Missing Summer Westerly Jet Barotropic Governor Effect Explains Climate Models—Observation Discrepancies in the Indian Monsoon Trends

Abstract The South Asian summer monsoon has exhibited a pronounced Northwest India‐Indo‐Gangetic Plains rainfall dipole since 1999, with northwest India experiencing a 24.6% increase, while rainfall in the Indo‐Gangetic Plain has decreased by 4.4%. This dipole pattern is absent in historical climate model simulations, and the underlying physical mechanisms responsible for it are not yet fully understood. Using synthesis of observational and assimilated data and climate model simulations, we demonstrate that this dipole is driven by North Atlantic Sea Surface Temperature (SST) changes, which are transmitted through a “barotropic governor mechanism” and modulate Asian jet stream dynamics. Enhanced barotropic energy conversion after 1999 generated momentum focusing in the jet core while weakening east‐west flanking regions. This fundamentally altered the monsoon circulation by modulating the local Hadley cell. We find that this barotropic governor activation coincides with the North Atlantic “cold blob” attributed to a slowdown in the Atlantic Meridional Overturning Circulation (AMOC). Coupled climate simulations over the historical era systematically fail to reproduce North Atlantic SST changes; thus, this atmospheric dynamical mechanism instead showed reversed barotropic energy conversion patterns and failed to reproduce this key teleconnection mechanism. Prescribed SST experiments forced by observed North Atlantic SST changes support the proposed mechanism, successfully reproducing both observed jet dynamics and rainfall trends. The identification of a North Atlantic‐Asian teleconnection pathway modulated by the barotropic governor effect directly links the behavior of the AMOC and monsoon tipping elements to each other.