South China Sea Summer Monsoon Index (SCSSMI)

 The South China Sea summer monsoon index (SCSSMI) is defined as an area-averaged seasonally (JJAS) dynamical normalized seasonality (DNS) at 925 hPa within the South China Sea monsoon domain (0°-25°N, 100°-125°E) (Li and Zeng, 2002, 2003, 2005).

The South China Sea Summer Monsoon Index (SCSSMI)

The normalized time series of SCSSMI (1948-2023).

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

From a global perspective, the South China Sea summer monsoon (SCSSM) might make a substantial contribution to the interannual variability of precipitation during boreal summer. During boreal summer, the SCSSM exhibits strongly positive correlations with rainfall over the warm pool of the western Pacific, North Pacific between 10°N and 25°N, South American monsoon regime and southeast South Pacific, and is well negatively correlated with precipitation over a broad domain extending from the Arabian Sea across the tropical Indian Ocean to Indonesia and Malaysia, a region extending from the Central American across the Caribbean Island countries to the North Atlantic Ocean between 10°N and 20°N and the Indian Ocean regions adjacent to the south and west Australia. The global teleconnection pattern is remarkably strong.

 Correlation maps between the seasonal (June-September) rainfall and SCSSMI ( 1979-97).

The shaded areas indicate significant at the 95% confidence level.

The correlations between the seasonal (June-September) sea level pressure anomalies and SCSSM appear a remarkable global-scale teleconnection pattern. The worldwide surface pressure changes connected with the changes in the strength of SCSSM. The map shows significant negative correlations with the SCSSM over almost the entire Pacific Ocean and significant positive correlations with the SCSSM over the Southern Indian Ocean and the North America-tropical Atlantic Ocean. In the strong (weak) SCSSM stages large shifts in atmosphere mass occur so that the sea level pressure decreases (increases) over almost the entire Pacific Ocean and increases (decreases) over almost the entire Indian Ocean and the North America-Atlantic Ocean. Very large anomalies in the atmospheric circulation and in the world weather are associated with significant changes in the SCSSM. Therefore, the SCSSM influences the global precipitation possibly through the global-scale teleconnection.

Correlation maps between the seasonal (June-September) sea level pressure anomalies and SCSSMI ( 1958-97).

The shaded areas indicate significant at the 95% confidence level. The map shows that global shifts of atmospheric mass take place during strong SCSSM anomalies period.

Correlation between precipitation and the SCSSMI (shading) and regression of 850-mb winds against the monsoon index (vectors) for June, July, August, and September 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)