Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 106

northwest coast to the southeast subtropical region and a southwards shift of the
Atlantic ITCZ. This variation leads to increased rainfall over northern and
eastern Brazil.
One final example illustrating the use of the NCEP/NCAR reanalysis data
deals with the climatology of the spring monsoon in South China (Yan 2002 ).
The results indicate that the spring monsoon season in South China occurs in
April and May, a finding supported by both seasonal and interannual variation of
circulation and precipitation patterns. The interannual variation of the spring
monsoon rainfall in South China relates primarily to the anomalous circulation
over the North Pacific Ocean. This is related to the westerly jet stream over
North Asia, the polar vortex, and sea surface temperature anomalies in the
Pacific. It is shown that changes in the Asian tropical atmospheric circulation
have little influence on the spring monsoon in South China.
4.5 The Northern Hemisphere
4.5.1 Middle-latitude circulation: evolution of a concept
In 1735 George Hadley provided a hypothesis that was to become the starting
point of succeeding models of the general circulation of the atmosphere. In an
attempt to explain the trade winds observed and reported by mariners, Hadley
suggested that air heated at the equator would rise and, at some high elevation,
spread toward the poles. The return flow near the surface would complete the
cell and hence explain the trades. Accordingly, a convective cell (a Hadley Cell)
was identified in each hemisphere. Subsequent observations of surface pressure
and wind patterns negated the idea of a single huge cell in each hemisphere and
saw the creation of the three-cell model in which three cells (Hadley, Ferrel, and
Polar) were identified in each hemisphere.
General circulation models from the 1800s are not markedly different from
the general circulation models used in many textbooks of today. Superficially,
the three-cell model appears to explain observed surface phenomena; equatorial
low pressure, trade winds, the subtropical high pressure, mid-latitude surface
westerlies, subpolar lows, and polar easterlies. However, the three-cell model
does not consider many variables including seasonal variations.
First, consider data applicable to meridional circulation (Figure 4.1 ). Winter
meridional circulation in the NH shows a definitive transfer between the equator
and about 308 N, the Hadley Cell, but poleward of 308 N there is no clearly
defined pattern (Palm´n and Newton 1969 ). The meridional circulation of the
Ferrel Cell is some 80% weaker than the Hadley Cell, while a polar meridional
circulation is not evident at all. In the extratropical areas of the Ferrel and Polar
Cells, zonal mass transfer is, in fact, an order of magnitude larger than meridio-
nal transfer during the winter season. During summer NH meridional circulation
patterns are even weaker.