Transcript Sys-2 Daisyworld [text KKC, pp.23-31]

Present-Day climate variability
Objectives:
• ENSO mode
• AO and NAO mode
• PDO
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• Anomaly : The deviation from the mean. To calculate SST
anomalies, the long-term mean for a specific point in the
ocean is subtracted from the current value. A negative value
indicates that the current value is cooler (smaller) than
usual, while a positive value indicates that the current value
is warmer (larger) than usual.
For example:
•
•
•
The Nino 3.4 value for December 2003
26.9 °C
The long-term mean for the Nino 3.4 region
26.5 °C
Anomaly = current value - mean = 26.9 - 26.5 = 0.4
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ENSO MODE
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• South American fishermen have noticed the
appearance of warm waters in the eastern Pacific
Ocean along the coast of Ecuador and Peru. As
the phenomenon typically becomes apparent
around Christmas, the name "El Niño", or the
Christ Child was eventually used.
• La Niña: A cooling of the ocean surface off the
western coast of South America, occurring
periodically every 4 to 12 years and affecting
Pacific and other weather patterns.
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• Recognizing El Niño
El Niño can be seen in Sea Surface Temperature in the
Equatorial Pacific Ocean
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ENSO
•
A scientific definition of ENSO events: When the three-month
running mean of the SST anomalies in the Nino 3.4 region
are greater than or equal to 0.5°C, there is a good chance of
an El Niño event taking place. When the anomalies are
smaller than or equal to -0.5°C, there is a good chance of a
La Niña event taking place.
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Scientists who studied El Niño
• Sir Gilbert Thomas
Walker
• Credited with the
discovery of El
Niño
• Identified the
Walker
Circulation
• His findings
represented
invaluable steps
forward
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Scientists who studied El Niño
•
Dr. Klaus Wyrtki is now
best known for his
• ENSO research from the
•
1970s to 1993.
Developing
breakthroughs in
understanding and
forecasting El Niño
• Establishing the tide
gage network that
provided the essential
oceanographic data set.
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Scientists who studied El Nino
•
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Jacob Bjerknes
• Took Walker’s idea of El
Niño one step further
and suggested that
many long-term
variations in the world’s
climate may be due to
large-scale interactions
between the oceans
and the atmosphere
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• Irregularity of ENSO
(1) noise internal to either the atmosphere
or ocean
(2) inherent nonlinearity of the coupled
atmosphere/ocean system (or in the
coupling itself);
(3) changes in the external forcing;
(4) interactions between ENSO and the
seasonal cycle
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• Seasonal Cycle and ENSO
Is the seasonal cycle necessary for ENSO to be realized?
Not likely: Many models without an annually varying sun
have proven successful at simulating interannual
variability that is ENSO-like.
Is the seasonal cycle fundamental to the irregularity in the
ENSO events? Many models are run without an annually
varying sun and the ENSO events that they produce occur
irregularly.
However, ENSO is sufficiently tied to the annual cycle that
it is useful to think of a canonical ENSO event, formed by
compositing observations fixed to the calendar year.
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Effect of ENSO on Climate
•
In the Tropics, El Niño episodes are associated with increased rainfall
across the east-central and eastern Pacific and with drier than normal
conditions over northern Australia, Indonesia and the Philippines.
Elsewhere, wetter than normal conditions tend to be observed 1)
during December-February (DJF) along coastal Ecuador, northwestern
Peru, southern Brazil, central Argentina, and equatorial eastern Africa,
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2) Wetter during June-August (JJA) in the intermountain regions of the
United States and over central Chile. Drier than normal conditions generally
observed over northern South America, Central America and southern Africa
during DJF, and over eastern Australia during JJA.
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La Niña
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La Niña
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Flooded area in Lakeport, California as a result of the 1998 El Nino
event. (Federal Emergency Management Agency)
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Bush fire in Australia as a result of the 1998 El Niño event. (Photo
courtesy of Fred Hoogervirst/Panos Picture/London)
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Temperature
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Difference in maximum
Snow depth in millimeters
between El Niño and La
Nina and Neutral years
El
ELNiño
Niño
La Niña
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The map shows typical summer
precipitation response following
the onset of El Niño. The numbers
indicate the rate of precipitation
(mm/day) by which the
precipitation departs from the
seasonal normal.
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Significant impacts of ENSO on Canadian natural resources and
environment have been documented in a variety of areas
including water resources, agriculture, forestry, fisheries, power
utilities, coastal zones and other climate sensitive sectors of the
Canadian economy.
(1) the fishery of British Columbia.
(2) forestry
…
Hsieh and Tang, 2001. Interannual variability of accumulated snow in the Columbia basin, British Columbia. Water Resources
Res. 37: 1753-1760.
Hsieh, W.W., B. Tang and E.R. Garnett, 1999. Teleconnections between Pacific sea surface temperatures and
Canadian prairie wheat yield. Agricul. Forest Meteorol. 96: 209-217.
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• During El Nino, as the sea surface temperature (SST) is
above normal off B.C., the returning Fraser River sockeye
salmon (with yearly landed value of about $600 million)
favor traveling via the northern route of Johnstone Strait
instead of the more commonly used southern route of
Juan de Fuca Strait. Many fishermen with fishing permits
on the west coast of Vancouver Island suffered heavy
losses in 1997 as the sockeye bypassed their area.
• Other impacts occur in forestry.
During El Nino,
temperatures in the BC interior, especially in winter, are
above normal and summer precipitation is typically below
normal. Mountain pine beetle and forest fires are the two
major natural disturbance agents in interior forests. A
warm winter climate is favorable to mountain pine beetle
survival and has recently led to a severe increase in
lodgepole pine mortality. At the same time fire risk
increase under warm and dry summer conditions.
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• 1997-1998: El Niño induced mild weather
helped to significantly reduce motor vehicle
accidents on B.C. roads. The ICBC reported
that it enjoyed a substantial economic
benefit this winter. The insurance
corporation realized a saving of $3 million
per day owing to the mild winter weather.
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• 2002-2003: El Nino spawned storms brought
damaging winds to southwestern B.C. in
early January. Winds gusting over 110 km/h
toppled two mobile towers, each worth
about $15 million, at the shipping docks
near Vancouver.
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• How can sea surface temperatures in the
tropical Pacific Ocean have any bearing on
the weather that occurs in northern
America?
i) Atmospheric circulation
ii) Oceanic Circulation
iii) Atmospheric teleconnection
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Atmospheric circulation
1) a pronounced eastward extension of the midlatitude jet
stream to the extreme eastern Pacific, and 2) an
equatorward shift of the jet streams over the eastern
Pacific. Overall, these conditions reflect an increasing
zonally uniform distribution of both temperature and
winds across the Pacific basin, and are a major factor
affecting the winter weather patterns and storm tracks in
the middle latitudes over both North and South America.
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Oceanic circulation
• The warm water propagates poleward along
the west coast of North America by
coastally trapped Kelvin waves; encounters
the steep Mendocino escarpment off
California (about 41N) and is capable of
propagating past the escarpment (Allen and
Hsieh 1997). Propagation north of 41N was
observed during an El-Nino (Huyer and
Smith 1985).
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• Atmospheric Teleconnection
PNA (Pacific-North American Pattern)
AO (Arctic Oscillation)
NAO (North Atlantic Oscillation)
PDO (Pacific Decadal Oscillation)
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How is ENSO currently detected and predicted?
•
•
•
•
•
Satellites provide data on tropical rainfall, wind, and ocean
temperature patterns, as well as changes in conditions for hurricane
formation.
Ocean buoys help to monitor sea-surface and upper ocean
temperatures.
Radiosondes help to monitor global weather and climate patterns,
and to monitor and predict El Niño and La Niña influences on U.S.
weather. High-density surface data network helps to monitor and
predict El Niño and La Niña influences on U.S. weather.
Super computers are used to gather all of the weather data around the
world and put it into useful formats used by scientists. They also run
sophisticated computer models to help scientists better understand
and predict El Niño and La Niña.
An entire suite of diagnostic and prediction tools run on high-speed
computers that allow El Niño and La Niña to be monitored in near-real
time.
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• Predictions of ENSO
(1) statistical models
(2) dynamical models
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PNA
PNA: 4 centers:
Hawaii(20N,160W);
North Pacific
Ocean (45N
165W); Alberta
(55N 115W); and
the Gulf Coast
region of USA
(30N 80W)
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Red, black, and blue contours on the maps indicate positive,
zero, and negative values, respectively
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The PNA is associated with a Rossby wave pattern with
centers of action over the Pacific and over N. America.
PNA index = ½ [ Z (20N,160W)-Z(45N,165W)
+Z(55N, 115W)-Z(30N, 85W)]
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• PNA is one of the most prominent modes
of low-frequency variability in the
Northern Hemisphere extratropics. The
positive phase of the PNA pattern features
above-average heights in the vicinity of
Hawaii and over the intermountain region
of North America, and below-average
heights located south of the Aleutian
Islands and over the southeastern United
States.
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• The positive phase of the PNA pattern is associated with
above-average temperatures over western Canada and
the extreme western United States, and below-average
temperatures across the south-central and southeastern
U.S. The PNA tends to have little impact on surface
temperature variability over North America during
summer. The associated precipitation anomalies include
above-average totals in the Gulf of Alaska extending into
the Pacific Northwestern United States, and belowaverage totals over the upper Midwestern United States.
• Although the PNA pattern is a natural internal mode of
climate variability, it is also strongly influenced by the El
Niño/ Southern Oscillation (ENSO) phenomenon. The
positive phase of the PNA pattern tends to be associated
with Pacific warm episodes (El Niño), and the negative
phase tends to be associated with Pacific cold episodes
(La Niña).
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AO
AO is the dominant
mode of meanmonthly sea level
pressure variability
over the Northern
Hemisphere with
an out-of-phase
relation between the
sea level pressure over
the Arctic basin and
that at the midlatitudes (Thompson
and Wallace 1998).
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AO
Index
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• What is the NAO ?
Sometimes AO is also referred to as the
North Atlantic Oscillation (NAO) due to its
strong manifestation over the Atlantic
sector.
A large-scale mode of natural climate
variability having large impacts on weather
and climate in the North Atlantic region and
surrounding continents.
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• The NAO is the dominant mode of winter
climate variability in the North Atlantic
region ranging from central North America
to Europe and much into Northern Asia. The
NAO is a large scale seesaw in atmospheric
mass between the subtropical high and the
polar low. The corresponding index varies
from year to year, but also exhibits a
tendency to remain in one phase for
intervals lasting several years.
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North Atlantic Oscillation (positive)
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• Positive NAO Index
•
•
The Positive NAO index phase shows a stronger than usual subtropical high
pressure center and a deeper than normal Icelandic low.
The increased pressure difference results in more and stronger winter storms
crossing the Atlantic Ocean on a more northerly track.
This results in warm and wet winters in Europe and in cold and dry winters in
northern Canada and Greenland
The eastern US experiences mild and wet winter conditions
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NAO negative phase
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•
•
•
•
•
•
Negative NAO Index
The negative NAO index phase
shows a weak subtropical high
and a weak Icelandic low.
The reduced pressure gradient results in fewer and weaker winter
storms crossing on a more west-east pathway.
They bring moist air into the Mediterranean and cold air to northern
Europe
The US east coast experiences more cold air outbreaks and hence
snowy weather conditions.
Greenland, however, will have milder winter temperatures
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•PDO (Pacific Decadal Oscillation)
PDO is a long-term ocean fluctuation of the
Pacific Ocean. The PDO waxes and wanes
approximately every 20 to 30 years.
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Typical wintertime Sea Surface Temperature (colors),
Sea Level Pressure (contours) and surface wind stress
(arrows) anomaly patterns during warm and cool
phases of PDO
• Warm phase
Cold phase
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• Fisheries scientist Steven Hare coined the
term "Pacific Decadal Oscillation" (PDO) in
1996 while researching connections
between Alaska salmon production cycles
and Pacific climate. PDO has since been
described as a long-lived El Niño-like
pattern of Pacific climate variability because
the two climate oscillations have similar
spatial climate fingerprints, but very
different temporal behavior.
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Percent change in mean catches of four Alaskan
salmon stocks following major PDO sign changes
in 1947 and 1977. salmon stock1947 step1977
step
• western Alaska sockeye
• central Alaska sockeye
• central Alaska pink
• southeast Alaska pink
- 37.2% +242.2%
-33.3%
-38.3%
-64.4%
+220.4%
+251.9%
+208.7%
Published in the Bulletin of the American Meteorological Society
78: 1069-1079, 1999.
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