Atmospheric and Oceanic Sciences

Abstract Heinrich Stadials provide insights into a potential future weakening of the Atlantic Meridional Overturning Circulation (AMOC), yet the mechanisms driving AMOC weakening during these intervals remain elusive. While Heinrich Stadials are associated with iceberg-discharge events over the North Atlantic (i.e., Heinrich events), AMOC weakening is known to precede Heinrich events. Here we run freshwater hosing experiments to show that contemporaneous Northeast Pacific iceberg-discharge events (i.e., Siku events), which consistently occur at the onsets of Heinrich Stadials, could trigger AMOC weakening by transporting freshwater to the North Atlantic deepwater formation regions. This initial weakening may subsequently be amplified by further meltwater release from the British and Laurentide Ice Sheets, induced by stepwise subsurface warming in the northeastern and northern North Atlantic. Our findings suggest that Siku events may have played a critical role in preconditioning the North Atlantic for Heinrich events, underscoring the interconnected nature of the global ocean–cryosphere system.

The El Niño–Southern Oscillation (ENSO) periodicity governs the recurrence of its warm and cold phases, shaping the climate variability and the frequency of extremes, yet remains poorly constrained in climate model projections. Here we assess, using coordinated multi-model large-ensemble simulations, the forced modulation of ENSO periodicity from the last millennium to the future and its global imprint. ENSO periodicity tends to shorten under future warming, and this periodicity shift extends to tropical climate modes and propagates globally through teleconnections, a phenomenon we refer to as ENSO frequency entrainment. The frequency entrainment appears to be ubiquitous, evident in forced responses in last-millennium simulations and under nonstationary, nonmonotonic CO2 forcing, as well as in periodicity modulation driven by internal multi-decadal variability. Changes in future ENSO periodicity may alter the frequency of local climatic events globally, potentially favoring more frequent extreme events. Multi-model large-ensemble simulations agree that El Niño cycles may become shorter under future warming, shifting climate rhythms around the world through teleconnections and potentially increasing the frequency of regional extreme events.

Abstract Projections of future changes in tropical cyclone (TC) rainfall are critical for understanding evolving flood risks and infrastructure impacts under climate change. This study couples a physics-based Tropical Cyclone Rainfall (TCR) model with the Columbia HAZard (CHAZ) model, a statistical–dynamical TC downscaling framework, which generates synthetic storm tracks and intensity, to produce large synthetic ensembles of TC generated rainfall downscaled from 12 CMIP6 models. TC rainfall is simulated in the North Atlantic basin based on the CMIP6 historical and future simulations under three Shared Socioeconomic Pathways (SSP2-4.5, SSP3-7.0, SSP5-8.5). Projections from this integrated framework are evaluated against the Geophysical Fluid Dynamics Laboratory (GFDL) Rainfall Climatology and Persistence model (R-CLIPER), a statistical TC rainfall model that provides an alternative to the dynamical simulations. TCR projects widespread increases in TC rainfall, with the strongest changes along the US East and Gulf coasts and for major hurricanes (Categories 3–5). By late century, average rainfall increases reach 40–100% in TCR compared to 30–60% in R-CLIPER, with the largest increases in the Northeast regions. Extreme 24-hour rainfall increases by up to 55% in TCR, roughly twice the magnitude simulated by R-CLIPER. Key drivers include higher TC intensities, increased atmospheric moisture, and projected slower translation speeds, which together contribute to increasing flood risks, especially when combined with more frequent sequential TC events.

Why Antarctica became glaciated ∼34 million years ago (Ma) remains debated, as relatively warm climates and sea temperatures appear inconsistent with ice sheet formation. Although a critical decline in CO 2 is considered primarily responsible, evidence suggests that other factors were important, too. We investigated whether regional topographic uplift, rooted in Jurassic continental breakup and mantle-surface feedbacks, enabled nucleation of the East Antarctic Ice Sheet (EAIS). By integrating geodynamic-topographic models with ice sheet and energy balance models, we show that progressive plateau growth in East Antarctica, including Eocene uplift of the Gamburtsev Mountains, pushed landscapes above the threshold for ice sheet nucleation by ∼45 Ma. Uplift enabled EAIS growth under warmer-than-expected climates, producing hemispheric asymmetry in early glaciation and reconciling Oligocene polar warmth with the onset of the modern icehouse world.

Reliable drought prediction is crucial for early warning and decision-making regarding drought-related disasters. Flash droughts, characterized by their rapid onset and severe impacts, are challenging to predict and present critical challenges to water resource management. Specifically, streamflow flash droughts (SFDs) are significant as they directly influence water supply for irrigation, industry, and domestic use. Traditional hydrologic models and drought early warning systems are primarily designed to capture slowly evolving drought conditions and often fail to represent rapid transitions in streamflow that characterize the onset of SFDs. Despite increasing attention to the identification and characterization of SFDs, their prediction remains unexplored. This study evaluates the ability of different deep learning architectures, especially the Temporal Fusion Transformer (TFT), in comparison to the widespread baseline Long Short-Term Memory (LSTM) model and a two-source LSTM incorporating static catchment attributes, to predict SFDs based on streamflow percentiles. The model training and performance evaluation were conducted using hydroclimatic datasets from 671 Catchment Attributes and Meteorology for Large-sample Studies (CAMELS) catchments in the contiguous United States. Performance was evaluated using standard accuracy and event detection metrics. Results show that the TFT achieved the highest overall performance, with a median KGE of 0.87, RMSE of 0.13, MAE of 0.10, and a correlation coefficient of 0.91, outperforming the LSTM with static features (median KGE = 0.81) and the baseline LSTM (KGE = 0.78). In terms of detecting SFDs, the TFT showed the highest median detection rate and lowest miss rate across all models. These improvements were consistent across most catchments and hydroclimatic regions, indicating robust performance gains relative to LSTM-based approaches, although some regions with complex hydrologic behavior remained challenging for all models. While model performance varied across hydroclimatic regions, particularly in data-sparse or hydrologically complex areas, the integration of static attributes and attention-based mechanisms consistently improved predictive skill. These findings demonstrate that Transformer-based models can improve prediction of rapid streamflow changes associated with the onset of SFDs; however, the results are based on deterministic predictions and do not explicitly quantify predictive uncertainty. These findings contribute new insights into the spatial patterns and physical drivers of model performance under non-stationary climate conditions.

Abstract Atmospheric rivers (ARs) over western North America drive extreme precipitation and flood hazard, yet their intensity upper bounds remain poorly constrained by the short observational record. We combine a differentiable global climate model (~1.4°) with high-resolution dynamical downscaling (~0.11°) to construct physically plausible storylines of five unprecedented AR events in British Columbia. By optimizing minimal perturbations to historical initial conditions, we generate events that maximize integrated vapor transport (IVT) and exceed the observational record under present-day conditions. Pseudo-global warming perturbations under SSP5–8.5 provide a second pathway through end-of-century warming. Both approaches amplify AR intensity through distinct mechanisms: the optimization primarily modifies the wind field, while the pseudo-global warming signal primarily increases atmospheric moisture. These contributions act on largely independent components, and when applied simultaneously, the combined effect is nearly additive, with differences generally below 15%. Non-linear effects, though small, are always positive, and within these storylines the most extreme physically plausible ARs arise from compounding both drivers. The pathways differ in precipitation efficiency: the dynamical amplification largely preserves the conversion of moisture transport to precipitation, whereas the thermodynamic amplification reduces it by up to 29%. Bounding the upper tail of AR hazard may therefore require accounting for both dynamical variability and thermodynamic change.

Escalating emissions of chlorinated very short-lived substances (Cl-VSLSs), which remain unregulated under the Montreal Protocol, could delay the recovery of the global ozone layer. In this study, we developed a comprehensive sectoral and regional bottom-up emission inventory of the two most abundant Cl-VSLSs, dichloromethane (CH2Cl2) and chloroform (CHCl3), from 2010 to 2023 in China, alongside projections to 2035. Our best estimates indicate an upward trend in historical emissions for both substances, with CH2Cl2 emissions more than doubling since 2010, and both species are projected to grow substantially by 2035 under the Business-As-Usual (BAU) scenario. The implementation of best practicable technologies is projected to mitigate cumulative CH2Cl2 and CHCl3 emissions by approximately 50% and 7%, respectively, over 2024–2035. Crucially, projections indicate that the CFC-11 equivalent emissions of CH2Cl2 in China would exceed the country’s aggregate emissions of all hydrochlorofluorocarbons (HCFCs) annually from 2024 onwards under the BAU scenario. This study underscores the need for integrated domestic regulations and international negotiations to address the increasing threat from Cl-VSLSs to stratospheric ozone recovery. Emissions of unregulated chlorinated substances, dichloromethane and chloroform, have risen substantially from 2010 to 2023 in China. Dichloromethane emissions alone are projected to surpass all regulated hydrochlorofluorocarbons in China in ozone-depletion impact. Adopting advanced control technologies could substantially reduce future cumulative emissions.

El Niño-Southern Oscillation (ENSO) is one of the key drivers of global climate anomaly, and its evolution during geological history is crucial for understanding the natural variability of the climate system. The Middle Miocene Climatic Optimum (MMCO, 16.9–14.7 Ma) was a period of relatively warm global climate. During this period, the contraction and closure of tropical seaways exerted profound impacts on global ocean circulation and climate. Using idealized sensitivity experiments with the FGOALS-g3 model, we artificially close the Indonesian Seaway to isolate its mechanistic influence on ENSO dynamics. Although this extreme scenario does not reflect the actual open-gateway conditions of the Miocene, it allows us to clearly identify the seaway’s role in modulating tropical climate variability. The results demonstrate that the closure of the Indonesian Seaway exerts a significant impact on ENSO, increasing ENSO amplitude by 140%, an impact far exceeding that of the Panama and Tethys Seaways. Diagnostics based on the mixed-layer heat budget equation reveal that the enhancement of ENSO amplitude is primarily driven by the strengthened zonal advection feedback. The closure of the Indonesian Seaway drives the formation of an “El Niño-like” mean state in the tropical Pacific, which facilitates the eastward propagation and amplification of sea surface temperature anomalies and wind stress anomalies. These findings underscore the critical role of the Indonesian Seaway in modulating ENSO variability during the MMCO and provides mechanistic insights into the potential amplifying effect of the weakening Indonesian Throughflow on ENSO under future global warming scenarios.

Abstract. Climate change is driving wildfires to higher elevations, yet the hazard cascades that follow the burning of pristine tropical mountain ecosystems remain largely unexplored. Here, we analyse the long-term cascade following a February 2012 wildfire that burned 31 km2 of forest and wetland in Uganda's Rwenzori Mountains National Park, including sections above 3800 m elevation with no major fire history in 12 000 years. Combining remote sensing, humanitarian records, field surveys and interviews, we document ten major floods since 2012, including two debris floods that required large-scale humanitarian responses. Post-fire increases in erosion and mass movement have widened the River Nyamwamba sevenfold since 2012, breaching copper-cobalt mine tailings and mobilising an estimated 744 000 t of waste into the river. Slow vegetation recovery at high altitudes and positive feedbacks between hazards have prolonged this high-risk state. These findings point to an urgent need to understand where emergent tropical mountain fires can occur, how their impacts cascade downstream, and where early interventions can reduce risk.