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1840
1860
1880
1900
1920
1940
1960
1980
2000
Figure 4.7 Seasonal
values of the Transpolar
Index (TPI), calculated from
MSLP data from Hobart,
Australia, and Stanley,
Falkland Islands, compared
to a base period of 1951-80.
(From Jones et al.
1999
,
Figure 6, Copyright John
Wiley & Sons Ltd., with
permission)
3.0
2.0
1.0
0.0
-1.0
-2.0
TPI - Summer (DJF)
-3.0
3.0
2.0
1.0
0.0
-1.0
-2.0
TPI - Autumn (MAM)
-3.0
3.0
2.0
1.0
0.0
-1.0
-2.0
TPI - Winter (JJA)
-3.0
3.0
2.0
1.0
0.0
-1.0
-2.0
TPI - Spring (SON)
-3.0
1840
1860
1880
1900
1920
1940
1960
1980
2000
indicating that meridional circulation was more important than in the previous
period. A reversion to dominant zonal flow has occurred since 1980.
Karoly et al.(
1996
) measured variance of SH mid-latitude circulation asso-
ciated with several indices. They discovered that ENSO was still the most
important factor (29% of variance explained) with the TPI second (16% variance
explained). No other index of measurement was significant. The authors state
that the TPI is independent of ENSO, and therefore a reasonable indicator of
changes in the mid-latitude zonal pressure gradient linked to the importance of
Rossby waves 1 and 3.
The TPI does not have a strong correlation with temperature variations on a
regional scale. It does, however, show significant negative correlations (approx
0.3 to
0.5) with temperature in southern South America, in summer and
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