60-year trends of δ18O in global precipitation reveal large scale hydroclimatic variations
Graphical abstract
Introduction
Atmospheric oscillations across the planet have a strong impact on hydroclimate processes at different spatial scales (Baldini et al., 2008; Kurtz, 2015). These global oscillations have frequencies ranging from weeks to multidecadal periods; some of them can be predicted whereas others cannot (Mantua and Hare, 2002; Baldini et al., 2008; Kurtz, 2015; Comas-Bru et al., 2016). The North Atlantic Oscillation (NAO), the Pacific Decadal (PDO), and Atlantic Multi-decadal (AMO) are the most important decadal and multi-decadal oscillations that control inter-annual climate variability at global scales (Sutton and Hodson, 2005; Kurzt 2015; Wang et al., 2017).
The NAO is known to modulate inter-annual atmospheric variability in the Northern Hemisphere and is a key driver of climate and weather in mid- to high- latitudinal North Atlantic regions, but at unpredictable periodicities from multi-daily to decadal (Baldini et al., 2008). The NAO “index” is determined from normalized pressure anomalies at the Azores and Iceland and is often interlinked to East Atlantic influence. Weak NAO activities (NAO-) deviate air masses towards the Mediterranean and eventually back to Central Europe, as observed in the decades before the late 1980's. Strong NAO activities (NAO+) diminished the influence of Atlantic moisture over the past few decades (Comas-Bru et al., 2016; Deininger et al., 2016). These variations in NAO activities are related to changes in air temperature and precipitation in a site and time-specific way (Comas-Bru et al., 2016; Deininger et al., 2016). NAO has also been linked to cyclonic activities (Vb-cyclones) in the Northern Hemisphere (Hofstätter and Blöschl, 2019).
In the Southern Hemisphere, Pacific atmospheric air mass circulation is largely associated with the El Niño-Southern Oscillation (ENSO), which brings hotter and drier weather during El Niño phase whereas cooler and wetter conditions prevail during La Niña phase (Mantua and Hare, 2002). Some scientists relate ENSO to PDO and consider PDO to be a long-lived El Niño-like pattern of Pacific climate variability, but others consider both as time independent modes with distinctive spatial and temporal characteristics of North Pacific temperature variability (Mantua and Hare, 2002). In any case, both oscillations are important determinants of climate in the mid-latitudes and may be responsible for air and sea water temperature rise since the early 1980's (Mantua and Hare, 2002). The AMO has a strong impact on global air mass circulation and particularly on rainfall patterns, acting with various frequencies and different regional patterns (Sutton and Hodson, 2005; Wang et al., 2017).
Stable isotope ratios (18O/16O, 2H/H) of precipitation collected from stations worldwide are well-known sensitive indicators of hydroclimatic variations, and depending on the precipitation collection frequency (e.g. events, monthly composites) and timeframe (years, decades) can be used as proxies to study large scale atmospheric circulations (Baldini et al., 2008; Liu et al., 2010; Kurtz, 2015; Comas-Bru et al., 2016). Stable isotopes in precipitation may also reflect decadal and lower frequency atmospheric oscillations (e.g. El Nino, El Nina, NAO, Pacific North American) (Liu et al., 2011; Brown et al., 2006), however, long isotope time series are generally needed to detect multi-decadal climatic oscillations (Kurtz, 2015; Gao et al., 2016). Additionally, the isotope composition of precipitation driven by global air mass circulation may be further modified by local and regional hydroclimatic processes, such as sub-cloud evaporation, convective and moisture recycling processes (Dansgaard, 1964; Rozanski et al., 1982, Rozanski et al., 1992; Araguás-Araguás et al., 2000; Brown et al., 2006; Aggarwal et al., 2012, Aggarwal et al., 2016; Wang et al., 2017). Accordingly, stable isotopes (δ18O and δ2H) have been used as long-term (hundreds to thousands of years) proxies of hydroclimate variation in ice cores, tree rings, speleothems, etc. (Raymo et al., 2006; Azzoug et al., 2012; Finkenbinder et al., 2016). Paleoclimate proxies integrate past precipitation events over various time frames; however, many proxies cannot be used for detecting distinctive changes in hydroclimate variations over monthly or annual decadal scales.
To gain a better understanding of hydroclimate variations over the past decades, we focused on long-term (60-year) records and trends of δ18O in precipitation as well as air temperature and precipitation for 21 meteorological stations worldwide located in diverse climate and geographical zones. The isotope and climatic data were used to detect hydroclimate variations that influence the isotope composition of global precipitation. Our findings contribute to reducing uncertainties in our understanding of global long-term temporal variations of stable isotopes in precipitation and offer some directions for hydrological predictions and climatic modelling. Based on our results we were able to identify gaps in the organization of future global observation strategies and proposed helpful directions for the long-term monitoring of stable isotopes in precipitation.
Section snippets
Study area and data collection
Long-term world-wide records of stable isotopes in monthly composites of precipitation and associated meteorological data were obtained from the International Atomic Energy Agency (IAEA) Global Network for Isotopes in Precipitation (GNIP) (IAEA/WMO, 2020). We selected a subset of 21 GNIP stations, whose data spanned the isotopic observation period from 1960 to 2018 (Fig. 1) and had <50% of data gaps across the time period. These long-term GNIP stations included inland and continental regions in
Continental stations in Europe and Northern America
GNIP stations located on the American and European continents in the Northern Hemisphere (Groningen, Stuttgart, Vienna, Ankara, Antalya, Gibraltar, Grimsel, Bern and Ottawa) had different climate types (Fig. 1, Fig. 2). While Valentia is an island in the North Atlantic, similarly to Dansgaard (1964) we considered it to be in this group due to the proximity to the European continent. Groningen, Stuttgart, Valentia and Vienna stations are in temperate fully humid climates (Cf) with mean annual
Recommendations
The analysis of the long-term GNIP data series for δ18O in precipitation revealed both complex and diverse isotopic patterns and trends that could not be simplistically related to climate type or “climate change”. This large diversity in long-term global δ18O variations is perhaps better explained by the sensitivity of δ18O to these and other regional and global hydroclimate processes, which are not easily unified in time or space without continuous or higher-frequency isotope monitoring. While
Summary
While longer, more frequent, and complete δ18O (and δ2H) observations are needed, the detected trends, break points with shifts, and polynomial trends are in good agreement with known changes in large scale climate oscillations modes. We suggest that with improved and continuous coordinated long-term monitoring, isotopes in precipitation are an excellent proxy of past, present and even future large-scale hydroclimate processes, such as atmospheric oscillations acting in Northern and Southern
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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