Geoscience Reference
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of the station-based index (top panel) is that a longer record can be obtained (back
to 1864). The time series structure from the three approaches is very similar for the
period of overlap. As discussed shortly, the PC1 for the Northern Hemisphere has
come to be known as the Arctic Oscillation or Northern Annular Mode.
Figure 4.18
shows SLP patterns, composite anomalies and composite differ-
ences for the region north of 50°N based on extremes of Hurrell's NAO index. Data
are drawn for the years 1947-
1996
. Immediately evident is that the SLP signa-
ture, while strongest in the vicinity of the Icelandic Low (a maximum difference of
22 hPa between extreme positive and negative states), extends well into the Arctic
Ocean. Over the pole, there is a composite difference of about 8 hPa. Building on
previous discussion, the Icelandic Low, although weaker under the negative NAO/
AO, is also shifted to the southwest. The low composite also shows a separate weak
closed low over the Barents Sea.
Recall that one of the earliest-noted manifestations of the NAO/AO is the “see-
saw” in SAT over Europe and northeastern North America. Under the positive mode,
the SLP pattern associated with the strong Icelandic Low will tend to advect cold
high latitude air over Greenland and eastern Canada, consistent with the tendency
for negative temperature anomalies in these areas. By comparison, an opposing
strong advection of warm lower-latitude air over northern Europe and Scandinavia
is consistent with positive temperature anomalies in this area. Under the negative
mode with a weak and southward-shifted Icelandic Low, winds over Greenland and
eastern Canada are weaker (positive temperature anomalies). Winds over northern
Europe and Scandinavia, while still southerly, are also much weaker than under the
positive NAO mode, and have opposing temperature anomaly signals.
The map of correlations between the winter (November-February) NAO index
and surface air temperatures (based on the NCEP/NCAR reanalysis) provided as
Figure 4.19
reveals that the “seesaw” is just part of a larger-scale organization of
anomalies; when the NAO is in its positive phase, temperatures tend to be below
normal over a larger area of the northern North Atlantic and northeastern North
America, and above normal over a wide swath of northern Eurasia. The opposite
pattern holds when the NAO is in its negative phase. Based on data through 1994,
Hurrell (
1996
) concluded that the NAO accounts for 31 percent of the interannual
variance of Northern Hemisphere extratropical temperatures over the past sixty
years (through 1994) while the El-Nino Southern Oscillation (ENSO) accounts for
16 percent. The latter reflects the ENSO link with the strength of the Aleutian Low
and amplitude of the downstream Pacific North America (PNA) mid-tropospheric
wavetrain over North America that we will be briefly reviewing later.
It should come as no surprise that the NAO/AO is allied with pronounced signals
in cyclone frequency. Serreze et al. (
1997
) inspected the NAO time series from 1966
through 1993. For each cold-season month (October through March), they extracted
the seven years with the most positive and most negative NAO index values. For
each month and seven-year period, they extracted records of cyclone events over
the North Atlantic, using an automated detection and tracking algorithm applied
to SLP fields (see
Chapter 4
). The positive minus negative NAO difference field in