change in the pattern, with heavy rain and flooding a regular feature of the middle months of the year.
Overall temperatures were the fourth warmest on record in 2016, capping off Australia’s hottest decade. We track these events and more in the Bureau of Meteorology’s annual climate summary released today.
Dry to start
At the start of 2016, many parts of Australia were significantly affected by drought. Long-term drought had existed since 2012 through much of inland Queensland and adjacent northern areas of New South Wales, while shorter-term drought affected Tasmania, central and western Victoria, and parts of South Australia.
While some rain fell between January and April in these areas, it was generally not enough to have much impact on the rainfall deficiencies. Tasmania was hit especially hard, with low water storages restricting hydroelectric production, and long-lived and extensive bushfires in central and western parts of the state a feature of the summer period.
January to April, normally the wettest time of the year across Australia’s far north, was also much drier than normal with rainfall well below average in the Kimberley, the Northern Territory Top End, and on Cape York Peninsula.
It was the least active Australian tropical cyclone season since comprehensive satellite records began in 1970, with only three cyclones in the region, none of them severe, and only one of which made landfall.
The rains are here
Widespread heavy rains began in May – something well predicted by seasonal forecast models – as the El Niño ended and conditions in the Indian Ocean became very favourable for Australian rainfall, with unusually warm waters between Western Australia and Indonesia. Each month from May to September was wetter than average across most of the continent, with heavy rains extending into areas such as inland Queensland where the winter is normally the driest time of the year.
The wet conditions culminated in September, when nationally averaged rainfall was nearly three times the average. It was the wettest September on record for New South Wales and the Northern Territory, and in the top four wettest for every state except Western Australia and Tasmania.
May to September was the wettest on record over Australia, with some locations in inland New South Wales breaking previous records for the period by nearly 200 millimetres. Rainfall returned to more normal levels in eastern mainland Australia from October onwards, although Tasmania remained wet, and a tropical low brought widespread heavy rains extending from the Kimberley south through central Australia as far south as South Australia and Victoria in the year’s final days.
Despite flood damage in places and some rain-affected harvests, the wet conditions were generally positive for agriculture. They also led to large increases in water storage levels in many areas, especially in the Murray-Darling Basin and in Tasmania.
Flooding and storms were also a feature of this period. In early June, an East Coast Low affected almost the whole east coast from southern Queensland southwards.
Northern Tasmania saw some of its most severe flooding on record, and the Sydney region suffered significant coastal erosion with some property damage. The heavy September rains led to major flooding on several inland rivers, particularly the Lachlan River in central New South Wales, and went on to produce the highest flood since the early 1990s on the Murray River in South Australia as the waters moved downstream.
An intense low-pressure system in South Australia at the end of September caused major wind and flood damage there. In Tasmania, which had further flooding in November, the seven months from May to November were the wettest on record, after the seven months from October 2015 to April 2016 had been the driest on record.
Over Australia as a whole, it was the 17th wettest year on record with rainfall 17% above the long-term average. Tasmania had its second-wettest year on record, despite the dry start, and South Australia its fourth-wettest. Below-average rainfalls in 2016 were largely confined to parts of the northern tropics, coastal areas of southern Queensland and northern New South Wales, and some parts of coastal Western Australia. Heavy rains in the year’s final week were enough to lift Adelaide to its second-wettest year on record, while Uraidla, in the Adelaide Hills, had the largest annual rainfall total at any South Australian site since 1917.
The heat is on
It was the fourth-warmest year on record for Australia, with temperatures 0.87℃ above average nationally, 0.33℃ short of the record set in 2013.
The year got off to a very warm start; it was the warmest autumn on record for Australia, and the first half of the year was also the warmest on record, although there were no individual heatwaves on the scale of those experienced in 2013 or 2014.
The second half of the year was less warm. During the wet months in mid-year, heavy cloud cover led to cool days but warm nights, then a cool October resulted in spring temperatures almost exactly matching the long-term average. A warm start and cooler finish is typical of a post-El Niño year as rainfall typically changes from below to above average.
It was the warmest year on record in many parts of the northern tropics, along much of the east coast, and in parts of Tasmania. Darwin, Brisbane, Sydney and Hobart all had their warmest year on record. The warmth on land in these coastal areas was matched by warmth in the oceans.
Sea surface temperatures in the Australian region were the warmest on record, with the first half of the year especially warm. The record warm waters contributed to extensive coral bleaching on the Great Barrier Reef, and also affected fisheries in Tasmania.
Temperatures were closer to average in other parts of the country, including inland areas of the eastern states, South Australia and most of Western Australia. In a few parts of southern Western Australia, which had its coldest winter since 1990, temperatures in 2016 were slightly below average (one of only a handful of land areas in the world where this was the case), and there was some frost damage to crops in what was otherwise a very productive year for Australia’s grain-growers.
2016 continues a sequence of years with Australian temperatures well above average. While 2016 did not set a record, the last four years all rank in Australia’s six warmest, and the last ten years have been Australia’s warmest on record. 2016 is also almost certain to be the hottest year on record globally.
It’s the same old story: with 2016 on track to become the hottest year on record globally, and record-breaking heat already evident around the world, Australia has just experienced its hottest autumn on record.
The Bureau of Meteorology has reported that for average temperatures across Australia, this has been the hottest March-May period ever recorded – beating the previous record, set in 2005, by more than 0.2℃.
Within this period, March was also the hottest on record, while April and May were each the second-warmest in a series extending back to 1910.
Why so hot?
El Niño events tend to cause warmer weather across the east and north of Australia and the major El Niño of 2015-16 undoubtedly contributed to the extreme temperatures experienced across these areas.
However, climate change also played a significant role in our warmest autumn. Previous work, led by ANU climatologist Sophie Lewis, indicates that the human influence on the climate has made a record-breakingly hot autumn roughly 20 times more likely.
In other words, without climate change we would be much less likely to experience autumns as warm as this one has been in Australia.
How we’ll remember autumn 2016
In the past few months, Australia has seen many extreme hot weather events. Melbourne experienced its warmest March night on record, while Sydney had a run of 39 days with daytime highs above 26℃, as the summer heat continued long into March.
But it’s the coral bleaching event on the Great Barrier Reef that will likely linger in our memories the longest. Some 93% of the reef was found to be affected by bleaching and recent surveys have revealed that more than one-third of coral in the northern and central parts of the reef have died.
Without climate change, a bleaching event like this would be virtually impossible.
The extreme heat over Australia this autumn and the associated damage to the reef are also having an effect on the election campaign. As public concern over the future of the reef grows, the parties are being asked to defend their climate change policies.
Both major parties have made election commitments to the reef, with the Coalition announcing an extra A$6 million to tackle crown-of-thorns starfish (adding to a further A$171 million committed under the 2016 budget), and Labor an extra A$377 million over five years (A$500 million in total). While both Labor and the Coalition aim to improve water quality in the reef through their policies, the coral bleaching and death this year is linked with warm seas.
Whether we’ll be able to save parts of the reef largely depends on whether we reduce our greenhouse gas emissions and manage to prevent the rising trend in temperatures from continuing.
As the summer ends, heat is dominating the meteorological landscape, with the warmest month ever recorded and the drought continuing unabated in California. At the same time, it is clear that an El Niño is building that is expected to culminate in the fall and last until the winter, with the possibility of it becoming a “mega” El Niño.
The hope in California is that the large amounts of precipitation usually associated with extreme El Niño events would lessen the impacts of the state’s multi-year drought by partly refilling reservoirs and groundwater, even as scientists caution that this might not happen to the degree needed to alter the present situation.
What drives the El Niño weather pattern and what do scientists know about El Niño under man-made greenhouse warming?
A tropical Pacific phenomenon with global influence
To be clear, El Niño is a tropical Pacific phenomenon, even though it represents the strongest year-to-year meteorological fluctuation on the planet and disrupts the circulation of the global atmosphere. When sea surface temperature changes – or anomalies – in the eastern equatorial Pacific exceed a certain threshold, it becomes an El Niño.
What are the mechanisms behind El Niño? In normal conditions in the tropical Pacific, the trade winds blow from east to west, driving ocean currents westwards underneath. These currents transport warm water that is heated by low-latitude solar radiation and eventually piles up in the western Pacific. As a result, heat accumulates in the upper ocean.
The warm water evaporates from the ocean surface, and the light, warm and humid air rises, leading to deep convection in the form of towering cumulonimbus clouds and heavy precipitation. As this air ascends, it reaches upper levels of the troposphere and returns eastwards to eventually sink over the cooler water of the eastern Pacific. This east-west (zonal) circulation is called the Walker Circulation.
What happens to the atmosphere and the ocean during El Niño?
This circulation gets disrupted every few years by El Niño or enhanced by La Niña, the opposite effect. This periodic, naturally occurring phenomenon is called the El-Niño Southern Oscillation (ENSO).
During the typical El Niño, the warm phase of that oscillation, the trade winds weaken, and episodic westerly wind bursts in the western equatorial Pacific generate internal waves into the ocean. These waves trigger the transport of the warm water from the west to the east of the basin.
This induces a reduction of the upwelling (upward motion) of cold water in the east, at the equator and along the coast. It also creates warm sea surface temperature anomalies along the equator from the international dateline in the Pacific to the coast of South America.
As the central part of the Pacific warms up during El Niño, the atmospheric convection that normally occurs over the western warm pool migrates to the central Pacific. That transfer of heat from the ocean to the atmosphere gives rise to extraordinary rainfall in the normally dry eastern equatorial Pacific. Warm air then flows from the west, feeding this convection and further weakening the east-west-flowing trade winds. This leads to further warming as this feedback loop amplifies the phenomenon and ensures that deep atmospheric convection and rainfall patterns are maintained in the central equatorial Pacific. El Niño eventually ends when changes in the ocean cause negative feedbacks that reverse the dynamics that create the El Niño effects.
How can El Niño affect weather in United States and rainfall in California?
In association with El Niño, the heat redistribution in the ocean creates a major reorganization of atmospheric convection, severely disrupting global weather patterns from Australia to India and from South Africa to Brazil.
What explains the specific effect on the US and California, however, is a particular type of connection – called extratropical teleconnections – between the heating generated by El Niño and North America. This heating excites wave trains, or groups of similar-sized atmospheric waves, that propagate northward, connecting the central equatorial Pacific to North America. This shifts the subtropical jet stream northward and induces a series of storms over California and the southern US, in general. The increased precipitation that ensues seems to only occur during a strong El Niño.
While El Niños have a rather “typical” signature in the tropics, their impacts over North America vary because other influences act in temperate climates. Nevertheless, most El Niño winters are mild over western Canada and parts of the northern central United States, and wet with anomalous precipitation over the southern United States from Texas to Florida.
How might El Niño evolve under man-made greenhouse warming?
Scientists are now studying the diversity in El Niño behavior – strong and weak ones, changes in duration, and the different regions for the maximum SST anomalies. Are these changes to El Niño related to global warming? It is too early to say.
For one thing, there is significant natural variability in the Pacific over the decade-length and longer time scales, which could be masking changes driven by global warming.
Climate models do suggest that the mean conditions in the Pacific will evolve toward a warmer state. That means sea surface temperatures are likely to rise and the trade wind to weaken, which could lead to a more permanent El Niño state and/or more intense El Niño events.
Some climate model projections, together with reconstructions of past El Niños, provide empirical support for more extreme El Niño events under greenhouse warming. They also point toward an eastward shift of the center point where heat from the ocean transfers to the air. This would mean an eastward shift of extratropical rainfall teleconnections, the phenomenon responsible for weather changes in North America, including more rain in the West.
But models diverge in their predictions of whether and how the teleconnections’ intensity will change. So there is no simple answer to how precipitation will change in California in association with changes of El Niño related to greenhouse warming.
A complex phenomenon with many tricks for scientists
Will the sensitivity of the atmosphere to the primary mechanism at the heart of El Niño – that is, feedback between the higher sea temperatures and slowing trade winds, leading to atmospheric convection over the central Pacific – continue in the future?
It was not maintained during 2014, when otherwise favorable conditions for a big El Niño were present. In that case, persistent deep convection did not occur in the central Pacific, and the usual strong interaction between the atmosphere and the ocean there failed to play its normal role in anchoring the convection and heat transfer.
These results show us that we still have much to learn. This is true despite the dramatic scientific progress that has been accomplished over the last few decades regarding El Niño and ENSO cycles, including new theories, sophisticated seasonal forecasting models and extensive observation systems.
Our ability to predict El Niño and the potential connections between increasing greenhouse gases and El Niño is still limited by the complexity of the ENSO dynamics, as exemplified by the failed prediction of a 2014 El Niño. In the meantime, we can look forward to a winter when El Niño, perhaps even a mega El Niño, will dominate the weather discussion.
Before now no one had looked at how far back in time we could go and still link these weird weather events and record-breaking climate extremes to our influence on the climate.
Our study, published in Geophysical Research Letters, addressed the question of when climate change started altering the influence of record hot years and summers in a way we can detect. We looked at five regions of the world, as well as the whole globe.
We’ve been changing the weather for a long time
Human-made climate change has been influencing heat extremes for decades, with many past records directly attributable to the effect we have had on the climate.
The map above shows how many record-breaking hot years and summers we can attribute to climate change in different parts of the world.
We found the last 16 record-breaking hot years globally up to 2014 were made more likely because of climate change (we didn’t include 2015 – the current holder of the hottest year record – because we performed our analysis before the end of last year). The earliest year we found that humans had contributed to temperature records was 1937. Since then every record-breaking year globally (all 15 following 1937 that were the hottest up to that time) can be attributed to climate change.
The record-breaking hot years the world has experienced in the last couple of decades would have been almost impossible in a world without climate change.
Even on a regional scale we see many record-breaking hot years and summers where the fingerprint of climate change is clear. The last six record hot years and three record hot summers in Australia were made more likely by the human influence on the climate.
Hot and cold
In other regions of the world, human activities have also had a cooling effect through increasing aerosols in the atmosphere (due to industrial activity and the emissions of particulates from cars and airplanes, for example). Here fewer record heat events show as clear a human influence. But even in areas such as eastern Asia the overall human influence on the climate has increased the likelihood of hot years and summers back to the 1980s.
Central England has had no record-breaking hot summers since the extreme heat of 1976. Any future hot summer that breaks that record will be associated with climate change.
In fact, for all the regions of the world we studied, the influence of climate change on record-breaking heat extremes has been increasing through time. For instance, we find that the Australian record hot year of 2013 was 22 times more likely due to climate change, whereas if we look back at the 1980 record we find only a three-fold increase in likelihood of that record from human influences (see here for an explanation of how these studies are done).
As we have exerted a greater influence on the climate, by increasing the concentration of greenhouse gases in the atmosphere, the likelihood of record-breaking heat extremes has increased.
This study shows that the human influence on record-breaking hot extremes extends back many decades, especially when we look at the globe and Australia. While our aerosol emissions have delayed a clear human influence appearing in other areas of the world, the fingerprint of climate change has become clear over recent decades as the warming influence of greenhouse gases has overtaken the cooling influence of aerosols.
Reduce emissions and we should get fewer records
Other factors also influence the likelihood of record heat events occurring – for example, El Niño events. We looked at only the human influence in this study. Undoubtedly natural variability was important in many of these events occurring; by isolating the human influence we found that climate change played a major role in many record heat events as well.
Any hot years or summers in the near future in these regions will be strongly linked with climate change. This analysis was conducted before the end of last year, so although we didn’t include it we would expect a similar result for the 2015 global heat record. Already, 2016 is expected to challenge that record.
Whether we continue to experience such frequent record-breaking heat extremes will in part depend on whether we reduce the influence on the climate as the Paris agreement sets out to do.
If we reduce our emissions, we may not see as many future record hot summers and years. The impacts these events have on society and the environment may be reduced.