In this study, an East Asian summer monsoon intensity index is defined and the summer monthly indices of the East Asian monsoon are calculated for the period 1873–1989. Studies of the climate change of the index indicate that around 1918 the East Asian monsoon experienced an abrupt change, from weak to strong. This abrupt change occurred synchronously with that of the Northern Hemisphere air temperature. Studies of the relation of this East Asian summer monsoon index to the rainfall and temperature over China by using the data for the recent 40 years shows that when a strong monsoon occurs, China will experience an extensive high temperature in summer. The relationship between the East Asian summer monsoon and China's summer rainfall manifests itself on the north–south position of the rain belt. The position of the summer belt is northward when the monsoon is strong and southward when the monsoon is weak. Wet years over the middle and lower reaches of Chingjiang River are related to a weak monsoon whereas with a strong summer monsoon, dry years are often observed.

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Dry/wet features for the globe and the Northern and Southern Hemispheres are investigated in terms of 1920-2000 June-August (JJA) global land precipitation data. The weighted mean JJA precipitation anomaly index and the weighted mean JJA dry/wet area index are used to describe the extent of global dryness/wetness. It is pointed out that 1988 (1930) was the globally wettest (driest) year in 1920-2000, and 1954 (1976) was the second wettest (driest). The dryness/wetness of the globe and Northern and Southern Hemispheres has shown distinctive interdecadal changes: during the 1920s, global dryness occurred in JJA frequently; from the 1920s to the 1940s global wetness occurred infrequently in JJA; the 1950s-1960s was a period of frequent global JJA wetness; and the 1970s to 2000 was a period of frequent global JJA dryness/wetness, with the number of dry years greater than that of wet years. The dry/wet features of the Northern Hemisphere are comparatively consistent with those of the globe, but there is no obvious relation between JJA mean precipitation anomalies of the two hemispheres. The analyses of the simultaneous and last winter sea-surface temperature anomalies of global dry/wet years reveal that there is clear correlation of JJA global dry/wet change with sea-surface temperature variations and El Niño-southern oscillation (ENSO) events. Global JJA dryness occurs when the summer ENSO (El Niño) is weaker, and global JJA wetness occurs when the summer ENSO is strong. The largest difference between the last winter sea-surface temperature fields for global dry and wet years is that the sea-surface temperature in the North Pacific and North Atlantic for wet years is substantially higher than that for dry years.

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The secular variation over the global land annual precipitation (GLAP) fields for 1948-2000 is investigated. Evidence suggests that the GLAP abruptly reduced around 1978 and experienced a decrease of 0.54 mm/a, on average. Fuzzy clustering is used to group the annual precipitation over commonly-used 36 latitudes belts into six belts for examining its secular variation. The results show that, except northern high-latitudes (60°--90°N) where it is slightly increased, the precipitation is decreased, especially in 35°N-35°S, and the diminution is the maximum of 0.98 mm/a at tropics. Moreover, identification of 1948-2000 flood/drought years on a global basis is made, showing noticeable interdecadal variations. During the years of warm (cold) events the precipitation decreases (increases) by 15.4 mm (14.4 mm) per year, on average. It can be assumed that global warming and frequent ENSO occurrences from the end of the 1970s maybe serve as the root cause of the diminution.