Synoptic-scale precursors of landslides in the western Himalaya and Karakoram.

In the Upper Indus Basin (UIB), precipitation associated with synoptic-scale circulations impinges on the complex and steep orography of the western Himalaya and Karakoram. Heavy rainfall often falls over the foothills, frequently triggering landslides there. This study explores the role of these synoptic-scale circulations - extratropical western disturbances (WDs) and tropical depressions (TDs) - in producing the conducive conditions necessary to trigger landslides, using data from the NASA Global Landslide Catalog and WD and TD track databases. During the winter (October to April), UIB landslides peak in February and occur at a rate of 0.05 day-1, 61% of which are associated with the passage of a WD. They are most common when a WD is located within a few hundred kilometres of 30°N, and significantly rarer if the WD is north of 40°N. WDs provide moist southwesterly flow from the Arabian Sea and Mediterranean Sea to the UIB, resulting in large-scale precipitation, but landslide probability is not related to WD intensity. Non-WD winter landslides are associated with small-scale orographic precipitation that we hypothesise is due to cloudbursts. During the summer (May to September), UIB landslides peak in August and occur at a rate of 0.11 day-1, 60% of which are associated with TD activity. Many of these TDs are found over central India, slightly south of the climatological monsoon trough, where they provide strong monsoonal southeasterlies to the UIB flowing along the Himalayas. Increased landslide frequency is also associated with TD activity over the southern Bay of Bengal (BoB), and it is hypothesised that this is related to monsoon break conditions. Landslide frequency is significantly correlated with TD intensity. Non-TD landslides are associated with a northwestward extension of the monsoon trough, providing southeasterly barrier flow to the UIB. Implications for forecasting and climate change are discussed.