Water isotopes, climate variability, and the hydrological cycle: recent advances and new frontiers

Many paleoclimate reconstructions have been published with varying levels of metadata descriptions, and interpreting results has often required specialist knowledge of the environmental drivers of individual proxy archives. New global paleoclimate networks are closing this usability gap in the existing suite of paleoclimate reconstructions. Water isotope-based paleoclimate databases include Iso2k (figure 5), which includes all proxy types and provides comprehensive metadata allowing efficient proxy intercomparison (Konecky et al 2020). Proxy-specific databases such as SISAL (speleothems) (Atsawawaranunt et al 2018) and CoralHydro2k (corals) (Walter et al 2022) allow for investigation of climate changes through a proxy-specific lens. Such databases are well-suited for evaluating hydroclimate processes through time and space by facilitating large-scale syntheses, model-data intercomparison, and paleoclimate data assimilation (Paleo-DA). Indeed, the newly launched Past Global Changes (PAGES) Phase 4 effort is focused on reconstructing hydroclimate variability over the Common Era using these and other hydroclimate databases. Recent efforts to improve and develop new PSMs (e.g. Evans et al 2013, Dee et al 2015a, 2018b, Konecky et al 2019, Malevich et al 2019) further complement the development of these databases, allowing the climate information contained in the records to be more explicitly resolved.

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Rapid advances in stable water isotope measurement technology have positioned us to evaluate water cycle processes controlling climate variability in greater detail, and to refine interpretations of paleoclimate data. In addition, the establishment of large water isotope databases have positioned water isotopes to contribute significantly to our understanding of internal climate variability and forced change. Galewsky et al (2016) reviewed recent advances with respect to the measurement of water vapor. These include advances in remote sensing of water vapor isotopologues from space that have supported the development of multi-year to multi-decadal gridded datasets, providing important context for understanding spatial variability in hydrologic processes and atmosphere's water balance (Galewsky et al 2016, Shi et al 2022, Bailey et al 2017, Tuinenburg et al 2015, Lacour et al 2018, Risi et al 2012a, Risi et al 2012, etc). They also include the widespread adoption of cavity-ringdown spectroscopy and off-axis integrated cavity output spectroscopy instruments, which have high throughput and are designed to measure both water vapor and liquid water samples (Gupta et al 2009). These analyzers have several advantages over traditional mass spectrometry techniques. They are field-deployable, which has supported the recent proliferation of in-situ land-based, ship, and aircraft measurements of vapor stable isotope values to investigate water cycle processes (Galewsky et al 2016, Bailey et al 2022). They also facilitate high-resolution measurements of isotope ratios in ice cores (e.g. at sub-annual resolution, Jones et al 2018) when paired with continuous flow melt systems, creating opportunities to investigate climate variability at even finer resolution.

Large databases of modern isotopic observations are ushering in a new era of global water cycle investigations, enabling new developments in process-oriented studies across large geographic areas, more robust validations of isotope-enabled model simulations, and improved reconstructions of past climate. Several observational databases have paved the way, including the WISER water isotopes database, which includes river water and precipitation data from the foundational Global Network of Isotopes in Precipitation (GNIP) program, and the Goddard Institute of Space Studies (GISSs) Global Seawater $\delta$ ¹⁸O database (Rozanski et al 1993, Schmidt 1999, LeGrande and Schmidt 2006). A large number of new water isotope observational datasets have since been produced, which have been incorporated into numerous databases, including the Water Isotopes Database (Putman and Bowen 2019), which is merging with IsoBank (Pauli et al 2015, 2017), the Stable Water Vapor Isotope Database (Wei et al 2019)), the CISE-L'OCEAN Seawater Isotope Database (Reverdin et al 2022) and the PAGES CoralHydro2k Seawater Isotope Database (expected in 2023, DeLong et al 2022). In addition, the EarthChem geochemical data repository has recently been expanded to facilitate the archival of seawater isotope datasets with standardized metadata, and allows researchers to obtain DOIs for submitted seawater isotope records (DeLong et al 2022). For a more detailed review of current observational isotope databases we refer the reader to (Bowen et al 2019).

Water isotope modeling efforts have expanded at a similarly rapid pace, reflecting the growing valuation of isotopic tracers for improving climate model physics, chemistry, and biology and their unique contributions to improving paleoclimate reconstructions from proxy records. Isotope-enabled models now include not just atmosphere-only and fully-coupled GCMs, but also regional-scale models (Sturm et al 2005, Pfahl et al 2012, Stevenson et al 2015, Yoshimura and Kanamitsu 2010), cloud-resolving models, and large-eddy simulations (Blossey et al 2010, Moore 2016, Smith et al 2006, Lee et al 2012, Torri 2022; see Box 2, Bailey et al 2021). Higher resolution models are an important tool for process-level studies and can be used to (dynamically) down-scale GCM results to aid in the interpretation of paleoclimate proxies collected at specific locations. In some cases, high-resolution models are necessary to simulate smaller-scale atmospheric phenomena such as the mesoscale organization of convection, shown to influence isotope ratios in some proxy archives (Maupin et al 2021). Finally, water isotope enabled simulations are poised to generate significant understanding of future anthropogenic changes to the global hydrological cycle and atmospheric circulation. Incorporation of stable water isotope tracers in simulations spanning the 21st century, for example, could elucidate the Walker Circulation's response to tropical Pacific warming (e.g. Dee et al 2018a), changes in moisture supply to atmospheric river events (Nusbaumer and Noone 2018), shed light on how climate change affects low-cloud and vertical mixing feedbacks in climate models (e.g. Hu et al 2022), and help quantify changes in moisture transport length scales across the mid-latitudes and Arctic (Singh et al 2016b, Bailey et al 2019).

Given these recent developments in our ability to measure isotope ratios across space and time, the integration of water isotopes in models of various resolution and complexity, and improved access to both modern and paleoclimate records through continued reconstruction efforts and improved archival practices, this is an opportune moment for the scientific community to take advantage of the full potential of water isotope tracers to tackle both long-standing questions and emerging grand challenges central to understanding and predicting climate change.

Ice cores constitute our most direct archive of past precipitation, modified by post-deposition processes. Long polar ice core $\delta$ ¹⁸O and $\delta$ ²H records from Greenland and Antarctica—spanning thousands to hundreds of thousands of years—have provided foundational constraints on the timing of glacial and interglacial cycles and their orbital pacing (see review by Brook and Buizert (2018)). They have provided key insights into the temporal relationships between greenhouse-gas forcing and temperature (e.g. Knutti et al 2004, EPICA Community members 2006). And, they are one of several water isotope proxies that have helped characterize periods of abrupt climate change, including Dansgaard–Oeschger (DO) events—characterized by a rapid warming of the Northern Hemisphere, followed by gradual cooling—and Heinrich events, or cooling events in which large freshwater inputs into the North Atlantic from melting ice sheets alter the density-driven thermohaline circulation of the ocean (Grootes et al 1993, EPICA Community members 2004, Jouzel and Masson-Delmotte 2007, Landais et al 2018).

Ice core stable isotope records have been viewed traditionally as a local paleothermometer, since isotopic fractionation is sensitive to the temperature at last saturation—in this case, the temperature of condensation near the ice accumulation region. While this interpretation is not exact, data assimilation analyses, theoretical modeling, and GCM experiments suggest that it is a useful concept (Jouzel et al 1997, Masson-Delmotte et al 2008, Noone 2008, Steiger et al 2017, Brook and Buizert 2018).

Continued research, leveraging modern observations and isotope-enabled simulations, has created a more nuanced understanding of isotopes as indicators of past shifts in moisture source and moisture cycling. Recent analyses of ice core records from West Antarctica, for instance, have used isotopic information to elucidate the coupled atmosphere-ocean response to DO events. Using the distinct phasing of $\delta$ ¹⁸O records from Greenland and $\delta$ ¹⁸O and d-excess records from Antarctica, these critical records have demonstrated that DO events are characterized by a rapid equatorward shift in the moisture source for Antarctic precipitation. These shifts occur as the atmospheric circulation responds to Northern Hemisphere warming and the Intertropical Convergence Zone migrates north (Markle et al 2017). The fast dynamic response of the atmosphere is followed by a slower warming, driven by Southern Hemisphere sea surface temperatures, that lags by about 200 years. Other studies have suggested a role for Southern Ocean temperatures in influencing the strength of the Atlantic Meridional Overturning Circulation, or AMOC, and thus the duration of DO warm periods (Buizert and Schmittner 2015). Isotopic data thus elucidate the distinct yet linked timescales over which the atmosphere and ocean respond to and influence abrupt climate changes.

Water isotope records have also demonstrated important shifts in the moisture source of Arctic precipitation during DO events (Sime et al 2019). Data from Greenland ice cores, in conjunction with model simulations, for example, show that seasonal precipitation isotope ratios and seasonal precipitation amounts vary independently—a clear example of water isotopes providing additional constraints beyond what basic hydrological variables provide. These findings suggest sea ice loss during DO events increases the proportion of Greenland precipitation that originates from local oceanic evaporation. Moreover, these shifts in evaporative source help establish the positive, albeit heterogeneous, ${\text{ }}\delta$ ¹⁸O-temperature relationship across Greenland. The importance of evaporative site changes to the hydroclimate state has emerged as a prominent theme in other modeling experiments that include water isotopic tracers, or that 'tag' and explicitly track moisture that has evaporated from predefined regions, both for the Arctic and Antarctic (Noone 2004, Singh et al 2016a, 2016b, Nusbaumer and Noone 2018, Siler et al 2021).

In addition to recording abrupt changes in mean climate state and associated mean moisture source patterns, long, high-resolution ice core records have also been used to identify abrupt changes in the variability of the climate system through spectral analyses (e.g. Jones et al 2018, Grisart et al 2022). Careful dating of the timing of these changes, matched with other paleoclimate archives (e.g. speleothems), can isolate likely drivers for subsequent evaluation in numerical model experiments. This approach has helped reveal the importance of northern hemisphere ice sheet topography in influencing the zonal position of deep convection in the tropical Pacific, which, in turn, modifies Southern Hemisphere high-latitude stationary eddies via Rossby wave trains (Jones et al 2018). Other studies, using multi-proxy syntheses combined with model simulations, reinforce the importance of ice sheet extent in controlling tropical hydroclimate variability (DiNezio and Tierney 2013, DiNezio et al 2018, Windler et al 2020). DiNezio and Tierney (2013), for instance, argued that resultant variations in sea level and, ultimately, continental shelf exposure during the Last Glacial Maximum (LGM) (∼40kyr ago) inhibited moisture convergence and dried the western Pacific Warm Pool. These findings not only indicate that isotopic records provide critical insight into changes in the general circulation, but they also emphasize the importance of accurately representing ice sheet dynamics so that the effects of sea level rise are incorporated accurately into projections of future atmospheric circulation.

Inverse correlations between ice core aerosol concentrations and $\delta$ ¹⁸O provides further evidence that ice core isotope records reflect variations in global circulation patterns. Markle et al (2018) interpreted this anti-correlation as showing that as atmospheric circulation slows, in response to a weaker meridional temperature gradient, precipitation intensifies globally, effectively scavenging the atmosphere of dust, sea salt, and other particles. The idea that dust accumulation in ice cores is regulated by the global hydrologic cycle, rather than changes in source emissions, represents a major shift in perspective (see Fischer et al 2007). However, the sensitivity of Antarctic isotope records to the general circulation is supported by modern observations and modeling studies (Noone 2008), and modern isotopic observations support the covariance between precipitation scavenging and aerosols (Bailey et al 2015). Knowledge gained about such covariant processes from ice cores creates valuable opportunities to test the coupling between components of today's Earth system models and how these components respond jointly to change.

Speleothem calcite formation in karst environments provides a wide range of climate information on timescales ranging from decades to hundreds of thousands of years (see review by Lachniet 2009, Cheng et al 2016). As groundwater drips into caves, the calcite that precipitates from the dripwater forms the speleothem; its $\delta$ ¹⁸O record thus represents a smoothed time series of amount-weighted precipitation isotope ratios from the local region—with less smoothing in wetter environments, where rainwater flushes groundwater reservoirs more frequently (e.g. Moerman et al 2014). The environmental conditions of the cave can also affect the calcite $\delta$ ¹⁸O but, again, are less important in wetter regions. These and other details are discussed in considerable detail in (Lachniet 2009).

Because of the strong connection to precipitation $\delta$ ¹⁸O, speleothems formed at low latitudes provide a means to study the dynamics controlling the strength and seasonality of monsoons, their role in global hydroclimate, and their sensitivity to external and internal forcings. Once considered indicators of monsoonal precipitation amount, speleothem $\delta$ ¹⁸O records are now frequently interpreted as reflecting variations in atmospheric circulation and its associated impacts on moisture source, moisture transport, and upstream water cycling—with changes in source encompassing both the location and thermodynamic conditions of the evaporation site (e.g. Pausata et al 2011, Battisti et al 2014, Chen et al 2016, Hu et al 2019). Multi-proxy syntheses (e.g. DiNezio and Tierney 2013, DiNezio et al 2018), modern precipitation collections, and isotopically-enabled modeling experiments (e.g. LeGrande and Schmidt 2009, Chiang et al 2015, 2020, Hu et al 2019) have greatly enhanced our ability to resolve this more nuanced picture of monsoon dynamics. Indeed, simulations combining isotopic tracers and water tags have even made it possible to distinguish the individual effects of source, transport, and cycling on the South Asian and East Asian Summer Monsoons (SASM, EASM) (Tabor et al 2018, Hu et al 2019). While earlier papers emphasize the importance of upstream rainout (e.g. Liu et al 2014, Cai et al 2017), more recent analyses suggest shifts in source location dominate the EASM hydroclimate response (Hu et al 2019, Chiang et al 2020), providing a useful analogy for thinking about evaporative site changes with anthropogenic warming (see Singh et al 2016b). That said, these analyses remind us that hydroclimate variations seldom occur in isolation, especially when they are tightly linked to large-scale circulation changes and global energetics.

Some of the most coherent variability across speleothem records is caused by the wobbling of Earth's axis, or precession, and its impact on summer insolation, which modifies meridional temperature gradients, the position of the ITCZ, and the strength of subtropical highs (Hai et al 2012, Battisti et al 2014, Liu et al 2014). Both the Asian Monsoon and South American Summer Monsoon are stronger when summer insolation is high. However, millennial-scale forcings, such as meltwater pulses associated with Heinrich events and DO events, also imprint strongly on speleothem records (see also section 3.1.1, Hai et al 2012, Liu et al 2014). Since these events correspond with changes in the AMOC, they provide a useful lens through which to examine and benchmark simulations of ties between polar change and tropical climate. For example, freshwater inputs into the North Atlantic associated with Heinrich events slow the AMOC, resulting in Northern Hemisphere cooling. This increases the meridional temperature gradient—just like when summer insolation is low—causing the Inter-Tropical Convergence Zone (ITCZ) to contract southward and alter hydroclimate across the tropical belt. The opposite occurs when AMOC strengthens. This coherent tropical response supports the idea of a Global Monsoon governed by seasonal changes in atmospheric circulation on multiple timescales (Hai et al 2012, Battisti et al 2014, Trenberth et al 2006). Moreover, high-resolution records make it possible to precisely identify the timing and duration of tropical hydroclimate shifts relative to high-latitude temperature changes and meltwater pulses over multi-decadal to millennial timescales—for example, linking speleothem records from Borneo (Carolin et al 2013) and East Asia (Rhodes et al 2015) to Greenland ice core reconstructions.

Despite the importance of high-latitude change in influencing low-latitude climate, speleothems and multi-proxy studies suggest localized tropical changes can also play a significant role in regulating individual monsoon systems (e.g. Cruz et al 2009). For example, Liu and Battisti (2015) found that during low summer insolation periods (and Northern Hemisphere cooling), the southward movement of the Atlantic ITCZ is enhanced by the strengthening of the African winter monsoon, which, in turn, is induced by anomalous north African cooling. Similarly, Jian et al (2022) argued that upper ocean heat convergence in the Indo-Pacific Warm Pool accompanies surface wind changes during high summer insolation periods. The resulting rise in ocean heat content enhances northward latent heat transport and provides a coupled atmosphere-ocean link between the Asian Monsoon, ENSO, and the Indian Ocean dipole (IOD) (see section 3.2 for more on modes of climate variability).

Recent water isotope work has also revealed a key role for the mid-latitude jet in influencing the Asian Monsoon, particularly its East Asian component (Chiang et al 2015, 2020, Zhang et al 2018). Like the ITCZ, the jet responds to meridional temperature gradients, and its position influences how westerly winds interact with the Tibetan Plateau, generating stationary eddies downstream. When the temperature gradient is strong (e.g. during low summer insolation or glacial periods), the westerlies tend to remain south of the plateau, blocking low-level southerly flow, which otherwise carries highly cycled, low $\delta$ ¹⁸O moisture toward NE China in summer (see Liu et al 2014). The mean EASM moisture source change, as does the speed with which the moisture source rotates counterclockwise from the south toward the Pacific, shortening characteristic stages of the monsoon's development. Associated variations in the progression and reach of the main monsoon rainbelt are captured by the geographic diversity of isotopic signals in speleothems and modern Asian precipitation (Liu et al 2020). The jet position thus regulates the seasonality of EASM precipitation (Chiang et al 2020), providing a unique benchmark for models aiming to simulate seasonal shifts in monsoonal dynamics and even jet position.

The oxygen isotope composition of the carbonate shells of marine microfauna (principally foraminifera and also coccolithophores) that are preserved in marine sediments provide some of the most foundational records of past climate variations spanning the last ~100 million years (e.g. Zachos et al 2001, Westerhold et al 2020). These records have been critical in establishing the close coupling between global temperature, ice volume, sea level, and atmospheric greenhouse gas concentrations on glacial/interglacial timescales (Imbrie et al 1992, Lisiecki and Raymo 2005). When calcium carbonate is crystallized slowly in water under equilibrium conditions, ¹⁸O is slightly concentrated in the carbonate relative to the water. This process is temperature dependent, with the fractionation factor decreasing as temperature increases (Urey 1947). Oxygen isotopes in marine carbonates thus provide a record of past temperature and seawater $\delta$ ¹⁸O variations (the latter of which covaries with global ice volume in shells of deep sea-dwelling benthic foraminifera). This relationship has led to the establishment of $\delta$ ¹⁸O in marine carbonates as one of the most widely used tools in paleoclimatology.

The partitioning of trace elements such as Mg into carbonate minerals is also a function of temperature, and Mg/Ca ratios in calcitic foraminifera have been widely applied as a paleotemperature proxy since the mid-1990s (Nuernberg 1995). Paired Mg/Ca and $\delta$ ¹⁸O measurements in foraminifera permit the explicit reconstruction of both temperature and seawater $\delta$ ¹⁸O variations. In surface-dwelling (planktonic) foraminifera, $\delta$ ¹⁸O values are sensitive to surface water fluxes (precipitation and evaporation) and ocean mixing via their impacts on seawater $\delta$ ¹⁸O. Such paired analyses in planktonic foraminifera are thus a valuable way of assessing past hydroclimate variability and its response to forcing on centennial to orbital timescales (e.g. Lea et al 2000, Arbuszewski et al 2013, Jacobel et al 2016).

Marine sediments also contain organic matter from photosynthesizing organisms that serve as important records of past hydroclimate change. Organic hydrogen preserved in marine sediments serves as a proxy for past hydroclimate changes, as lipid $\delta$ ²H values derived from photosynthesizing organisms record variations in $\delta$ ²H of their environmental waters (since water is the primary hydrogen source of photosynthesizing organisms and their biosynthetic products). The hydrogen isotopic composition of aquatic and terrestrial lipids in marine sediments provide reconstructions of seawater $\delta$ ²H (e.g. Pahnke et al 2007) and precipitation $\delta$ ²H (Tierney and deMenocal 2013), respectively. Hydroclimate reconstructions derived from the leaf waxes of terrestrial plants have provided a variety of key insights into past hydroclimate variations, including the identification of wet-to-dry transitions in tropical Africa over the past several thousand years the intensity, onset and duration of the green Sahara (Tierney et al 2017b) and potential climate forcing of migrations out of Africa ∼200,000 years ago (Tierney et al 2017a). At higher latitudes, leaf wax reconstructions have been used to evaluate forced changes in modes of climate variability like the North American Monsoon during the Pliocene and LGM (Bhattacharya et al 2017, 2022) and polar climate variability during the Holocene (Corcoran et al 2021).

On land, lake sediment records are an important source of high resolution (typically decadal to centennial) paleo-hydroclimatic information that spans centuries to millennia. As lakes have a global distribution, from the tropics to the high latitudes, such records provide crucial terrestrial water cycle information when other proxy archives, such as trees and ice, are not available. Hydroclimate reconstructions have been derived from a range of disparate isotope proxies in lake sediments, including the $\delta$ ²H values of aquatic and terrestrial lipids (Sachse et al 2012), the $\delta$ ¹⁸O values of authigenic and biogenic carbonates (e.g. Hodell et al 1991, Leng and Marshall 2004, Zhang et al 2011, Bhattacharya et al 2015, Wyman et al 2021), the $\delta$ ¹⁸O values of the siliceous frustules of diatoms (Dodd and Sharp 2010) and the $\delta$ ¹⁸O, $\delta$ ¹⁷O, and $\delta$ ²H values of gypsum hydration waters (Evans et al 2018). The unique combination of robust age control (up to annual resolution when annual sediment laminae, or varves, are present), along with the numerous other physical, geochemical, and biological measurements that typically accompany stable isotope measurements from lake sediments, renders lake sediment isotope records especially valuable as indicators of past climate and environmental change.

Although the temperature dependence of isotope fractionation between lake water and carbonate imparts a temperature signal in lake carbonate isotope values, stable isotope measurements from lake sediments are more commonly interpreted as proxies for the $\delta$ ²H and $\delta$ ¹⁸O values of lake water (Ito 2001; Leng and Marshall 2004). In turn, these lake water isotope values reflect the balance and isotopic values of inputs into a lake system, namely precipitation, runoff, surface inflow and groundwater inflow, and outputs from the lake system, which include evaporation, runoff, surface outflow and groundwater outflow (Gonfiantini 1986, Gat 1995). If supplied with constraints on these values, isotope mass-balance hydrologic modeling can be used to improve understanding of the climatic signals archived in lake sediment isotope values (Gibson et al 2015).

Lakes without surface inflow or outflow, or closed basin lakes, simplify this mass balance, and thus are a frequent focus of paleolake studies (Talbot and Kelts 1990). In these studies, isotope records are frequently interpreted as reflecting precipitation minus lake water evaporation (Leng and Marshall 2004). One recent methodological advance of note used a lake catchment isotope mass balance model to demonstrate the predominance of precipitation in setting lake $\delta$ ¹⁸O values in the Pacific Northwest (Steinmann et al 2013). Given the extra degree of freedom provided by isotopic data, variability in precipitation isotope values are also often implicated in driving lake water isotope variability (e.g. Richey and Sachs 2016). Additionally, proxy system modeling for lake sediment isotope proxies have also been developed for the community in recent years, for both carbonate $\delta$ ¹⁸O and leaf wax $\delta$ ²H values (Jones et al 2018, Dee et al 2018b, Konecky et al 2019). Such process-based approaches permit more nuanced inquiries into the drivers of lake sediment-based $\delta$ ¹⁸O and $\delta$ ²H values.

Lake sediment isotope records are a temporal 'bridge' between longer records of orbital climate variability and shorter proxy archive records that capture seasonal to interannual timescales. The sedimentation rates of most lakes produce records that are decadally to multi-decadally resolved, but interpretations of annual variability are possible, if varves (annual laminae) are present. Recently, stable isotope records developed from lakes in 'gold' locales, sensitive to particular modes of the climate system, as well as networks of stable isotope records have been used to reconstruct the history of large-scale features and modes of the climate system. In the tropics and subtropics, salient examples include interpretations of shifts in ENSO frequency and intensity (Atwood and Sachs 2014, Yamoah et al 2016, Rodysill et al 2019) the meridional location of the ITCZ (Konecky et al 2013, Richey and Sachs 2016), Pacific Walker Circulation strength changes (Konecky et al 2013, Wyman et al 2021), as well as numerous records of past monsoon circulation variability (e.g. Zhang et al 2011, Anderson 2012, Bird et al 2011, Dixit et al 2014). In the mid-latitudes of the Northern Hemisphere, lake sediment isotope records have provided evidence of drought and pluvials resulting from past multidecadal variability in the Pacific North American (PNA) pattern (Bird et al 2017), and the North Atlantic Oscillation (NAO) (Diefendorf et al 2006, Auger et al 2019, Zhao et al 2022a). In the mid to high latitudes of the Southern Hemisphere, lake sediment isotope records have been applied to track westerly wind shifts (Moy et al 2008). In sum, lake sediment isotope records provide valuable information on major climate modes over past centuries to millennia that capture the high degree of natural variability inherent in the climate system and water cycle.

The majority of water isotope-based reconstructions on interannual to multidecadal timescales are derived from the stable isotope measurementsof ice cores and coral skeletons, with the latter providing records that cover the last several millennia in tropical locations such as the central equatorial and southwest Pacific (see review by Thompson, 2022). Many coral species precipitate aragonite skeletons, with annual banding similar to tree rings. Variations in sea surface temperature exert a first-order control on the coral aragonite${\text{ }}\delta$ ¹⁸O, but a secondary and critical control is the ambient seawater $\delta$ ¹⁸O during coral growth. The relative balance of temperature and seawater $\delta$ ¹⁸O influences on coral records varies widely across tropical ocean basins and depends on the relative magnitudes of sea surface temperature and seawater $\delta$ ¹⁸O variability, as well as their covariance, in the location of coral growth (Russon et al 2013, Thompson 2022).

Because seawater $\delta$ ¹⁸O depends on both surface freshwater fluxes and advection or mixing of oceanic water masses (Stevenson et al 2015, 2018, Conroy et al 2017, LeGrande and Schmidt 2006), coral records with stronger sensitivity to seawater δ¹⁸O create opportunities to evaluate past ocean circulation (e.g. Conroy et al 2023), much as proxies recording precipitation $\delta$ ¹⁸O provide a means to evaluate past atmospheric circulation (LeGrande and Schmidt 2006). Moreover, with seawater δ¹⁸O and salinity both sensitive to freshwater inputs—and salinity strongly correlated with large-scale changes in evaporation and precipitation (e.g. Durack et al 2012)—some coral $\delta$ ¹⁸O records are well-suited for evaluating changes in salinity and large-scale atmospheric moisture balance (Reed et al 2022, Thompson et al 2022). Given the need for improved constraints on regional patterns of both ocean temperature and hydrology, the practice of separating the temperature component of the coral isotope signal from the seawater δ¹⁸O component, by pairing the coral $\delta$ ¹⁸O with Sr/Ca temperature proxies, has become increasingly common (McCulloch et al 1994, Walter et al 2022).

Because of where they grow and their high growth rates, coral isotope records have proven particularly useful for reconstructing the past interannual behavior of tropical climate, including one of the most globally important modes of variability, ENSO (Cobb et al 2013). While evidence from long coral reconstructions suggests anthropogenic warming is indeed leaving a mark in coral networks across the western Pacific, western Atlantic, and Indian Oceans (PAGES Oceans2k, Tierney et al 2015, Thompson 2022), one especially salient finding is that ENSO variability may be increasing with anthropogenic forcing (Cobb et al 2013, Li et al 2013, Grothe et al 2020, Zhu et al 2017)—a finding that is also substantiated by East Antarctic ice core records from the past five centuries (Rahaman et al 2019). Corals from the central Pacific suggest that ENSO's variance over the last 50 years is 25% stronger than it was over the [last millennium (Grothe et al 2020). Other high-resolution coral reconstructions studies have provided evidence for multi-decadal changes in ENSO variability over the last millennium (e.g.,\ Lawman et al 2020). Millennial analyses of Indian Ocean coral $\delta$ ¹⁸O suggest that the IOD has also become more variable and that positive IOD events have become more intense in recent decades (Abram et al 2020); broadly speaking, such long records indicate an important coherence between Pacific and Indian Ocean variability.

In addition to revealing changes in ENSO intensity, coral networks have provided insight into the changing 'flavors' of El Niño events. Central Pacific El Niños—events which tend to be weaker and centered farther west—are contribute to drought conditions in south Asian monsoon regions (Kumar et al 2006), among other important distinctions from their Eastern Pacific counterparts in terms of teleconnection impacts. Analyses of corals over the past four centuries suggest that Central Pacific El Niños may have become more frequent compared to Eastern Pacific El Niños during the 20th century (Freund et al 2019). However, Central Pacific El Niños do not appear to have become more intense in the way that Eastern Pacific El Niños have (Freund et al 2019, Grothe et al 2020). Beyond changes in interannual variability, ensembles of coral δ¹⁸O records can also capture longer climate trends, such as 20th century warming and freshening trends in the tropical Pacific (Hitt et al 2022).

Finally, while providing long records on tropical hydroclimate change, coral $\delta$ ¹⁸O also provides information about ENSO's response to short-term forcings. Dee et al (2020), for example, used monthly-resolved coral records from the central Pacific to show that the ENSO response to volcanic forcings appears to be weaker than previously suggested by tree ring-based proxy synthesis products—a finding that stands in direct contrast to many model simulations.

The cellulose of tree ring wood expands paleoclimate information from tree rings, providing rich water isotope hydrology information with seasonal-to-annual resolution (McCarroll and Loader 2004, Schubert and Hope Jahren 2015). The isotopic composition of the soil and xylem water, evapotranspiration during photosynthesis, and isotopic back-diffusion between the leaf/needle and xylem water all contribute to the final measured $\delta$ ¹⁸O signal. Thus, tree ring cellulose oxygen isotopes may commingle variations in precipitation, evaporation, and soil and groundwater processes (Roden and Ehleringer 2000, Roden et al 2000, 2002, Anderson et al 2002, Barbour et al 2002, Evans 2007)

Wood cellulose reconstructions have enabled high-resolution paleoclimate reconstructions in tropical latitudes, where other proxy data at annual resolution are generally sparse. Reconstructions using tree ring width, for example, rely on strong seasonality, which is often absent at low latitudes. By contrast, the oxygen isotope composition in wood cellulose houses information about changes in the $\delta$ ¹⁸O of precipitation, capturing changes in wet- and dry-season rainfall and moisture supply. The $\delta$ ¹⁸O of wood cellulose has been used to explore hydroclimate and ENSO variability (Boysen et al 2014) in the tropical Americas (Anchukaitis et al 2008, Anchukaitis and Evans 2010, Rodriguez‐Caton et al 2022) and in Southeast Asia (Chenxi et al 2011, Zhu et al 2012). Cellulose reconstructions have also been used to capture high-resolution changes in the Asian Summer Monsoon system (Xu et al 2018). These records broadly indicate that stable oxygen isotopes in cellulose are dominated by interannual hydroclimate variability rather than by large global warming trends, and the seasonal resolution of these records accurately captures both wet and dry periods associated with extreme ENSO events and record monsoon years, often tied to both human and ecological perturbations (e.g. Anchukaitis and Evans 2010). Long reconstructions from multiple oxygen isotope chronologies offer evidence for a post-industrial weakening of the Indian Summer Monsoon (Xu et al 2018). Given recent advances in extraction methodology (Andreu-Hayles et al 2019, Switsur and Waterhouse n.d.2020), oxygen isotopes in tree wood cellulose have proven to be powerful tools for capturing past hydroclimate extremes (e.g. drought, pluvials) on seasonal-to-interannual timescales (Zhao et al 2022b, Szejner et al 2021, Nagavciuc et al 2022).

Ice cores with annual and sub-annual resolution have provided critical context for rates and amplitudes of anthropogenic climate change at high latitudes and polar amplification (as summarized in Jones et al 2001, Thompson et al 2003, Abram et al 2013, and many others). For example, recently, Hörhold et al (2023), used water isotopes in ice cores to demonstrate that current temperatures in central Greenland exceed those of any other time in the last millennium.

High resolution ice core data also help elucidate the dynamical conditions that enhance the strength of tropical-polar teleconnections associated with modes of climate variability like ENSO (Patterson et al 2005, Schneider et al 2012, Crockart et al 2021). Indeed, many West Antarctic isotope records are broadly correlated with tropical SSTs on multi-year timescales (Schneider and Steig 2008, Okumura et al 2012). During the positive phase of ENSO, mid-latitude eddies, including the polar jet stream, tend to weaken (Xichen et al 2021). This reduces vertical mixing and advection of evaporated water from the mid-latitudes and creates stronger moisture dependencies between the tropics and the poles (Noone 2008). Moreover, as tropical deep convection shifts eastward, Rossby wave trains develop, whose downstream high pressure anomalies alter warm air advection near the West Antarctic coast (Okumura et al 2012, Jones et al 2018, Xichen et al 2021). Specifically, it is the zonal position of convection that appears to influence the efficacy with which the waves permeate mid-latitude wind barriers and propagate poleward. These teleconnections highlight the correlations between high polar isotope ratios, high polar temperatures, and weakened global circulation during El Niño events.

Other modes of variability regulate the strength of these correlations. For instance, West Antarctic ice records from the first half of the 20th century show an anomalously large positive shift in isotope ratios associated with the 1939–1942 El Niño, which occurred during a period in which the Southern Oscillation Index (SOI) was in phase with the Southern Annular Mode, or SAM (Schneider and Steig 2008)—the dominant mode of high-latitude circulation variability in the Southern Hemisphere. (Gregory and Noone 2008) argued that even though SOI and SAM have opposing effects on West Antarctic isotope ratios, the two patterns of variability must be positively correlated in order for ENSO teleconnections to reach the poles effectively. Other studies have demonstrated qualitatively similar teleconnection sensitivities to SAM in other Antarctic locations (Fogt and Bromwich 2006, Kino et al 2021). Ice core isotopic records thus provide two important opportunities. First, they reinforce the idea that models must accurately simulate tropical climate and its teleconnections in order to predict Antarctic temperature and sea ice changes (Steig et al 2013, Holland et al 2019). Second, they provide an opportunity to test our understanding of how the strength and position of tropical SSTs influence mid- and high-latitude circulation anomalies and the ability of Rossby wave trains to reach higher latitudes.

Low-latitude ice core isotopic records, such as from the Quelccaya ice cap in Peru(well-known for its strong 19th–20th century anthropogenic trend, Thompson 2000, Thompson et al 2003) also record higher $\delta$ ¹⁸O values during El Niños (Thompson and Mosley-Thompson 2013, Thompson et al 2017). Multi-year snow sampling and modeling efforts have confirmed that these signals are the result of changes in the South American Monsoon, which reduce precipitation over the Amazon and allow a greater proportion of isotopically heavy water to reach the western Amazon and high Andes (Hurley et al 2019b).

As modern water isotope observations—including precipitation samples and remote-sensing retrievals—grow in number, they are becoming increasingly used to evaluate modes of climate variability and their teleconnections. Through the GNIP, precipitation collections extend back to the 1960s in some locations, and remotely sensed H²HO-H2O pairs are available from the early 2000s onward (see Section 2.2.2). These records provide both higher temporal resolution and greater spatial coverage than proxy networks. They also extend our ability to investigate teleconnection patterns to many mid- and high-latitude locations. Examples include the PNA pattern (Birks and Edwards 2009, Liu et al 2011, 2013, Liu et al 2014); the NAO (Sodemann et al 2008), ENSO and the IOD (Sutanto et al 2015, Dee et al 2018a, Moerman et al 2014, Moerman et al 2013).

In addition to capturing interannual variations in tropical Pacific dynamics associated with ENSO, water isotopic information has been used to study variations in the strength of the Walker Circulation on decadal timescales (Dee et al 2018a, Falster et al 2021). Indeed, (Dee et al 2018a) used simulations to show that as the Walker Circulation slows, $\delta$ ²H in the mid-troposphere decreases in the west Pacific (the region of large-scale upward motion) and increases in the east Pacific (the region of large-scale subsidence), capturing the changing vertical mass flux. Falster et al 2021 subsequently showed that the changing water vapor signal leaves an imprint on local precipitation isotope ratios, which can be detected across the Pacific basin.