62
Chapter 4: The Instrumental Data Record
4.3.2 Precipitation
Analyses of precipitation data have tended to focus on regional rather than
hemispheric scales. The reasons for this stern from two factors; precipitation
data have much greater spatial variability compared to temperature and precipitation data from oceanic areas are virtually non-existent. For most areas
of the world, precipitation variability, both in space and time, is so large that
it will be generally impossible to detect changes until significant impacts will
have been feIt in the agricultural and water resource sectors.
Various regional time se ries were developed for the IPCC Scientific Assessments (Folland et al. 1990, 1992). All of the studied regions in North America, Eurasia, Africa and Australia show large year-to-year variability in either
annual or growing-season precipitation totals. All show marked decadal-scale
variation but with little long-term change on the century timescale. There
are two major exceptions to this. Average annual series for the former Soviet
Union show a gradual increase in precipitation since the beginning of this
century (Figure 4.4). Most of the increase has occurred in the non-summer
season. Some of the increase may be related to the underestimation of snowfall during winter but even allowing for this a significant increase over the
USSR south of 70° N is still evident (Groisman et al. 1991).
The most significant and dramatic changes in precipitation have occurred
over the Sahel region of Subsaharan Africa. Here the reduction of precipitation during the rainy season in re cent years has been highly statistically
significant. At some sites in the region there has been a decline of about 30%
between averages for the two standard WMO periods of 1931-60 and 1961-90.
The steep change would be even greater if the periods 1941-70 and 1971-90
were compared as the 1960s were relatively wet. Figure 4.5 plots the average
time series for region using the two standard WMO reference periods. Using
the most re cent period changes the character of the time series from the Sahel
drought of the last 20 years (relative to 1931-60) to the Sahel wet period of
the 1920s to the 1960s (relative to 1961-90). The difference between the two
analyses is not a step change because of the use of standardized anomaIies.
Interannual variability is also greater at most stations during the 1961-90
period compared to 1931-60
4.3.3 Pressure
The interrelationships of the climate system mean that past changes in the
patterns of temperature and precipitation patterns will undoubtedly also be
accompanied by changes in circulation throughout the troposphere. Changes
that have been noted range from local-scale studies such as the decline in
westerlies over the British Isles (Larnb, 1972) to much larger regional scale
indices such as the North Atlantic Oscillation (van Loon and Rogers, 1978),
North Pacific winter pressure (Trenberth, 1990) and various Southern Hemisphere indices (including the Southern Oscillation) (Karoly, 1994).
Chapter 4: The Instrumental Data Record
4.3.2 Precipitation
Analyses of precipitation data have tended to focus on regional rather than
hemispheric scales. The reasons for this stern from two factors; precipitation
data have much greater spatial variability compared to temperature and precipitation data from oceanic areas are virtually non-existent. For most areas
of the world, precipitation variability, both in space and time, is so large that
it will be generally impossible to detect changes until significant impacts will
have been feIt in the agricultural and water resource sectors.
Various regional time se ries were developed for the IPCC Scientific Assessments (Folland et al. 1990, 1992). All of the studied regions in North America, Eurasia, Africa and Australia show large year-to-year variability in either
annual or growing-season precipitation totals. All show marked decadal-scale
variation but with little long-term change on the century timescale. There
are two major exceptions to this. Average annual series for the former Soviet
Union show a gradual increase in precipitation since the beginning of this
century (Figure 4.4). Most of the increase has occurred in the non-summer
season. Some of the increase may be related to the underestimation of snowfall during winter but even allowing for this a significant increase over the
USSR south of 70° N is still evident (Groisman et al. 1991).
The most significant and dramatic changes in precipitation have occurred
over the Sahel region of Subsaharan Africa. Here the reduction of precipitation during the rainy season in re cent years has been highly statistically
significant. At some sites in the region there has been a decline of about 30%
between averages for the two standard WMO periods of 1931-60 and 1961-90.
The steep change would be even greater if the periods 1941-70 and 1971-90
were compared as the 1960s were relatively wet. Figure 4.5 plots the average
time series for region using the two standard WMO reference periods. Using
the most re cent period changes the character of the time series from the Sahel
drought of the last 20 years (relative to 1931-60) to the Sahel wet period of
the 1920s to the 1960s (relative to 1961-90). The difference between the two
analyses is not a step change because of the use of standardized anomaIies.
Interannual variability is also greater at most stations during the 1961-90
period compared to 1931-60
4.3.3 Pressure
The interrelationships of the climate system mean that past changes in the
patterns of temperature and precipitation patterns will undoubtedly also be
accompanied by changes in circulation throughout the troposphere. Changes
that have been noted range from local-scale studies such as the decline in
westerlies over the British Isles (Larnb, 1972) to much larger regional scale
indices such as the North Atlantic Oscillation (van Loon and Rogers, 1978),
North Pacific winter pressure (Trenberth, 1990) and various Southern Hemisphere indices (including the Southern Oscillation) (Karoly, 1994).