Contributed by Xu Qu

According to the method of seasonal prediction of summer monsoon rainfall in central-east China developed by Xu and Yang (1997), seasonal predictions on 2 recent summers (1998-99) have obtained considerable success through correctly predicting the severe summer floods of mid-lower Yangtze Basin in 1998-99 with summer drought trend of the north China in 1999 (Xu, 1998,1999). The skill scores of this method will be summarized in the near future. Seasonal predictions for summer 2000 are made by using the same method on the basis of regression equations trained on data from a 20-year period:1979-1998. The predictors are selected from a series of high correlation regions of 4 monthly climate fields (Xu,1999) for January-March preceding the summer monsoon. The selected predictors were then combined to form multivariate linear equations. The process for selecting the predictors and predictands was described in June 1997 of this Bulletin, which further showed that skillful forecasts can be made only in the subsequent 2 years of the base period; i.e., avoiding predicting a year away from the 20-year development period. According to the forecasting experience in recent years, a few of the seasonal predictions made last summer failed. Consequently, predictions made for summer 2000 are made using the predictive equations trained on data from a new period: 1980-1999, which are now marked by the symbol (*) in the following.

Predictions for Summer 2000

The forecast was made on April 18, 2000. Most predictands listed below were shown in the forecast issued in 1999 (ELLFB June, 1999). The others differ from the above predictands only in different forecasted periods. For example, Huai68r is the average rainfall anomaly of the same 8 stations (as Huai67r) in Huaihe basin during June-August and etc.

2. Meiyu: Percentage of total rainfall in 5 stations located in the mid-lower Yangtze basin during the period of Meiyu: 150% (Flooding).

3. Onsemei: Date of onset of the Meiyu, which will come in the third pentad of June 2000 (Early).
4. Outmei: The first day after the end of Meiyu, which will come in the mid July (Slightly late).
5. Snmei: Percentage of total rainfall in 5 stations located in the southern part of Jiangsu province during the Meiyu period: 158% (Flooding).
6. Jnmei: Percentage of total rainfall in 5 stations located in the middle part of Jiangsu province during the Meiyu period: 83% (Near normal or slightly less)* .
7. Huai67r: The average rainfall anomaly of 8 stations evenly distributed in Huaihe basin (32-34N, east of 112E) during June-July: -251.5 (Drought)*.
8. Huai68r: The average rainfall anomaly of the same 8 stations as Huai67r in Huaihe basin during June-August: -30.6 (Near normal).
9. Nor78r: Averaged mid summer (July-August) rainfall anomaly in 10stations evenly distributed in north China (36-41N, east of 110E): -250.4 (Drought).
10. Nor68r: Averaged summer (June-August) rainfall anomaly in the same 10 stations as above in north China: -99.0 (Less than normal).
11. Yanlsr: Summer averaged rainfall anomaly in 4 stations in the lower Yangtze basin (31.0-32.8N, east of 117E): +67.3 (Above than normal).
12. Yanmsr: Summer averaged rainfall anomaly in 4 stations in the middle Yangtze basin (29-31N, 111.5-117.0E): +470.4(Flooding).
13. Jrs67r: Averaged rainfall anomaly in 7 stations evenly distributed in Jiannan region, south of the mid-lower Yangtze (27-29N, east of 112E) during June-July: +686.9(Flooding).
14. Jrs68r: Averaged rainfall anomaly of the same 7 stations as above in Jiannan region during June-August: +528.7(Flooding).

The forecast indicates that the summer rainfall in central-east China will be similar to 1998, 1999 and 2000, with drought in the north and flooding in the south. The summer flooding areas are still located in mid Yangtze basin and Jiannan region, the Meiyu will come early, but its ending date will near normal, positive anomalies of Meiyu rainfall are forecasted in the whole mid-lower Yangtze basin. Large areas north of 33N including the north China, the mid-lower Yellow River basin and Huaihe basin will be still suffered from summer drought, though the drought extent will be weaker than last summer. It is notable that the mid-lower Yangtze basin have experienced successive summer flooding since 1995, breaking the general rule of climate statistics, which reveal some possible strong external forcings have played the key role in suppressing the northward movement of summer monsoon rainy belt of east China, even surpassing the influence of LaNiña. If so, such external forcings may be the severe atmospheric pollution of east China with large amounts of releasing SO2 (up to 20 million tons per year since 1992) and the southward expansion of the desert in whole northern China due to overgrazing.

References:

Xu Qun and Yang Qiuming, 1997: Seasonal prediction of summer monsoon rainfall in central-east China. Experimental Long-Lead Forecast Bulletin, 6(2), 74-79.

Xu Qun, 1998: Seasonal predictions of summer monsoon rainfall in central-east China. Experimental Long-Lead Forecast Bulletin, 7(2), 79-80.

Xu Qun, 1999: Seasonal predictions of summer monsoon rainfall in central-east China. Experimental Long-Lead Forecast Bulletin, 8(2), 60-61.