Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 111

2.8

Figure 4.4 Global annual
mean and five-year running
mean precipitation change.
(Source: NASA:GISS)

2.7

2.6

Annual mean

2.5

5-year mean

1900 1920 1940 1960 1980 2000

the 1970s, the precipitation was above the mean; it then declined until the mid
1990s. Hence, for the twentieth century, this study found variations, but essen-
tially from about 1915 there has been no net change. Given that the NH is the
land hemisphere, such a finding would seem significant to areas such as
agriculture and insurance (see Section 8.4 ).
The graphical expression of global precipitation trends is shown in Figure 4.4 .
The NH data are graphed in Figure 4.5 , which, for comparison, also shows
other world areas. These graphs were derived from the CRU TS 2.0 data set
that comprises 1200 monthly grids of observed climate for the period
1901-2000. Available from the Tyndall Centre for Climate Change Research
(www.cru.uea. ac.uk/~timm/grid/CRU_TS_2_0_text.html) the data set covers
the global land surface at 0.58 resolution. The data are also readily available from
other sources.
In another study of global precipitation variations for the twentieth century,
Dai et al.( 1997 ) used statistical analysis and found the leading EOF to be ENSO-
related precipitation variations in low latitudes. The second EOF demonstrated
an increasing global linear trend for the world; the EOF trend is seen as a long-
term increase in North America, mid- to high-latitude Eurasia and, in the SH, in
Argentina and Australia. Interestingly, these researchers found that the NAO
accounts for some 10% of winter variance over regions surrounding the North
Atlantic Ocean. During high-index NAO winters, northern Europe, the eastern
United States and the Mediterranean experience above normal precipitation.
Below normal occurs in southern Europe and eastern Canada.

Precipitation patterns
Figure 4.6 provides the twentieth century precipitation trend for the United
States based upon the CRU TS data-base. While this does provide an overall