Western disturbances (WDs) are synoptic-scale systems embedded in the subtropical westerly jet, often associated with extreme precipitation events (EPEs) in Pakistan and north India. Here, we use an objective feature-tracking algorithm on reanalysis data to develop a 37-year catalogue of over 3000 systems. This catalogue is used to study the three-dimensional structure of WDs, revealing a complex asymmetrical structure that suggests strong baroclinicity and orographic interaction. k-means clustering is then used to separate the catalogue into different WD classifications based on dynamical features and precipitation footprint, from which we show firstly that heavier rainfall is strongly coupled with winds aloft, and secondly that there are several important precipitation-generating mechanisms.

We then explore the relationship between WDs and precipitation; using a simple attribution technique we show that more than half of the winter precipitation over South Asia is likely caused by WDs, but more importantly that over 85% of winter EPEs in Pakistan and north India are directly attributable to WDs, typically as part of an extensive upper-level Rossby wave train. These events are explored in more detail using moisture trajectory analysis.

Next, we apply the tracking algorithm to the output of CMIP5 models; firstly, we look at the historical experiments and evaluate the models' performance in representing WD structures and precursor mechanisms; secondly, we look at the RCP scenario experiments to determine and understand the fate of WDs in future climate projections. We find that they are projected to fall in frequency by over 10% by the end of the century in the RCP8.5 scenario. Finally, we use a high-resolution model to determine the impact of global warming on WD moisture budgets and use this information to constrain EPEs in a future climate.