Abrupt Change of Drorght/Flood for the Last 522 Years in the Middle Reaches of Yellow River

The summer dryness/wetness grades series for the last 522 years (1470~1991) in the middle reaches of Yellow River are analysed by T test. The results indicate that there are two abrupt changes with 100-year scale in this region, around 1650’s (from drought to flood) and in the early 1760’s (from flood to normal). There are also jumps of 20~30 year scale within the 100-year scale variations. It is also found that the drought/flood in the region are associated with the evolution of atmospheric active centers and the Sea Surface Temperature Index (SSTI) of the equatorial East Pacific.