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5 Impact of extratropical dynamical variability upon
hemispheric-mean temperature
In this section we will consider the premise that the time-mean hemispheric circulation during a particular month, season or decade is capable of influencing the hemispheric-mean temperature observed during that
time interval, from which it follows that part of the temporal variability
of hemispheric-mean temperature is "dynamically induced": i.e., induced
by changing circulation patterns such as those described in the previous
section. Two specific examples come to mind:
• the North Atlantic Oscillation (Fig. 10) influences surface air temperatures over a large expanse of the Northern Hemisphere. "High
index" periods (as defined by the strength of the westerlies along the
node in the dipole pattern in the sea-level pressure field over the North
Atlantic) tend to be characterized by above normal temperatures over
Europe and most of Russia and the northeastern United States, and
below normal temperatures over Greenland and Labrador (van Loon
and Rogers 1978, Hurrell 1995). Since the area of positive temperature anomalies is much larger than the area of negative anomalies, one
might expect the index of the NAO to be positively correlated with
hemispheric-mean surface air temperature .
• the positive polarity of the PN A pattern (as defined by the sign of the
'center of action' in the 500-mb height field over western Canada) is
characterized by above normal temperatures over Alaska and western
Canada and below normal temperatures over the southeastern United
States. Since the positive anomalies tend to be much larger than the
negative ones, one might expect the positive polarity of the PNApattern to favour above normal hemispheric-mean temperature.
Implicit in both these examples is the presumption that anomalies over
the continents are more influential in determining the hemispheric-mean
temperature than anomalies over the oceans. The emphasis on land areas
is justified by the fact that the temporal variances of surface air temperature over the continental interiors tend to be larger, by a factor of 3 or
more, than those over the oceans. This inequality holds for the diurnal
cycle, for the climatological-mean annual annual march, and for the yearto-year variability. It is a consequence of the much larger heat capacity of
5 Impact of extratropical dynamical variability upon
hemispheric-mean temperature
In this section we will consider the premise that the time-mean hemispheric circulation during a particular month, season or decade is capable of influencing the hemispheric-mean temperature observed during that
time interval, from which it follows that part of the temporal variability
of hemispheric-mean temperature is "dynamically induced": i.e., induced
by changing circulation patterns such as those described in the previous
section. Two specific examples come to mind:
• the North Atlantic Oscillation (Fig. 10) influences surface air temperatures over a large expanse of the Northern Hemisphere. "High
index" periods (as defined by the strength of the westerlies along the
node in the dipole pattern in the sea-level pressure field over the North
Atlantic) tend to be characterized by above normal temperatures over
Europe and most of Russia and the northeastern United States, and
below normal temperatures over Greenland and Labrador (van Loon
and Rogers 1978, Hurrell 1995). Since the area of positive temperature anomalies is much larger than the area of negative anomalies, one
might expect the index of the NAO to be positively correlated with
hemispheric-mean surface air temperature .
• the positive polarity of the PN A pattern (as defined by the sign of the
'center of action' in the 500-mb height field over western Canada) is
characterized by above normal temperatures over Alaska and western
Canada and below normal temperatures over the southeastern United
States. Since the positive anomalies tend to be much larger than the
negative ones, one might expect the positive polarity of the PNApattern to favour above normal hemispheric-mean temperature.
Implicit in both these examples is the presumption that anomalies over
the continents are more influential in determining the hemispheric-mean
temperature than anomalies over the oceans. The emphasis on land areas
is justified by the fact that the temporal variances of surface air temperature over the continental interiors tend to be larger, by a factor of 3 or
more, than those over the oceans. This inequality holds for the diurnal
cycle, for the climatological-mean annual annual march, and for the yearto-year variability. It is a consequence of the much larger heat capacity of