Two Moisture Transport Routes Revealed by Western Disturbance Precipitation Isotopes in Pakistan

A two-year daily precipitation isotope profile of four stations in Pakistan from south to north has been presented and analyzed here to understand how western disturbance (WD) system affects precipitation isotope composition. Vorticity and circulation fields of WD events are investigated, with a composite analysis based on precipitation isotope values. There is no direct relation between WD intensity and precipitation isotopes, but the position of WD matters instead. We find two significant moisture transport routes according to the composite analysis. The south route, usually accompanied with a trough on the Arabian Sea, deeply going through the sea and passing the Tibetan Plateau (TP) from south to north, corresponds to depleted precipitation isotopes and a WD whose center locates more southernly. While the north route, usually accompanied with a more western trough, slightly grazing the sea or not going to the sea at all, bypassing the TP and entering from the north, corresponds to enriched precipitation isotopes and a northerly WD center. In addition, a series of Lagrange air mass back trajectories are executed. Whether in summer or in winter, it does not become moist until the air mass reaches near the ocean. The cluster mean of trajectories with enriched precipitation isotopes locates more northward and further inland than the cluster with depleted isotopes, which confirms the previous circulation analysis results. The two routes revealed by this study, especially the north route, may have more implications for paleoclimate reconstructions on northern Tibetan Plateau.