Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 125

This creates in winter, for example, shifts in the circulation zones. During
sunspot maxima conditions, circulation zones are shifted northward, zonal
flow enhanced, coastal rain over Australia increases, and winds tend to be
calm across the middle of the continent. Comparing this situation to Table 4.2 ,
it is in general equivalent to the zonal conditions in the left-hand column. When
sunspots are at minima, the circulation zones shift southward, and the calm area
occurs over southern Australia. Table 4.2 suggests that this should represent
enhanced meridional circulation.
However, the problems with the argument are illustrated in the correlation
analysis between the sunspot cycles and the pressure time series over Australia,
South America, and Southern Africa. Only the last shows statistical significance,
despite apparent visual relationships (Figure 4.10b ). Thus Thresher's results
show some evidence of a relationship between sunspots and circulation vari-
ations, but there is a lack of consistency, and an apparent dependence on spatial
location. The controversy about sunspots as a potential forcing mechanism will
continue, with more extensive research leading to a better understanding.
4.7 Chapter summary
Geographically, the mid-latitudes in both hemispheres are located between 35
and 608. However, the extent of land mass in the NH compared to ocean surface
in the SH creates major contrasts in circulation strength and influence. In the
NH, the circulation is dominated by troughs and ridges at three locations around
the globe. High mountain ranges such as the Rocky Mountains in the United
States have an important influence. In the SH, aside from the Andes in South
America, there is little influence from terrain. Instead, the ocean, and its con-
trasts with ice from Antarctica, is the major influence. The circulation anchor for
the SH mid-latitude westerlies is the East Antarctic plateau.
The NAO has a strong influence on seasonal and annual temperature and
precipitation patterns in the NH. In the SH, the TPI has a much weaker influence,
and regional contrasts are a more important determinant of the relationship
between zonal and meridional flow.
NCEP/NCAR reanalysis has proven to be an extremely useful tool to provide
detailed information about precipitation, temperature, and circulation patterns
and trends in both hemispheres. Correlations between circulation areas in the
SH, for example, have clearly established relationships between centers in the
South Atlantic, south Indian, and Tasman Sea areas, associated with variations
in Rossby wave number 3. The latter also determines the extent of zonality in
the Australia-New Zealand region, and has been linked to glacial advances on
the South Island of New Zealand, the occurrence of drought in Southern Africa,
and the level of precipitation in East Antarctica.
The role of precipitation in the global climate system is providing a wealth of
research studies. Until fairly recently, the analysis of precipitation was hindered