Underlying physical mechanisms of winter precipitation extremes over India's high mountain region

This figure illustrates the underlying physical processes leading to extreme precipitation events over the western Himalayas during the winter season. Note that this graphical figure may not represent the actual political boundaries/orography. image Extreme precipitation events (EPEs) are among the most pervasive weather hazards in the western Himalayan region (WHR), posing widespread damage to life, infrastructure, and agriculture. This study investigates the synoptic and large-scale characteristics linked to winter precipitation extremes over the WHR. EPEs are identified as events surpassing the 95th percentile threshold. A composite analysis is employed using two reanalyses-the fifth-generation European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5) and the Indian Monsoon Data Assimilation and Analysis (IMDAA)-to elucidate the synoptic conditions conducive to EPEs. Our findings suggest that EPEs in the WHR are linked to an intensified subtropical westerly jet, characteristically shifted to south than normal. Enhanced kinetic energy in the upper troposphere, attributed to increased baroclinic instability, reinforces moisture convergence and strengthens synoptic-scale circulation, triggering deep convection and supporting EPEs. Notably, the interplay of pronounced Rossby waves sinking over the WHR and regional orography significantly modulates the intensity of western disturbances (WDs). Employing clustering analysis, we observed that the strongest EPEs are linked to anomalous vorticity in the upper to middle troposphere, together with deep convection via strengthened WDs, suggesting the potential role of large-scale influences. Using Lagrangian method, we identify that the Arabian Sea is the primary moisture source for EPEs in the WHR. We further delved into the role of large-scale connections and EPEs through quasi-resonant amplification (QRA) analysis. The findings unveil distinct QRA fingerprints in meridional temperature gradients along with notably magnified, quasi-stationary midlatitude planetary waves characterized by zonal wave numbers 6/7/8 (baroclinic waves) contributing to EPEs. Overall, our results highlight the underlying physical mechanisms for winter precipitation extremes, emphasizing QRA's role in amplifying planetary waves and promoting EPEs, underscoring the WHR's vulnerability to evolving climatic conditions.