Synoptic and Large-Scale Drivers of Extreme Precipitation Events in the Western Himalaya

This study investigates the synoptic and large-scale atmospheric circulation associated with extreme precipitation events (EPEs) that occurred during the period 1979-2023 in the Western Himalaya (WH). These EPEs are defined as days when the mean precipitation exceeds the 99th percentile threshold of daily precipitation for each month from 1979 to 2023 across all grid points in the Indian Himalayan region (Karakoram, WH, Central and Eastern Himalaya). The weather regimes associated with these events are then classified using K-means clustering of geopotential height at 500hPa and vertical integrated moisture flux components. We have identified six clusters and determined that EPEs linked to four of these clusters predominantly occur during the monsoon months, whereas the other two clusters are characterized by WD (Western Disturbance)-driven EPEs that appear in the winter months. The EPEs in cluster-1 are mainly driven by the low-pressure system (LPS) in the Bay of Bengal and Rossby-wave breaking (atmospheric blocking by Siberian anticyclone) in the upper-atmosphere along with the midlatitude forcing of North Pacific Oscillation (NPO) (positive phase). The EPEs in clusters 2 and 5 resulted from a break in the monsoon caused by the northward displacement of LPS close to the Himalayan foothills, along with an omega type of blocking with a strong anticyclone over the WH, which is located between two cyclones. The midlatitude forcings of the negative phase of NPO and the negative phase of ENSO during EPEs in clusters 2 and 5, respectively, support the occurrence of EPEs in the WH. The EPEs in cluster-6 occurred due to incursion of a WD into the WH, along the northward migration of LPS in the break-monsoon phase over the WH; tropical forcing of positive phase of ENSO promotes the EPEs in this cluster. The WDs-driven clusters (cluster-3 and 4) mainly support higher amount of precipitation over the WH, and account for 80-95% of mean precipitation over the region, primarily driven by subtropical jet stream dynamics and upper-level trough over the WH. The EPEs in cluster-3 are linked with positive phase of North Atlantic Oscillation, while weaker Tibetan anticyclonic circulation is observed in the cluster-4 compared to cluster-3.