The relationship between the late spring North Atlantic Oscillation (NAO) and the summer extreme precipitation frequency (EPF) in the middle and lower reaches of the Yangtze River Valley (MLYRV) is examined using an NECP/NCAR reanalysis dataset and daily precipitation data from 74 stations in the MLYRV. The results show a significant negative correlation between the May NAO index and the EPF over the MLYRV in the subsequent summer. In positive EPF index years, the East Asian westerly jet shifts farther southward, and two blocking high positive anomalies appear over the Sea of Okhotsk and the Ural Mountains. These anomalies are favorable to the cold air from the mid-high latitudes invading the Yangtze River Valley (YRV). The moisture convergence and the ascending motion dominate the MLYRV. The above patterns are reversed in negative EPF index years. A wave train pattern that originates from the North Atlantic extends eastward to the Mediterranean and then moves to the Tibetan Plateau and from there to the YRV, which is an important link in the May NAO and the summer extreme precipitation in the MLYRV. The wave train may be aroused by the tripole pattern of the SST, which can explain why the May NAO affects the summer EPF in the MLYRV.
The seasonal forecasting skill with respect to the South Asian summer monsoon(SASM) was compared between the European Commission FP7 project(ENSEMBLES) and the Development of a European Multimodel Ensemble System for Seasonal to Interannual Prediction project(DEMETER). The Webster-Yang index(WYI) was chosen to represent the intensity of the SASM. First, the authors compared the ability to forecast the zonal wind at 850 h Pa(U850) and 200 h Pa(U200) between ENSEMBLES and DEMETER models. The results indicated that the models from the European Centre for Medium-Range Weather Forecasts, International Organization(ECMWF) and UK Met Office(UKMO) in ENSEMBLES possess greater skill in seasonally forecasting the JJA(June, July, and August) U850, U200, and U850 minus U200 than in DEMETER. Compared to in DEMETER, the JJA U200 and U850 minus U200 forecasting skill was greater for the model from MétéoFrance(MF) in ENSEMBLES over most of the SASM region. The three coupled models(ECMWF, MF, and UKMO), especially the UKMO model in ENSEMBLES, all demonstrated improved skill in their seasonal forecasts compared to in DEMETER with respect to the interannual variability of the SASM. The three ENSEMBLES models also showed better ability in forecasting the sea surface temperature anomalies(SSTAs) over the eastern equatorial Pacific and North Indian Ocean, and more accurately reproduced the large-scale atmospheric circulation and precipitation over northern India, which are related to the SASM. It seems that the couple between the atmospheric system and external forcing of ENSMBLES over Indian Ocean and Pacific is better than that of DEMETER.
The authors used an atmospheric general circulation model(AGCM) of European Centre Hamburg Model(ECHAM5.4) and investigated the possible impacts of eastern Pacific(EP) and central Pacific(CP) El Nio on the winter precipitation anomalies in South China.A composite analysis suggested much more rainfall during the mature phase of EP El Nio than in the case of CP El Nio,and their corresponding observed wet centers to be located in the southeast coast and the region to the south of the Yangtze River,respectively.Results obtained on the basis of model-sensitive run imply that the modelsimulated rainfall anomalies agree well with the observation,and the magnitude of simulated rainfall anomalies were found to be reduced when the amplitude of sea surface temperature anomaly(SSTA) forcing of EP and CP El Nio was cut down.These results imply that the rainfall anomaly in South China is very sensitive not only to the type of El Nio but also to its intensity.
