Frequency Conversion Of Drought-Flood Over The Eastern Parts Of Northwest China And Its Possible Causes

By using the drought and flood grades of18meteorology stations in eastern parts of the Northwest China from1470to2000, the sunspot number from1700to2000, the precipitation data at39stations from1960to2010, monthly mean values from the NCEP/NCAR I reanalysis data, through methods of Multi Taper Method-Singular Value Decomposition(MTM-SVD) et.al, interannual(interdecadal) frequency conversion of drought and flood in eastern parts of the Northwest China and their effect factors are studied. The results show that,(1) the prominent interdecadal period of drought and flood in eastern parts of the Northwest China is23-year and10-year, the significant period of23-year period appears in the eastern part of the area from1740s to the present day; the significant period of10-year period appears in the western part of the area from middle of17th century to early18th century,1930s to1970s. There is possible influence relationship about sun activation with drought and flood, large (small) sunspot number corresponding to drought (flood) in eastern parts of the Northwest China. When the23-year amplitudes of sunspot number increase (decrease), the23-year amplitudes of drought and flood in eastern part of the area decrease (increase); When the10-year amplitudes of sunspot number increase (decrease), the10-year amplitudes of drought and flood in western part of the area decrease (increase).(2)the prominent period of precipitation in the flood period in eastern parts of the Northwest China is2-3years, but its significance is changing with the varying of time, the significant period of3-year period appears between1958and1982, the significant period of2-year period appears between1990and1999, both of the amplitudes of2-year and3-year period show a trend of increase since the21st century, but neither of them is significant. In terms of four periods, precipitation in3-year significant period is the maximum, and the interannual fluctuation is large in this period, shows the most unstable precipitation;2-year significant period is the minimum precipitation period, but relatively stable; Precipitation in the transition period (1983-1989)and since the21st century(2000-2010) is less, the most stable precipitation is the period since the21st century.(3)Both sensible heat flux and precipitation have a significant quasi3-year period, and their coupling field also shows a significant3-year period. When the spring sensible heat flux become stronger (weaker) on eastern Plateau, there is convergence (divergence) of moisture content and steady updraft (downdraft), corresponding to more (less) precipitation over the eastern parts of Northwest China during rainy season. The most significant period of such quasi3-year period synergistic relationship appears during1960-1982, and there is an adjustment stage in1983-1990, the quasi3-year period appears again after1990s. This correspondence well explains the quasi3-year cycle in the first EOF mode between spring sensible heat flux over the Eastern Tibetan Plateau and the precipitation during rainy season over the eastern parts of Northwest China. The continuous heating process of the former can affect the latter, and mainly reflects in August.