Data and Model Used
Monthly mean circulation fields at several vertical levels
for the period January 1949 to December 1998 were obtained
from the NCEP/NCAR Reanalysis Project (Kalnay
et al. 1996). The seasonal mean (June-September) all India
monsoon rainfall (IMR) based on uniformly distributed rain
gauge stations over continental India (Parthasarathy et al.
1994, updated for the recent years from publications of India
Meteorological Department) is also used. The inter-annual
variations are described by the anomalies from the long term
mean of IMR. The Indian monsoon is considered 'strong'
('weak') if the normalized (by the inter-annual standard deviation)
anomalies is > •-1 (< -1). SST data for the period
January 1949 to December 1991 have been obtained from
the global analysis of SST created by Kaplan et al. (1997).
For the recent years we have added the Optimum Interpolation
SST (OISST) data set (Reynolds and Smith 1994).
We also use the monthly precipitation analysis created by
Xie and Arkin (1996) covering the period January 1979 to
December 1998. Table I lists 'strong' and 'weak' monsoon
years selected following the criterion described above and E1
Nifio and La Nifia years based on Nino3 (150W-90W,5S-5N)
SST anomalies together with a list of 'non-ENSO' 'strong'
and 'weak' monsoon years. In choosing 'non-ENSO' monsoon
years, the criterion of > i s.d (or < -1 s.d) was relaxed
to > 0.5 s.d (or < -0ø5 s.d)