Lijuan CHEN *， Wei GU ， Weijing LI Beijing Climate Center ， CMA * chenlj@cma

1. Why is the East Asian summer monsoon extremely strong in 2018? —— Collaborative effects of SST and snow cover anomalies LijuanCHEN*， Wei GU， Weijing LI Beijing Climate Center，CMA * chenlj@cma.gov.cn

2. Outline • Introduction • Data and methods • East Asian Summer Monsoon in 2018 • Collaborative effects of forcing factors in 2018 • Two kinds of typical years with combined influence of multi-factors • Summary and discussion

3. East Asian Monsoon Region Indian Monsoon Region Western Pacific Monsoon Region Earliest onset of Asian summer monsoon Asian summer monsoon in 2018 STRONG!

4. Data and method • Daily Data Set of Basic Meteorological Elements of China National Ground Weather Station (V3.0) issued by the National Meteorological Information Center (Ren et al., 2012) • NCEP/NCAR Reanalysis Project monthly dataset • OISSTv2 from NOAA • snow cover area data is obtained from the Rutgers University in the United States (http: / / climate. rutgers. EDU / snocover /) • composite analysis and linear regression analysis • the composite/regression analyses are focused on the period from 1982 to 2017

5. Fig. 1. Time Series of the (a) Shi, (b) Zhang and (c) Zhu EASM Indices (1981-2018) The Shi index (1996): the difference in normalized sea level pressure between 110°E and 160°E within the range of 20-50°N. 4th The Zhang index (2003): the difference in the mean 850hPa zonal wind between the eastern subtropical monsoon trough region (100-150°E, 10-20°N) and the East Asian subtropical region (100-150°E, 25-35 °N). 1st The Zhu index (2000): reflects the comprehensive east-west and north-south thermal differences over East Asia. 3rd

6. ACC of each index and precipitation (detrend) Shi et al.(1996) Zhang et al.(2003) 1981-2010 1951-2010 1951-1980

7. Fig. 2. (a) Difference in summer precipitation between strong and weak EASM years (mm, thick dashed line indicates the area where the difference exceeds the significance level of 0.1), (b) percentage precipitation anomaly (%) and (c) the anomaly of rainy days for the summer of 2018 based on Zhang index (2003), 0.7 standard deviation; 11 strong years:81, 84,85,86,90,94,97,01,02,04,12;11weakyears:83,87,88,93,95,96,98,03,07,10,13

8. Extreme daily precipitation stations in JJA Persistent rainfall anomaly pattern in JJA; Two main rainbelt in North and South China, less rainfall in the middle-lower reaches of Yangtze river June July August

9. Fig. 3. Average summer precipitation (mm) in Northwest China (blue line), North China (red line) and the Middle-Lower Reaches of Yangtze River (black line) for 1981-2018. -11% +14% +32% 1st Northwest China: including Xinjiang, Qinghai, Gansu, Ningxia and Shaanxi provinces; North China: including Beijing, Tianjin, Hebei and Shanxi provinces; Middle-Lower Reaches of Yangtze River: including Hubei, Anhui, Jiangsu, Shanghai, Zhejiang, Jiangxi and Hunan provinces

10. Fig. 4. Summertime geopotential height difference between 200hPa and 500hPa: (a) climatological value field and (b) 2018 anomaly field (gpm)

11. Fig. 5. Anomalous (a) 200hPa geopotential height (gpm) and winds (m.s-1), (b) 500hPa geopotential height (gpm) and 850hPa winds (m. s-1), (c) column-integrated water vapor flux (kg·m-1.s-1) and divergence (10-7kg · s-1.m-2) and (d) 500hPa vertical velocity (-300pa.s-1) in the summer of 2018

12. Fig. 6. Three index of the Western Pacific subtropical high in the summers of 1981-2018 Daily index of SH ridge line in JJA,2018 1st Meridional-vertical wind anomaly in summer 2018

13. Oct.,2017－Mar.,2018, weak, tradional La Nina event Fig. 7. (a) Niño 3.4 Index (℃) and SOI Index from September 2016 to August 2018 and (b) global SST anomaly in the 2017 / 2018 Winter

14. Eight summers:85,89,96,00,01,08,11,12 Fig. 8 Composite precipitation over eight summers that followed a La Niña Event from 1981 to 2017 (mm, no regions in the figure exceeding the 0.1 significance level) 5 of 8 years EASM index close to normal or weak

15. Fig. 10. Time series of (a) the Niño 3.4 index in the winter, (b) the IOBM index in the summer, (c) the NAT index in the spring and (d) the Tibetan snow cover index in the winter during 1982-2018 Fig.9. Lead-lag correlation coefficients of the Niño 3.4 index, IOBM index, NAT index, Tibetan snow cover index with the EASM index (three-month sliding average is performed prior to the calculation of correlation)

16. Fig. 11. Regressions of the 500hPa height (gpm) and 850hPa wind (m.s-1) anomalies in the summer against the (a) winter Niño 3.4 index, (b) summer IOBM index, (c) spring NAT index and (d) winter plateau snow cover index Zhang et al.,1996; Huang and Li,1987; Wang et al.,2000 Xie et al.,2009, 2016 Chen et al.,2000; Zhang and Tao ,2001; Peng et al.,2005; Ren et al.,2016 Gu et al.,2009; Zuo et al.,2013,2018

17. C A C Fig. 12. Sum of the regressed values of 500hPa height and 850hPa wind in the summer of 2018 calculated according to the linear regression equations using the Niño 3.4 index, the NAT index and the Tibetan snow cover area index from 1982-2017, respectively. Black contours represent 500hPa height anomaly (gpm), vectors represent 850hPa wind anomaly (m. s-1); shadings, red thick lines and blue thick lines indicate the areas where the correlations of the Niño 3.4 index, the NAT index, and the Tibetan snow cover index with the geopotential height anomaly exceeding the significant level of 0.1. A C

18. 1985 2018 C C Fig.13. 500hPa geopotential height (gpm) and 850hPa wind (m.s-1) anomalies in the summers of (a) 1985, (b) 1983 and (c) 1998. 1983 A typical years with combined influence of multi-factors: La Nina,NAT+,Snow cover-:1985,2018; El Nino,NAT-,Snow cover+: 1983,1998 1998 A

19. Fig. 14 Percentage precipitation anomalies (%) in the summers of (a) 1985, (b) 1983 and (c) 1998 1998 1985 1983 2018

20. Conclusion and Discussion • the EASM in the summer of 2018 was stronger than normal. • weakened Meiyu front, more rainfall in northern parts of China. • several key external forcing factors of the EASM, such as the La Niña event, the cool IOBM, the positive NAT and the less than normal snow cover over Tibetan all exhibited obvious anomalous features. The impacts of individual factors mentioned above were consistently favorable for the development of strong EASM. • Since 1981, the characteristics in 1985 were similar to those in 2018, while the features in 1983 and 1998 were opposite to those in 2018. The two groups of years further demonstrates the importance of consistent impacts of external forcing factors on the EASM, such as the tropical SST, the North Atlantic SST and the Tibetan snow cover. With Collaborative effects of the above external factors, the anomalous feature of the EASM would be more remarkable.

21. real time prediction in operation in NCC observation Prediction issued in late March,2018 ACC:0.3 (total 2000+ stations)

22. reanalysis B A A B B A A A B B

23. Prediction of EASM in 2018 by multi-models forecast from Mar.,2018 observation

24. Prediction of PSAC in 2018 by multi-models forecast from Mar.,2018 observation

25. Thanks for your attention Chen Lijuan, Gu Wei, Li Weijing. 2019. Why is the East Asian summer monsoon extremely strong in 2018? —— Collaborative effects of SST and snow cover anomalies. Journal of Meteorological Research ,DOI: 10.1007/s13351-019-8200-4