Lamont-Doherty Earth Observatory

Arctic sea ice extent grows from its September minimum through winter, influenced mainly by September sea ice conditions and atmospheric circulation during the ice‐growing season. However, the changing role of the two drivers in a warming climate remains unclear. Using large‐ensemble climate model simulations and Ridge Regression, this study quantifies the changing relative importance of the two drivers from 1861 to 2100. Up until recent years, low September sea ice largely enhanced sea ice growth via a negative feedback, as open water allows more freezing when the water is still cold enough. However, this negative feedback weakens with rising Arctic air and ocean temperatures that increasingly limit and delay sea ice freezing. Atmospheric circulation will relatively play an increasing role in driving sea ice growth in the next few decades. These findings are useful in guiding future studies for improving Arctic sea ice seasonal forecasts and long‐term projections.

On behalf of AGU, the scientific community, and the editorial team of Journal of Geophysical Research: Machine Learning and Computation, we extend our sincere gratitude to the reviewers who dedicated their time and expertise to evaluating manuscripts for us in 2024. Scientific research can now be communicated in various ways, yet peer review remains the cornerstone of scholarly publishing. We deeply appreciate the reviewers who devoted hours to reading and providing insightful feedback. The high quality of our published papers is a testament to their commitment to this vital community service.

Rangelands provide significant environmental benefits through many ecosystem services, which may include soil organic carbon (SOC) sequestration. However, quantifying SOC stocks and monitoring carbon (C) fluxes in rangelands are challenging due to the considerable spatial and temporal variability tied to rangeland C dynamics as well as limited data availability. We developed the Rangeland Carbon Tracking and Management (RCTM) system to track long‐term changes in SOC and ecosystem C fluxes by leveraging remote sensing inputs and environmental variable data sets with algorithms representing terrestrial C‐cycle processes. Bayesian calibration was conducted using quality‐controlled C flux data sets obtained from 61 Ameriflux and NEON flux tower sites from Western and Midwestern US rangelands to parameterize the model according to dominant vegetation classes (perennial and/or annual grass, grass‐shrub mixture, and grass‐tree mixture). The resulting RCTM system produced higher model accuracy for estimating annual cumulative gross primary productivity (GPP) (R² > 0.6, RMSE <390 g C m⁻²) relative to net ecosystem exchange of CO2 (NEE) (R² > 0.4, RMSE <180 g C m⁻²). Model performance in estimating rangeland C fluxes varied by season and vegetation type. The RCTM captured the spatial variability of SOC stocks with R² = 0.6 when validated against SOC measurements across 13 NEON sites. Model simulations indicated slightly enhanced SOC stocks for the flux tower sites during the past decade, which is mainly driven by an increase in precipitation. Future efforts to refine the RCTM system will benefit from long‐term network‐based monitoring of vegetation biomass, C fluxes, and SOC stocks.

El Niño is generally phase‐locked to the boreal winter but displays significant variability in its onset timing, contributing to its diverse climate impacts. The physical mechanisms driving this variability remain inadequately understood. This study demonstrates that onset of El Niño events can occur over a broad range of months from March to September, with its onset timing closely linked to the precondition of oceanic recharged state and the occurrence of westerly wind bursts (WWBs) in the preceding spring. A stronger recharged state and increased frequency of WWBs promote earlier onset by efficiently transporting warm water to the equatorial eastern Pacific. Supporting evidence from MIROC6 simulations and a conceptual model underscores the crucial roles of both the recharged state and WWBs in determining the timing of El Niño onset. These results enhance our understanding of El Niño dynamics and hold important implications for seasonal climate prediction.

The atmospheric chemistry of volatile organic compounds (VOC) has a major influence on atmospheric pollutants and particle formation. Accurate modeling of this chemistry is essential for air quality models. Complete representations of VOC oxidation chemistry are far too large for spatiotemporal simulations of the atmosphere, necessitating reduced mechanisms. We present Automated MOdel REduction version 2.0 (AMORE 2.0), an algorithm for the reduction of any VOC oxidation mechanism to a desired size by removing, merging, and rerouting sections of the graph representation of the mechanism. We demonstrate the algorithm on isoprene (398 species) and camphene (100,000 species) chemistry. We remove up to 95% of isoprene species while improving upon prior reduced isoprene mechanisms by 53-67% using a multi-species metric. We remove 99% camphene species while accurately matching camphene secondary organic aerosol production. This algorithm will bridge the gap between large and reduced mechanisms, helping to improve air quality models.

Because of the limited length of observed tropical cyclone (TC) data and low confidence in modeling TC genesis frequency, it has been difficult to understand why the genesis frequency of global TCs has remained nearly invariant while regional variations are highly pronounced. Investigating the co-variability of TC genesis frequency between different regions may shed light on this question. Here, we identify that TC genesis frequency varies out of phase between the eastern and western regions of the western North Pacific (WNP). Such a seesaw relationship could be explained by the east-west dipole patterns of low-level vorticity and mid-tropospheric relative humidity in the WNP, associated with variations in atmospheric conditions. Composite analyses and numerical experiments show that a combination of cooling of the WNP and warming of the North Indian Ocean (NIO) exerts a significant influence on the seesaw pattern. Our study adds more evidence to believe on number conservation of worldwide TC genesis.

Frictional interfaces are found in systems ranging from biological joints to earthquake faults. When and how these interfaces slide is a fundamental problem in geosciences and engineering1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19–20. It is believed that there exists a threshold shear force, called static friction, below which the interface is stationary4,10, despite many studies suggesting that this concept is outdated1,21, 22, 23, 24, 25, 26, 27–28. By contrast, rate-and-state friction formulations1,26,27 predict that interfaces are always sliding²⁹, but this feature is often considered an artefact that calls for modifications³⁰. Here we show that nominally stationary interfaces subjected to constant shear and normal loads, with a driving force that is notably below the classically defined static friction for which creep is known to occur9,27, 28–29, are sliding, but with diminishingly small rates down to 10⁻¹² m s⁻¹. Our precise measurements directly at the interface are enabled by digital image correlation18,31,32. This behaviour contradicts classical models of friction but confirms the prediction of rate-and-state friction1,26,27. The diminishing slip rates of nominally stationary interfaces reflect interface healing, which would manifest itself in higher peak friction in subsequent slip events15,27,33, such as earthquakes and landslides, substantially modifying their nucleation and propagation and hence their hazard3,12,13,34.

We present Bedmap3, the latest suite of gridded products describing surface elevation, ice-thickness and the seafloor and subglacial bed elevation of the Antarctic south of 60 °S. Bedmap3 incorporates and adds to all post-1950s datasets previously used for Bedmap2, including 84 new aero-geophysical surveys by 15 data providers, an additional 52 million data points and 1.9 million line-kilometres of measurement. These efforts have filled notable gaps including in major mountain ranges and the deep interior of East Antarctica, along West Antarctic coastlines and on the Antarctic Peninsula. Our new Bedmap3/RINGS grounding line similarly consolidates multiple recent mappings into a single, spatially coherent feature. Combined with updated maps of surface topography, ice shelf thickness, rock outcrops and bathymetry, Bedmap3 reveals in much greater detail the subglacial landscape and distribution of Antarctica’s ice, providing new opportunities to interpret continental-scale landscape evolution and to model the past and future evolution of the Antarctic ice sheets.

This study investigates surface weathering and sediment preservation at Table Mountain, a high‐elevation, hyperarid, polar landscape in the Transantarctic Mountains. We report cosmogenic nuclide concentrations (¹⁰Be and ²⁶Al) in quartz from bedrock surfaces, erratic boulder lag, and cobbles embedded within Sirius Group sediments to quantify erosion rates. In situ ¹⁰Be and ²⁶Al depth profiles from a 2.95 m permafrost core in the Sirius Group further constrain surface erosion rates and elucidate landscape stability. Measured ¹⁰Be and ²⁶Al concentrations from two sandstone bedrock surfaces adjacent to Sirius Group sediments give erosion rates of 0.18–0.28 m/Myr. An erratic sandstone boulder within the lag above the Sirius Group yields erosion rates of ∼0.42 ± 0.03 m/Myr, whereas two cobbles embedded within the Sirius Group yield higher rates of 0.81–1.12 m/Myr. Depth profiles of in situ ¹⁰Be and ²⁶Al indicate no vertical mixing of Sirius Group permafrost since deposition. Depth profile models are best explained by erosion rates of 0.53+0.13/−0.12 m/Myr, and an exposure age of 0.78+0.06/−0.08 Ma. We view the model “age” to represent the ∼0.8‐million‐year time‐scale for surface lowering equivalent to one attenuation length of cosmic ray production to achieve steady‐state conditions. Continual exhumation of embedded clasts from within the Sirius Group results in an accumulation of clasts forming the observed erosional lag deposit covering the landscape. Our erosion rates of the Sirius Group surface based on in situ ¹⁰Be and ²⁶Al depth profiles are an order‐of‐magnitude larger than those based on meteoric ¹⁰Be infiltration and further clarification is required.

Compound events (CEs) are attracting increased attention due to their significant societal and ecological impacts. However, their inherent complexity can pose challenges for climate scientists and practitioners, highlighting the need for a more approachable and intuitive framework for detecting and visualising CEs. Here, we introduce the Compound Events Toolbox and Dataset (CETD), which provides the first integrated, interactive, and extensible platform for CE detection and visualisation. Employing observations, reanalysis, and model simulations, CETD can quantify the frequency, duration, and severity of multiple CE types: multivariate, sequential, and concurrent events. It can analyse CEs often linked to severe impacts on human health, wildfires, and air pollution, such as hot-dry, wet-windy, and hot-dry-stagnation events. To validate the performance of CETD, we conduct statistical analyses for several high-impact events, such as the 2019 Australian wildfires and the 2022 European heatwaves. The accessibility and extensibility of CETD will benefit the broader community by enabling them to better understand and prepare for the risks and challenges posed by CEs in a warming world.

Plain Language Summary During summer 2023, multiple heat waves affected Mexico and Texas and contributed to hundreds of heat‐related fatalities and thousands of heat‐related emergency‐room visits. Particularly notable was an unusually intense and persistent early‐season heat wave in June, when numerous locations exceeded their all‐time record highs. This heatwave was the hottest, largest, and longest‐lasting heatwave to affect the Mexico‐Texas region in the observational record spanning 1940–2023. In this study, we quantify the influence of atmospheric circulation and soil moisture on the heatwave intensity. We find that these factors together account for most of the extreme temperature anomaly at the peak of the heatwave, with most of the remainder explained by long‐term warming. We also find that June 2023‐like circulation patterns will not occur more frequently but are projected to become nearly 2°C hotter than present by the mid‐21st century. The hottest simulated day with these patterns could produce widespread temperatures hotter than 50°C (122°F) across south Texas. Although these temperatures have a low probability of occurrence, they represent physically plausible conditions that could threaten human survivability. Such low‐likelihood, yet high‐risk scenarios can inform disaster preparedness and adaptation planning efforts.

Mantle processes control plate tectonics and exert an influence on biogeochemical cycles. However, the proportion of mantle sampled in-situ is minimal, as it is buried beneath igneous crust and sediments. Here we report the lithological characteristics of two mantle sections from an embryonic ocean drilled by the International Ocean Discovery Program (IODP) in the Tyrrhenian Sea. Contrary to the mantle drilled at Mid Ocean Ridges (MORs) and hyperextended passive margins, our findings reveal exceptionally heterogeneous and fertile mantle lithologies, ranging from fertile lherzolites to depleted harzburgites and dunites, interlayered with pyroxenites. Plagioclase- and clinopyroxene-rich layers, hydrous potassic magmatic veins, and mafic intrusions indicate substantial mantle refertilization and delayed inception of magmatic crust. We propose that magma-poor rifts do not require a chemically depleted mantle, too refractory to melt. Deep lithospheric processes such as mantle refertilization and prolonged lithospheric thinning delayed melt focusing and the formation of a steady-state spreading center.

Spring persistent rainfall is a unique climate phenomenon that prevails in East Asia today, providing precious water resources to this densely populated region. However, its Cenozoic history and underlying mechanisms remain poorly understood. Here we show that the spring persistent rainfall in East Asia has emerged since the Miocene, whereas it previously flourished in North America during the Eocene, as revealed by climate models integrated with climate proxies. The contrasting evolution of spring persistent rainfall in East Asia and North America is determined by paleogeography and further influenced by CO2-induced warming. The uplift of the Tibetan Plateau and the westward drift of the Rocky Mountains have triggered a mid-latitude Rossby wave train since the Miocene, altering the position and intensity of the subtropical highs and thus rainfall patterns. Our results illuminate the Cenozoic evolution of spring persistent rainfall, with implications for the spring climate under the extreme future warming.

Hercules Dome is a prospective ice‐core site due to its setting in the bottleneck between East and West Antarctica. If ice from the last interglacial period has been preserved there, it could provide critical insight into the history of the West Antarctic Ice Sheet. The likelihood of a continuous, well‐resolved, easily interpretable climate record preserved in ice extracted from Hercules Dome depends in part on the persistence of ice‐flow dynamics at the divide. Significant changes in ice drawdown on either side of the divide, toward the Ross or Ronne ice shelves, could change the relative thickness of layers and the deposition environment represented in the core. Here, we use radar sounding to survey the ice flow at Hercules Dome. Repeated radar acquisitions show that vertical velocities are consistent with expectations for an ice divide with a frozen bed. Polarimetric radar acquisitions capture the ice‐crystal orientation fabric (COF) which develops as ice strains, so it depends on both the pattern of ice flow and the time over which flow has been consistent. We model the timescales for COF evolution, finding that the summit of Hercules Dome has been dynamically stable in its current configuration, at least over the last five thousand years, a time period during which the Antarctic ice sheet was undergoing significant retreat at its margins. The evident stability may result from a prominent bedrock ridge under the divide, which had not been previously surveyed and has therefore not been represented in the bed geometry of coarsely resolved ice‐sheet models.

On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts in 2024. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. We received 5,225 submissions in 2024, and 5,597 reviewers contributed to their evaluation by providing 9,697 reviews in total. We deeply appreciate their contributions. We would also like to acknowledge the passing of our beloved colleague, Harihar Rajaram. An AGU Fellow and longtime affiliate of AGU's Hydrology Section, Hari was the Editor‐in‐Chief of Geophysical Research Letters since 2019, a former editor on Water Resources Research, and served on the AGU Publications Committee.

El Niño–Southern Oscillation (ENSO) is an oscillation of the ocean–atmosphere system in the tropical Pacific, which is argued to be energized by high-frequency stochastic atmospheric disturbances. Among these disturbances, westerly wind bursts (WWBs) play a crucial role in the development of El Niño by generating eastward-propagating downwelling Kelvin waves and suppressing the thermocline in the central-eastern equatorial Pacific. The present work elucidates distinct seasonal evolutions of WWBs during cyclic and noncyclic El Niño events, and their association with the local sea surface temperature anomalies (SSTAs). For noncyclic El Niño events, WWBs prevail over the western-central equatorial Pacific during spring of the developing year, accompanied by local warming SSTAs. In contrast, active WWBs cannot be observed until the developing summer for cyclic El Niño events. Significant differences in high-frequency WWBs and associated local deep convection appear in the developing spring season of noncyclic and cyclic El Niño events. These differences are closely linked to local SSTAs in the western-central equatorial Pacific via the stimulation of atmospheric deep convection, preceding the full manifestation of ENSO-associated large-scale SSTAs in the central-eastern tropical Pacific. The observed difference in WWBs for noncyclic and cyclic El Niño events and its association with the western-central equatorial Pacific SSTAs is realistically reproduced in a coupled general circulation model. This study enhances our comprehension of El Niño development by illustrating the intricate connection between WWBs and El Niño evolution from the ENSO cycle perspective.

This study aims to understand the mechanisms of the activation and evolution of the marine heatwave (MHW) that occurred in the Gulf of Mexico (GoM) during summer 2023. We quantified contributions of the thermodynamic processes that transformed surface waters in the GoM into an unprecedented large volume of extremely warm water (>31.8°C) $( > 31.8{}^{\circ}\mathrm{C})$. Through water mass transformation analysis of reanalyses data, we find that the genesis of this MHW was due to the compounding effect of anomalously warm winter surface water priming the region for a MHW, coupled with greater exposure to strong solar radiation. Transformation due to total surface fluxes (sensible and latent heat, solar and longwave radiation) contributed to the MHW volume at a peak rate of 17.0 Sv (106 ${10}^{6}$ m3 ${\mathrm{m}}^{3}$ s−1 ${\mathrm{s}}^{-1}$ = Sv), while the residual term (including mixing) countered the effect by 22.3 Sv at its peak. Total transformation during this 2023 MHW peaked at 4.9 Sv.