Effects of ENSO Phase-Switching on Rainy-Season Precipitation in North China

The changing characteristics of Niño3.4 index during 1961-2014 corresponding to much more (less) RSPNC (rainy-season precipitation in North China) were statistically analyzed using RSPNC station data and NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalyzed data, etc. It was found that most of the years with much more RSPNC were coincident with the ENSO phase-switching years from El Niño to La Niña, and most of the years with much less RSPNC corresponded to the years when the sea surface temperature anomalies over the central and eastern tropical Pacific changed from negative to positive for El Niño phase. In the years with much more RSPNC, days with regional mean rainfall more than 10 mm over North China during July and August accounted for about 25% of the total days, and days with regional mean rainfall between 4-10 mm also accounted for about 25% of the total. Both are far more than that in the years with much less RSPNC. The earlier than normal northward shift of the NWPSH (northwestern Pacific subtropical high), the evident maintenance of NWPSH at the location more north than its normal position, and the northward shift of the East Asian subtropical jet stream were major reasons for the frequent and heavy rainfall in July and August in the years with much more RSPNC. Compared with the El Niño decaying years, the years when ENSO switched from El Niño to La Niña had stronger positive geopotential height anomaly over 500 hPa near the Japan Sea during July and August, which was more in accordance with the typical circulation pattern in higher RSPNC years. In addition, compared with the El Niño decaying years, the years when ENSO switched from El Niño to La Niña had stronger Walker circulation during July and August. As a result, the East Asian subtropical jet stream shifted northward more significantly, which was also in favor of more precipitation over North China. The phenomenon inspired us to pay close attention to the changing speed of ENSO event in the past and future, which might be helpful for forecasting rainy-season precipitation in North China.