Maarten H. P. Ambaum and Brian
J. Hoskins
University of Reading, UK
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PNA
NAO |
The PNA is associated with a Rossby wave pattern with centres of action over the Pacific and over N. America. The NAO is associated with a north-south orientated dipolar structure in the pressure field over the Atlantic. A useful definition of the two patterns is in terms of EOFs of the streamfunction at 850 hPa. The first EOF represents the PNA and the second EOF the NAO. Both patterns correlate well with more traditional definitions of these patterns, such as teleconnections. First two EOFs for the DJFM monthly mean streamfunction at 850 hPa. These EOFs explain 29% and 14% of the variance respectively. The contour interval is 5x105 m2s-1. |
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PNA
NAO |
The effects of the PNA and NAO on the zonal winds at 250 hPa are remarkably similar even though the index timeseries here are orthogonal by definition. Both patterns correspond to a strengthening of the subtropical jet over the Atlantic and Arabia and a strengthening and Northward shift of the main jet off the east coast of N. America. The PNA pattern is also associated with a weakening and broadening of the Pacific jet, whereas the NAO only weakly influences the Pacific region. Monthly mean zonal wind field regressed on the normalised timeseries of the PNA (top panel) and NAO (lower panel). The contour interval is 1 ms-1. |
Increasing either
the NAO or PNA index leads to similar effects in the Atlantic.
Increasing these indices
strengthens and splits the jets in theAtlantic.
The Pacific jet weakens
and broadens on increasing PNA index.
The NAO does not affect
the Pacific jet.
The polar night jet
strengthens on increasing NAO index.
The PNA index affects
the polar night jet in the Pacific sector only.
The NAO and PNA patterns both represent a tendency for the zonal winds at 35N and 55N to anticorrelate. This tendency for anticorrelation appears over both the Atlantic and Pacific basins and the Arctic Oscillation is a reflection of this.
The presence of centres of action over both the Atlantic and the Pacific basins
is not a consequence of a correlation between lower tropospheric fields in the
Atlantic and the Pacific.
The covariance matrix for the mean sea-level pressure in the three centres of
action is:
where the rows/columns represent the Azores, Arctic and Pacific centres of action respectively. The first eigenvector, explaining 59% of the variance in this three component system is (0.38,-0.86,0.33), which corresponds to the Arctic Oscillation. The correlations between the Pacific and the other centres of action are however not significantly different from zero.
What do the words
`annular' and `mode' mean?
The zonal mean zonal
winds at 35N and 55N tend to anticorrelate in both ocean basins in association
with the PNA and NAO patterns. Is this a coincidence or is it a sign of annular
behaviour?
Zonally symmetric
anomalies affect climatological features at different latitudes in different
zonal sectors differently. Should annular behaviour be zonally organised along
latitude circles or along climatological streamlines?
The PNA and NAO patterns
both result in anticorrelation of the zonal mean zonal winds at 35N and 55N.
The effects of the
PNA and NAO patterns in the Atlantic sector on the zonal mean zonal winds are
very similar.
The Arctic Oscillation
reflects the tendency in both ocean basins for anticorrelation between the zonal
mean zonal winds at 35N and 55N.
The mean sea-level
pressure in the Pacific is not significantly correlated with the mean sea-level
pressure over Iceland or the Azores, so the Arctic Oscillation cannot be interpreted
as a teleconnection.
Zonally symmetric
anomalies act on zonally asymmetric climatology leading to significant differences
in behaviour in the two ocean basins.
This work was done as part of the EC-funded project STOEC. The STOEC webpage can be found on http://www.met.rdg.ac.uk/cag