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Seasonal (DJF,MAM,JJA,SON) indices of the NAO are based on the difference of
normalized sea level pressure (SLP) between Ponta Delgada, Azores and
Stykkisholmur/Reykjavik, Iceland since 1865. The SLP anomalies at each station
were normalized by division of each seasonal mean pressure by the long-term mean
(1865-1984) standard deviation. Normalization is used to avoid the series being
dominated by the greater variability of the northern station. Positive values
of the index indicate stronger-than-average westerlies over the middle latitudes.
The station data were originally obtained from the
World Monthly Surface Station Climatology.
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The annual index of the NAO is based on the difference of normalized sea level pressure (SLP)
between Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland since 1865. The SLP anomalies
at each station were normalized by division of each annual mean pressure by the long-term mean
(1865-1984) standard deviation. Normalization is used to avoid the series being dominated by the
greater variability of the northern station. Positive values of the index indicate stronger-than-average
westerlies over the middle latitudes.
The station data were originally obtained from the
World Monthly Surface Station Climatology.
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The station based index of the NAO is useful for extending the
record back in time. However, such a simple index might not be the
optimal representation of the spatial pattern associated with it. The
principal component (PC) time series of the leading EOF of seasonal
(December through March) SLP anomalies over the Atlantic sector
(20-80N, 90W-40E) serves as an alternative index (see below and Hurrell 1995).
It is shown in color, and the station based index is given by the
thick black line. The correlation between the two is 0.92 over
1899-2004. The black dots on the EOF panel show the location
of the stations used in the Extended Winter Station Based Index.
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The DJF principal component timeseries is shown below in color,
and the station based index is given by the thick black line.
The correlation between the two is 0.86 over the period 1899-2003. The black dots on the
EOF panel show the location of the stations used in the
DJF Station Based Indices.
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The MAM principal component timeseries is shown below in color,
and the station based index is given by the thick black line.
The correlation between the two is 0.89 over the period 1899-2002. The black dots on the
EOF panel show the location of the stations used in the
MAM Station Based Indices.
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The JJA principal component timeseries is shown below in color,
and the station based index is given by the thick black line.
The correlation between the two is 0.53 over the period 1899-2002. The black dots on the
EOF panel show the location of the stations used in the
JJA Station Based Indices.
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The SON principal component timeseries is shown below in color,
and the station based index is given by the thick black line.
The correlation between the two is 0.75 over the period 1899-2002. The black dots on the
EOF panel show the location of the stations used in the
SON Station Based Indices.
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The principal component (PC) time series is shown below in color,
and the station based index is given by the thick black line.
The correlation between the two is 0.90 over 1899-2002.
The black dots on the EOF panel show the location of the stations used in the
Annual Station Based Indices.
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The NAM (or Arctic Oscillation) is defined as the first EOF of NH
(20-90N) winter SLP data (see below). It explains 23% of the extended winter
mean (December-March) variance, and it is clearly dominated by the
NAO structure in the Atlantic sector. Although there are some subtle
differences from the regional pattern over the Atlantic and Arctic,
the main difference is larger amplitude anomalies over the North
Pacific of the same sign as those over the Atlantic. This feature
gives the NAM a more annular (or zonally-symmetric) structure.
For more information, see
http://horizon.atmos.colostate.edu/ao/.
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