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In early 2020, an international team set out to investigate trade wind cumulus and their coupling to the large-scale circulation through the field campaign EUREC4A: ElUcidating the RolE of Clouds‐Circulation Coupling in ClimAte. Focused on the western tropical Atlantic near Barbados, EUREC4A deployed a number of innovative measurement strategies, including a large network of water isotopic collections, to study the tropical shallow convective environment. The goal of the isotopic measurements was to elucidate processes that regulate the hydroclimate state – for example, by identifying moisture sources, quantifying mixing between atmospheric layers, characterizing the microphysics that influence the formation and persistence of clouds and precipitation, and providing an extra constraint in the evaluation of numerical simulations. During EUREC4A, researchers deployed seven water vapor isotopic analyzers on two aircraft, on three ships, and at the Barbados Cloud Observatory (BCO). 

In tropical paleoclimate studies, paleo‐precipitation is often reconstructed from proxies via the “amount effect,” that is, the empirical inverse relationship between local precipitation amount (P) and the oxygen isotopic composition of precipitation (δ¹⁸O_P). However, recent research has illustrated numerous microphysical and dynamical controls on δ¹⁸O_Pthat do not necessarily covary with P, complicating the reconstruction of circulation features like the Intertropical Convergence Zone. Here we introduce a new conceptual and statistical model for δ¹⁸O_Pthat better captures the physical foundations for δ¹⁸O_Pas a tracer of hydrological balance. We find that bulk precipitation microphysics and cloud type exert comparable influences on δ¹⁸O_P. Moisture transport plays an important secondary role in regions of deep atmospheric convection such as the Intertropical Convergence Zone and Indo‐Pacific Warm Pool. Our findings help reconcile conflicting interpretations of Intertropical Convergence Zone excursions, and provide a firm physical grounding for more nuanced, accurate interpretations of past hydroclimate using water isotope proxies.

 

Explosive volcanic eruptions are one of the largest natural climate perturbations, but few observational constraints exist on either the climate responses to eruptions or the properties (size, hemispheric aerosol distribution, etc.) of the eruptions themselves. Paleoclimate records are thus important sources of information on past eruptions, often through the measurement of oxygen isotopic ratios (δ¹⁸O) in natural archives. However, since many processes affectδ¹⁸O, the dynamical interpretation of these records can be quite complex. Here we present results from new, isotope‐enabled members of the Community Earth System Model Last Millennium Ensemble, documenting eruption‐inducedδ¹⁸O variations throughout the climate system. Eruptions create significant perturbations in theδ¹⁸O of precipitation and soil moisture in central/eastern North America, via excitation of the Atlantic Multidecadal Oscillation. Monsoon Asia and Australia also exhibit strong precipitation and soilδ¹⁸O anomalies; in these cases,δ¹⁸O may reflect changes to El Niño‐Southern Oscillation phase following eruptions. Salinity and seawaterδ¹⁸O patterns demonstrate the importance of both local hydrologic shifts and the phasing of the El Niño‐Southern Oscillation response, both along the equator and in the subtropics. In all cases, the responses are highly sensitive to eruption latitude, which points to the utility of isotopic records in constraining aerosol distribution patterns associated with past eruptions. This is most effective using precipitationδ¹⁸O; all Southern eruptions and the majority (66%) of Northern eruptions can be correctly identified. This work thus serves as a starting point for new, quantitative uses of isotopic records for understanding volcanic impacts on climate.

 

Because of the pervasive role of water in the Earth system, the relative abundances of stable isotopologues of water are valuable for understanding atmospheric, oceanic, and biospheric processes, and for interpreting paleoclimate proxy reconstructions. Isotopologues are transported by both large‐scale and turbulent flows, and the ratio of heavy to light isotopologues changes due to fractionation that can accompany condensation and evaporation processes. Correctly predicting the isotopic distributions requires resolving the relationships between large‐scale ocean and atmospheric circulation and smaller‐scale hydrological processes, which can be accomplished within a coupled climate modeling framework. Here we present the water isotope‐enabled version of the Community Earth System Model version 1 (iCESM1), which simulates global variations in water isotopic ratios in the atmosphere, land, ocean, and sea ice. In a transient Last Millennium simulation covering the 850–2005 period, iCESM1 correctly captures the late‐twentieth‐century structure of δ¹⁸O and δD over the global oceans, with more limited accuracy over land. The relationship between salinity and seawater δ¹⁸O is also well represented over the observational period, including interbasin variations. We illustrate the utility of coupled, isotope‐enabled simulations using both Last Millennium simulations and freshwater hosing experiments with iCESM1. Closing the isotopic mass balance between all components of the coupled model provides new confidence in the underlying depiction of the water cycle in CESM, while also highlighting areas where the underlying hydrologic balance can be improved. The iCESM1 is poised to be a vital community resource for ongoing model development with both modern and paleoclimate applications.