Winter and Spring Atmospheric Rivers in High Mountain Asia: Climatology, Dynamics and Variability

Abstract:

Atmospheric Rivers (ARs) occur on average 30 days a year in High Mountain Asia (HMA), and have been shown to be associated with precipitation extremes and flooding in Nepal and the Bay of Bengal. The goal of this research is to explain the dynamical mechanisms associated with the development of HMA ARs in an effort to determine the hydrological impacts in a region critical to water resources for hundreds of millions of people. Principal component analysis is applied to HMA AR days using 500 hPa geopotential height and winds across the region 0°N to 50°N and 20°W to 110°W. Cluster analysis defined by the first two principal components uncovered three subtypes of AR events with distinct synoptic characteristics for winter (DJF) and spring (MAM) seasons. Composite analysis shows that one DJF HMA AR subtype results in enhanced southwesterly integrated vapor transport (IVT) and precipitation in the Karakorum that is associated with a deep trough in 250 hPa heights and anomalous anticyclonic circulation over northwest India. The other two DJF HMA AR subtypes resulted in enhanced precipitation and IVT in central and eastern Himalayas related to eastward propagating synoptic storms. Composite analysis of MAM HMA AR subtypes show anomalous precipitation, particularly east of Bangladesh and strong southwesterly IVT across the Bay of Bengal and the South China Sea, but these events are associated with weaker troughs and ridges compared to DJF HMA ARs. This research also investigates interannual and multidecadal variations in HMA ARs and examines their relationship with El Niño Southern Oscillation, and Pacific Decadal Oscillation. Finally, this research identifies changes in intensity and frequency in HMA ARs and their associated vertically integrated moisture transport in recent decades.