About

Sara Sanchez is an Assistant Professor in the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder. Her research focuses on climate dynamics, paleoclimate, tropical climate variability, and isotope geochemistry. She is involved in advancing understanding of atmospheric and oceanic processes through her scientific investigations.

Research topics

• Climatology
• Geology
• Environmental science
• Meteorology
• Mathematics
• Statistics
• Oceanography
• Geography
• Machine Learning
• Computer Science

Selected publications

• The PAGES CoralHydro2k Seawater <i>δ</i> ¹⁸ O Database: a FAIR-aligned compilation of seawater <i>δ</i> ¹⁸ O data to uncover “hidden” insights from the global ocean

  Earth system science data · 2026-03-12 · 1 citations

  articleOpen access

  Abstract. The stable isotope values of seawater (δ18O and δ2H) provide valuable information on the exchange of water between the ocean, atmosphere, and cryosphere and on ocean mixing processes. As such, observational seawater δ18O and δ2H data place powerful constraints on hydrologic changes in the modern ocean. Seawater δ18O data are also essential for calibrating paleoclimate proxies based on the δ18O of marine carbonates and are an increasingly critical diagnostic tool for assessing model performance and skill in isotope-enabled climate models. Despite their broad value, no centralized and actively-curated database for this type of data exists, even though a growing number of new seawater δ18O datasets have been generated over the last decade. As such, many seawater δ18O datasets remain “hidden”. To improve the accessibility of seawater δ18O data for the Earth Science research community, the Past Global Changes (PAGES) CoralHydro2k project has created a new, machine-readable, and metadata-rich database of observational seawater δ18O data, paired with seawater δ2H and salinity data, that is compliant with findability, accessibility, interoperability, and reusability (FAIR) standards for digital assets. The data has been collected from public databases and repositories, direct researcher data submissions, scientific papers, and student theses. In total, the PAGES CoralHydro2k Seawater δ18O Database contains over 18 600 data points with extensive metadata that makes the database suitable for a myriad of research applications. For hidden data, we searched for and included all datasets within the global ocean. For public data, our data collation efforts were focused on the upper 50 m from 35° N to 35° S (to aid in CoralHydro2k's seawater δ18O reconstruction studies using δ18O and Sr/Ca in tropical-subtropical coral skeletons). We also provide a set of best practices to the community for reporting seawater isotope data in the future. The database is available on the NOAA NCEI World Data Service for Paleoclimatology landing page: https://www.ncei.noaa.gov/access/paleo-search/study/34575 (last access: 11 February 2026; https://doi.org/10.25921/ap7d-2k16, Atwood et al., 2026). A Seawater Oxygen Isotopes Community was also developed within the EarthChem Library (https://www.earthchem.org/communities/seawater-oxygen-isotopes/, last access: 20 February 2026) to help researchers submit new datasets and obtain a dataset DOI. This template is aligned with the CoralHydro2k Seawater δ18O Database to facilitate future updates to the database.

  PublisherOA PDFDOI

• Paleo data assimilation of coral δ18O Part 2: 20th century trends and variability of the tropical Pacific

  2025-12-18

  article1st authorCorresponding

  Limited instrumental observations have created uncertainties in the rates and patterns of historical variations in tropical sea-surface temperature, precipitation, and salinity. Coral archives are well suited to monitor past variations in the tropical oceans; in particular, the oxygen isotope composition (δ18O) has the unique potential to provide untapped insight into historic changes in the tropical hydrological cycle. We build upon Part 1 to emphasize the challenges and opportunities of using coral δ18O with the Last Millennium Ensemble paleoclimate data assimilation (paleo-DA) framework. We evaluate the sensitivity of our reconstructions to specific choices of paleo-DA components, such as climate model priors, coral δ18O proxy system models, and salinity calibration products, and reflect on the combination of methodological choices that are required to best incorporate coral δ18O with a paleo-DA approach. We then evaluate trends and variability across the global tropics, finding evidence for strengthening Walker Circulation and an intensification of the hydrological cycle. However, diagnosing the relative rates of warming across the equatorial Pacific is highly challenging, likely due to the high covariability of sea-surface temperature and salinity anomalies across the global tropics. We also find that the hydrological cycle response to ENSO has intensified and marginally shifted eastward over the 19th and 20th centuries. This work emphasizes a need for greater monitoring and improved simulations of δ18Osw and salinity, and illustrates the potential for using coral δ18O to evaluate past hydroclimate variability with paleo-DA.

  PublisherDOI

• The PAGES CoralHydro2k Seawater δ ¹⁸ O Database: A FAIR-aligned compilation of seawater δ ¹⁸ O data to uncover 'hidden' insights from the global ocean

  2025-08-27

  articleOpen access

  Abstract. The stable isotope values of seawater (δ18O and δ2H) provide valuable information on the exchange of water between the ocean, atmosphere, and cryosphere and on ocean mixing processes. As such, observational seawater δ18O and δ2H data place powerful constraints on hydrologic changes in the modern ocean. Seawater δ18O data are also essential for calibrating paleoclimate proxies based on the δ18O of marine carbonates and are an increasingly critical diagnostic tool for assessing model performance and skill in isotope-enabled global climate models. Despite their broad value, no centralized and actively-curated database for this type of data exists, even though a growing number of new seawater δ18O datasets have been generated over the last decade. As such, many seawater δ18O datasets remain ‘hidden’. To improve the accessibility of seawater δ18O data for the Earth Science research community, the Past Global Changes (PAGES) CoralHydro2k project has created a new, machine-readable, and metadata-rich database of observational seawater δ18O data, paired with seawater δ2H and salinity data, that is compliant with findability, accessibility, interoperability, and reusability (FAIR) standards for digital assets. The data has been collected from public databases and repositories, direct researcher data submissions, scientific papers, and student theses. In total, the PAGES CoralHydro2k Seawater δ18O Database contains over 18,600 data points with extensive metadata that makes the database suitable for a myriad of research applications. For hidden data, we searched for and included all datasets within the global ocean. For public data, our data collation efforts were focused on the upper 50 m from 35° N to 35° S (to aid in CoralHydro2k’s seawater δ18O reconstruction studies using δ18O and Sr/Ca in tropical-subtropical coral skeletons). We also provide a set of best practices to the community for reporting seawater isotope data in the future.

  PublisherOA PDFDOI

• A Critical Role for North Pacific Meridional Mode in the ENSO Response to Orbital Precession

  Journal of Climate · 2025-09-09 · 1 citations

  articleSenior author

  Abstract Orbital precession modifies the intensity of the annual cycle at millennial time scales and is a major external driver of El Niño–Southern Oscillation (ENSO) variability in both proxy records and climate model simulations. We examine precession’s influence on ENSO through a subtropical pathway, the Pacific meridional mode (PMM), using a suite of NCAR Community Earth System Model, version 1.2 (CESM1.2), experiments that simulate five precessional extremes: perihelion at autumnal equinox (AE), winter solstice (WS), vernal equinox (VE), summer solstice (SS), and zero eccentricity (E0). We investigate mechanisms that may moderate the PMM’s influence on ENSO such as the strength of midlatitude stochastic forcing via the North Pacific Oscillation, changes in the climatological mean state, and the wind–evaporation–sea surface temperature (SST) (WES) feedback. We find that orbital precession strongly influences PMM variability, the PMM’s ability to trigger El Niño events, and ENSO diversity. Precessional extremes characterized by a more southerly intertropical convergence zone (ITCZ) and stronger trade winds (WS and AE) have more variable PMM behavior and a PMM that is more effective at triggering El Niño events, particularly central Pacific events. Precessional extremes characterized by a more northerly ITCZ and weaker trade winds (SS and VE) have reduced PMM variability and a PMM that is a less-reliable precursor to El Niño events. The PMM response to precession is driven by variations in surface wind fields that moderate the strength of WES feedback, the mechanism by which PMM anomalies grow and propagate. Understanding the sensitivity of ENSO to subtle shifts in the mean state contextualizes past variability and aids in anticipating future change. Significance Statement Orbital precession alters the seasonal distribution of solar insolation around Earth and has vast impacts on the climate system including El Niño–Southern Oscillation (ENSO). Here, we examine how precession influences ENSO through a subtropical ENSO precursor: the Pacific meridional mode (PMM). We use climate models to test how the influence of warm PMM events on El Niño events may vary under extreme states of orbital precession. We find precession moderates the variability of the PMM, the ability of the PMM to trigger El Niño events, and the spatial diversity of El Niño events. These results help us to interpret past climatic changes and understand the sensitivity of the tropical Pacific to small variations in external forcing.

  PublisherDOI

• Paleo data assimilation of coral δ18O Part 1: Best practices and uncertainties

  2025-12-18

  article1st authorCorresponding

  Uncertainties in early 20th century observations limit understanding of the full range of internal climate variability and muddle understanding of climate system response to external forcing. Coral archives can provide high-resolution information about historic sea-surface temperatures (SSTs) and hydroclimatic change over the tropical oceans. Paleoclimate data assimilation (paleo-DA) can extract information from these valuable records and generate it to gridded, full-field reconstructions. However, reasonable choices in the paleo-DA methodology can influence the reconstructed climate variability. Here, we outline the challenges and opportunities for using the oxygen isotope composition of corals (δ18O) to reconstruct temperature and hydrological cycle variability with paleo-DA using the Last Millennium Reanalysis offline framework. Using a fixed proxy network of coral δ18O and Sr/Ca that spans the 20th century, we evaluate the sensitivity of reconstructions to the choice of coral δ18O proxy system models (PSMs), the choice of instrumental sea surface salinity (SSS) products used with the PSMs, and the choice of a climate model priors, ultimately testing 28 distinct reconstruction approaches. The choice of coral δ18O PSM and climate model prior can generate inconsistencies in the reconstructions. This influence is likely due to sparse observations of δ18Osw and SSS limiting “training” data, the nonstationary relationship between SSS and δ18Osw, and the strong covariability of SST and SSS across the global tropics. Given these limitations, we advise the use of a high resolution climate model prior, but the “best” configuration of PSM, SSS product and prior is dependent on the question and region of interest.

  PublisherDOI

• Decadal oscillations in the ocean’s largest oxygen-deficient zone

  Science · 2024-11-28 · 10 citations

  article

  The impact of global warming on the ocean’s oxygen-deficient zones (ODZs) is uncertain, partly because of a lack of data on past changes. We report monthly resolved records of coral skeleton–bound nitrogen isotopes (CS-δ 15 N) to reconstruct denitrification in the Eastern Tropical North Pacific (ETNP) ODZ over the last 80 years. The data indicate strong decadal variation in ETNP denitrification, with maxima during the cool North Pacific phase of Pacific Decadal Variability. The maxima in denitrification (and thus oxygen deficiency) were likely due to stronger upwelling that enhanced productivity leading to greater oxygen demand in the thermocline. Prior findings of multidecadal-to-centennial ODZ trends were likely biased by this variability. ODZ evolution over the next century will depend on how global warming interacts with the decadal oscillations.

  PublisherDOI

• Distilling the Evolving Contributions of Anthropogenic Aerosols and Greenhouse Gases to Large‐Scale Low‐Frequency Surface Ocean Changes Over the Past Century

  Geophysical Research Letters · 2024-11-22 · 5 citations

  articleOpen accessSenior author

  Anthropogenic aerosols (AER) and greenhouse gases (GHG)-the leading drivers of the forced historical change-produce different large-scale climate response patterns, with correlations trending from negative to positive over the past century. To understand what caused the time-evolving comparison between GHG and AER response patterns, we apply a low-frequency component analysis to historical surface ocean changes from CESM1 single-forcing large-ensemble simulations. While GHG response is characterized by its first leading mode, AER response consists of two distinct modes. The first one, featuring long-term global AER increase and global cooling, opposes GHG response patterns up to the mid-twentieth century. The second one, featuring multidecadal variations in AER distributions and interhemispheric asymmetric surface ocean changes, appears to reinforce the GHG warming effect over recent decades. AER thus can have both competing and synergistic effects with GHG as their emissions change temporally and spatially.

  PublisherOA PDFDOI

• Uncovering 'Hidden' Insights from the Ocean in the PAGES CoralHydro2k Seawater &amp;#948;18O Database&amp;#160;

  2024-03-08

  preprintOpen accessCorresponding

  The oxygen isotope ratio (&amp;#948;18O) of seawater is a powerful tracer of the global water cycle, providing valuable information on the exchange of water between the ocean, atmosphere, and cryosphere as well as on ocean mixing processes. As such, observational seawater &amp;#948;18O data place powerful constraints on hydrologic changes in the modern ocean, are essential for calibrating paleoclimate proxies based on the &amp;#948;18O of marine carbonates, and are an increasingly critical diagnostic tool for assessing model performance and skill in isotope-enabled global climate models. In recognition of the broad value of seawater &amp;#948;18O data to the Earth science community and the growing number of new seawater &amp;#948;18O data sets that have been generated over the last decade, we launched the PAGES CoralHydro2k Seawater &amp;#948;18O Database Project in 2020 to recover &amp;#8216;hidden&amp;#8217; seawater oxygen isotope data sets. We have collated these records and combined them with public data to create a new, machine-readable, and metadata-rich database that aligns with findability, accessibility, interoperability, and reusability (FAIR) standards for digital assets.Here, we present a summary of our crowdsourcing efforts and description of the database to date, and report initial findings from the new database. The database consists of over 19,000 observations of seawater &amp;#948;18O with more than 50 metadata fields. We compare seawater &amp;#948;18O variability from the database to that simulated by a suite of isotope-enabled climate models and to seawater &amp;#948;18O reconstructions derived from coral records and find substantial differences at annual to decadal timescales across different data sets. Lastly, we discuss the potential for future investments in water isotope observation networks to tackle 21st century science questions related to ocean changes in the past, present, and future.

  PublisherDOI

• A Critical Role for Meridional Modes in Determining the Equatorial Pacific Response to Orbital Precession

  2024-03-08

  preprintOpen accessSenior authorCorresponding

  Understanding external drivers of the El-Nino Southern Oscillation (ENSO) is essential for predicting its future evolution. Orbital precession has been identified as a driver of ENSO variability through both proxy records and climate model simulations, yet the exact mechanics remain unclear. This orbital cycle moderates the seasonal timing of insolation relative to Earth's revolution around the Sun, thereby adjusting the magnitude of the seasonal cycle experienced by each hemisphere. Here, we analyze output from a suite of simulations in NCAR CESM 2.1.1 designed to analyze ENSO under different precessional extremes that significantly modify the meridional temperature gradients and the cold tongue seasonal cycle in the Pacific ocean. Variations in orbital precession have a strong impact on the magnitude, periodicity, and spatial expression of tropical Pacific variability. We find a critical role for both the North and South Pacific Meridional Modes (NPMM and SPMM) in explaining changes in ENSO and decadal variability by propagating subtropical anomalies to the equatorial Pacific along with a shift in the meridional structure of equatorial winds. As an example, when the perihelion of orbit occurs during boreal winter creating a dampened (strengthened) seasonal cycle in the Northern (Southern) Hemisphere, the SPMM becomes significantly more active while the NPMM weakens. This precessional state experiences a shift toward amplified decadal variability and a greater prevalence of Eastern El Nino events in comparison with the other orbital configurations tested. Understanding the precessional control of tropical variability via subtropical pathways may help explain developments that have occurred in the past, as well as future changes which may be observed due to shifts in meridional temperature gradients.

  PublisherDOI

• Distilling the evolving contributions of anthropogenic aerosols and greenhouse gases to historical low-frequency surface ocean changes

  2024-07-17

  preprintSenior author

  Anthropogenic aerosols (AER) and greenhouse gases (GHG) – the leading drivers of the forced historical change – produce different large-scale climate response patterns, with varying trend pattern correlations from negative to positive over the past century. To understand what caused the time-evolving comparison between GHG and AER responses, we apply a joint low-frequency component analysis on global sea-surface temperature and sea-surface salinity response over 1921-2020 from CESM1 single-forcing large ensemble simulations. While GHG response is well-described by its first leading mode, AER response consists of two distinct modes. The first one features global AER increase and global cooling, opposite to GHG-induced warming. The second mode features multidecadal variations in AER distributions, where the recent shift from North America/western Europe to southeast Asia emissions drives regional changes enhancing the GHG effect. We argue that AER can have both competing and synergistic effects with GHG, as their emissions change temporally and spatially.

  PublisherDOI

Frequent coauthors

• Émilie Pauline Dassié

  Centre National de la Recherche Scientifique

  9 shared

• Antonietta Capotondi

  Cooperative Institute for Research in Environmental Sciences

  9 shared

• Sebastian Milinski

  8 shared

• K. M. Cobb

  Providence College

  8 shared

• C.D. Charles

  Scripps Institution of Oceanography

  7 shared

• Nicola Maher

  7 shared

• Malte F. Stuecker

  University of Hawaii System

  7 shared

• Jessica A. Hargreaves

  University of Bremen

  6 shared