Epochal changes in Indian monsoon-ENSO precursors

Precursors for the Indian monsoon are known to be highly epoch specific in their skills in predicting the monsoon on seasonal time scales. We show that the various precursors are correlated with the monsoon, only when they are correlated with ENSO, which happened in the recent period 1951 - 1990, but not in the 1990's and the period 1911-50. This accounts for the skill in monsoon prediction during 1951-90. We find that ENSO and its precursors tend toward higher amplitude and 3 - 5 year periods in the 1951 - 1990 epoch, and toward decreased amplitude and 5 - 7 year periods in the 1911 - 1950 epoch. We argue that the shift to lower frequency and amplitude in the earlier epoch diminished the association between the monsoon precursors and ENSO, leading to diminished skill in predicting the monsoon in that epoch. However, the simultaneous relationship between the monsoon and ENSO has been stable over the past 140 years, suggesting that the monsoon - ENSO teleconnections are robust once ENSO is established. Changes in the frequency of ENSO have implications for statistical prediction schemes for ENSO and the monsoon.