Interdecadal variability of summer rainfall in Northwest China and its possible causes

Abstract The present study investigates interdecadal variability of summer rainfall in Northwest China using station observations for the period 1951–2000 and the ERA‐40 reanalysis for the period 1958–2000. It is found that Northwest China summer rainfall experienced an obvious interdecadal increase around 1978. Analyses show that summer circulation, temperature and potential vorticity anomalies over Asia vary vastly before and after 1978. During the period 1978–2000, cyclonic and anti‐cyclonic circulation anomalies dominate over Northwest and North China, respectively. These are associated with the anomalous ascending motion over Northwest China, and their anomalous descending motion over North China. The temperature over Northwest China shows an obvious decrease at 300 hPa, but increase near the surface after 1978. This enhances convective instability, which contributes to the strengthening of ascent and the increase of summer rainfall in Northwest China during 1978–2000. Copyright © 2009 Royal Meteorological Society