  Large socio-economic impact of the Indian Summer Monsoon (ISM) extremes
motivated numerous attempts at its long range prediction over the past century.
However, a rather estimated low potential predictability limit (PPL) of
seasonal prediction of the ISM, contributed significantly by 'internal'
interannual variability was considered insurmountable. Here we show that the
'internal' variability contributed by the ISM sub-seasonal (synoptic +
intra-seasonal) fluctuations, so far considered chaotic, is partly predictable
as found to be tied to slowly varying forcing (e.g. El Nino and Southern
Oscillation). This provides a scientific basis for predictability of the ISM
rainfall beyond the conventional estimates of PPL. We establish a much higher
actual limit of predictability (r~0.82) through an extensive re-forecast
experiment (1920 years of simulation) by improving two major physics in a
global coupled climate model, which raises a hope for a very reliable dynamical
seasonal ISM forecasting in the near future.
