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Pacific North American (PNA) Index

[1958-present monthly mean time series |  Sorted Time series]

  • The PNA or Pacific North American Pattern is characterized by atmospheric flow in which the west coast of North America is out of phase with the Eastern Pacific and Southeast United States. It tends to be most pronounced in the winter months. In it's most positive phase as shown below for January geopotential height anomalies,

    it is characterized by "wavy" flow over the continent with increased temperatures and decreased storminess in the Northwest and cold temperatures in the Southeast. The negative phase, shown below for January,

    is characterized by a more zonal (east/west) flow across the US with decreased temperatures and increased precipitation in the Northwest and increased temperatures to the southeast.

  • There are several definitions of the pattern. The standard one is from Wallace and Gutzler (1981) where they defined the index as being:
    time(year)=1/4*[z(20N,160W)-z(45N,165W)+z(55N,115W)+z(30N,85W)]
    (where the height anomalies are normalized for a particular season)
    Other definitions (including the one for the time series I use) are the result of a mathematical pattern analysis. Note that the pattern is not identical for different months and so the latter definitions may be better.

  • Suggestions for interactive analysis:

    1. Temperature and Precipitation over the US (http://www.cdc.noaa.gov/USclimate/USclimdivs.html).
      Look at precipitation and temperature years where the index is very high or low. Start with one month or season. How similar are the temperatures and precipitation patterns during different years? Do the precipitation and temperature seem to be related? Try looking at different months during one year. How does the pattern change? Are the patterns linear? That is, is the temperature or precipitation of the opposite sign during opposite phases of the PNA? How much of the variation between different PNA years might be due to sampling (the fact that there are only relatively few years with data).
    2. Correlations with temperature and precipitation over the US (http://www.cdc.noaa.gov/USclimate/Correlation/).
      Correlate the PNA index with temperature and precipitation. How do the patterns compare with composites for high and low years? How do the patterns change through the different months of the year. Do the patterns change if you choose different sets of years?
    3. Global Atmospheric Variables (http://www.cdc.noaa.gov/Composites/)
      Given the height anomaly patterns shown in the above figures, what winds would you expect (look at the geostrophic equation). What about vertical motion? How would that relate to precipitation? Do you get the precipitation in the composites (look at anomaly AND mean fields). How do the temperatures relate to the winds?

  • References