Abstract
Abstract. Precipitation in the Upper Indus Basin is triggered by orographic interaction and the forced uplift of a cross-barrier moisture flow. Winter precipitation events are particularly active in this region and are driven by an approaching upper-troposphere western disturbance. Here statistical tools are used to decompose the winter precipitation time series into a wind and a moisture contribution. The relationship between each contribution and the western disturbances are investigated. We find that the wind contribution is related not only to the intensity of the upper-troposphere disturbances but also to their thermal structure through baroclinic processes. Particularly, a short-lived baroclinic interaction between the western disturbance and the lower-altitude cross-barrier flow occurs due to the shape of the relief. This interaction explains both the high activity of western disturbances in the area and their quick decay as they move further east. We also revealed the existence of a moisture pathway from the Red Sea to the Persian Gulf and the north of the Arabian Sea. A western disturbance strengthens this flow and steers it towards the Upper Indus Plain, particularly if it originates from a more southern latitude. In cases where the disturbance originates from the north-west, its impact on the moisture flow is limited, since the advected continental dry air drastically limits the precipitation output. The study offers a conceptual framework to study the synoptic activity of western disturbances as well as key parameters that explain their precipitation output. This can be used to investigate meso-scale processes or intra-seasonal to inter-annual synoptic activity.
Highlights
The Upper Indus Basin (UIB, Fig. 1) is a mountainous region that differs from the rest of the Indian subcontinent in the large amount of precipitation it receives outside of the summer monsoon season (56 % or 505 mm between October and May for ERA5 in the period between 1979 and 2018; Baudouin et al, 2020b)
Winter precipitation events in the UIB are in general related to the passing of extra-tropical, synoptic-scale disturbances often originating from the Mediterranean Sea, the Black Sea, or the Caspian Sea and are referred to as western disturbances (WDs; Dimri et al, 2015; Hunt et al, 2018a)
High UV700 is associated with a negative geopotential anomaly with the minimum located at 36◦ N, 66◦ E, just north of the Hindu Kush, northwest of the UIB (Fig. 3b), and near the tropopause, at around 300 hPa (Fig. 4b), in agreement with previous studies (Hunt et al, 2018a; Midhuna et al, 2020)
Summary
The Upper Indus Basin (UIB, Fig. 1) is a mountainous region that differs from the rest of the Indian subcontinent in the large amount of precipitation it receives outside of the summer monsoon season (56 % or 505 mm between October and May for ERA5 in the period between 1979 and 2018; Baudouin et al, 2020b). Much of this precipitation falls as snow at altitude during the coldest part of the season (Hewitt, 2011; Dahri et al, 2018; Baudouin et al, 2020b).
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