Winter and spring atmospheric rivers in High Mountain Asia: climatology, dynamics, and variability

Abstract

Atmospheric rivers (ARs) that reach the complex terrain of High Mountain Asia (HMA) cause significant hydrological impacts for millions of people. While ARs are often associated with precipitation extremes and can cause floods and debris flows affecting populated communities, little is known about ARs that reach as far inland as HMA. This paper characterizes AR types and investigates dynamical mechanisms associated with the development of ARs that typically affect HMA. Combined empirical orthogonal function (cEOF) analysis using integrated water vapor transport (IVT) is applied to days where an AR reaches HMA. K-means cluster analysis applied to the first two principal components uncovered three subtypes of AR events with distinct synoptic characteristics during winter and spring months. The first subtype increases precipitation and IVT in Western HMA and is associated with a zonally oriented wave train propagating within the westerly jet waveguide. The second subtype is associated with enhanced southwesterly IVT, anomalous upper-level cyclonic circulation centered on 45$$^\circ $$ ∘ E, and precipitation in Northwestern HMA. The third subtype shows anomalous precipitation in Eastern HMA and southwesterly IVT across the Bay of Bengal. Interannual variations in the frequency of HMA ARs and relationships with various teleconnection patterns show that western HMA AR subtypes are sensitive to well-known remote large-scale climate factors, such as the El Niño Southern Oscillation, Arctic Oscillation, and the Siberian High. These results provide synoptic characterization of the three types of ARs that reach HMA and reveal the previously unexplored significance of their contribution to winter and spring precipitation.

Influence of the freezing level on Atmospheric Rivers in High Mountain Asia: WRF case studies of orographic precipitation extremes

“…HMA AR events were identified and classified between 1979 and 2015 based on ERA5 according to the criteria developed in Nash et al (2021) (see Nash et al (2021) for more details on identifying HMA ARs). Characteristics of each event were based on the location of above-average precipitation during each HMA AR Type and include northwestern HMA (66-74°E and 37-40°N), western HMA (71-79°E and 32-37°N), and eastern Himalaya (90-100°E and 24-30°N) (see Fig.…”

“…IVT, a variable widely used in the detection and characterization of ARs (Guan and Waliser, 2015;Nash et al, 2021;Rutz et al, 2014), was calculated by taking the 3-hourly model data, interpolating u and v wind components (m s -1 ), and water vapor mixing ratio (kg kg -1 ) between 200 hPa and the surface (see Appendix A for equations). We also examined the vertical fluxes of water vapor, or the water vapor flux at each pressure level, to identify the height at which the majority of the moisture is located within each AR.…”

“…To detect ARs, we use the Tracking Atmospheric Rivers Globally as Elongated Targets (tARget) algorithm version 3 which was applied to global, 6-hourly ERA-Interim data from 1979 to 2015 (Guan and Waliser, 2019). This AR detection algorithm is useful for the HMA region as it detects ARs via relative IVT intensity thresholds, which is particularly useful during the winter in HMA, as there is, on average, little to no moisture (Nash et al, 2021). Nash et al (2021) identified three main types of ARs that reach HMA in winter and spring months using tARget v3.…”

“…This AR detection algorithm is useful for the HMA region as it detects ARs via relative IVT intensity thresholds, which is particularly useful during the winter in HMA, as there is, on average, little to no moisture (Nash et al, 2021). Nash et al (2021) identified three main types of ARs that reach HMA in winter and spring months using tARget v3. We use the resulting classification of HMA AR types in this study to focus on Northwestern and Western HMA AR Types that resulted in extreme precipitation.…”

“…In western HMA, Norris et al (2019) identified positive trends of cloud ice and liquid cloud, indicating the higher frequency of extratropical cyclones in recent years. Nash et al (2021) demonstrated that of the three types of HMA ARs, Northwestern and Western HMA ARs are primarily associated with extratropical cyclones, where the warm, moist air from the AR is advected in the area ahead of the cold front.…”