Recent precipitation trends

Rain and snow are pivotal variables in the hydrologic cycle, but reliable estimates of global precipitation are difficult to achieve. An obvious contributor to the difficulty in achieving accurate quantitative documentation of global precipitation is that precipitation is discontinuous in time and space. The variation in the spatial character of precipitation is exacerbated by the fact that most oceanic and unpopulated land areas are inadequately represented in existing data (Xie and Arkin, 1997). In addition, errors and inhomogeneities in precipitation measurements and time-limited observational programs contribute to uncertainty in the record. Consequently, global precipitation time-series commonly represent land areas only (IPCC, 2001). Efforts to combine various data sources that include both land and ocean areas have produced promising results, but these data sets are limited to the last two decades of the twentieth century (Huffman et al., 1995; Xie and Arkin, 1997).

8.11.1 Global precipitation

The time-series of annual precipitation for the twentieth century displays a slight increasing trend of 0.89 mm per decade for land precipitation (New et al., 2001) and 2.4 mm per decade for global precipitation (Dai et al., 1997; IPCC, 2001). This increasing trend is accompanied by changes in precipitation characteristics (Trenberth et al., 2003) and a tendency toward increases in the frequency of high precipitation amounts (Katz et al., 2002; Fowler and Kilsby, 2003). These conditions are evident in Australia, the UK, the United States, and in Germany in the fall and spring (Kundzewicz, 2002). The first and last 20 years of the land surface record (Fig. 8.13) are dominated by relatively dry years, and

1900 1920 1940 1960 1980 2000

Year

Fig. 8.14. Annual precipitation for the contiguous United States for 1900-2005. (Data courtesy of NOAA's National Satellite and Information Service and the National Climate Data Center from their website at http://www.ncdc.noaa.gov/oal/climate/ research/cag3/na.html.)

sharp increasing precipitation trends characterize the mid 1940s to mid 1950s and the mid 1960s to mid 1970s. Global precipitation for 1979-2002 shows increases in some regions and decreases in others, but the global average change is near zero (Smith et al., 2006). Time-series for selected latitude zones display a variety of trends. Substantial increasing precipitation trends characterize the Northern Hemisphere middle and high latitudes, and strong trends in moisture recycling are especially evident over northern Europe and North America (Dirmeyer and Brubaker, 2006). Precipitation trends for the Southern Hemisphere and the tropics in both hemispheres are relatively flat, but they display decreasing trend tendencies in recent years (Dai et al., 1997). One exception is a precipitation increase in the lowland tropics of South America (Villalba et al., 1998). In a long-term view, a precipitation reconstruction for central Chile indicates precipitation was greater in the nineteenth century than in the twentieth century (LeQuesne et al., 2006).

Important year-to-year differences in annual precipitation for the contiguous United States (Fig. 8.14) are related to shifts in rainfall patterns, increases in annual precipitation, and changes in the seasonal distribution of precipitation related to ocean-atmospheric influences. On decadal time scales, precipitation is more sensitive than temperature to atmospheric circulation variations. The average precipitation for 1900 to 2005 is 80 cm, but a trend toward greater precipitation and greater precipitation variability is indicated by the data. Precipitation increases by 3 mm per decade and the coefficient of variation is 2-times larger for the first half of the record compared to the second half. The wettest year is 1983 (95.7 cm), and 12 of the 16 years with precipitation greater

100 -50

1870 1890 1910 1930 1950 1970 1990 Year

Fig. 8.15. Annual precipitation for Long Beach, Washington, and Portland, Maine, for 1870-2002. The broken horizontal lines are the 1971-2000 average for each station. (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http:// cdiac.ornl.gov/epubs/ndp/ushcn/usa_monthly.html.)

than 86 cm occur after 1952. The driest year is 1917 (66 cm), and 8 of the 13 years receiving less than 73 cm occur before 1952.

8.11.2 Individual station precipitation

Station data reveal that the Northern Hemisphere mid-latitude increasing precipitation trend reported by Dai et al. (1997) is not evident at all locations. Portland, Maine (44° N), and Long Beach, Washington (46° N), on the east and west coasts, respectively, of North America have data available as early as 1870 (Fig. 8.15). The average annual precipitation at Portland is 109 cm and the average annual precipitation at Long Beach is 190 cm. Portland has an increasing trend of 16 mm per decade, but Long Beach has an increase of less than 1 mm per decade. Furthermore, the data for the two stations display little similarity in the occurrence of wet and dry years. The wettest year at Portland is 1983 which recorded 169 cm of precipitation and the driest year is 64 cm received in 1941. The wettest year at Long Beach is the 283 cm received in 1968, and the driest year is 1929 when only 109 cm were recorded. Both stations display groups of years predominantly wet or dry, but most of these groupings do not occur at the same time in the station's records. The one evident exception is the period of predominantly dry years from 1903 to 1930. However, predominantly dry years continue to 1952 at Long Beach while wet years predominate from 1931 to 1945 at Portland. These two stations highlight that spatial and temporal precipitation trend differences occur at specific locations even in a latitudinal zone characterized by a strong increasing precipitation trend.

1870 1890 1910 1930 1950 1970 1990 Year

Fig. 8.15. Annual precipitation for Long Beach, Washington, and Portland, Maine, for 1870-2002. The broken horizontal lines are the 1971-2000 average for each station. (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http:// cdiac.ornl.gov/epubs/ndp/ushcn/usa_monthly.html.)

Mechanisms responsible for the contrasting patterns of wet and dry years at east and west coast locations are difficult to identify with a high level of confidence. The dynamic behavior of the climate system and the influence of ocean-atmosphere interactions on hydroclimate variability are primary considerations, but the complex interaction of these processes eludes precise explanation. Long-term precipitation is a useful summary indicator of the intensity of the hydrologic cycle, and details of the distribution of precipitation over time and space are likely to be the most important issues in determining impacts of precipitation changes (Allen and Ingram, 2002).

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