Recognition of two dominant modes of EASM and its thermal driving factors based on 25 monsoon indexes

Based on three reanalysis datasets—ERA-Interim, NCAR–NCEP and JRA-55—the classification of 25 commonly used indexes of the East Asian summer monsoon (EASM) was investigated. The physical nature of two categories of monsoon index, together with their circulation pattern, climate anomalies, and driving factors, were investigated. Results suggest that the selected 25 monsoon indexes can be classified into two typical categories (Category I and II), which are dominated by interannual and decadal variabilities of the EASM, respectively. The anomalous circulation patterns and summer rainfall patterns related to the two categories of index also exhibit evident differences. Category I is closely linked to the low-latitude circulation system and the anomalous circulation pattern is a typical East Asia–Pacific teleconnection pattern. The summer rainfall anomaly exhibits a typical tripole pattern. However, Category II mainly reflects the impacts of the middle–high latitude circulation system on the summer monsoon and is closely linked to a typical Eurasian teleconnection pattern, which corresponds to a dipole of summer rainfall anomalies. Further analysis suggests that the underlying thermal driving factors of the two categories of monsoon are distinct. The main driving factors of Category I are the tropical sea surface temperature anomalies (SSTAs), especially ENSO-related SSTAs in the preceding winter and summer SSTAs in the tropical Indian Ocean. The winter signal of Category II summer monsoon anomalous activity mainly originates from the polar region and the middle and high latitudes of the Eurasian continent. Category II monsoon activity is also associated with summer SSTAs in the equatorial central Pacific.