Flood Seasonality Over the Third Pole Region Modulated by Upper Level Moisture Transport

Abstract

The nexus between atmospheric moisture transport and basin‐scale flood response is still lacking over the Third Pole (TP) region, despite projected increases in extreme rainfall under a changing climate. Based on long‐term daily streamflow observations, we show that peaks‐over‐threshold floods over the Yarlung Zangbo River (YZR) basin show two temporal clusters, that is, July and late August, with the flood magnitudes of the second cluster larger than the first by 20%. These floods are resultant from a mixture of flood‐producing storms with contrasting storm motion and moisture transport pathways. A suite of coupled atmospheric‐hydrological modeling experiments further quantify the impacts of different moisture transport pathways on the space‐time rainfall structure and flood response. A 50% reduction of upper level moisture transport (i.e., through the upslope of the Himalayas) leads to decreased flood peak magnitudes by 30% at the basin outlet for the August 1998 floods over the YZR basin. Upper‐level moisture transport is associated with the notable low pressure over the central‐east India and westward extension of West Pacific Subtropical High that enhances convergence of water vapor fluxes from Bay of Bengal toward TP. The magnitude of impact can potentially alter the seasonal clustering of high‐flow events over TP. Our analyses contribute to improved characterization of flood risk over TP, and advances in flood frequency hydrology.

Heterogeneity in Spatiotemporal Variability of High Mountain Asia's Runoff and Its Underlying Mechanisms

“…The suppressed convection over the western Pacific and MC contributes to the development of WPSH (T. Li et al., 2017). Thus, the IPO affects hydroclimate in HMA through modulating in the position and strength of the WPSH (L. Yang et al., 2022; Y. Zhao & Zhou, 2021). During the positive phase of IPO, the WPSH tends to be stronger and farther westward to the southeastern edge of HMA, which promotes the water vapor transport from the Indian Ocean to the central and east HMA, leading to ascending motion and thus increasing precipitation and runoff in this region (Figure S8b in Supporting Information ).…”

“…The suppressed convection over the western Pacific and MC contributes to the development of WPSH (T. Li et al, 2017). Thus, the IPO affects hydroclimate in HMA through modulating in the position and strength of the WPSH (L. Yang et al, 2022;Y. Zhao & Zhou, 2021).…”