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