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

Chapter 12: 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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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 Nino
• Jacob Bjerknes
• Took Walker’s idea of El
Niño one step further and
suggested that many longterm 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);
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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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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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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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Temperature
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Difference in
maximum Snow depth
in millimeters between
El Niño and La Nina
and Neutral years
EL
El Niño
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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El Niño/ La Niña: SST and
Pacific Jet Stream
El Niño features a weakerthan-average equatorial
cold tongue.
La Niña features a
stronger-than-average
equatorial cold tongue.
Positive SST anomalies
are usually largest in the
eastern equatorial Pacific.
Negative SST anomalies
are usually largest in the
central equatorial Pacific.
North Pacific jet stream is
shifted southward and is
stronger than average
over the eastern Pacific
and southern US.
North Pacific jet stream is
shifted northward and is
weaker than average over
the eastern Pacific and
southern US.
There is considerable event-to-event variability in the intensity of
these departures from normal (see following slides for El Nino).
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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 nearreal time.
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• Predictions of ENSO
(1) statistical models
(2) dynamical models
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Niño Indices: Recent
Evolution
The latest weekly SST
anomalies are positive
(between +0.8C and
+1.2C) in all of the Niño
regions.
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Recent Evolution of Equatorial
Pacific SST Departures
During September 2005- January 2006
SST anomalies decreased and belowaverage temperatures developed
throughout most of the central and
eastern equatorial Pacific.
In February 2006 positive SST
anomalies developed in the extreme
eastern equatorial Pacific, similar to
what occurred during the La Niña
years 1999, 2000 and 2001.
Time
Since May 2006, positive SST
anomalies have increased across the
equatorial Pacific between 160E and
the South American coast.
Longitude
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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, respectively39
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 below-average totals
over the upper Midwestern
United States.
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• 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 mean-monthly 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 stock 1947 step 1977 step
• western Alaska sockeye
- 37.2% +242.2%
• central Alaska sockeye
• central Alaska pink
• southeast Alaska pink
-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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