Weakening of Triennial Oscillation of the Indian Summer Monsoon Rainfall (at 1° × 1° Gridded Scale) Under Future Global Warming

India's agriculture is a significant component of the Gross National Product (GNP) and is largely dependent on the Indian summer monsoon. This makes an accurate prediction of rainfall a key factor in improving agricultural production. A statistical cycle, widely known to have a strong 2.85‐year period and will therefore be called a triennial oscillation here, plays a vital role in such predictions. In the present work, we examine the spatial distribution of this short‐period oscillation at 1° × 1° (lat./long.) resolution using 1,260‐month data (1901–2005). A power spectral analysis of the observed data set shows that the statistically significant triennial oscillation (at 95% confidence level) is present over 54% of the total number of 354 grids across India, which covers 69.03% area. Projections of selected data from models used in Coupled‐Model Inter‐comparison Project Phase 5 predict weakening of this oscillation (in amplitude) by 2100. The projections indicate reduced confidence levels of this oscillation at 80%, 85%, and 90%, respectively, in 3%, 20%, and 62% of the selected 54% grids, which covers  41.3% area. A weakened triennial monsoon cycle will have a severe impact on agriculture and water resource management, particularly over the southwest‐coastal, northern, and northeast‐central parts of India.