Climate Weather Nat Haz L3 2011 - School of GeoSciences · Climate & Weather-related Natural...

Climate & Weather-related Natural

Hazards - Lecture 3

David Stevenson 1

Tropical Cyclones

(continued from L1)


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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)


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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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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 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)

Climate Weather Nat Haz L3 2011 - School of GeoSciences · Climate & Weather-related Natural...

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Transcript of Climate Weather Nat Haz L3 2011 - School of GeoSciences · Climate & Weather-related Natural...