Atmos. Chem. Phys., 20, 10493–10511, 2020 https://doi.org/10.5194/acp-20-10493-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Moisture origin as a driver of temporal variabilities of the water vapour isotopic composition in the Lena River Delta, Siberia Jean-Louis Bonne1, Hanno Meyer2, Melanie Behrens1, Julia Boike2,3, Sepp Kipfstuhl1, Benjamin Rabe1, Toni Schmidt2, Lutz Schönicke2, Hans Christian Steen-Larsen4,5, and Martin Werner1 1Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, 27515 Bremerhaven, Germany 2Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, 14401 Potsdam, Germany 3Geography Department, Humboldt-Universität zu Berlin, 10099 Berlin, Germany 4Geophysical Institute, University of Bergen, 5020 Bergen, Norway 5Bjerknes Centre for Climate Research, 5020 Bergen, Norway Correspondence: Jean-Louis Bonne ([email protected]) and Martin Werner ([email protected]) Received: 15 October 2019 – Discussion started: 6 January 2020 Revised: 28 May 2020 – Accepted: 5 July 2020 – Published: 9 September 2020 Abstract. In the context of the Arctic amplification of cli- the Atlantic sector, while dry and cold air with isotopically mate change affecting the regional atmospheric hydrological depleted moisture is generally associated with air masses cycle, it is crucial to characterize the present-day moisture moving slowly over northern Eurasia. sources of the Arctic. The isotopic composition is an impor- tant tool to enhance our understanding of the drivers of the hydrological cycle due to the different molecular characteris- tics of water stable isotopes during phase change. This study 1 Introduction introduces 2 years of continuous in situ water vapour and pre- cipitation isotopic observations conducted since July 2015 The amplitude of climate change in Arctic regions is likely in the eastern Siberian Lena delta at the research station on to affect the atmospheric hydrological cycle, as sea ice is re- Samoylov Island. The vapour isotopic signals are dominated treating and the temperatures are increasing, modifying the by variations at seasonal and synoptic timescales. Diurnal sources of evaporation and the saturation vapour pressure of variations of the vapour isotopic signals are masked by syn- the atmosphere. However, changes in the evapotranspiration optic variations, indicating low variations of the amplitude of Arctic regions during recent decades are poorly known so of local sources at the diurnal scale in winter, summer and far (Vihma et al., 2015). 18 autumn. Low-amplitude diurnal variations in spring may in- Isotopic concentrations are commonly reported as δ O 2 18 dicate exchange of moisture between the atmosphere and the and δ H, representing for the relative abundances of H2 O 2 16 snow-covered surface. Moisture source diagnostics based on and H H O, respectively, compared to the most abundant 16 semi-Lagrangian backward trajectories reveal that different isotopologue H2 O. As isotope fractionation occurs during air mass origins have contrasting contributions to the mois- phase changes in water, isotopes are largely used as tracers of ture budget of the Lena delta region. At the seasonal scale, the hydrological processes in the atmosphere and can be in- the distance from the net moisture sources to the arrival site terpreted as proxies for past temperature variations in differ- strongly varies. During the coldest months, no contribution ent types of climatic archives, such as ice cores, speleothems from local secondary evaporation is observed. Variations of or ice wedges, which are specific ground ice features found the vapour isotopic composition during the cold season on in permafrost environments. 18 2 the synoptic timescale are strongly related to moisture source The δ O and δ H of precipitation and vapour at the global regions and variations in atmospheric transport: warm and scale are primarily determined by the cooling-induced equi- isotopically enriched moist air is linked to fast transport from librium distillation of the moisture from the source towards the measuring site. To the first order, this distillation affects Published by Copernicus Publications on behalf of the European Geosciences Union. 10494 J.-L. Bonne et al.: Moisture origin and temporal variabilities in the Lena delta, Siberia 18 2 16 both H2 O and H H O similarly, causing a global linear dis- tions as well as strong diurnal variations in summer on the tribution of δ2H versus δ18O with a slope close to 8 (Craig, first- and second-order vapour isotopic signals. To explain 1961). Additional phase change processes occurring during the lack of relationship between the vapour isotopic signal non-equilibrium, namely kinetic fractionation processes such and meteorological parameters (humidity and temperature) as fast evaporation from the ocean surface or snow crys- in summer, a strong contribution of continental recycling and tal formation, are visible in deviations from this relationship local evapotranspiration has been suggested (Butzin et al., (Dansgaard, 1964; Jouzel and Merlivat, 1984; Merlivat and 2014). These observations were also used to test the ability Jouzel, 1979). To study the impacts of kinetic fractionation, of an isotope-enabled atmospheric general circulation model the second-order parameter deuterium excess, hereafter d- to reproduce the water vapour isotopic composition and have excess, has been defined as the deviation from this δ2H to shown an excellent correlation between simulation and water δ18O relationship (Dansgaard, 1964). vapour δ2H values measured at the surface (Gribanov et al., Present-day observations of vapour and precipitation 2014). events refine the understanding of various phases of the at- In this study, we focus on water vapour isotopic observa- mospheric water cycle and their imprint on water isotopic tions continuously performed from July 2015 to June 2017 compositions (Galewsky et al., 2016). Such studies focus on at the research station on Samoylov Island in the Lena delta exchange processes between the atmosphere and the Earth’s (72◦ 220 N, 126◦ 290 E) in the Russian Arctic. This station is at surface over open oceans (Benetti et al., 2014, 2017; Bonne higher latitude and much further east than the Kourovka and et al., 2019; Zannoni et al., 2019), ice sheets (Madsen et al., Labytnangi stations and will therefore be more representa- 2019; Steen-Larsen et al., 2014), sea ice surfaces (Bonne et tive of an Arctic continental climate. We assess the variations al., 2019) or continents (Bastrikov et al., 2014). Other studies of water vapour isotopic composition at different timescales, focus on the atmospheric transport of moisture and show that from the seasonal to the synoptic and diurnal scales, and re- typical vapour isotopic signals can be associated with dis- late them to the variations of local meteorological param- tinct patterns of moisture origins (Bonne et al., 2014, 2015; eters. In order to interpret these variations in a larger re- Guilpart et al., 2017; Kopec et al., 2014; Steen-Larsen et al., gional context, we use atmospheric transport simulations to- 2013, 2015). However, the systematic relationship between gether with moisture source diagnostics. These simulations water isotopes and atmospheric moisture transport remains help identify the main moisture advection regimes of the re- uncertain in many conditions and locations. Recently, water gion and decipher the imprint of local versus remote moisture vapour isotopic observations have also been showing great sources on the locally observed water vapour isotopic com- potential when used to benchmark the simulations of the hy- position. drological cycle in general circulation models equipped with water isotopes (Steen-Larsen et al., 2017). In the eastern Arctic region, water isotopic measurements 2 Method of precipitation samples have been conducted at a land sta- tion along the Lena River (Zhigansk; 66.8◦ N, 123.4◦ E; 2.1 Study area 92 m a.s.l.) during multiple years (2004–2008) and have been combined with water vapour isotopic observations based on The observations presented in this study have been conducted discrete samples during a ship campaign in the eastern Arctic in the Lena River Delta, north-east Siberia (Fig. 1a), at the Ocean for a period extending from before the sea ice mini- research station on Samoylov Island (72◦ 220 N, 126◦ 290 E) mum to the beginning of the sea ice growth season (Kurita, (Fig. 1b, c). The Lena River flows from its source in the 2011). This study revealed that higher values of d-excess Baikal Mountains towards north into the Laptev Sea where were observed at the land station in mid-Autumn for air it forms a large delta 150 km in diameter. In its northern masses originating from the Arctic Ocean region compared part, the river flows alongside the Verkhoyansk Mountains to air masses originating from lower latitudes. A hypothesis on its eastern side (see Fig. 1). Samoylov Island consists on to explain these high d-excess values was that evaporation its western part of a modern floodplain and on its eastern part over the ocean was taking place during the sea ice growth of a Holocene terrace characterized by polygonal tundra and season at very low relative humidity: the dry air masses larger water bodies (see Fig. 1c and Supplement Fig. S1). originating from sea ice covered areas enhance evaporation With a mean annual temperature below −12 ◦C, minimum with strong kinetic fractionation when passing over the open winter air temperatures below −45 ◦C and summer air tem- ocean. peratures that can exceed 25 ◦C, the region has a typical More recently, continuous water vapour isotopic obser- Arctic continental climate (Boike et al., 2013, 2019a). Over vations have been conducted in western Russia at the the 2002–2017 period, average annual rainfall was about Kourovka observatory (Bastrikov et al., 2014; Gribanov et 169 mm, and the winter snow cover was thin (∼ 0:3 m), with al., 2014) and at Labytnangi (Konstantin Gribanov and Jean a maximum recorded of about 0.8 m in 2017.