Isotopic composition of daily precipitation along the southern foothills of the Himalayas: impact of marine and continental sources of atmospheric moisture

Abstract

Abstract. The flow of the Himalayan rivers, a key source of fresh water for more than a billion people primarily depends upon the strength, behaviour and duration of the Indian summer monsoon (ISM) and the western disturbances (WD), two contrasting circulation regimes of the regional atmosphere. An analysis of the 2 H and 18 O isotope composition of daily precipitation collected along the southern foothills of the Himalayas, combined with extensive backward trajectory modelling, was used to gain deeper insight into the mechanisms controlling the isotopic composition of precipitation and the origin of atmospheric moisture and precipitation during ISM and WD periods. Daily precipitation samples were collected during the period from September 2008 to December 2011 at six stations, extending from Srinagar in the west (Kashmir state) to Dibrugarh in the east (Assam state). In total, 548 daily precipitation samples were collected and analysed for their stable isotope composition. It is suggested that the gradual reduction in the 2 H and 18 O content of precipitation in the study region, progressing from δ 18 O values close to zero down to ca. −10 ‰ in the course of ISM evolution, stems from regional, large-scale recycling of moisture-driven monsoonal circulation. Superimposed on this general trend are short-term fluctuations of the isotopic composition of rainfall, which might have stem from local effects such as enhanced convective activity and the associated higher degree of rainout of moist air masses (local amount effect), the partial evaporation of raindrops, or the impact of isotopically heavy moisture generated in evapotranspiration processes taking place in the vicinity of rainfall sampling sites. Seasonal footprint maps constructed for three stations representing the western, central and eastern portions of the Himalayan region indicate that the influence of monsoonal circulation reaches the western edges of the Himalayan region. While the characteristic imprint of monsoonal air masses (increase of monthly rainfall amount) can be completely absent in the western Himalayas, the onset of the ISM period in this region is still clearly visible in the isotopic composition of daily precipitation. A characteristic feature of daily precipitation collected during the WD period is the gradual increase of 2 H and 18 O content, reaching positive δ 2 H and δ 18 O values towards the end of the period. This trend can be explained by the growing importance of moisture of continental origin as a source of daily precipitation. High deuterium-excess (d-excess) values of daily rainfall recorded at the monitoring stations (38 cases in total, range from 20.6 to 44.0 ‰) are attributed to moisture of continental origin released into the atmosphere during the evaporation of surface water bodies and/or soil water evaporation.

Onset of summer monsoon in Northeast India is preceded by enhanced transpiration

“…Analysis of stable water isotopes in rivers and lake water across the globe have been used to partition various fluxes within the hydrological cycle 10,22 . Isotopic analysis of precipitation has been widely carried out to understand moisture sources 18,23–26 . Using observations of δD and δ 18 O in precipitation and comparing them with Global Meteoric Water Line (GMWL) provides information about source and transport of moisture over a region 27,28 .…”

Measurement report: Spatial variability of northern Iberian rainfall stable isotope values – investigating atmospheric controls on daily and monthly timescales

“…Prior to conducting correlation analysis at daily timescales, we removed the seasonal cycle of the variables by subtracting their monthly averages to avoid sympathetic seasonal correlations (e.g., Kawale et al, 2011;Rozanski et al, 1993) (Table 3a). To establish correlations on the monthly scale with meteorological variables (Table 3b), δ 18 O p monthly averages weighted by the amount of precipitation were calculated using the following formula:…”

“…In addition, to date, the majority of empirical studies of meteorological controls over δ 18 O p are based on event, daily or monthly time series from individual locations (Moreno et al, 2014;Smith et al, 2016). The scarce studies dealing with multiple sampling locations span areas under the influence of the same climatic regime (Baldini et al, 2010;Jeelani et al, 2018). This approach raises concerns about the spatial representativeness of the resulting statistical models and the mechanisms behind the identified relationships in areas as complex as the IP.…”

Synoptic processes of winter precipitation in the Upper Indus Basin

“…The upper-troposphere disturbance characterising a WD is embedded in the subtropical westerly jet (SWJ; Dimri et al, 2015;Hunt et al, 2018a). The SWJ characterises the northern edge of the Hadley circulation (Krishnamurti, 1961). In Asia, the Tibetan Plateau disrupts the SWJ.…”

“…The jet oscillates between two stable states: one north of the Tibetan Plateau, always present in summer, and one south of it, reached only in winter (Schiemann et al, 2009). Furthermore, in winter, the SWJ is also split into two climatological jet streaks of higher intensity (Krishnamurti, 1961;Schiemann et al, 2009): one over the Arabian Peninsula (Arabian jet; Yang et al, 2004;de Vries et al, 2016) and the other over East Asia (East Asian jet; Xueyuan and Yaocun, 2005). It has been argued that the position and strength of the SWJ influence WD intensity at the intra-seasonal and inter-annual scale (Filippi et al, 2014;Dimri et al, 2015;Hunt et al, 2018a;Ahmed et al, 2019).…”