Definition of Monsoon Indices  Asian summer monsoon These three monsoon indices provide a succinct description of the Asian summer monsoon circulation variability for the broad scale South Asian monsoon, ISM, and WNP-EASM, respectively. In order to conveniently measure Asian summer monsoon circulation anomalies, Webster and Yang (1992) proposed a circulation index, the Webster- Yang index (WYI), which is defined by the vertical zonal wind shear between 200 and 850 hPa (u 200-u 850) averaged over the region (40 ºE-110 ºE, 0-20 ºN). The WYI describes the broad-scale South Asian monsoon variability that is primarily driven by two convective heat sources, one lies in the Bay of Bengal, and the other in the vicinity of Philippines. Wang and Fan (1999) found that the variations of the two convective heat sources reflect, respectively, the variations of the Indian summer monsoon (ISM) and the western North Pacific-East Asian summer monsoon (WNP-EASM); but they are not significantly correlated on the interannaul and decadal time scales. Thus, Wang and Fan (1999) proposed two indexes that quantify the variability of the ISM and WNP-EASM, respectively. The ISM index (IMI) may be defined by the 850 hPa zonal wind averaged over (5 ºN-15 ºN 40 ºE-80 ºE) minus that averaged over (20 ºN-30 ºN, 60 ºE-90 ºE) (Fig. 2). This meridional shear of zonal winds depicts the intensity of the Indian monsoon trough and associated southwesterly monsoon. Similarly, the WNP-EAM index (WPEMI) can be defined by the 850hPa wind speed averaged over (5 ºN-15 ºN, ºE) minus that over (20 ºN-30 ºN, ºE). The IMI and the WPEMI are highly representative of the dominant EOF modes of the low-level circulation anomalies over the ISM (30 ºE-100 ºE, 0-30 ºN) and the WNP-EASM ( ºE, 0-40 ºN) regions with correlation coefficients of 0.72 and 0.88, respectively for the period of (Wang et al. 2001). The regional monsoon index, RM2 is index for east Asia monsoon region defined by Lau et al.(2000) obtained from upper circulation connected with rainfall. To identify circulation features that are most strongly correlated with rainfall over 5– 25 º N, 100–130 º E for Southeast Asia, it is defined as u200mb[40–50 o N, 110– 150 o E] minus u200mb[25–35 o N, 110–150 o E] based on the correlation map between rainfall and 200mb zonal wind field.  Australian summer monsoon McBride et al (1995) proposed a circulation index measuring the Australian summer monsoon variability using 850 hPa zonal wind anomalies averaged over (0-10 ºS, 120 ºE-150 ºE), which is referred to as Australian monsoon index (AUSMI).

IndexRegionreferenceFocus WY U850(0-20N E)-U200(0-20N E) Webster and Yang 1992 Broad scale South Asian monsoon IMI U850(5-15N 40-80E)-U850(20-30N E) Wang and Fan 1999Indian summer monsoon WPEMI U850(5-15N E)-U850(20-30N E) Wang and Fan 1999Western Pacific-East Asian monsoon RM2 U200(42-52N E)-U200(27-37N E) Lau et al. 2000Regional scale East Asian monsoon AUSMIU850(0-10S E )Mcbride et al. 1995Australian summer monsoon Definition of Monsoon Indices

Summer Monsoon Indices Using climatology for anomaly WYI IMI WPEMI RM2 AUSMI (Here, 1950 means DJF)