climate weather nat haz l3 2011 - school of geosciences · climate & weather-related natural...
TRANSCRIPT
Climate & Weather-related Natural
Hazards - Lecture 3
David Stevenson 1
Tropical Cyclones
(continued from L1)
Climate & Weather-related Natural
Hazards - Lecture 3
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Regions of FormationQ. Why do they form only these regions of the tropics and during these periods?
Conditions for Tropical Cyclone Formation
They form only over oceanic regions with
sea-surface temperatures (SSTs) are greater than 26.5°C
They do not form within 5°of the equator due to the negligible Coriolis force there
They form in regions where vertical wind
shear between the surface and upper
troposphere is low (less than ~23 mph or 10 m/s)
Climate & Weather-related Natural
Hazards - Lecture 3
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Evaporation and temperature
•Evaporation increases rapidly as temperature increases
•Evaporation = energy in the form of latent heat that fuels the cyclone
Regions and seasons T>26.5oC
Orange/yellow regions - tropics between June and December
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Reasons why cyclones do not form in certain tropical regions
Cold currents
Without the Coriolis force, surface winds
cannot gain sufficient rotation to converge and the low pressure of the disturbance
cannot be maintained
Large values of vertical wind shear disrupt
the formation of a tropical cyclone by
interfering with the organization of deep
convection around the cyclone centre
Path of Isabel
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Tropical cyclone dissipationTypical lifetime is less than 1 week
Record: hurricane John (1994) 31 days
Weaken rapidly when they lose their heat source:
Reach more northerly locations and cooler waters
Travel over land: a) energy source removed b) friction at land-surface decreases surface winds causing central pressure to rise
Encounter large vertical shear e.g. in mid-latitude jet-stream
Prediction is still very difficult
The 8-day ensemble forecast shows large uncertainties in the path of Hurricane Ivan.
Ultimate path in black. Operational path in red. Note tendency for clustering of tracks. 5 members to east, 4 members to west, 1
ensemble member in the middle.
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Hazards - Lecture 3
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Hurricane forecasting
The “Cone of uncertainty"-also known colloquially as the “Cone of death" It represents the forecasted track of the centre of a hurricane and the likely error in the forecast track based on predictive skill of past years as well as
details about the storm
Tropical Cyclones: SummarySize: ~500 km
Lifetime: ~1 week
To form: SST > 26.5°C, >5°N/S, low vertical wind shear
Dissipate over land/cool ocean/mid-latitudes
Flooding from storm surge greatest hazard
Probabilistic forecasting a few days ahead
Will TCs increase with climate change?
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Climate change,climate variability
and natural hazards
What is climate change?What is climate variability?
Climate is the average weather at a given point and time of year, over a long period (typically 30 years).
We expect the weather to change a lot from day to
day, but we expect the climate to remain relatively
constant.
If the climate doesn’t remain constant, we call it
climate change.
The key question is what is a significant change –and this depends upon the underlying level of climate
variability
Crucial to understand difference between climate
change and climate variability…
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Global meansurfacetemperature
Time/years0 300
Temperature
Time
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Climate & Weather-related Natural
Hazards - Lecture 3
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Climate change and variability
IPCC(2007) finds that ‘warming of the climate system is
unequivocal’ and ‘changes in the global climate system during the 21st century…would very likely* be larger than
those observed during the 20th century’*‘very likely’ means >90% probability
i.e. we are pretty sure climate is changing/will change
IPCC makes no clear statement on changes in climate variability – we don’t know if it has changed or if it is
likely to change in future.
i.e. we are unsure if climate variability is changing/will change
El Niño-Southern Oscillation (ENSO)�
The major mode of global climate variability.
It is naturally occurring (evidence back >125,000 yrs ago)�
Ocean-Atmosphere interaction in the tropical Pacific
Winds, ocean temperatures, cloud and rainfall patterns all change
Occurs every 2-7 years, lasts 9-12 months (up to 2 years)�
Irregular – initiation not understood
During a strong El Nino (e.g. 1997/98), global temperatures can rise, by up to about 0.3 °C
Impacts:
Peruvian fishing & seabirds
Coral bleaching due to high sea temperatures
South American rainfall
Droughts in Africa/Indonesia – also promotes fires
Modulates strength of tropical storms
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Sea surface temperatureanomalies
This is a snapshot
of sea surface temperature in the
tropical Pacific
The anomaly is
how it differs from
average values.
Current SST anomaly
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‘Normal’
vs.
El Nino
conditions
www.pmel.noaa.gov/tao/elnino/
Normal
El Nino
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A: Cold water off Peru – not El Nino
B: Warmer water than usual at
150m depth in W. Pacific
January 1997
February 1997
C: Warm water spreads across
the Pacific beneath the surface
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D: Warm water reaches surface in the central Pacific – this starts an interaction with the winds: Easterly winds weaken, and this tends
to make the ocean warmer: positive feedback
April 1997
May 1997
Warm water reaches the surface
off Peru, and warm water in
the central Pacific expands
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September 1997
Strong El Nino is underway –
sea surface is 2-4°C warmer
across half the Pacific
January 1998
El Nino fully underway.
Note the cold water at depth
spreading from the W. Pacific
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March 1998
El Nino has started to shrink – peak temperature anomaly 5°C, compared to 11°C in January
May 1998
El Nino almost over, after about 1 year of elevated sea-surface temperatures.
Note the expanding cold anomaly: this heralds
La Nina, the opposite phase of El Nino.
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Pacific SST anomalies since 1982
El Nino
La Nina
Current ENSO forecast
http://iri.columbia.edu/climate/ENSO/
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Does ENSO affect hurricanes?
Yes – less Atlantic hurricanes in El Nino years.
Thought to be mainly because high level winds increase, making it harder for hurricanes to form.
ENSO impacts
The most famous impacts of El Niño events are:
Damages from floods and landslides caused by very high rainfall in Peru and southern California
Collapse of the Peruvian anchovetta fisheries because of warmer coastal waters.
Forest fires in Indonesia that have caused serious air pollution problems (1997/8 event see later)�
Crop failures and sometimes famine from droughts in southern Africa
http://iri.columbia.edu/climate/ENSO/societal/impact/index.html
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Summary of 1997/8 El Niño impacts
NB More hurricanes in N. Pacific, less in N. Atlantic
El Niño 1997/8 wildfires
Wildfires in Borneo and Sumatra
Anthropogenic - fires for land clearing intensified due to dry conditions associated with an El Niño event that commenced in June 1997
Area burned: 70-100,000 km2
Fires out of control in Sept 1997, halted during monsoon rains in November
Commenced again in Feb and lasted until rains in April 1998
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ENSO and climate variability
ENSO impacts are only a subset of the
impacts of year-to-year global climate
variability.
At most ENSO may be responsible for
about 50% of seasonal climate variability
in some regions. In most regions of the
world its influence is small or non-existent.
North Atlantic Oscillation (NAO)�
Main expression of climate variability in NW Europe
Reflects strength of westerly winds off the Atlantic
NAO +Mild, wetwinter
NAO –Cold, drywinter
www.met.rdg.ac.uk/cag/NAO/
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North Atlantic Oscillation (NAO)�
Main expression of climate variability in NW Europe
Reflects strength of westerly winds off the Atlantic
NAO +Mild, wetwinter
NAO –Cold, drywinter
www.met.rdg.ac.uk/cag/NAO/
Winter-time (JFM) NAO index 1950-2010
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao_index.html
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Summary of climate variability and its influence on natural hazardsClimate always shows some level of natural variation –even without man-made climate change
ENSO is the major mode of global climate variability
ENSO clearly influences the distribution of drought, floods and hurricane activity
Understanding and predicting ENSO can be important for warnings of some hazards, e.g. drought in S. Africa
NAO is a more important index of climate variability in NW Europe:
+ve NAO: Mild, wet winters over UK
-ve NAO: Cold, dry winters over UK (e.g. 2010)