Download What caused the extreme weather of 2010

The La Niña of
2010-2011 and its
impact on Australia
Nicholas Klingaman
NCAS-Climate and Walker Institute
© University of Reading 2011
www.reading.ac.uk
Introduction and Outline
• Review of the extreme weather in Australia in
summer 2010-2011
• Brief introduction to Australian climate
• The Rockhampton and Brisbane floods
– Heavy rain falling on already-saturated soils
– The weather systems responsible for the flooding
• “Is this climate change?”
• The La Niña of 2010-2011 (and now 2011-2012)
– What are La Niña and El Niño?
– How does La Niña cause flooding in Australia?
• Severe Tropical Cyclone Yasi
• Summary and conclusions
Extreme weather events
in 2010-2011
• December 2010: Central Queensland flooded, with
particularly severe flooding in Rockhampton
• January 2011: Toowoomba and Brisbane flooded;
also flooding in Sri Lanka, Philippines and Brazil
• February 2011: Severe Cyclone Yasi strikes Queensland
between Cairns and Townsville.
Source: telegraph.com.au
Source: guardian.co.uk
Brief introduction to
Australian climate
• Australia’s coasts receive
the vast majority of the rainfall.
• About 70% of Australia receives
less than 500 mm of rainfall
per year (about half of
Reading’s average rainfall).
• The northern third of Australia
receives nearly all of its rainfall in
summer (December-February).
• The southern third of Australia
receives most of its rainfall in
winter (June-August).
Source: bigthink.com
The Rockhampton Floods
• The wettest December on record
in Queensland; the second-wettest
on record in Australia (since 1900).
• Much of Queensland received
more than twice its average rainfall.
• The wet December followed the
wettest spring on record in Australia.
The Rockhampton Floods
• Exceptionally heavy
rain fell between Christmas
and New Year’s Eve:
• Rockhampton: 320 mm
• Carnarvon: 265 mm
on 27 December alone
• Roughly five times the
average rainfall for
December
• Daily rainfall records fell
along the Carnarvon Hills to
the west of Rockhampton
• Thousands of square
kilometres flooded.
The Rockhampton Floods
• The proximate cause of
the heavy rainfall was
the combination of
• Low pressure to the
north, along the
Queensland coast
• High pressure to the
south, in the Tasman
Sea
• The combined circulation
around these systems
directed warm, moist ocean
air onshore, which
subsequently rose over the
Carnarvon Hills, producing
heavy rainfall
The Brisbane Floods
• Southeastern Queensland received
heavy rain 6-12 January 2011
• Brisbane: 267 mm
• Toowoomba: 345 mm
• Flash flooding struck Toowoomba
on 10 January
• Similar onshore wind pattern to
Rockhampton floods
The Brisbane Floods
• Clouds and precipitation
“streaming” in from the
Pacific on 10 January.
• Large southward dip in the
monsoon trough brought
tropical moisture as far
south as Melbourne, causing
localized flooding.
• Tasmania experiences its
wettest January on record.
The Brisbane Floods
2011
• Brisbane City gauge
reached a height of 4.46
metres, well below the
record highest floods.
• The Wivenhoe Dam was
constructed in the wake of
the 1974 floods
(5.45 m) to protect the city.
• Preliminary results: Without
the Wivenhoe Dam, the river
level at the City gauge
would have been
6.5 metres (Neville Nichols,
Monash University)
The cost of the
Australian floods
15,000 homes destroyed
35 people killed
Climate change and extremes
• In the wake of the Queensland floods, many asked:
“Is it climate change?”
… or:
“Would the Queensland floods have occurred if humans were
not warming the climate through greenhouse-gas emissions?”
• Climate change will affect the overall number and strength of
extreme weather events.
• Two slightly easier and much more useful questions:
“Will floods in Queensland become more frequent?”
“Will floods in Queensland become more intense?”
Climate change and
extreme rainfall
• For the tropics as a whole,
the frequency of extreme
rainfall is expected to
increase with warming
temperatures.
• Clausius-Clapeyron:
the atmosphere has a
greater capacity for water
vapour at higher
temperatures.
• This does not mean that
global warming will be
every rainfall event
stronger, or that total
rainfall will increase
everywhere.
Changes in frequency of extreme rainfall
in the tropics with warming ocean temperatures
Figure from Allan et al. (2010, Env. Res. Lett.)
Climate change and
Queensland extreme rainfall
• For Queensland,
doubling CO2
in a climate model
increases the
frequency of
100 mm/day rainfall
by 50%:
• Present-day:
1 in 670 days
• Double CO2:
1 in 402 days
• Doubling CO2 also
also increases the
intensity of a 1-in-200
Frequency of different amounts of rainfall in Queensland
day event by 20%:
during summer, for present-day and 2xCO2 climates
• Present-day: 54 mm
• Double CO2: 65 mm
Brief introduction to
El Niño and La Niña
• La Niña and El Niño are naturally
occurring phenomena, in which
ocean temperatures in the equatorial
Pacific cool and warm, respectively.
• Events often develop in
June-September, peak in DecemberFebruary, then decay in March-May.
• La Niña and El Niño are episodic
(irregular). El Niño does not
necessary follow La Niña; neutral
conditions can prevail for years.
• The strength and position of warm
ocean temperatures shifts rainfall
patterns around the world.
Development of the
2010-2011 La Niña event
Brief introduction to
El Niño and La Niña
El Niño
La Niña
Departures of ocean temperatures in the
central equatorial Pacific from average conditions
(Source: KNMI Climate Explorer from UK Met Office data)
There are insufficient reliable data to determine whether
El Niño and La Niña are changing with global warming.
• On average,
El Niño and La Niña
are responsible
for approximately
25% of the
year-to-year
variations in
Queensland rainfall.
• Strong La Niña
events result in
large floods in
Queensland, but
stronger El Niño
events do not result
in stronger
droughts.
Queensland annual (May-Apr) rain (mm)
Historical relationships between
La Niña and Australian rainfall
Strong
Weak Strong
Weak
La Niña La Niña Neutral El Niño El Niño
1974/75
2010/11
Nino-4 index (degrees Celsius)
Historical relationships between
La Niña and heavy rainfall
• La Niña increases by 20-70% the risk of heavy rainfall in Queensland
• El Niño reduces by 10-40% the risk of heavy rainfall in Queensland
La Niña
minus
neutral
years
Difference
in the
number
of
1-in-30 day
rainfall
events
per
season
El Niño
minus
neutral
years
Climate change or
natural variability?
• Scientists expect warming
temperatures to increase the
frequency and intensity of extreme
rainfall, particularly in the tropics.
Climate
change
More
heavy
rainfall
Strong
La Niña
Heavier
rainfall
• There is a strong historical link
between La Niña and strong
monsoon seasons in Australia,
including with the frequency of
extreme rainfall.
• There is no robust evidence that
global warming has influenced the
frequency or intensity of La Niña; the
2010-11 event was strong, but not
outside the range of past events.
Severe Cyclone Yasi
• Severe Tropical
Cyclone Yasi was
the strongest
cyclone to strike
Queensland in at
least 100 years.
• The cyclone struck
between Cairns and
Townsville with
winds of 180 mph
(290 km/h) and a
seven-metre-high
storm surge.
• One death and
AU$3.5 billion
(£2.27 billion) in
damages
Severe Cyclone Yasi
Yasi was
“Australia’s Katrina”
The track of Severe Cyclone Yasi as it
passed across northern Queensland
Historical relationships between
La Niña and tropical cyclones
Observed tropical-cyclone tracks during the seven
El Nino and La Nina events from 1979-2009.
• Compared to El Niño, La Niña doubles the risk of a tropical cyclone
striking eastern Australia (Callaghan and Power, 2010)
• All years in which more than one tropical cyclone has struck eastern
Australia have been La Niña years (Callaghan and Power, 2010).
Historical relationships between
La Niña and tropical cyclones
Changes in tropical-cyclone track and genesis locations for
El Niño years minus La Niña years
Summary and conclusions
• Rainfall records fell across Queensland in
December 2010 and January 2011.
• A persistent pattern of onshore easterly winds
produced heavy rainfall in coastal regions.
• La Niña is associated with stronger summer
monsoons in Australia and more frequent tropical
cyclones.
• Warm ocean temperatures around Australia,
enhanced by La Niña, provided moisture and energy
to amplify cyclones and monsoon depressions.
• It is difficult to separate the effects of natural variability
and climate change on a single extreme event.
Climate change and
tropical cyclones
• In a warmer world,
we expect to see
fewer tropical
cyclones.
• The decrease may
not be evenly
spread across the
globe; some regions
may have more
cyclones.
• The intensity of
tropical cyclones,
particularly the
strongest ones, is
expected to
increase.
Temperature
changes over
1979-1999
Estimates from three
models of the change
in the number of days
with a tropical cyclone:
2081-2100 minus
1981-2000.
From Thorne
From Lavender
et al.
et al. (2010)
(2011).
Climate change or
natural variability?
• Climate change is expected to
reduce the number, but increase
the intensity of tropical cyclones
in the Southwest Pacific.
Climate
change
More
tropical
cyclones
Strong
La Niña
Stronger
tropical
cyclones
• La Nina increases the number of
landfalling tropical cyclones in
eastern Australia, but has no
effect on their overall intensity.
• Was Yasi “enhanced” by global
warming?
Would Yasi have even formed in
a warmer world?
In an El Nino year, would Yasi
have simply have moved out to
sea?
Should we blame ourselves?
• Humans are definitely warming the climate through
emissions of greenhouse gases.
• A warming climate is affecting and will affect weather
patterns, especially the number and strength of extreme
weather events.
• The effects of climate change are being seen and
will be seen in the overall behaviour of extreme events.
• Natural variations, like La Nina, still have an important role
in determining when and where extreme weather will occur.
• For individual events, trying to separate natural variations
from climate change is not easy and (in my opinion)
not particularly useful for adapting to climate change.