East Asian Summer Monsoon Index (EASMI)

 The East Asian summer monsoon (EASM) index is defined as an area-averaged seasonally (JJA) dynamical normalized seasonality (DNS) at 850 hPa within the East Asian monsoon domain (10°-40°N, 110°-140°E) (Li and Zeng, 2002, 2003, 2005). There is an apparent negatively correlation between the EASMI and summer (JJA) rainfall in the middle and lower reaches of the Yangtze River in China, indicating drought years over the valley are associated with the strong EASM and flood years with the weak EASM.

The East Asian Summer Monsoon Index (EASMI)

The normalized time series of EASMI for the period of 1948-2022.

The summer here is JJA. The thick solid lines indicate 9-year Gaussian-type filtered values.

Correlation maps between the EASMI and summer (JJA) rainfall over (a) China (1958-99) and (b) the East Asian (1977-97). The shaded areas indicate significant at the 95% confidence level.

Correlation between precipitation and the EASMI (shading) and regression of 850-mb winds against the monsoon index (vectors) for June, July, and August from 1979 to 2006, respectively. The shaded areas and vectors indicate significant at the 95% confidence level.

Related References

• Li, J. P., J. Feng, and Y. Li, 2011: A possible cause of decreasing summer rainfall in northeast Australia. Int. J. Climatol., 31, doi: 10.1002/joc.2328. PDF
• Li, J. P., Z. W. Wu, Z. H. Jiang, and J. H. He, 2010: Can global warming strengthen the East Asian summer monsoon?. J. Climate, 23, 6696-6705. PDF
• Feng, J., J. P. Li, and Y. Li, 2010: A monsoon-like Southwest Australian circulation and its relation with rainfall in Southwest Western Australia. J. Climate, 23: 1334-1353. PDF
• Wang, B., Z. W. Wu, J. P. Li, J. Liu, C. P. Chang, Y. H. Ding, and G. X. Wu, 2008: How to measure the strength of the East Asian summer monsoon. J. Climate, 21, 4449-4462. PDF
• Li, J. P., and Q. C. Zeng, 2005: A new monsoon index, its interannual variability and relation with monsoon precipitation. Climatic and Environmental Research, 10(3): 351-365. PDF　
• Li, J. P., and Q. C. Zeng, 2003: A new monsoon index and the geographical distribution of the global monsoons. Adv. Atmos. Sci., 20, 299-302. PDF 　
• Li, J. P., and Q. C. Zeng, 2002: A unified monsoon index. Geophysical Research Letters, 29(8), 1274, doi:10.1029/2001GL013874. PDF 
• Zeng, Q. C., and  J. P. Li, 2002: On the Interaction between Northern and Southern Hemispheric Atmospheres and the Essence of Tropical Monsoon. Chinese Journal of Atmospheric Sciences, 26, 207-226. PDF
• Li, J. P., and Q. C. Zeng, 2000: Significance of the normalized seasonality of wind field and its rationality for characterizing the monsoon. Science in China (D), 43(6): 647-653. PDF 

Subroutine for Dynamical Normalized Seasonality (DNS): seasonality(mx,my,v1,v2,v3,v4,fai0,grdy,suv)