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Analysis of the 140-year historical record suggests that the inverse relationship between the El Nino-Southern Oscillation (ENSO) and the Indian summer monsoon (weak monsoon arising from warm ENSO event) has broken down in recent decades. Two possible reasons emerge from the analyses. A southeastward shift in the Walker circulation anomalies associated with ENSO events may lead to a reduced subsidence over the Indian region, thus favoring normal monsoon conditions. Additionally, increased surface temperatures over Eurasia in winter and spring, which are a part of the midlatitude continental warming trend, may favor the enhanced land-ocean thermal gradient conducive to a strong monsoon. These observations raise the possibility that the Eurasian warming in recent decades helps to sustain the monsoon rainfall at a normal level despite strong ENSO events.
Most parts of India receive a major proportion of their annual rainfall during the summer (June to September) monsoon season. Extreme departures from normal seasonal rainfall, such as large-scale droughts and floods, seriously affect agricultural output and regional economies. By the early 1900s, investigators had identified the two largescale forcings still thought to be most important for predicting monsoon anomalies: Himalayan/Eurasian snow extent (1) and the ENSO cycle (2). The former is generally believed to provide an indication of the premonsoon thermal condition over the Asian land mass. Warmer conditions are thought to aid the buildup of a strong land-sea thermal gradient during the summer (3, 4). ENSO, the largest known climatic forcing of interannual monsoon variability, acts through the eastwest displacement of large-scale heat sources in the tropics (5). Numerous studies (6) have shown a significant simultaneous association between the monsoon rainfall over India and ENSO indices. However, secular variations in the relationships between monsoon rainfall and its predictors have also been noted (7). These variations have been found to be linked to changes in ENSO characteristics such as amplitude and period (8).
Almost all the statistical seasonal prediction schemes of monsoon rainfall rely heavily on the change in magnitude in various ENSO indices (8, 9) from winter [December to February (DJF)] to spring [March to May (MAM)]. Numerical general circulation models (GCMs) are also used for seasonal rainfall prediction. The monsoon simulated in these GCMs is more sensitive to the sea surface temperatures (SSTs) specified...