Geoscience Reference
In-Depth Information
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Figure 4.19.
Spatial pattern of the linear correlation between the winter (November-
February) averaged NAO index and surface air temperature, based on data from the
NCEP/NCAR reanalysis for the 1960-2010 period (courtesy of A.P. Barrett, NSIDC,
Boulder, CO).
cyclone counts from that study is reproduced in
Figure 4.20
. There is an obvious
poleward (equatorward) shift in cyclone activity between the positive (negative)
NAO phases, broadly consistent with the different patterns of SLP associated with
NAO extremes. For the region corresponding to the climatological center of the
Icelandic Low, cyclone events are more than twice as common under positive NAO
extremes as compared to negative extremes. Systems found in this region during the
positive phase are also significantly deeper than their low NAO counterparts.
There is little evidence of any preferred time scale of variability in the NAO.
The studies of Hurrell and van Loon (
1997
) and E. Cook, R. D'Arrigo, and K.
Briffa (
1998
) document concentrations of spectral power in the NAO at 2.1, 8,
and 24 years, but as noted by Hurrell et al. (
2003
) there are no significant peaks.
Furthermore, large changes can occur from one winter to the next, and there is con-
siderable variability within a given winter season. Although this behavior is consis-
tent with the notion that much of the variability in the NAO is from processes inter-
nal to the atmosphere (Hurrell et al.,
2003
), one can expect surface influences. One
such forcing that can operate on seasonal and shorter time scales is the distribution
of sea ice. Some modeling studies (e.g., Alexander et al.,
2004
; Deser et al.,
2004
)
