Prolonged dry spells in recent decades over north‐central China and their association with a northward shift in planetary waves

摘要: Summer rainfall in north-central China, accounted for 70% of the annual rainfall, is sensitive to anomalies East Asian summer monsoon (EASM) and westerlies due lying EASM marginal belt. durations frequency dry spells were investigated using daily data from 332 weather stations, European Centre Medium-range Weather Forecasting (ECMWF) re-analysis Climate Research Unit (CRU) surface temperature data. The results demonstrate that exhibits an ‘early wetting later drying’ pattern with a weak increasing June dramatic decreasing trend July August (JA). Decreased JA associated prolonged spells; exceeding 7 days increase, especially 15 days. This increase spell duration positively correlated northward shift planetary waves expressed by 5700-gpm isoline at 500 hPa westerly jet 200 hPa, which lead China close left entrance positive vorticity convergence region on high level, it helpful downward movements less rainfall. affected boundary forcing. Dramatic increases over Tibetan Plateau (TP) result large gradient thermal contrast between TP mid-high latitude, likely strengthening widening South High (SAH) pressure gradient. These conditions contribute prolonging spells.