Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 116

about 0.3 ( > 0.4 at about 308 S just south of Madagascar). Correlations with
Southern African rainfall, on the slightly more tropical east coast, are on the
order of 0.5. Despite its importance as the dominant mode of variability in the
hemisphere Karoly et al.( 1996 ) and Bigg et al.( 2003 ) suggest that the role of
ENSO is probably overemphasised.
4.6.2 The influence of jet streams
The jet streams in the middle and upper troposphere in the SH mid-latitudes are
not as affected by terrain compared to the NH. They are important steering
mechanisms for surface cyclones and anticyclones. Jets are normally strongest in
the winter, globally averaging > 40m s 1 , compared to about 10 m s 1 in summer.
However, Trenberth ( 1987 ) states that, unlike the NH, zonal mean winds repre-
senting jets can be stronger in the summer in some parts of the SH atmosphere. He
illustrates this point by comparing vertical wind shears in the 40-508 S latitudinal
band between the hemispheres. In winter, the results are comparable, 19.8m s 1 in
the SH compared to 20.6m s 1 in the NH. In summer the mean jet is much weaker
in the NH, 10.2m s 1 compared to 20.8m s 1 in the SH.
In the southwest Pacific, jets in winter can occur near 408 S (subtropical jet)
and 60-658 S (polar front jet) (Sinclair 1996 ). Trenberth ( 1987 ) emphasizes the
importance of a split jet occurring during stronger meridional flow, but there is
considerable variation depending on the circumstances.
4.6.3 General circulation
Analysis of surface pressure distributions and variations is the main approach
taken to describing mid-latitude general circulation variability in the SH, and the
importance of zonal flow. One approach was developed in the early 1980s by
Pittock ( 1984 ), and adopted by Jones et al.( 1999 ). This is the Trans-Polar Index
(TPI), designed to measure the influence of Rossby wave number 1. The TPI is a
similar calculation to the SOI in the SH tropics, in that it compares pressure at
two widely spaced locations, Hobart, Tasmania, in Australia, and Stanley in the
Falkland Islands. While these locations may not be ideal, the results of TPI
calculations for the four seasons, reproduced in Figure 4.7 , provide some
indication of the relative strength of zonal and meridional circulation.
It is immediately apparent from Figure 4.7 that there is considerable varia-
bility in the TPI values between seasons and between years. Positive values
indicate higher pressure in Hobart compared to Stanley and a stronger zonal
index. Negative values indicate higher pressure in Stanley than Hobart and
stronger meridional circulation. The solid line in Figure 4.7 is a 10-year
Gaussian filter that provides an indication of trends. In autumn for example,
the TPI values are mainly positive between 1900 and 1940, suggesting dominant
zonal circulation. Between 1940 and 1980, greater negative years existed,