10% coefficient of variation (COV) in All India Monsoon Rainfall (AIMR) assumes paramount importance as the government classifies years with AIMR above 110% as surplus and below 90% as deficit, shaping critical decisions. While most Indian regions exhibit a 20-40% COV for grid-level monsoon rainfall, the mystery surrounding AIMR’s 10% COV persists. An examination of 40 years of gridded monsoon rainfall data exposes the negative covariance between Central India (CI) and Northeast India (NE) constraining the COV of AIMR to approximately 10%. Utilizing Lagrangian backtracking of moisture, we found that during CI’s deficit rainfall years, NE experiences surplus with 61.69% moisture from terrestrial sources. In CI’s surplus years, major fraction of the moisture supply from ocean is confined to CI, resulting in NE facing deficit rainfall. Our findings emphasize the non-intuitive processes leading to the 10% COV in AIMR cautioning against relying on this value for scientific studies or planning.
