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Northern Hemispheric flow Indices that may influence the Flow


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Title: Northern Hemispheric flow Indices that may influence the Flow

Northern Hemispheric flow Indices that may
influence the Flow
  • Richard H. Grumm
  • National Weather Service Office
  • State College PA
  • PSU 497A Lecture Series
  • Version 3.0

  • Large scale Indices that affect the flow and
    climatology over North America and Eurasia
  • North Atlantic Oscillation (NAO)
  • Pacific Decadal Oscillation (PDO)
  • Pacific North American pattern (PNA)
  • East Pacific Index (EP)
  • Definitions
  • Impacts on weather
  • and a tad of history..

North Atlantic OscillationNAO-flow Index
  • A measure of the flow over the Atlantic Basin
  • Typically, the value is computed using MSLP
  • over Iceland and the Azores
  • high NAO associated with deep Icelandic low
  • low or negative NAO associated with ridge over
    Iceland and Norway
  • Periods of low NAO have been linked to cold in
    Northern Europe and North America
  • the Little Ice age 1300-1860

  • Strictly the index should only be applied in the
    cold season
  • Recent low/negative NAO
  • the winter of 1995-96
  • Winter Oct-Jan 2002-2003
  • Historically
  • NAO use to fluctuate more then in recent times
  • we know this from history
  • wine harvest dates (later was a cold year)
  • wheat harvests and periods of European Famine
  • wheat and other grains failed or produced bad
    crops in cold wet years.
  • shipping and fishing industries back to Nordic
    and Basque fishing times.
  • They followed the water-temperature sensitive Cod

NAO-Positive Phase
  • 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.
  • Wetter in western Europe
  • keeps cold air out of Europe
  • Stronger Atlantic Jet into Western Europe
    (similar to EP in Pacific!)
  • North American Weather
  • in cold and dry winters in northern Canada and
  • The eastern US experiences mild and wet winter

NAO-Positive Phase
NAO-Negative Phase
  • shows a weak subtropical high and a weak
    Icelandic low.
  • A reduced pressure gradient results in fewer and
    weaker winter storms crossing on a more west-east
  • Moist Atlantic air moves into the Mediterranean
  • cold air to northern Europe
  • associated with many record cold European winters
  • US east coast experiences more cold air outbreaks
    and snowy weather conditions.
  • Greenland, has milder winter temperatures
  • during the warm conditions 800-1250 AD negative
    NAO's may have allowed farming and fishing in

NAO-Negative Phase
NAO in recent history
Recent NAO 2002-03
Positive Phase NAO height anomalies
Trough/Ridge Enhances Atlantic Jet (Green)
Pacific Decadal OscillationPDA/NPO Oceanic Index
  • The Pacific Decadal Oscillation (PDO) is an index
    of long-term variability of the sea-surface
    temperatures of the North Pacific Ocean.
  • reflects the dominant mode of SST over the North
    Pacific Ocean.
  • The PDO can impact the climate.
  • A characteristic that distinguishes the PDO from
    ENSO is that 20th century PDO events have tended
    to persist for 20-to-30 years, while ENSO events
    have typically persisted for 6 to 18 months.
  • also known as the North Pacific Oscillation (NPO)
    and the terms can be used interchangeably.

Phases of the PDO/NPO
  • the high phase of the NPO is the term used to
    describe a warm PDO and the low phase of the NPO
    is the term used to describe the cold PDO.
  • Recent research suggests a clear link of the PDO
    to the ENSO. Some believe the sign of the PDO may
    be linked or driven by the longer term ENSO cycle.

PDO SST Examples
The cold phase
  • Cold PDO regimes prevailed from 1890-1950
  • cold phase index is positive
  • the PDO was higher then average from 1920 to
  • cold phase may behave like a weak La Nina
  • note colder water in tropical Pacific dominates
  • may enhance impacts of La Nina when they are in
  • The cold phase occurs when there is warmer water
    over the western and central Pacific associated
    with a deepened Aleutian low.
  • Colder water over eastern and tropical Pacific

The warm phase
  • Warm PDO regimes dominated from 1950 through the
  • During the warm phase, the PDO is negative.
  • On a 30 year time scale, the PDO was lower than
    average from 1950 to 1980.
  • A negative PDO may act like an El Nino.
  • The warm phase occurs when there is colder water
    over the western and central Pacific.

The warm phase
  • Warmer and drier winters in the northern and
    wetter and cooler winters observed in the
    southern in the United States.
  • El Nino is associated with a negative SOI, weak
    tropical easterlies and warm ENSO3 SSTs.

PDO SST/wind vectors/MSLP anomalies
PDO-ENSO Similarities SST/wind vectors/MSLP
PDO values 1900-2000
East Pacific Index (EP)
  • Pattern is evident all months except August and
  • A north-south dipole of height anomalies over the
    eastern North Pacific.
  • The northern center is located in the vicinity
    of Alaska and the west coast of Canada
  • the southern center is of opposite sign and is
    found near/east of Hawaii.
  • May have a link to the PDO and ENSO
  • Weak correlation with the PNA pattern too.

Positive EP
  • Deeper than normal trough near the vicinity of
    the Gulf of Alaska/ western North America, and
  • positive height anomalies are observed farther
  • a pronounced northeastward extension of the
    Pacific jet stream toward western North America,
  • enhanced westerlies over the Pacific northwestern
    US, northern California, and sometimes
    southwestern British Columbia.
  • Last persistent positive phase of the EP from

Positive EP Maps
Negative phase of the EP
  • split-flow over the eastern North Pacific,
  • reduced westerlies throughout the region.
  • a confinement of the climatological mean Pacific
    trough to the western North Pacific
  • May result in a blocking flow configuration
    farther east.
  • Persistent negative phase occurred early 1992 to
  • A period dominated by ENSO warm episode in
    the equatorial Pacific,
  • two distinct periods of mature ENSO conditions.

EP in recent history
EP and PNA in recent time Courtesy of the cpc website EP and PNA in recent time Courtesy of the cpc website EP and PNA in recent time Courtesy of the cpc website
Month-Year EP PNA
Dec 2002 0.40 1.1
Nov 2002 -0.7 1.6
Oct 2002 -1.4 0.8
Mar 2002 0.4 -2.1
Feb 2002 0.6 -0.7
Jan 2002 0.5 -0.3
Dec 2001 1.4 0.3
Pacific North American Patten
  • Flow Pattern over North America and adjacent
    pacific ocean
  • referred to as the PNA
  • The standard one is from Wallace and Gutzler
    (1981) where they defined the index as being
    zzonal component of wind
  • time(year)z(20N,160W)-z(45N,165W)z(55N,115W)
    z(30N,85W)/ 4

PNA Description
  • The PNA pattern is one of the most prominent
    modes of low-frequency variability in the
    Northern Hemispheric extra-tropics
  • appearing in all months except June and July.
  • The PNA pattern reflects a quadripole pattern of
    height anomalies,
  • anomalies of similar sign located south of the
    Aleutian Islands and over the southeastern United
  • Anomalies with sign opposite to the Aleutian
    center are located in the vicinity of Hawaii, and
    over the intermountain region of North America
    (central Canada) during the Winter and Fall

(No Transcript)
Positive PNA-January
  • Several large scale Indices affect the flow and
    climatology over North America
  • North Atlantic Oscillation (NAO)
  • Pacific Decadal Oscillation (PDO)
  • Pacific North American pattern (PNA)
  • East Pacific Index (EP)
  • The NAO relates to the flow over the Atlantic
  • Modulates Atlantic jet stream into Europe
  • This relates to the storm track
  • The EP relates to the flow over the Pacific
  • Modulates the Pacific Jet stream into North
  • This relates to the storm track

  • Pacific North American pattern (PNA)
  • Quardripole over Pacific/North America
  • Positive PNA is associated with
  • trough over the North Pacific and southern US
  • Also, a strong ridge over western North America
  • Cold in eastern US
  • Negative Phase
  • Trough over western North America and central
  • Ridge over southern US and Gulf of Alaska
  • Warm in eastern US

PDO Summary
  • Relates to ENSO cycles
  • Frequency of El Nino can be related to PDO phase
    during the past 30-40 years.
  • PDO can interfere both constructively and
    destructively with ENSO.
  • PDO may not be independent of ENSO
  • High NPO (warm phase)
  • Cold water in north/central Pacific
  • Warm water along west coast NOAM (fishing
    industry named phases so obvious name is not
  • May act like a Positive El Nino trough in eastern
  • Low NPO (cold phase)
  • Warm water in north/central Pacific
  • La Nina like impact connect with weaker storm
    tracks, farther north dry-nation

Web resources/Credits
  • NAO
  • http//
  • http//
  • http//
  • http//
  • http//
  • http//

Climatic Indicesitalics-NAO Bold EP
  • 2002 1 0.4 1.5 -9.9 0.1 0.5
  • 2002 2 1.6 1.2 -9.9 0.8 0.6
  • 2002 3 0.5 0.5 -9.9 1.0 0.4
  • 2002 4 2.2 -0.4 0.2 -0.1 0.9
  • 2002 5 -0.7 -9.9 0.3 -0.3 0.3
  • 2002 6 0.4 -9.9 2.0 0.4 0.6
  • 2002 7 0.7 -9.9 0.6 -0.5 1.0
  • 2002 10 -1.4 1.6 -9.9 2.5 -1.4
  • 2002 11 -0.5 2.0 -9.9 0.6 -0.7
  • 2002 12 -1.3 1.0 -9.9 1.4 0.4

  • Rogers, J.C., 1997 North Atlantic storm track
    variability and its association to the North
    Atlantic Oscillation and climate variability of
    Northern Europe. Journal of Climate 10(7),
  • Hurrell, J.W., 1995 Decadal trends in the North
    Atlantic Oscillation and relationships to
    regional temperature and precipitation. Science
    269, 676-679.
  • Wallace, J.M. and David S. Gutzler,
    1981"Teleconnections in the Geopotential Height
    Field during the Northern Hemisphere Winter" Mon.
    Wea. Review,109,784-812.
  • Teleconnections Linking Wolrdwide Climate
    Anomalies. ed. M.H. Glantz, R.W. Katz and N.
    Nicholls, Cambridge University Press, 1991.
  • Rogers, J.C. and H. Van Loon, 1979 "The Sea-Saw
    in winter temperatures between Greenland and
    Northern Europe. Part II. Some atmospheric and
    oceanic effectes in middle and high latitudes."
    Mon Wea. Rev. ,107, 509-519.