UC Santa Barbara

Westerly disturbances are the primary climatic influence within High Mountain Asia during winter, producing over half of annual precipitation in 4-6 events per winter season and supplying essential water resources for large populations across Asia. This research examines High Mountain Asia’s hydroclimate, focusing on the relationship between westerly disturbance dynamics, the mechanisms that drive orographic precipitation, and their variability on intraseasonal and interannual scales. The first chapter establishes that extreme winter precipitation events in High Mountain Asia are primarily attributable to combined contributions from dynamical forcing and moisture availability during westerly disturbance interaction with regional topography. A novel wave-tracking algorithm was developed to provide an inventory of location, timing, intensity, and duration of westerly disturbance events, allowing for a comprehensive study of the mechanisms that drive orographic precipitation, on an individual event basis and in the aggregate. In the second chapter, westerly disturbances are investigated using extreme event composites to identify significant influence of global atmospheric variability over westerly disturbance dynamics and moisture availability, focusing on tropical forcing by the Madden Julian Oscillation on intraseasonal timescales and the El Nino Southern Oscillation on interannual scales. This work demonstrates that El Nino simultaneously enhances the strength of the storm track and moisture availability to westerly disturbances. Contrastingly, during Madden Julian Oscillation propagation there is a transition in the balance of contributions from moisture availability and dynamical forcing to orographic precipitation. The third chapter of this dissertation employs a mesoscale model to perform a set of modified topography experiments in which extreme precipitation events in High Mountain Asia that were related to westerly disturbances are simulated at 6km resolution with native model topography and with smoothed topography taken from a global circulation model. These experiments illustrate that topographic smoothing fundamentally alters the dynamic and thermodynamic mechanisms that produce orographic precipitation during westerly disturbances, and identifies important deficiencies in the ability of models with coarse topographic resolution to simulate High Mountain Asia weather and climate. Collectively, the three chapters of this dissertation give novel insight into the dynamics of westerly disturbances, how these systems generate extreme precipitation events in High Mountain Asia, and their relationships with global atmospheric variability. These findings advance the scientific community’s understanding of weather and climate in High Mountain Asia and improve the potential for evaluating the current state and future fate of regional water resources.