Zhengyu Liu

• Ph.D
• Professor (Full) at The Ohio State University

Resolving discrepancies in long-term Holocene temperature trends between climate models and proxy records is essential to improve future climate projections. However, uncertainties in paleoclimate reconstructions limit their ability to constrain models. This study compares ice core-derived oxygen isotope records with isotope-enabled climate simulat...

Generally, the interaction between the East Asian subtropical westerly jet (EASWJ) and the East Asian summer monsoon (EASM) is regarded as a critical dynamic factor in the evolution of precipitation patterns in China. Using simulation results from the transient climate evolution since the last glacial maximum, this study applies the multivariate em...

Abundant proxy records suggest a profound reorganization of the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM, ~21,000 y ago), with the North Atlantic Deep Water (NADW) shoaling significantly relative to the present-day (PD) and forming Glacial North Atlantic Intermediate Water (GNAIW). However, almost all...

We use a recent compilation of global mean sea surface temperature changes (ΔGMSST) over the last 4.5 Myr together with independent proxy-based reconstructions of bottom water or deep ocean temperatures to infer changes in mean ocean temperature (ΔMOT). We find that the ratio of ΔMOT/ΔGMSST, which is also a measure of ocean heat storage efficiency,...

Proxy reconstructions suggest that increasing global mean sea surface temperature (GMSST) during the last deglaciation was accompanied by a comparable or greater increase in global mean ocean temperature (GMOT), corresponding to a large heat storage efficiency (HSE; ∆GMOT/∆GMSST). An increased GMOT is commonly attributed to surface warming at sites...

Upwelling-induced productivity in the Arabian Sea infers an out-of-phase relation between the Indian summer monsoon and Northern Hemisphere summer insolation at the precession band but an in-phase relation at the obliquity band. These records contrast speleothem oxygen records showing an in-phase relation between the monsoon and summer insolation i...

The interaction between sea surface temperatures (SST) and surface heat flux (SHF) is vital for atmospheric and oceanic variabilities. This study investigates SST‐SHF relationship in the framework of a coupled oscillatory model, extending beyond previous research that predominantly used AR‐1 type simple stochastic climate models. In contrast to the...

El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual climate variability in the tropical Pacific, whose nature nevertheless may change significantly in a warming climate. Here, we show that the predictability of ENSO may decrease in the future. Across the models in the Coupled Model Intercomparison Project Phase 6 (CMIP6), we fin...

Sea surface temperature (SST) variability on decadal timescales has been associated with global and regional climate variability and impacts. The mechanisms that drive decadal SST variability, however, remain highly uncertain. Many previous studies have examined the role of atmospheric variability in driving decadal SST variations. Here we assess t...

The spreading pathway of the North Atlantic Deep Water, which is the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), determines how climate change signals are transported throughout the global ocean. The North Atlantic Deep Water is suggested to be transported from the subpolar Atlantic to the subtropics in the western basin b...

For Paleoclimate data assimilation (PDA), a hybrid gain analog offline ensemble Kalman filter (HGAOEnKF) is proposed. It keeps the benefits of the analog offline ensemble Kalman filter (AOEnKF) that constructs analog ensembles from existing climate simulations with joint information of the proxies. The analog ensembles can provide more accurate pri...

The quaternary climate is characterised by glacial-interglacial cycles, with the most recent transition from the last glacial maximum to the present interglacial (the last deglaciation) occurring between ~ 21 and 9 ka. While the deglacial warming at southern high latitudes is mostly in phase with atmospheric CO2 concentrations, some proxy records h...

Previous studies have indicated that the extratropics can influence ENSO via specific processes. However, it is still unclear to what extent ENSO is influenced by the extratropics in observation. Now we assess this issue by applying the regional data assimilation (RDA) approach in an advanced model, the GFDL CM2.1. Our study confirms a strong extra...

A reliable projection of future risk of decadal megadrought is crucial to adaption and mitigation over eastern China in future climate changes. However, it’s difficult to forecast the time of megadrought, which is dominated by internal variability of the model. Using a 50-member ensemble of simulations from the Community Earth System Model Version...

The Atlantic meridional overturning circulation (AMOC) is predicted to weaken under global warming. Whether it is caused by heat flux or freshwater flux is under debate. Here we separate these two processes in changing the AMOC under global warming. The simulated AMOC is weakened during the first 600 years and then gradually recovered to its initia...

Very high tropical alpine ice cores provide a distinct paleoclimate record for climate changes in the middle and upper troposphere. However, the climatic interpretation of a key proxy, the stable water oxygen isotopic ratio in ice cores (δ ¹⁸ O ice ), remains an outstanding problem. Here, combining proxy records with climate models, modern satellit...

Coupled data assimilation (CDA), which combines coupled models and observations from multiple Earth system domains, plays a critical role in climate studies by producing a four-dimensional estimation of Earth system states. Traditional ensemble Kalman filter (EnKF) CDA algorithms, while convenient to implement in multiple DA components in a coupled...

Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation and precipitation δ ¹⁸ O ( δ ¹⁸ O p ) response have remained controversial, and their response mechanisms are unclear. In particular, it is not clear whether the regional pre...

The role of ocean forcing on Atlantic multidecadal variability (AMV) is assessed from the (downward) heat flux–SST relation in the framework of a new stochastic climate theory forced by red noise ocean forcing. Previous studies suggested that atmospheric forcing drives SST variability from monthly to interannual time scales, with a positive heat fl...

The long-term response of the Atlantic meridional overturning circulation (AMOC) to anthropogenic forcing has been difficult to detect from the short direct measurements available due to strong interdecadal variability. Here, we present observational and modeling evidence for a likely accelerated weakening of the AMOC since the 1980s under the comb...

Understanding the hydroclimate representations of precipitation δ ¹⁸ O ( δ ¹⁸ O p ) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), ch...

The relationship between the East Asian summer monsoon (EASM) and East Asian winter monsoon (EAWM) across time scales has been an interesting topic for decades. In this study, we quantitatively investigate the EASM–EAWM relationship at the obliquity time scales using a set of accelerated transient simulations. By comparing different indices defined...

The double-Intertropical Convergence Zone (ITCZ) bias has been an outstanding problem among climate models for two decades. However, it remains unclear how much of this tropical bias is attributed to the extratropics and tropics itself, respectively. Applying Regional Data Assimilation (RDA) method, we follow up a previous study in a more advanced...

Abrupt changes in the Atlantic meridional overturning circulation (AMOC) are thought to affect tropical hydroclimate through adjustment of the latitudinal position of the intertropical convergence zone (ITCZ). Heinrich Stadial 1 (HS1) involves the largest AMOC reduction in recent geological time; however, over the tropical Indian Ocean (IO), proxy...

Heinrich Stadial 1 (HS1) was the major climate event at the onset of the last deglaciation associated with rapid cooling in Greenland and lagged, slow warming in Antarctica. Although it is widely believed that temperature signals were triggered in the Northern Hemisphere and propagated southward associated with the Atlantic meridional overturning c...

A reliable projection of future risk of decadal megadrought event is crucial for adaption and mitigation over eastern China to future climate changes. However, large uncertainties are induced by emission scenarios, model structures, and internal variability in current model simulation. Using a 50-member ensemble of simulations from the Community Ea...

Most recent studies on Meiyu over the middle and lower reaches of the Yangtze River (MLRYR) have focused on its interannual variability or the mechanism of certain abnormal events. The influence and physical mechanism of solar radiation intensity on the interdecadal frequency of strong Meiyu events over the MLRYR during historical periods were inve...

During the last deglaciation Earth’s climate experienced strong and abrupt variations, resulting in major changes in global temperature, sea level, and ocean circulation. Although proxy records have significantly improved our understanding of climate during this period, questions remain regarding the connection between ocean circulation evolution a...

The primary sources of predictability of the Earth climate system come from initial conditions and external radiative forcings, which is a joint initial-boundary-value problem. It is crucial to clearly understand the contributions of initial conditions and external forcings (i.e. initial-value and boundary-value) to the total predictability, for wh...

In this paper, we investigate the relationship between El Niño-Southern Oscillation (ENSO) spring persistence barrier (PB) and predictability barrier (PD) and apply it to explain the interdecadal modulation of ENSO prediction skill in anomaly correlation coefficient (ACC). Previous studies showed that a longer persistence (i.e., autocorrelation) te...

A reversal of zonal sea surface temperature (SST) gradient in the equatorial Atlantic is a common bias in climate models. Studies to investigate the origin of this bias mainly focused on the tropics itself. Applying the regional data assimilation method in the GFDL CM2.1 model, we investigate the combined and respective influences of the northern a...

Plain Language Summary Orbital‐induced insolation variations play an important role in driving monsoon changes at geological time scales. One key issue that has remained outstanding is whether the monsoon is driven by local insolation from the corresponding hemisphere or remote insolation from the other hemisphere. For the first time, this work sys...

Plain Language Summary The Afro‐Asian summer monsoon response to solar insolation has been studied extensively both in observation evidences and model simulations. However, whether the Afro‐Asian summer monsoon is driven by local insolation from the Northern Hemisphere (NH) and remote insolation from the Southern Hemisphere (SH) still remains an op...

Plain Language Summary Dust in the atmosphere reflects and absorbs sunlight, reducing the shortwave radiation reaching the surface, while the dust deposited on snow and ice reduces the surface albedo and increases the shortwave radiation received at the surface. Our previous work (Zhang et al., 2021) showed dust reduction during the interglacial pe...

The primary sources of predictability of the Earth climate system come from initial conditions and external radiative forcings, which is a joint initial-boundary-value problem. It is crucial to clearly understand the contributions of initial conditions and external forcings (i.e. initial-value and boundary-value) to the total predictability, for wh...

At the Last Glacial Maximum (LGM), records suggest drier conditions in the northwest United States and wetter conditions in the southwest United States relative to present-day as well as widespread changes in the isotopic composition of water. However, the mechanisms responsible for these changes remain ambiguous. Here, we explore differences in we...

Plain Language Summary The Antarctic Circumpolar Current (ACC) in the Southern Ocean is important in the interbasin exchange of ocean properties through its horizontal circulation. Most of this flow is associated with seawater density gradients (the baroclinic transport). Therefore, a robust method that can be used to reconstruct the ACC baroclinic...

The deglacial hydroclimate in South China remains a long-standing topic of debate due to the lack of reliable moisture proxies and inconsistent model simulations. A recent hydroclimate proxy suggests that South China became wet in cold stadials during the last deglaciation, with the intensification proposed to be contributed mostly by the East Asia...

δ13C records from the mid-depth Atlantic show a pronounced decrease during the Heinrich Stadial 1 (HS1), a deglacial episode of dramatically weakened Atlantic Meridional Ocean Circulation (AMOC). Proposed explanations for this mid-depth decrease include a greater fraction of δ13C-depleted southern sourced water (SSW), a δ13C decrease in the North A...

The Indo-Pacific Warm Pool (IPWP) is the largest source of atmospheric heating and moisture on Earth, energizing the global moisture and energy budgets and controlling global ocean-atmosphere circulation. However, the mechanisms driving orbital-scale changes in hydroclimate and proxy records of precipitation isotopic composition remain poorly known...

In this paper, we investigate the relationship between upper ocean heat content (OHC) and El Niño-Southern Oscillation (ENSO) sea surface temperature (SST) anomalies mainly using the neutral recharge oscillator (NRO) model both analytically and numerically. Previous studies showed that spring OHC, which leads SST by 6-12 months, represents a major...

Plain Language Summary The Antarctic Intermediate Water (AAIW) is an important component of global ocean circulation, and processes involved in AAIW variability have been extensively studied, particularly for its role in ventilating CO2 from deep oceans to the atmosphere during the last deglaciation. However, there is scarce information about chang...

Antarctic paleotemperatures It has been widely thought that East Antarctica was ∼9°C cooler during the Last Glacial Maximum, close to the ∼10°C difference between then and now determined independently for West Antarctica. Buizert et al. used borehole thermometry, firn density reconstructions, and climate modeling to show that the temperature in Eas...

Abrupt climate changes during the last deglaciation have been well preserved in proxy records across the globe. However, one long-standing puzzle is the apparent absence of the onset of the Heinrich Stadial 1 (HS1) cold event around 18 ka in Greenland ice core oxygen isotope  18 O records, inconsistent with other proxies. Here, combining proxy rec...

In this paper, we investigate the potential factors that control the relationship between the El Niño-Southern Oscillation (ENSO) persistence barriers (PB) in sea surface temperature (SST) and ocean heat content (OHC) and apply it to explain observational ENSO PBs. With the addition of seasonal growth rate in SST in the neutral recharge oscillator...

In this study, we investigate a diurnal predictability barrier (DPB) for weather predictions using an idealized model and observations. This DPB is referred to a maximum drop of predictability (e.g., autocorrelation) at a particular time of the day, regardless of the initial time. Previous studies demonstrated that a strong seasonal cycle of El Niñ...

One long-standing issue in the paleoclimate records is whether East Asian Summer Monsoon peaked in the early Holocene or mid-Holocene. Here, combining a set of transient earth system model simulations with proxy records, we propose that, over northern China, monsoon rainfall peaked in the early Holocene, while soil moisture and tree cover peaked in...

Oxygen isotope speleothem records exhibit coherent variability over the pan-Asian summer monsoon (AM) region. The hydroclimatic representation of these oxygen isotope records for the AM, however, has remained poorly understood. Here, combining an isotope-enabled Earth system model in transient experiments with proxy records, we show that the widesp...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Plain Language Summary The mid‐Holocene (6,000 years ago) had slightly lower greenhouse gas concentrations and significantly higher Northern Hemisphere summer sunlight than preindustrial (1850 CE). There is also evidence for vegetation changes during the mid‐Holocene, most notably increased vegetation over the now arid Sahara. Most climate model si...

The Atlantic Meridional Overturning Circulation (AMOC) is an active component of the Earth’s climate system¹ and its response to global warming is of critical importance to society. Climate models have shown an AMOC slowdown under anthropogenic warming since the industrial revolution2–4, but this slowdown has been difficult to detect in the short o...

24 In this paper, we investigate the role of El Niño-Southern Oscillation (ENSO) 25 period in the spring persistence barrier (SPB) mainly using the neutral recharge 26 oscillator (NRO) model both analytically and numerically. It is suggested that a shorter 27 ENSO period strengthens the SPB. Moreover, in contrast to the strict phase locking of 28 t...

Reconstructing the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM) is essential for understanding glacial-interglacial climate change and the carbon cycle. However, despite many previous studies, uncertainties remain regarding the glacial water mass distributions in the Atlantic and the AMOC intensity. Here...

Stable water isotopologues in paleoclimate archives (δ18O) have been widely used as an indicator to derive past climate variations. The modern observed spatial δ18O-temperature relation in the middle and high latitudes has been used to infer the paleotemperatures changes from ice core data. However, various studies have shown that the spatial slope...

Plain Language Summary El Niño–Southern Oscillation (ENSO) spring persistence barrier (SPB) describes the feature that ENSO's prediction skills decease significantly in boreal spring. In statistical models, previous studies show that the ENSO growth rate may play an important role on the strength of ENSO SPB. However, ENSO may shift between two reg...

The effect of ice sheet topography on the East Asian Summer Monsoon (EASM) during the Last Glacial Maximum is studied using CCSM3 in a hierarchy of model configurations. It is found that receding ice sheets result in a weakened EASM, with the reduced ice sheet thickness playing a major role. The lower ice sheet topography weakens the EASM through s...

Recent studies have started to explore coupled data assimilation (CDA) in coupled ocean–atmosphere models because of the great potential of CDA to improve climate analysis and seamless weather–climate prediction on weekly-to-decadal time scales in advanced high-resolution coupled models. In this review article, we briefly introduce the concept of C...

Our understanding of past climate conditions largely comes from paleoclimate proxies, such as oxygen isotope ratios (δ18Oc) in marine fossils. The marine δ18Oc signal primarily reflects a mixture of seawater temperature and oxygen isotopic composition of seawater (δ18Ow) at the time of calcification. Knowledge of δ18Ow is critical for the interpret...

The seasonal response of the poleward extent of continental precipitation maxima to high obliquity and its controlling mechanisms are examined using the NCAR-CESM and Budyko-Sellers energy balance model. In CESM, the latitude of the poleward-most continental precipitation maximum (denoted as \({\phi }_{mp})\) migrates poleward with increased obliqu...

Recent studies proposed LACC (leading averaged coupled covariance) as an effective strongly coupled data assimilation (SCDA) method to improve the coupled state estimation over weakly coupled data assimilation (WCDA) in a coupled general circulation model (CGCM). This SCDA method, however, has been previously evaluated only in the perfect model sce...

The northern North Atlantic (NNA) subsurface temperature in response to the slowdown of the Atlantic meridional overturning circulation (AMOC) is crucial for ice sheet calving and recovery of the AMOC in Heinrich events. Paleoclimate proxies and modeling studies suggest that the NNA subsurface exhibits a robust warming during Heinrich 1, but with a...

The northern North Atlantic (NNA) subsurface temperature in response to the slowdown of the Atlantic meridional overturning circulation (AMOC) is crucial for ice sheet calving and recovery of the AMOC in Heinrich events. Paleoclimate proxies and modeling studies suggest that the NNA subsurface exhibits a robust warming during Heinrich 1, but with a...

The Atlantic Meridional Overturning Circulation (AMOC) variability is suggested to be incoherent between the subpolar and subtropical gyres in the Atlantic on interannual and even decadal time scales, questioning the representativeness of AMOC variability at a single latitude in modern observation and paleoreconstruction. Paleoreconstructions of th...

The Pacific–North American (PNA) teleconnection is one of the most important climate modes in the present climate condition, and it enables climate variations in the tropical Pacific to exert a significant influence on North America. Here, we show climate simulations in which the PNA teleconnection was largely distorted or broken at the Last Glacia...

Recent studies have demonstrated a significant control of extratropical atmospheric forcing on the tropical Pacific interannual variability. However, it remains unclear how the extratropical atmospheric signal is transferred equatorward via potential pathways. Here aided by regional coupled data assimilation and partial restoring techniques, the ex...

Plain Language Summary El Niño‐Southern Oscillation (ENSO) spring persistence barrier (SPB) describes the feature that ENSO's prediction skills decrease significantly during the boreal spring. Through statistical methods, previous studies show that from autumn to spring, the decline of ENSO's growth rate—how easily the climate anomalies can develop...

While various data assimilation algorithms based on Bayes’ theorem have been developed for state estimation, some of these algorithms have also been applied to model parameter estimation. Coupled model parameter estimation (CPE) adjusts model parameters using available observations; then, the observation-adjusted parameters can greatly mitigate the...

The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) platform is used to simulate Lagrangian trajectories of air parcels in East China during the summer monsoon. The investigation includes four distinct stages of the East Asian Summer Monsoon (EASM) during its seasonal migration from south to north. Correspondingly, the main water...

A stochastic climate model is used to explain the major features of seasonal phase-locking of climate variability in general. The model is the classical damped persistence model, generalized with seasonal cycles in the growth rate and noise forcing. Our theory predicts distinct phase-locking features for different seasonal forcing. With seasonal gr...

Three transient National Center for Atmospheric Research Community Climate System Model, version 3 model simulations were analyzed to study the responses of El Niño–Southern Oscillation (ENSO) and the equatorial Pacific annual cycle (AC) to external forcings over the last 300,000 years. The time‐varying boundary conditions of insolation, greenhouse...

The Pacific-North American (PNA) teleconnection is one of the most important climate modes in the present climate condition, and it enables climate variations in the tropical Pacific to exert significant impacts on North America. Here, we show climate simulations that the PNA teleconnection was largely distorted or broken at the Last Glacial Maximu...

O in foraminifera (δ¹⁸Oc) is a useful proxy for density, and the strength of the Atlantic Meridional Overturning Circulation (AMOC) can be reconstructed by the zonal density contrast in the Atlantic. However, whether the deglacial zonal δ¹⁸Oc contrast can represent the AMOC change is still unclear. δ¹⁸Oc contrast across the Florida Straits has been...

Modern El Niño-Southern Oscillation (ENSO) events are characterized by their phase locking of variability to the seasonal cycle and tend to peak at the end of calendar year. Here, we show that in an idealized NCAR-CCSM3 simulation of the climate of the last 300,000 years, ENSO seasonal phase locking is shifted periodically following the precessiona...

Neodymium (Nd) isotopic composition (ε Nd ) is an important tracer for water mass mixing and the reconstruction of past ocean circulation. To allow for a direct model-data comparison, we have implemented Nd isotopes in the ocean component of the Community Earth System Model (CESM1.3). The model is able to capture the major features of the observed...

Model projections of the near-future response to anthropogenic warming show compensation between meridional heat transports by the atmosphere (AHT) and ocean (OHT) that are largely symmetric about the equator1–3, the causes of which remain unclear. Here, using both the Coupled Model Intercomparison Project Phase 5 archive and Community Climate Syst...

We performed parameter estimation in the Zebiak-Cane model for the real-world scenario using the approach of ensemble Kalman filter (EnKF) data assimilation and the observational data of sea surface temperature and wind stress analyses. With real-world data assimilation in the coupled model, our study shows that model parameters converge toward sta...