Northern NSW is no stranger to floods, but this one was different


Joelle Gergis, University of Melbourne

The devastating flood damage wreaked by Tropical Cyclone Debbie has left many residents in northern New South Wales facing an enormous cleanup that could take months. The Conversation

Any Lismore local will tell you that flooding is a fact of life in the Northern Rivers. In the floods of 1954 and 1974, the Wilsons River rose to a record 12.17 metres. This time around, the river peaked at 11.59m, breaching the flood levee built in 2005 for the first time.

So what are the conditions that caused those historic floods? And are they any different to the conditions of 2017?

Like the current flood, cyclonic rains also caused the 1954 and 1974 events. But unlike those past events, both of which were preceded by prolonged wet weather, almost all of the extreme rainfall from ex-Tropical Cyclone Debbie fell within 24 hours.

More interesting still is the fact that we are not currently experiencing La Niña conditions, which have historically formed the backdrop to severe flooding in eastern Australia.

The 1954 flood was preceded by an east coast low from February 9-11, followed by a decaying tropical cyclone from February 19-22. Thirty people were killed as flood records were set in Lismore, Kyogle, Casino, Nimbin and Murwillumbah. Some places received more than 1,000mm of rain in 14 days.

In 1974, former Tropical Cyclone Zoe unleashed torrential rain over Lismore, Wyrallah and Coraki. From March 10-13, some stations received almost 1,000mm in just four days. One analysis described the flood as a once-in-70-year event.

This time around, the remains of Tropical Cyclone Debbie delivered extreme rainfall to northern NSW towns including Murwillumbah, Chinderah and Lismore, despite having crossed the coast several days earlier and more than 1,200km to the north. Floods as far apart as Rockhampton in central Queensland and northern New Zealand show the storm’s colossal area of influence.

During the event, 20 rainfall stations in Queensland and 11 sites in NSW recorded their wettest March day on record. Mullumbimby, in the Brunswick River catchment, received a staggering 925mm during March – over half the annual average in a single month – causing major flooding in the region.

The heaviest rainfall in the Wilsons River catchment was at Terania Creek, which received 627mm over March 30-31, 99% of it in the 24 hours from 3am on March 30. Lismore recorded 324.8mm of rain in the 18 hours to 3am on March 31, its wettest March day in more than 100 years. A little further out of town, floodwaters submerged the gauge at Lismore Airport, so unfortunately we do not have reliable figures for that site.

March 2017 rainfall across Australia. Tropical Cyclone Debbie’s track down the east coast is visible in the trail of above-average falls.
Bureau of Meteorology

The main difference between the current flooding and the 1954 and 1974 floods is that the previous events both occurred against a background of sustained La Niña conditions. These tend to deliver above-average tropical cyclone activity and high rainfall totals, which increase flood risk.

During the early 1970s, Australia experienced the longest period of La Niña conditions in the instrumental record. This unleashed phenomenal deluges across virtually the entire country. By the end of 1973, many catchments were already saturated as the wet season started early, culminating in the wettest January in Australia’s rainfall records.

In 1974 the Indian Ocean was also unusually warm (what meteorologists call a “negative Indian Ocean Dipole (IOD) phase”), further enhancing rainfall in the region. When negative IOD events coincide with La Niña conditions in the tropical Pacific, the warm sea temperatures reinforce one another, resulting in more evaporation and increased rainfall. This double whammy resulted in the exceptionally wet conditions experienced across the country during 1974.

In January 1974, the Northern Territory, Queensland and Australia as a whole recorded their wettest month on record, while South Australia and New South Wales recorded their second-wettest January on record. Torrential monsoon rains in the gulf country of Queensland transformed the normally dry interior into vast inland seas, flooding all the way to Lake Eyre in the arid zone of South Australia.

Vast swathes of Australia were much wetter than average during the mid-1970s.
Bureau of Meteorology

In contrast, Tropical Cyclone Debbie formed under neutral conditions, rather than during a La Niña. In fact, the Bureau of Meteorology is currently on El Niño watch, meaning that there is double the normal risk of an El Niño event bringing low rainfall and high temperatures to Australia by mid-2017.

So, unlike the 1950s and 1970s, the current flooding happened despite the absence of conditions that have driven major flooding in the past. It seems extraordinary that such a damaging cyclone could develop under these circumstances, and deliver such high rainfall over such a short time. This suggests that other factors may be at play.

A rapidly warming climate means that storms are now occurring in a “super-charged” atmosphere. As temperatures increase, so does the water-holding capacity of the lower atmosphere. The oceans are also warming, especially at the surface, driving up evaporation rates. Global average surface temperature has already risen by about 1℃ above pre-industrial levels, leading to an increase of 7% in the amount of water vapour in the atmosphere.

Ocean evaporation, before and after ocean warming.
Climate Council

Of course, it is hard to determine the exact impact of climate change on individual storms. However, climate scientists are confident about the overall trends.

Australia’s land and oceans have warmed by 1℃ since 1910, with much of this warming occurring since 1970. This influences the background conditions under which both extremes of the rainfall cycle will operate as the planet continues to warm. We have high confidence that the warming trend will increase the intensity of extreme rainfall experienced in eastern Australia, including southeast Queensland and northern NSW.

While it will take more time to determine the exact factors that led to the extreme flooding witnessed in March 2017, we cannot rule out the role of climate change as a possible contributing factor.

CSIRO’s latest climate change projections predict that in a hotter climate we will experience intense dry spells interspersed with periods of increasingly extreme rainfall over much of Australia. Tropical cyclones are projected to be less frequent but more intense on average.

That potentially means longer and more severe droughts, followed by deluges capable of washing away houses, roads and crops. Tropical Cyclone Debbie’s formation after the exceptionally hot summer of 2016-2017 may well be a perfect case in point, and an ominous sign of things to come.

Joelle Gergis, ARC DECRA Climate Research Fellow, School of Earth Sciences, University of Melbourne

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

Tracking the storm: the science behind Tropical Cyclone Debbie


Liz Ritchie-Tyo, UNSW

Tropical cyclone Debbie has made landfall in Queensland as a category 4 cyclone with winds of more than 150 kilometres per hour. The Conversation

The cyclone crossed the coast near Airlie Beach on Tuesday afternoon. Reports of wind gusts in excess of 200km per hour and rainfall of more than 200mm of rain have been made in some areas along the central Queensland coast.

The Bureau of Meteorology forecasted an average to above-average number of Australian cyclones in its October severe weather outlook. Australia receives 11 cyclones on average each year, with about four of those in Queensland. Debbie is the fifth cyclone of the season for Australia as a whole and the most intense of the season so far.

Anomalously high moisture, warm ocean temperatures, and low environmental pressures seem to have created the conditions that allowed TC Debbie to form and grow in intensity.

Perfect storm

Tropical cyclones are low pressure systems that form over warm tropical oceans. The warmth and moisture of the oceans are what gives a cyclone its energy. The low pressure, which meteorologists measure in “hectopascals”, draws in the surrounding warm, moist air, which then rises into deep thunderstorm clouds. As the air is pulled into the centre of low pressure, Earth’s rotation causes it to spin cyclonically and it continues to intensify.

TC Debbie formed at the eastern end of an active monsoon trough extending from the Indian Ocean across the top of Australia and into the Coral Sea. The monsoon trough is a region of low air pressure and thunderstorms that forms over northern Australia in the summer months, bringing with it the wet season. On March 22, a large region of active thunderstorms began to organise into a weather disturbance off the eastern tip of Papua New Guinea.

Over the following two days the thunderstorms organised about a circulation centre as sea level pressures began to drop and moist air converged into the area. By late on March 24 a tropical depression, a forerunner of a cyclone, had formed and begun to drift south, making a long S-shaped track.

Tropical Cyclone Debbie was named on March 25. It then came under the influence of the subtropical ridge, a zone of stable high pressure that gives much of Australia’s fine weather during the summer. This drove Debbie west-southwest towards the Queensland coast while it gradually intensified further.

Because of the relatively high amounts of moisture in the atmosphere, and relatively warm ocean waters, Debbie intensified to category 4 by 10 pm on March 27, with the strongest wind gusts reaching 225-280km per hour. On Tuesday afternoon Debbie was a strong category 4 cyclone with a central pressure of 943 hectopascals and surface sustained winds of 185 kilometres per hour. The Bureau of Meteorology downgraded TC Debbie to a category 3 at 4:00 pm EST.

To put Debbie in context, there has been only one cyclone since 1980 to have made landfall in Queensland with a lower central pressure. That was Yasi in 2011.

Of the 46 cyclones to have made landfall in Queensland since 1980, only three others arrived at the coast with pressures of less than 960 hectopascals: Dominic in 1982, Winifred in 1986, and Ingrid in 2005.

Predicting cyclones

Tropical cyclone forecasters use a variety of tools to forecast the storm’s track, intensity, storm surge, and rainfall. Because it is difficult to obtain observations of wind at the ocean’s surface under a cyclone, meteorologists have developed tools based on satellite imagery to estimate a storm’s intensity, location, and where the strongest and most destructive winds are found.

Several models are also used to aid in making forecasts – from the complex numerical weather prediction models, to statistical models. Models start by using observations of the atmosphere, and then use these data to make a forecast.

Depending on their level of complexity the models can predict the future track, intensity, rainfall, wave height, and/or storm surge. The forecasters access all of this information to then make their forecast.

Cyclone forecasts have improved considerably over time. In particular, track forecasts have improved so that the 48-hour forecast is now more accurate than the 24-hour ones were back in the early 1990s. Track forecasting has become so reliable that the US National Hurricane Centre now produces 120-hour track forecasts.

Intensity forecasts have improved more slowly, but as models have become more refined and satellite technology has improved, the ability of forecasters to accurately estimate and predict intensity is also getting gradually better.

The prediction of rainfall, the extent of the damaging wind field, and storm surge forecasts are also slowly improving. Now that they are receiving more attention, we can expect considerable improvements in these over the next decade.

Liz Ritchie-Tyo, Associate Professor, School of Physical, Environmental, and Mathematical Sciences, UNSW

This article was originally published on The Conversation. 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.

Vanuatu Faces Food Shortages After Cyclone Pam Devastates Crops


TIME

Survivors of Cyclone Pam on the South Pacific islands of Vanuatu are bracing for a lack of food over the coming months because crops were destroyed in the recent storm.

Much of the archipelago’s population relies on subsistence farming and when the monster cyclone ripped through the country last week it wiped out livelihoods as well as homes.

“There’s always a lot of attention in the beginning, the first few weeks of a big disaster. But now, we’re looking at a hunger gap over the next three to six months,” said World Vision’s Emergency Operations Manager in Vanuatu, Alex Snary.

Aid agencies are rushing to deliver desperately needed supplies, especially to communities on the remote outer islands, which are still out of contact.

At least 11 people were killed in the disaster and 3,3000 displaced.

“It’s almost miraculous [on Tanna Island] that there isn’t a large number of casualties, given…

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Aid Agencies Struggle to Reach Affected in Vanuatu After ‘Monster’ Cyclone Pam


TIME

Vanuatu’s President Baldwin Lonsdale says the tiny South Pacific island nation has lost much of its development due to the havoc wreaked over the weekend by “monster” Cyclone Pam.

“It’s a setback for the government and for the people of Vanuatu. After all the development that has taken place, all this development has been wiped out,” he told the Associated Press in Japan attending a U.N. conference on disaster-risk reduction. “We will have to start anew again.”

Lonsdale went on to say that climate change contributed to the devastation as the low-lying islands of the Pacific are suffering from rising sea levels.

International aid agencies along with military personnel from Australia, France and New Zealand have arrived in Vanuatu to assess the damage and deliver much needed aid and supplies.

Oxfam, who led coordination efforts in preparing for the storm, said 90% of housing in the capital, Port Vila, has…

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