chapter - Atmospheric and Oceanic Science

In a large part of this region, most of the increase occurred during the 1960s,
and it seems to have been associated with a reduction of the meridional gradient of
surface temperature, which probably caused a southward shift of the regional circulation.
Consistently, the leading principal component of annual precipitation correlates
with the meridional gradient of temperature at interannual as well as interdecadal
timescales (Barros and Doyle 1996). Another strong precipitation increase
was observed during the late 1970s. This correlates with an increase in the subtropical
temperature of the Southern Hemisphere and a decrease of the Southern
Oscillation Index (Barros and Doyle 1996).
5.3.1. Total Precipitation
a. Annual trends
Regional precipitation trends
Southern South America is the region of the World with greatest increase of
annual precipitation during the 20th Century (Giorgi 2003). However, West of the
Andes Mountains the precipitation trend was markedly negative (Fig. 5.1). Each
continent depicts the opposite precipitation trend (negative to the West and positive
to the East) at subtropical latitudes. The only exception is South Africa, where the
land extent at subtropical latitudes is small. On the other hand, South America is the
continent with the largest contrast. Barros (2004) has suggested that the reason for
these changes may be related to the poleward shift of the subtropical anticyclones
observed in the last few decades (Gillet et al. 2003; Escobar et al. 2003).
85S
55N
30N
10N
10S
30S
55S
85S
-40% -40% -30% -20% -10% 0% 10% 20% 30% 40% 50%
Fig. 5.1. Annual global precipitation trends (%) in the twentieth.
[Source: IPCC 2001]
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