48
the leading EOF (Fig. 8) and it contains elements of the PNA pattern
in its negative polarity. The "Greenland pattern" resembles the NAO in
its negative polarity and the "Rockies" cluster contains elements in common with the PNA pattern (Fig. 9) in its positive polarity. The "Alaska"
and "Greenland" clusters are associated with high amplitude ridges or
'blocking episodes' in two regions of the hemisphere that most frequently
experience this phenomenon (Dole and Gordon 1983, Knox and Hay 1985),
and the "Rockies" cluster is characterized by a strong jet stream, displaced
southward of its climatological mean latitude over the central Pacific, with
a strongly diffluent flow downstream leading into a longwave ridge that
appears to be anchored to the Rockies.
The frequency of occurrence of the clusters is relatively sensitive to the
clustering algorithm: in this particular analysis, the "Alaska," "Greenland," and "Rockies" clusters account for roughly 9% , 9% , and 13% of
winter days, respectively, leaving 69% of the days unclassified. In Kimoto
and Ghil's analysis, the corresponding frequencies of occurrence are 6% ,
5% , and 7% and another 10% of the days are assigned to a fourth cluster,
not shown here.
The clusters can be identified with distinctive weather anomalies in
certain regions of the hemisphere. The "Alaska" cluster is associated with
unseasonably mild weather over much of Alaska and it accounts for most of
the major cold air outbreaks over the U.S. Pacific Northwest. The "Greenland" cluster is characterized by cold weather over the northeastern U.S.
and southeastern Canada and Scandinavia, and the "Rockies" cluster by
mild, dry weather over most of the western United States and the interior
of western Canada.
The dates of occurrence of the three clusters, shown in Fig. 12, match
up well with significant events in the wintertime climate records for these
regions. For example, the 1949-50 winter, during which the "Alaska" cluster was prevalent, was unseasonably cold in the U.S. Pacific Northwest.
The relatively high frequency of occurrence of the "Greenland" cluster in
1955-70, 1977-80, and 1985-87 matches up well with the dips in the NAO
index in Fig. 10, and the increased frequency of occurrence of the "Rockies"
cluster after 1911 reflects the ENSO-like interdecadal variablity discussed
in section 8.
It is notable that the dominant structures in the wintertime mean 500mb height field do not resemble pure spherical harmonic functions and their
projections upon latitude circles do not resemble pure sinusoidal waves
the leading EOF (Fig. 8) and it contains elements of the PNA pattern
in its negative polarity. The "Greenland pattern" resembles the NAO in
its negative polarity and the "Rockies" cluster contains elements in common with the PNA pattern (Fig. 9) in its positive polarity. The "Alaska"
and "Greenland" clusters are associated with high amplitude ridges or
'blocking episodes' in two regions of the hemisphere that most frequently
experience this phenomenon (Dole and Gordon 1983, Knox and Hay 1985),
and the "Rockies" cluster is characterized by a strong jet stream, displaced
southward of its climatological mean latitude over the central Pacific, with
a strongly diffluent flow downstream leading into a longwave ridge that
appears to be anchored to the Rockies.
The frequency of occurrence of the clusters is relatively sensitive to the
clustering algorithm: in this particular analysis, the "Alaska," "Greenland," and "Rockies" clusters account for roughly 9% , 9% , and 13% of
winter days, respectively, leaving 69% of the days unclassified. In Kimoto
and Ghil's analysis, the corresponding frequencies of occurrence are 6% ,
5% , and 7% and another 10% of the days are assigned to a fourth cluster,
not shown here.
The clusters can be identified with distinctive weather anomalies in
certain regions of the hemisphere. The "Alaska" cluster is associated with
unseasonably mild weather over much of Alaska and it accounts for most of
the major cold air outbreaks over the U.S. Pacific Northwest. The "Greenland" cluster is characterized by cold weather over the northeastern U.S.
and southeastern Canada and Scandinavia, and the "Rockies" cluster by
mild, dry weather over most of the western United States and the interior
of western Canada.
The dates of occurrence of the three clusters, shown in Fig. 12, match
up well with significant events in the wintertime climate records for these
regions. For example, the 1949-50 winter, during which the "Alaska" cluster was prevalent, was unseasonably cold in the U.S. Pacific Northwest.
The relatively high frequency of occurrence of the "Greenland" cluster in
1955-70, 1977-80, and 1985-87 matches up well with the dips in the NAO
index in Fig. 10, and the increased frequency of occurrence of the "Rockies"
cluster after 1911 reflects the ENSO-like interdecadal variablity discussed
in section 8.
It is notable that the dominant structures in the wintertime mean 500mb height field do not resemble pure spherical harmonic functions and their
projections upon latitude circles do not resemble pure sinusoidal waves