Present-Day climate variability

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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 NiñoEl 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
  0.5C, there is a good chance of an El Niño event
  taking place. When the anomalies are smaller than
  or equal to -0.5C, 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

• Jacob Bjerknes
• Took Walkers idea of El Niño one step further
  and suggested that many long-term variations in
  the worlds 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 Niño
EL 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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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.
Atmospheric circulation
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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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PNA (Pacific-North American Pattern) AO (Arctic
Oscillation) NAO (North Atlantic Oscillation)
PDO (Pacific Decadal Oscillation)

• Atmospheric Teleconnection

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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 4 centers Hawaii(20N,160W)North Pacific
Ocean (45N 165W) Alberta (55N 115W) and the
Gulf Coast region of USA (30N 80W)
PNA
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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 below-average 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 is the dominant mode of mean-monthly sea
levelpressure variability over the Northern
Hemisphere withan out-of-phase relation between
the sea level pressure over the Arctic basin and
that at the mid-latitudes (Thompson and Wallace
1998).
AO
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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 - 37.2 242.2
• central Alaska sockeye -33.3 220.4
• central Alaska pink -38.3 251.9
• southeast Alaska pink -64.4 208.7
• Published in the Bulletin of the American
  Meteorological Society
• 78 1069-1079, 1999.