The relationship between the North Pacific Oscillation (NPO) in May and the summer drought/flood in North China was analysed using the NCEP/NCAR month by month reanalysis data from 1948 to 2011 including sea level pressure, 500 hPa geopotential height, specific humidity(8 layers) and wind field(8 layers). The North Pacific Oscillation Index(NPOI) which reflects the changes of the sea level pressure in North Pacific was calculated firstly. Then the correlation coefficient between NPOI and Palmer Drought Severity Index(PDSI) which represents the situation of summer drought/flood in North China from 1960 to 2011 was also calculated. The results show that there is a positive correlation between NPO and summer drought/flood in North China. When in positive NPOI years, PDSI in summer is higher than normal, and North China has more summer floods; otherwise, in negative NPOI years, PDSI in summer is lower than normal, and North China has more summer droughts. In order to explain the positive correlation between NPO and summer drought/flood in North China, wind vector field on 850 hPa and geopotential height field on 500 hPa were composite analysed, respectively, and compare the different intensity, area and position of western boundary of Western Pacific Subtropical High(WPSH) when NPO is in different strength(weak and strong). The possible circulation mechanism is that when in positive(negative) NPOI years, on lower layer(850 hPa wind field), the common action of anti-cyclonic(cyclonic) in Ural Mountains, the cyclone(anti-cyclonic) of Lake Baikal, and the anti-cyclonic(cyclonic) anomaly circulation in West Pacific have strengthened(weakened) the convergence of south-west warm and humid air flows of low layer in North China region. And also, when in positive(negative) NPOI years, 500 hPa geopotential height anomaly shows‘+-+’(‘-+-’) wave train, and WPSH is stronger(weaker) shifting northwest(southeast) than normal years, cold and warm air activities make more(less) rainfall in North China. Vertically intergraded moisture fluxes was calculated, in order to show water transportation of summer drought/flood in North China and study the relationship between NPO in May and sea surface temperature in winter by using composition method. In the difference of vertically intergraded moisture fluxes field in summer, the moisture transport from the Arabian Sea and Bay of Bengal and the West Pacific make North China the positive anomaly of water vapor moisture, so there is more rainfall in North China; In divergence field, the water vapor divergence is negative in North China, so it is moisture sink and there is more rain. And also the Arabian Sea, Bay of Bengal, the south of Indo-China Peninsula to the South China Sea and the Western Pacific are the water vapor divergence centers, which are important vapor source for North China. The SST in winter in Pacific Ocean has forcing role on NPO in May, which influences summer drought/flood in North China.
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