Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 113

image, studies of identified regions show that area differences do occur. In North
America, there has been a gradual increase in precipitation since the 1920s for
large parts of the country. Increases of more than 20% have occurred in north-
western and eastern Canada, and in parts of the US Gulf coast. Decreases have
occurred over much of the High Plains and California (Karl and Knight 1998 ).
However, in a study of 105 years of data, from 1895 to 1999, Garbrecht and
Rossel ( 2002 ) showed that in the central and south Great Plains above normal
precipitation occurred in the last two decades of the twentieth century. During
this 20-year span, there was a reduction in the number of dry years, and an
increase in the number of wet years.
Yan ( 2002 ), in a study of temporal and spatial patterns of seasonal precipita-
tion variability in China, identified four distinct precipitation regions through
cluster analysis. In examining trends and variability of each region, none were
found to have significant increasing or decreasing trends for the period 1951-99.
In relating the role of large-scale circulation patterns in precipitation variabil-
ity in the identified regions, Yan found that the Arctic and Polar-Eurasia
Oscillations were positively associated with winter precipitation. Other telecon-
nections indices, including the Southern Oscillation, appeared to have negligible
effects.
Apart from precipitation trends and spatial patterns, considerable interest has
been afforded extreme precipitation events. The IPCC (Cusbasch et al. 2001 ) has
noted that there is likely to be an increase in heavy and extreme precipitation
events in those regions where the trend shows increasing precipitation. This, as
noted earlier (Figure 4.5 ), is largely in the mid to high latitudes of the NH.
Kunkel et al.( 2003 ) designed a study to test whether extreme precipitation
events were increasing in the United States using daily precipitation observa-
tions for the period 1895-2000. They revealed that heavy precipitation frequen-
cies in the late nineteenth and early twentieth centuries were very similar to the
frequency that occurred in the 1980s and 1990s. The values reached a minimum
in the 1920s and 1930s. This finding indicates that for this study area, at least,
there has not been an increasing trend in extreme precipitation events when long-
term data series are used. Had the longer time series containing earlier observa-
tions not been used, then a trend might have been discerned. Clearly, the
significance of the data series used is of singular importance.
In a similar study, Michaels et al.( 2004 ) focused on trends of precipitation on
the 10 wettest days of the year locations in the contiguous United States. These
trends were then compared with trends in overall precipitation. They found that
for the region as a whole the trends of the two sets are not significantly different.
On a regional scale, only in the northeast and southeast does the rate of increase
of wettest days exceed the total; for the rest of the country, precipitation on the
wettest days is increasing at a lesser rate than the total precipitation increases.
The results obtained by Kanae et al.( 2004 ) of changes in hourly precipitation
at Tokyo also show that extreme precipitation events have not increased in