Atmospheric and Oceanic Sciences

Abstract Given sparse historical data in the deep and abyssal ocean, previously only multi‐decadal temperature trends have been estimated from observations there on a global scale. Full‐depth CTD sampling started circa 1970, with the first decadal global ship‐based survey occupied in the 1990s, and the first regional pilot array of Deep Argo floats started circa 2016. Here we fit second‐order polynomial functions versus time to all available full‐depth CTD profile temperature data in local spatial bins to estimate changes in the rates of these multi‐decadal temperature trends. We find a statistically significant increase of the heating rate of the abyssal (4,000–6,000 dbar) ocean, from 5.4 (±4.9) TW in 1988 to 20.2 (±3.9) TW in 2018. In contrast, we find no statistically significant change in the heating rate of the deep ocean, estimated at 29.4 (±22.1) TW in 1988 and 25.0 (±17.5) TW in 2018.

Abstract. The rapid evolution of satellite-borne Earth Observation (EO) systems has fundamentally revolutionized terrestrial monitoring, yielding comprehensive petabyte-scale archives. However, the immense computational resources and storage volumes required for global-scale analysis often preclude widespread use by many research teams, hindering broader scientific adoption and the execution of planetary-scale studies. To address these barriers, we present the Embedded Seamless Data (ESD), an ultra-lightweight, 30 m global Earth embedding database spanning the 25-year period from 2000 to 2024. By transforming high-dimensional, multi-sensor observations from the Landsat series (5, 7, 8, and 9) and MODIS Terra into information-dense, quantized latent vectors, ESD distils essential geophysical and semantic features into a unified latent space. Utilizing the ESDNet architecture and Finite Scalar Quantization (FSQ), the dataset achieves a transformative ∼ 340-fold reduction in data volume compared to raw daily archives. This compression allows the entire global land surface for a single year to be encapsulated within approximately 2.4 TB, enabling decadal-scale global analysis on standard local workstations. Rigorous validation demonstrates that ESD maintains high reconstructive fidelity to the original reflectance values across the spectral dimension, achieving a Mean Absolute Error (MAE) of 0.0130 (averaged over six spectral bands, including Blue, Green, Red, NIR, SWIR1, and SWIR2), a Root Mean Square Error (RMSE) of 0.0179, and a Correlation Coefficient (CC) of 0.8543. By condensing the annual phenological cycle into 12 temporal latent steps, the embeddings provide inherent denoising effects and a semantically organized latent space that outperforms raw reflectance data in downstream land-cover classification tasks, achieving a comparable and even higher overall accuracy of 79.74 % than the 76.92 % obtained using raw sensor fusion data on globally distributed land cover sample sets. With robust few-shot learning capabilities and longitudinal consistency across 25 years, the ESD product provides a versatile foundation for democratizing planetary-scale Earth system research and advancing next-generation geospatial artificial intelligence. The ESD dataset is freely available at https://doi.org/10.12436/iEarth.0000.20251229.000064.v1 (Chen, 2025).

Tropical cyclone rain rates are projected to increase under warming, yet many climate models produce weaker rain-rate responses than expected from Clausius–Clapeyron scaling, which predicts that atmospheric moisture-holding capacity increases by approximately 7% per degree of warming. Here we use large-ensemble simulations, satellite observations and reanalysis data to identify what suppresses these rain-rate increases below Clausius–Clapeyron expectations. We show that future increases in tropical cyclone rain rate are constrained by the amplified absolute dryness of the warming atmosphere, quantified by the saturation deficit. Whereas atmospheric moistening elevates column water vapour at near-Clausius–Clapeyron rates and storm intensification further boosts rain rate, these gains are systematically offset by saturation-deficit-driven reductions in precipitation efficiency through enhanced evaporation. The negative relationship between precipitation efficiency and column saturation deficit is corroborated by satellite-based precipitation observations. Although precipitation efficiency correlates positively with storm intensity, attribution analyses reveal that increases in column saturation deficit can dominate intensity effects under warming, leading to net declines in precipitation efficiency and smaller rain rates. Our results highlight increased absolute dryness as the principal thermodynamic limiter of future tropical cyclone rain rate, uncovering a mechanism overlooked by prior projections focused solely on storm intensification and moisture increases. Climate model ensembles suggest tropical cyclones will bring more rain as the world warms, but increases may be smaller than expected. Atmospheric dryness reduces how much rain storms produce, offsetting extra moisture and stronger storms.

The latest Intergovernmental Panel on Climate Change assessment report highlighted the potential for more than 15 m of global sea-level rise by 2300. In this study, we explore the implications for UK coastal flood exposure by combining national-scale flood modelling with physically-based storylines of UK sea-level rise. By 2100 all storylines show broadly similar results with at least an additional 0.5 million people exposed to the 1-in-200 year undefended flood extent, which represents a 25% increase compared to present day. Under the most pessimistic storyline by 2300 involving significant ice-sheet instability, an additional 13 million people could be exposed to the 1-in-200 year undefended flood event. This would imply the potential need for large-scale movement of populations and settlements away from the coast in the coming centuries. Given current global emissions pledges, exposure increases by 1.7 million people by 2300, however up to 1 million could be avoided if Paris Agreement targets for greenhouse gas emissions are met.

Abstract. We present a novel methodology, AEROCHEMPro/GRASP (AEROsol CHEMical PROfiling), for retrieving vertical concentration profiles of aerosol chemical species by synergistically combining co-located measurements from a multiwavelength lidar and a multi-angular polarimeter. AEROCHEMPro represents the first retrieval framework for remote sensing of the vertically-resolved aerosol chemical composition. It is based on an improved version of the GRASP (Generalized Retrieval of Aerosol and Surface Properties) chemical component framework. The methodology is developed within the context of the Atmosphere Observing System (AOS) international initiative, which proposes a spaceborne observing system to advance our knowledge of aerosols, clouds, convection, and precipitation. Moreover, the retrieval strategy remains broadly applicable to future satellite missions and observing systems involving combined lidar and multi-angular polarimeter aerosol remote sensing. Based on a statistically optimized and physically-constrained inversion, AEROCHEMPro/GRASP delivers three distinct aerosol vertical profiles: (i) a fine mode composed of black carbon, brown carbon, inorganic salts, and associated water uptake; (ii) a hydrophobic coarse mode representing mineral dust, decomposed into iron oxide and quartz species; and (iii) a hydrophilic coarse mode consisting of sea salt particles and their associated water content. The approach explicitly retrieves these three aerosol profiles, along with the fractional abundance of each of the six mentioned aerosol chemical species and their water content. This retrieval of aerosol chemical composition vertical profiles offers, for the first time, a direct observational link between aerosol optical measurements and their speciation resolved in altitude. We demonstrate the feasibility and performance of this technique through an innovative retrieval experiment, where synthetic lidar and polarimeter observations are generated using the MOCAGE chemical transport model and a comprehensive radiative transfer simulator (GRASP forward model). These pseudo-observations include multi-wavelength attenuated backscatter and depolarization ratios, along with polarized radiances across multiple viewing angles and spectral bands. Results from global-scale transects spanning marine, urban, dust, and complex mixture-dominated regions show that the retrieval captures with good fidelity the main features of vertical aerosol composition and their bulk optical properties. Fine-mode species are well retrieved, particularly in the boundary layer, even for atmospheres with complex mixtures of multiple aerosol species. Dust is accurately retrieved and well defined in terms of vertical extent, load, and composition, and sea salt concentrations are very well reproduced. Some limitations for deriving water content in the upper atmospheric layers are remarked. AEROCHEMPro/GRASP reliably derives optical properties such as aerosol optical depth (AOD), single scattering albedo (SSA), and lidar ratio (LR), demonstrating the approach's robustness. As this initial validation relies on a self-consistent simulation framework over a spatially uniform surface baseline, these results represent an optimistic, idealized theoretical performance limit. This new retrieval approach represents a significant advancement in spaceborne aerosol remote sensing, as it provides vertically resolved chemical speciation that is directly related to chemical transport model results. It offers new opportunities to improve our understanding of aerosol processes and their effects on climate and air quality.

Abstract Proxy records indicate a robust Atlantic Meridional Overturning Circulation (AMOC) during the Bølling–Allerød (BA) interstadial, despite substantial meltwater discharge. Using ocean circulation simulations, we demonstrate that enhanced tidal mixing, sustained by lower‐than‐modern sea levels, significantly strengthens the BA AMOC and increases its freshwater collapse threshold from 0.04 to 0.12 Sv. We find that the weak stratification characteristic of the BA period triggers a positive feedback that amplifies both tidal mixing and overturning. These results identify tidal mixing as a critical, yet previously underexplored, control on AMOC tipping behavior during abrupt climate transitions. Characterizing the AMOC's response to such forcings provides vital insights into the mechanisms governing AMOC stability and variability under future climate change scenarios.

Abstract The National Centre for Medium Range Weather Forecasting (NCMRWF) receives NOAA-21 Advanced Technology Microwave Sounder (ATMS), Cross-track Infrared Sounder (CrIS) data through the European Organisation for the Exploitation of Meteorological Satellites’ data dissemination system (EUMETCast). Necessary modifications were made to the NCMRWF Unified Model (NCUM) assimilation and forecasting system to assimilate NOAA-21 data, as well as similar data from S-NPP and NOAA-20. As an initial step, prior to the operational use of NOAA-21 data, background and analysis innovations from ATMS and CrIS observations were computed and compared with those from S-NPP and NOAA-20 satellites. The NOAA-21 ATMS and CrIS innovations were found to be comparable in magnitude to those from Suomi National Polar Orbiting Partnership satellite (S-NPP) and NOAA-20. Upon confirming the quality of NOAA-21 data using the Observation Processing System, and the impact of the data on the assimilation and simulation of two monsoon deep depression (DD) events over east India in September 2024 was evaluated by performing Observing System Experiments. Two sets of experiments were conducted: a control run, in which all observations except NOAA-21 data were assimilated to produce analysis files; and an experimental run, in which all observations were assimilated, including NOAA-21 ATMS and CrIS observations. The experiments focused on two recent DD events over east India, occurring during 8–11th September and 14–20th September 2024. Results indicate that assimilating NOAA-21 observations alongside the existing dataset led to distinct, positive improvements in the analyses and forecasts of both DD events. A maximum up to 6% improvement in equitable threat score at moderate thresholds of accumulated precipitation was achieved across the storm’s lifecycle in the experimental forecast. These findings highlight the notable contributions of NOAA-21 ATMS and CrIS data to improving analysis quality and forecast accuracy across multiple lead times.

Abstract Using a pan-Arctic coupled sea ice-ocean model, this study examines how Arctic river runoff influences sea ice distribution through a combination of thermodynamic and dynamic processes. Our results show that while runoff has little effect on total ice volume, it significantly redistributes ice thickness across the Arctic, with local changes exceeding 0.1 m (nearly 10%) in typical regions. We attribute this to a three‑stage mechanism. Direct thermodynamic effect, i.e., coastal melt from riverine heat and offshore protection from riverine freshwater, triggers initial changes in ice conditions that subsequently alter kinetic energy transfer from winds to sea ice. This modifies large-scale ice drift patterns, weakening the anticyclonic Beaufort Gyre and accelerating westward transport from the northern coast of Alaska to the East Siberian Sea. The resulting dynamic redistribution then activates a negative feedback whereby thinner ice promotes faster thermodynamic growth (and vice versa), partially offsetting the dynamic change. This mechanism may explain the spatial heterogeneity of sea ice variability in the context of ongoing ice retreat.

Early warning systems for weather-related hazards are critical tools for increasing societal resilience, but their effectiveness depends on reliability and ability to support decision-making in the face of uncertainty. While many operational contexts rely on expert judgment and predefined warning thresholds, decision-making processes rarely incorporate a formal assessment of the operational and economic impacts associated with the chosen actions. This study proposes a decision-theoretic approach for issuing flood warnings in Italy’s Sieve basin (Tuscany), integrating probabilistic precipitation forecasts from the national Civil Protection system with a statistical post-processing method to explicitly evaluate predictive uncertainty. The proposed framework introduces a loss function that adapts to multiple warning levels and accommodates different risk propensities (from risk-averse to risk-prone), effectively meeting the operational needs of diverse stakeholders. The system’s performance is evaluated by comparing a Bayesian decision scheme with one that explicitly accounts for precipitation forecast uncertainty, assessing cumulative economic costs against different baseline scenarios. Results demonstrate that adopting a Bayesian cost-based approach yields economic benefits and substantial improvements in the Relative Economic Value (REV). Furthermore, incorporating precipitation uncertainty drives the system toward more cost-effective warning strategies, reducing false alarms and expected losses.

Abstract The “salty-gets-saltier, fresh-gets-fresher” paradigm predicts that an intensifying hydrological cycle should freshen the climatologically fresh Southern Ocean. Here we show the opposite: sea surface salinity increased at ~0.03 decade − 1 across 40–50°S during 2004–2024, most coherently in the Pacific and Atlantic sectors. We attribute this salinification to the poleward expansion of the southern subtropical gyres, which advects saline subtropical water into latitudes of steepest meridional salinity gradient. The Subtropical and Subantarctic Fronts, tracked via the 35 and 34 isohalines, migrate poleward at unequal rates (–0.46° and –0.18° decade −1 ), narrowing the frontal corridor and sharpening the cross-frontal gradient. A mixed-layer budget shows that horizontal advection dominates the salinity trend, roughly tripling the opposing contribution from surface freshwater flux. Forced by poleward-intensifying westerlies and a positive Southern Annular Mode trend, this circulation-driven salinification demonstrates that ocean dynamics can override freshwater forcing, cautioning against interpreting salinity trends as direct fingerprints of the hydrological cycle.