3.3.2.2 Spatial Patterns of Precipitation Trends

The spatial patterns of trends in annual precipitation (% per century or per decade) during the periods 1901 to 2005 and 1979 to 2005 are shown in Figure 3.13. The observed trends over land areas were calculated using GHCN station data interpolated to a 5° × 5° latitude/longitude grid. For most of North America, and especially over high-latitude regions in Canada, annual precipitation has increased during the 105-year period. The primary exception is over the southwest USA, northwest Mexico and the Baja Peninsula, where the trend in annual precipitation has been negative (1 to 2% per decade) as drought has prevailed in recent years. Across South America, increasingly wet conditions were observed over the Amazon Basin and southeastern South America, including Patagonia, while negative trends in annual precipitation were observed over Chile and parts of the western coast of the continent. The largest negative trends in annual precipitation were observed over western Africa and the Sahel. After having concluded that the effect of changing rainfall-gauge networks on Sahel rainfall time series is small, Dai et al. (2004b) noted that Sahel rainfall in the 1990s has recovered considerably from the severe dry years in the early 1980s (see Section 3.7.4 and Figure 3.37). A drying trend is also evident over southern Africa since 1901. Over much of northwestern India the 1901 to 2005 period shows increases of more than 20% per century, but the same area shows a strong decrease in annual precipitation for the 1979 to 2005 period. Northwestern Australia shows areas with moderate to strong increases in annual precipitation over both periods. Over most of Eurasia, increases in precipitation outnumber decreases for both periods.

Figure 3.13. Trend of annual land precipitation amounts for 1901 to 2005 (top, % per century) and 1979 to 2005 (bottom, % per decade), using the GHCN precipitation data set from NCDC. The percentage is based on the means for the 1961 to 1990 period. Areas in grey have insufficient data to produce reliable trends. The minimum number of years required to calculate a trend value is 66 for 1901 to 2005 and 18 for 1979 to 2005. An annual value is complete for a given year if all 12 monthly percentage anomaly values are present. Note the different colour bars and units in each plot. Trends significant at the 5% level are indicated by black + marks.

To assess the expected large regional variations in precipitation trends, Figure 3.14 presents time series of annual precipitation. The regions are 19 of those defined in Table 11.1 (see Section 11.1) and illustrated in Figure 11.26. The GHCN precipitation data set from NCDC was used, and the CRU decadal values allow the reproducibility to be assessed. Based on this, plots for four additional regions (Greenland, Sahara, Antarctica and the Tibetan Plateau) were not included, as precipitation data for these were not considered sufficiently reliable, nor was the first part of the Alaskan series (prior to 1935). Some discrepancies between the decadal variations are still evident at times, mostly owing to different subsets of stations and some stations coming in or dropping out, but overall the confidence in what is presented is quite high. Figure 3.15 presents a latitude-time series of zonal averages over land.

In the tropics, precipitation is highly seasonal, consisting of a dry season and a wet season in association with the summer monsoon. These aspects are discussed in more detail in Section 3.7. Downward trends are strongest in the Sahel (see Section 3.7.4) but occur in both western and eastern Africa in the past 50 years, and are reflected in the zonal means. The downward trends in this zone are also found in southern Asia. The linear trends of rainfall decreases for 1900 to 2005 were 7.5% in both the western Africa and southern Asia regions (significant at <1%). The area of the latter region is much greater than India, whose rainfall features strong variability but little in the way of a century-scale trend. Southern Africa also features a strong overall downward trend, although with strong multi-decadal variability present. Often the change in rainfall in these regions occurs fairly abruptly, and in several cases occurs around the same time in association with the 1976–1977 climate shift (Wang and Ding, 2006). The timing is not the same everywhere, however, and the downward shift occurred earlier in the Sahel (see also Section 3.7.4, Figure 3.37). The main location with different trends at low latitudes is over Australia, but it is clear that large interannual variability, mostly ENSO-related, is dominant (note also the expanded vertical scales for Australia). The apparent upward trend occurs due to two rather wet spells in northern Australia in the early 1970s and 1990s, when it was dry in Southeast Asia (see also Section 3.7.2). Also of note in Australia is the marked downward trend in the far southwest characterised by a downward shift around 1975 (Figure 3.13).

Figure 3.14. Precipitation for 1900 to 2005. The central map shows the annual mean trends (% per century). Areas in grey have insufficient data to produce reliable trends. The surrounding time series of annual precipitation displayed (% of mean, with the mean given at top for 1961 to 1990) are for the named regions as indicated by the red arrows. The GHCN precipitation from NCDC was used for the annual green bars and black for decadal variations (see Appendix 3.A), and for comparison the CRU decadal variations are in magenta. The range is +30 to –30% except for the two Australian panels. The regions are a subset of those defined in Table 11.1 (Section 11.1) and include: Central North America, Western North America, Alaska, Central America, Eastern North America, Mediterranean, Northern Europe, North Asia, East Asia, Central Asia, Southeast Asia, Southern Asia, Northern Australia, Southern Australia, Eastern Africa, Western Africa, Southern Africa, Southern South America, and the Amazon.

At higher latitudes, especially from 30°N to 85°N, quite distinct upward trends are evident in many regions and these are reflected in the zonal means (Figure 3.15). Central North America, eastern North America, northern Europe, northern Asia and central Asia (east of the Caspian Sea) all experienced upward linear trends of between 6 and 8% from 1900 to 2005 (all significant at <5%). These regions all experience snowfall (see also Section 3.3.2.3) and part of the upward trend may arise from changes in efficiency of catching snow, especially in northern Asia. However, there is ample evidence that these trends are real (see Section 3.3.4), and they extend from North America to Europe across the North Atlantic as evidenced by ocean freshening, documented in Sections 5.2.3 and 5.3.2. Western North America shows longer time-scale variability, principally due to the severe drought in the 1930s and lesser events more recently. Note the tendency for inverse variations between northern Europe and the Mediterranean, associated with changes in the NAO (see Section 3.6.4). Southern Europe and parts of central Europe, as well as North Africa, are characterised by a drier winter (DJF) during the positive phase of the NAO, while the reverse is true in the British Isles, Fennoscandia and northwestern Russia.

Figure 3.15. Latitude-time section of zonal average annual anomalies for precipitation (%) over land from 1900 to 2005, relative to their 1961 to 1990 means. Values are smoothed with the 5-point filter to remove fluctuations of less than about six years (see Appendix 3.A). The colour scale is nonlinear and grey areas indicate missing data.

In the SH, Amazonia and southern South America feature opposite changes, as the South American monsoon features shifted southwards (see Section 3.7.3). This movement was in association with changes in ENSO and the 1976–1977 climate shift. The result is a pronounced upward trend in Argentina and the La Plata River Basin, but not in Chile (where the main declines in precipitation are evident in the austral summer (DJF) and autumn (MAM; Figure 3.13). Decadal-scale variations over Amazonia are also out of phase with the Central American region to the north, which in turn has out-of-phase variations with western North America, again suggestive of latitudinal changes in monsoon features. East and Southeast Asia show hardly any long-term changes, with both having plentiful rains in the 1950s. At interannual time scales there are a number of surprisingly strong correlations: Amazonia is correlated with northern Australia (0.44, significant at <1%) and also Southeast Asia (0.55, <1%), while southern South America is inversely correlated with western Africa (−0.51, <1%). The correlations are surprising because they are based on high-frequency relationships and barely change when the smoothed series are used.