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Coral Bleaching: The Equatorial-Refugia Hypothesis
No full textThe rising threat of marine heatwaves has led to numerous predictions that coral reefs, especially those near the Equator, will be severely degraded by the end of the current century. Yet, environmental conditions near the Equator may regionally moderate coral bleaching by reducing thermal stress during marine heatwaves. We deployed a Bayesian spatio-temporal model over Earth to examine which environmental conditions may characterize marine-heatwave refugia for coral reefs by testing the relationship between the severity of coral bleaching and a suite of temperature, hydrodynamic, topographic, atmospheric, and biological variables. The model considered the severity of coral bleaching as the proportion of bleached hard corals during 30,266 coral-reef surveys conducted at 8728 sites, at depths of up to 20 m, and located between 35° north and south of the Equator across 81 countries, from 2002 to 2020. Except for the eastern Pacific Ocean, the severity of coral bleaching during marine heatwaves was lower on equatorial reefs than on higher-latitude reefs, suggesting that marine-heatwave refugia for corals have been concentrated in the equatorial Coral Triangle region. Indeed, equatorial reefs in the Coral Triangle were, on average, exposed to the weakest marine heatwaves, potentially because they were shielded from extreme insolation by frequent cloud coverage in the Intertropical Convergence Zone. Coral bleaching may also be moderated during marine heatwaves on reefs that experience high wave energy, high current velocity, high cloud frequency, or turbidity. Coral bleaching was also less severe on reefs that historically endured frequent heatwaves than on reefs that were naïve to thermal stress. Based on modern and historical responses of coral reefs to acute thermal stress, we hypothesize that many equatorial reefs will continue to serve as marine-heatwave refugia for corals.We thank Sandra van Woesik and Chelsey Kratochwill for providing helpful editorial comments on the manuscript draft, Pallav Ray for an insightful discussion on cloud dynamics, and three anonymous reviewers for helpful comments and suggestions that improved the manuscript. We also thank Keith Van Graafeiland for deriving the tidal range data from the Finite Element Solution 2022 Tide product, which was (i) funded by the Centre National d'Etudes Spatiales, (ii) produced by the Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, NOVELTIS, and Collecte Localisation Satellites, and (iii) made freely available by Archivage, Validation et Interprétation des données des Satellites Océanographiques. The National Science Foundation (NSF) Graduate Research Fellowship Program provided funding to R.v.W. and Z.F. under award NSF DGE-2240237. NSF also provided funding to R.v.W. under award NSF OCE-2048319. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. This is contribution number 260 from the Institute for Global Ecology at the Florida Institute of Technology
High-Resolution Wind-Wave Modeling in the Al-Wajh Region
No full textWave modeling in shallow water has gained significant attention due to its intrinsic relation with coral reef ecosystems and coastal wave processes. Coral reefs act as natural breakwaters, modulating wave energy fluxes and shaping nearshore hydrodynamics, with direct implications for marine biodiversity, shoreline stability, and infrastructure resilience. Accurate prediction of these processes requires high-resolution numerical simulations that capture both regional forcing and fine-scale reef geometry.
In this study, we develop and validate a spectral wave modeling framework for the Al-Wajh lagoon using WAVEWATCH III on the Shaheen High-Performance Computer. We first corrected regional wind fields from the Arabian Peninsula Reanalysis to reduce systematic biases, achieving excellent agreement with scatterometer and SARAL altimeter observations. We then nested an unstructured grid model (50–500 m resolution) to resolve complex lagoon bathymetry and reef structures, successfully reproducing rapid wave height transitions and spatial patterns observed by Envisat. Spatial analysis quantified wave energy dissipation between 71.7% and 85.7%, directly attributed to reef-induced attenuation. The resulting high-resolution model delivers a robust tool for coastal management, ecological preservation, and infrastructure planning in coral-fringed environments, underscoring the necessity of precise input data and fine-scale modeling in forecasting shallow-water wave dynamics
STAR-RISs Versus Full-Duplex Decode-and-Forward Relaying: Which is Better?
No full textSimultaneous transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) technology was recently proposed to address the issue of the limited coverage area of RISs. This technology is capable of providing reflection and refraction for signals towards both sides of the surface. This letter compares STAR-RIS, conventional RIS and traditional half-duplex (HD) and full-duplex (FD) decode-and-forward (DF) relaying in terms of the rate and energy efficiency (EE). The simulation results showed that large surfaces and very high rates are needed in STAR-RISs to outperform the HD- and FD-DF relaying, where the EE is maximized by minimizing the total power
Interacting free boundaries in obstacle problems
No full textWe study obstacle problems governed by two distinct types of diffusion operators involving interacting free boundaries. We obtain a somewhat surprising coupling property, leading to a comprehensive analysis of the free boundary. More precisely, we show that near regular points of a coordinate function, the free boundary is analytic, whereas singular points lie on a smooth manifold. Additionally, we prove that uncoupled free boundary points are singular, indicating that regular points lie exclusively on the coupled free boundary. Furthermore, optimal regularity, nondegeneracy, and lower-dimensional Hausdorff measure estimates are obtained. Explicit examples illustrate the sharpness of assumptions.DJA thanks the Abdus Salam International Centre for Theoretical Physics (ICTP) for great hospitality during his research visits
The stability of shallow neural networks on spheres: A sharp spectral analysis
No full textWe present an estimation of the condition numbers of the \emph{mass} and \emph{stiffness} matrices arising from shallow ReLU neural networks defined on the unit sphere~. In particular, when is \emph{antipodally quasi-uniform}, the condition number is sharp. Indeed, in this case, we obtain sharp asymptotic estimates for the full spectrum of eigenvalues and characterize the structure of the corresponding eigenspaces, showing that the smallest eigenvalues are associated with an eigenbasis of low-degree polynomials while the largest eigenvalues are linked to high-degree polynomials. This spectral analysis establishes a precise correspondence between the approximation power of the network and its numerical stability
Assessment of piston and injector cap designs on the performance of a hydrogen direct-injection spark-ignition engine
No full textHydrogen is considered a critical solution in the transition to sustainable energy systems. This study provides the first comprehensive evaluation of the combined effects of piston geometry and injector cap design on the performance of a heavy-duty hydrogen direct-injection spark ignition engine using high-fidelity computational fluid dynamics simulations. Four piston geometries: ω-shaped, flat, pent-roof, and a hybrid of flat and pent-roof, were evaluated. Moreover, the hydrogen injector design was analysed by varying the number of cap holes (4-, 5-, and 6-hole) and the jet-included angle (±10˚), alongside two cap orientations (X and + ). The study found that different piston geometries significantly influenced hydrogen jet interaction with the piston wall and overall mixing. The flat piston produced a more homogeneous mixture before ignition, contributing to lower NOx emissions. Conversely, the bowl-shaped piston resulted in a strongly stratified mixture distribution and faster combustion, yielding the highest thermal efficiency while increasing NOx emissions. Although the + cap orientation was intended to guide the mixture toward the spark plug, it could not ensure a richer mixture at the spark plug. The 5-hole cap promoted a more uniform mixture and reduced NOx emissions. Furthermore, adjusting the jet-included angle by 10° led to more stratified mixing, leading to a slower combustion process and negatively impacting engine performance. Considering the best compromise between NOx emissions and fuel economy, the ω-shaped piston combined with a 5- or 6-hole cap injector exhibited superior performance over the 4-hole configuration, primarily in favor of the significantly reduced NOx emissions.This paper is based on work supported by Saudi Aramco Research and Development Center FUELCOM program under Master Research Agreement Number 6600024505/01. FUELCOM (Fuel Combustion for Advanced Engines) is a collaborative research program undertaking between Saudi Aramco and KAUST intended to address the fundamental aspects of fuel combustion in engines, and develop fuel/engine design tools suitable for advanced combustion modes. The computational simulations utilized the clusters at KAUST Supercomputing Laboratory. The authors thank Convergent Science Inc. for providing the CONVERGE license
Formation Kinetics and Yields of Secondary Organic Aerosol from Benzothiazoles Based on Oxidation Flow Reactor and Ambient Studies
No full textBenzothiazoles (BTHs) are an important class of emerging organic pollutants from volatile chemical product emissions which have been detected in atmospheric environments due to their extensive use, especially as vulcanization accelerators in tire production. However, studies of the atmospheric photochemical oxidation of BTHs remain very limited, hindering the assessment of their atmospheric impacts. Herein, we systematically investigated the reaction kinetics and yields of secondary organic aerosol (SOA) from benzothiazole (BTH, the parent compound of BTHs) photo-oxidation under various experimental conditions using an oxidation flow reactor. The rate constant for BTH reacting with OH radicals, (3.14 ± 0.20) × 10–12 cm3 molecule–1 s–1, was similar to that of single-ring aromatics like toluene, yet its SOA yield was comparable to that of bicyclic aromatics like naphthalene. Furthermore, we found that while NOx suppressed the BTH-derived SOA production, relative humidity enhanced its production. Further field measurements revealed significant SOA formation potential from BTH photo-oxidation in typical urban areas, comparable to that of well-known benzene or naphthalene. Gaseous BTH in an offshore atmosphere was also detected, indicating its potential impacts on marine environments. Our results elucidate the atmospheric photochemical processes of BTH, revealing its important but previously overlooked contribution to ambient SOA formation.This work was supported by the National Natural Science Foundation of China (NSFC) under Grant No. 42525301 and No. 42275124, the Key Research Program of Frontier Sciences from the Chinese Academy of Sciences (ZDBS-LY-DQC001), the Shaanxi Innovation Team for Science and Technology (2024RS-CXTD-40), the New Cornerstone Science Foundation through the XPLORER PRIZE, the Science and Technology Development Fund, Macao Special Administrative Region, China (File No. 0031/2023/AFJ) and a multiyear research grant (No. MYRG-GRG2024-00032-FST-UMDF) from the University of Macau
A Generalization of Cardy’s and Schramm’s Formulae
No full textWe study critical site percolation on the triangular lattice. We find the difference of the probabilities of having a percolation interface to the right and to the left of two given points (such that the union of the triangles intersecting the interface does not separate the points) in the scaling limit. This generalizes both Cardy’s and Schramm’s formulae. The generalization involves a new interesting discrete analytic observable and an unexpected conformal mapping.The second author is supported in part by the Center of Excellence for Generative AI at King Abdullah University of Science and Technology (KAUST). The third author is supported in part by the Swiss NSF, ERC Advanced Grants 340340 and 741487, and National Center of Competence in Research (NCCR SwissMAP). Section 3 was written entirely under the support of Russian Science Foundation grant
19-71-30002. Appendix A was written entirely under the support of the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-15-2022-287). The authors are grateful to I. Benjamini, O. Feldheim, G. Kozma, I. Novikov, D. Speyer, A. Ustinov for useful discussionsOpen access publishing provided by King Abdullah University of Science and Technology (KAUST
Optimization of Vertical Well Pattern and Hydraulic Fracture in Offshore Low-Permeability Reservoir: The Role of Low-Velocity Nonlinear Flow
No full textSummary
Offshore low-permeability oil reservoirs are an important component of global energy reserves, yet the drilling and fracturing costs are significantly higher than those onshore. Thus, offshore development typically requires fewer wells with higher production, which leads to larger well spacing, wider low-pressure gradient areas, and more obvious low-velocity nonlinear flow. Therefore, there is an urgent need to investigate the nonlinear flow on the well and the fracture design in offshore low-permeability reservoirs. Here, we first establish a reservoir simulation method with multimechanism, including low-velocity nonlinear flow, fracture conductivity reduction, matrix stress sensitivity, complex fault block, and fracture. A grid preprocessing technology is developed to implement embedded discrete fracture on corner point grid. Then, the nonlinear flow parameters are fitted based on experimental data. Finally, the optimal design of well and fracture is investigated. The results show that the staggered well pattern is first recommended, followed by opposite well pattern. As the matrix permeability increases, the recommended well spacing gradually increases, and the fracture penetration ratio and conductivity gradually decrease. The lower the matrix permeability, especially below 10×10−3 μm2, the more significant the nonlinear flow characteristics in the target reservoir block. When the matrix permeability is 2×103 μm2, compared with Darcy flow, the optimal well spacing, fracture penetration ratio, and conductivity differ by 50%, 17%, and 13.6%, respectively.This work was partly supported by the National Natural Science Foundation of China (52304045); Open Fund (PLN202340) of the National Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University); and Open Fund (2024-KFKT-08) of China National Petroleum Corporation Science and Technology Research Institute. We are grateful for anonymous reviewers' valuable comments and precious suggestions and the research group of the MATLAB Reservoir Simulation Toolbox
Effect of applied AC electric field on flame spread over electrical wire with cross-linked polyethylene insulation
No full textThe effect of applied AC electric field on flame spread over electrical wires with NiCr-core insulated by cross-linked polyethylene (XLPE) is experimentally investigated by varying the AC voltage and frequency. Results are compared with those for low-density polyethylene (LDPE) insulation, commonly studied in fire safety research. For the baseline case without applying electric field, XLPE-insulated case exhibits distinct behaviors such as flame splitting and a unique molten dripping via merging of newly-formed globular molten XLPE, which were not observed in LDPE-insulated one. Under applied electric fields, the flame spread rate (FSR) and molten insulation dynamics differ markedly between XLPE and LDPE. Two regimes of FSR behavior are identified for XLPE and three for LDPE, depending on voltage and frequency. At high voltage and frequency, induced magnetic fields promote flame vortex formation, increasing flame width and FSR, while excessive conditions lead to flame extinction through mass loss via electrospray and dielectrophoresis. Scaling analyses are applied to elucidate the underlying mechanisms. The flame spread rates are phenomenologically characterized depending on these various phenomena in terms of the frequency and voltage, especially emphasizing the electric field intensity on the unburned wire surface. The extinction conditions are correlated with AC voltage and frequency.This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-RS-2023–00241310). ZL was supported by School of Mining and Geomatics, Hebei University of Engineering