Definition of Monsoon Indices

1
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, 100-130 ºE)
  minus that over (20 ºN-30 ºN, 110-140 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 (100-170 ºE, 0-40 ºN)
  regions with correlation coefficients of 0.72 and
  0.88, respectively for the period of 1958-1997
  (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
  525 º N, 100130 º E for Southeast Asia, it is
  defined as u200mb4050oN, 110150oE minus
  u200mb2535oN, 110150oE 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).

2
Definition of Monsoon Indices
3
Summer Monsoon Indices
WYI
IMI
WPEMI
RM2
AUSMI (Here, 1950 means 1950-51DJF)
Using 1950-2001 climatology for anomaly