53
the ocean mixed layer, which renders sea-surface temperature and surface
air temperature relatively unresponsive to the thermal variability of the
overlying atmosphere on time scales of a few years or less.
If the above reasoning is correct, it should be possible for the hemispheric circulation to induce positive anomalies in hemispheric-mean temperature simply by redistributing heat in a manner so as to make tropospheric temperatures colder than normal over the oceans and warmer than
normal over the continents. This hypothesis can be verified by regressing the hemispheric field of 1000-500-mb thickness anomalies, a measure
of the temperature anomalies in the lower troposphere, upon the time
series of hemispheric-mean surface air temperature anomalies (hereafter
denoted TL), as shown in the middle panel of Fig. 13. The strong regression coefficients are largely restricted to the region poleward of 40oN.
The corresponding 500-mb height configuration, shown in the upper panel,
is reminiscent of the Rockies cluster in Fig. 11 and the positive polarity of the PN A pattern over that limited sector of the hemisphere. The
1000-mb height (or sea-level pressure) field shown in the lower panel is
characterized by negative anomalies over high latitudes, reminiscent of the
'high index' phase of the 'zonal index cycle' defined by Rossby and collaborators during the 1940's in connection with their efforts to develop a
method of making long-range weather forecasts. Namias (1980) obtained
rather similar looking patterns when he composited Northern Hemisphere
1000-700-mb thickness and 700-mb height anomalies for the nine warmest
winters (based on hemispheric-mean thickness) of the period 1951-78.
The patterns in Fig. 13 should not be interpreted as a 'normal mode'
or favoured pattern of variability of the hemispheric circulation in its own
right, in the same sense as the PNA pattern or the NAO. If it were, one
would expect 1000-500-mb thickness anomalies at the two positive 'centres
of action' over the Yukon and Siberia to be positively correlated with one
another but, in fact, they are not. This pattern has special significance only
in relation to hemispheric-mean temperature. The striking correspondence
between the thickness pattern and the land-sea distribution poleward of
400N suggests the notion of a cold ocean/warm land (COWL) index that
might be used to characterize the hemispheric circulation pattern observed
each month. A positive value of the index for a particular month would
be indicative of a thickness pattern characterized by positive anomalies
over the continents and negative anomalies over the oceans, and and vice
versa. Since the index is supposed to reflect the redistribution of heat by
the ocean mixed layer, which renders sea-surface temperature and surface
air temperature relatively unresponsive to the thermal variability of the
overlying atmosphere on time scales of a few years or less.
If the above reasoning is correct, it should be possible for the hemispheric circulation to induce positive anomalies in hemispheric-mean temperature simply by redistributing heat in a manner so as to make tropospheric temperatures colder than normal over the oceans and warmer than
normal over the continents. This hypothesis can be verified by regressing the hemispheric field of 1000-500-mb thickness anomalies, a measure
of the temperature anomalies in the lower troposphere, upon the time
series of hemispheric-mean surface air temperature anomalies (hereafter
denoted TL), as shown in the middle panel of Fig. 13. The strong regression coefficients are largely restricted to the region poleward of 40oN.
The corresponding 500-mb height configuration, shown in the upper panel,
is reminiscent of the Rockies cluster in Fig. 11 and the positive polarity of the PN A pattern over that limited sector of the hemisphere. The
1000-mb height (or sea-level pressure) field shown in the lower panel is
characterized by negative anomalies over high latitudes, reminiscent of the
'high index' phase of the 'zonal index cycle' defined by Rossby and collaborators during the 1940's in connection with their efforts to develop a
method of making long-range weather forecasts. Namias (1980) obtained
rather similar looking patterns when he composited Northern Hemisphere
1000-700-mb thickness and 700-mb height anomalies for the nine warmest
winters (based on hemispheric-mean thickness) of the period 1951-78.
The patterns in Fig. 13 should not be interpreted as a 'normal mode'
or favoured pattern of variability of the hemispheric circulation in its own
right, in the same sense as the PNA pattern or the NAO. If it were, one
would expect 1000-500-mb thickness anomalies at the two positive 'centres
of action' over the Yukon and Siberia to be positively correlated with one
another but, in fact, they are not. This pattern has special significance only
in relation to hemispheric-mean temperature. The striking correspondence
between the thickness pattern and the land-sea distribution poleward of
400N suggests the notion of a cold ocean/warm land (COWL) index that
might be used to characterize the hemispheric circulation pattern observed
each month. A positive value of the index for a particular month would
be indicative of a thickness pattern characterized by positive anomalies
over the continents and negative anomalies over the oceans, and and vice
versa. Since the index is supposed to reflect the redistribution of heat by