We investigate the generation and stability of ion acoustic waves (IAWs) in the ramp region of the quasi-perpendicular Earth’s bow shock. Using a fluid model, we derive the dispersion relation of IAWs, assuming a flat-top electron velocity distribution function typically observed at interplanetary and Earth’s bow shocks, along with jumps in plasma variables. Our findings show that these electrostatic modes are non-dispersive within the shock ramp, which is in agreement with in-situ observations from the Magnetospheric Multiscale Mission (MMS). The calculated frequencies and phase velocities align closely with MMS measurements. Furthermore, we find that the growth rate of IAWs is more strongly affected by the ion temperature jump across the shock than by the electron temperature jump. The methodology developed in this work can be extended to investigate the generation and stability of other wave modes in diverse space and astrophysical plasma environments.

Based on over 8 years of moored observation data near the Xisha Islands in the South China Sea (SCS), this study investigated the seasonal variability and vertical distribution of near-inertial waves (NIWs) and internal tides (ITs). It also investigated the impact of mesoscale cyclonic eddies on the downward propagation of NIWs, as well as the mechanism of nonlinear interactions between NIWs and ITs following typhoons. Both NIWs and ITs exhibited significant seasonal variations. Near-inertial kinetic energy (NIKE) was stronger in autumn and winter, while internal tide energy peaked in both summer and winter. Over the entire observation period, ITs were primarily dominated by low modes. In contrast to internal tides, which are dominated by low modes, NIWs exhibit high-mode structures under the influence of negative vorticity. During periods of positive vorticity, the modal structures of NIWs show significant variability. In addition, the study found that cold eddies can also facilitate the downward transfer of near-inertial energy. Taking the autumn of 2011 as an example, under the influence of positive vorticity, NIKE was transmitted downward to the maximum observed depth of 543m, contributing to deep-ocean mixing. The study also revealed that typhoon-induced NIWs, characterized by strong vertical shear, played a dominant role in the nonlinear interactions between NIWs and ITs, leading to the enhancement of nonlinear coupled waves.

Back-propagating earthquakes, characterized by a secondary front reversing into previously ruptured regions, have been recently documented due to advances in seismological techniques. While rupture complexity is often attributed to fault heterogeneity, its underlying physics remains unclear. Here, we employ earthquake simulations with rate-state friction to reveal that back-propagating fronts can occur on frictionally homogeneous faults if ruptures propagate unilaterally, for example when faults are subject to stress gradients due to lateral loading. Our analytical model demonstrates that unilateral propagation, coupled with velocity-dependent friction, precludes stable crack-like solutions at constant residual stress. Instead, ruptures alternate between pulse and crack-like mode, with back propagation generated at pulse-to-crack transitions. The crack-to-pulse transition is driven by velocity-weakening friction and happens when the slip rate becomes non-uniform and decreases below a specified threshold, a phenomenon that can occur under less stringent conditions without requiring heterogeneity in fault properties or stress. We find that back propagation is more likely to occur under conditions of slow rupture propagation and low stress drop. Observational examples of back propagation on simple faults align with our estimated conditions that favor this phenomenon. Our study presents a simple, fundamental model for back-propagating earthquakes, indicating that they may be more prevalent than previously acknowledged.

Time-lapse seismic full waveform inversion (FWI) estimates dynamic changes in subsurface properties using waveform data collected in seismic surveys conducted at multiple epochs. Since FWI problems are nonlinear and solutions are non-unique, uncertainties must be accounted for if decisions based on the results are to be trusted. Previous methods either produce unrealistically high uncertainty estimates for time-lapse changes, or incur extreme memory and computational requirements by inverting multiple datasets simultaneously. We propose a methodology for conducting time-lapse FWI, and quantifying uncertainties in the maximum a posteriori (MAP) solutions of time-lapse changes, particularly tailored for subsurface CO2 monitoring in carbon capture and storage scenarios. We first employ a fully nonlinear, variational Bayesian inversion to invert baseline seismic data. Random samples from the posterior distribution are refined to obtain a set of samples of the baseline MAP model manifold, which are used as strong prior information for the monitoring inversion. In the monitoring inversion we update baseline MAP samples to find MAP solutions for the monitoring data. In this step, we employ a linearised, target-oriented strategy because time-lapse changes caused by CO2 injection and migration are often small and may be spatially localised, further increasing the strength of prior information. We demonstrate that even if extremely sparse acquisition geometries are used in the monitoring survey to reduce cost as is necessary for monitoring stored carbon dioxide, this method estimates time-lapse variations and uncertainties that are sufficiently accurate to inform subsequent decision-making.

Anticipating the onset of the 2020/21 effusive-explosive eruptive sequence at La Soufrière volcano, St. Vincent was challenging despite the established monitoring networks in operation. Here, we integrate petrological data to decipher retrospectively signs of imminent eruption from available pre-eruptive monitoring data. Using diffusion chronometry, we estimated timescales over which magmas transported to the surface. We examined olivine crystals hosted in basaltic andesite scoria, categorising them into four groups based on their textures (euhedral to anhedral) and core compositions (Fo73-89). Multiply zoned olivine populations are tracked through a multi-stage journey from depth to surface corresponding to periods of magma ascent and accumulation years before eventual eruption. This correlates temporally with two phases of unrest from monitoring data: (1) a protracted priming phase (lasting more than a decade) manifesting in low-level seismicity, small but inherently significant crater transformations and an elevated CO2 degassing signal; and (2) a subsequent transition phase, initiating just over a year before eruption with the onset of geophysical unrest in the form of discrete episodes of elevated seismicity and volcano inflation. Our findings provide new insight into the dynamics of magma mobilisation at La Soufrière. We demonstrate that magmatic unrest in the roots of the sub-volcanic system precedes geophysical precursors by years, drawing connections between individually ambiguous surface signals over long timescales. Monitoring strategies optimised to detect early stages of magmatic unrest, such as identifying and locating rarer deep seismicity and routine sampling at the crater plume, could improve future responses to volcanic crises on St. Vincent.

This chapter presents important experimental and numerical findings regarding the dynamics of gravity currents in various stratified environments. First, the macro-and microstructure of gravity currents descending an inclined bed in linearly stratified environments were studied using high-speed camcorders and particle image velocimetry technology. Results indicate that the dynamics of gravity current are complicated by ambient stratification. Two equations with three fitted parameters from experimental data were proposed to predict the velocity profiles of gravity current. Secondly, gravity currents released from two-layer stratified locks were also experimentally studied, with emphasis on the effect of the initial height ratio (ℎ 𝑅) and lock aspect ratio (𝑅 𝑙) on the mixing process. Analysis reveals the importance of ℎ 𝑅 and 𝑅 𝑙 in determining the front velocity and mixing process within the gravity current. Finally, the dynamics of turbidity currents in linearly stratified environments were numerically simulated with the aim of elucidating the effects of ambient stratification, bed slope, and particle settling velocity on the evolution process of the currents. The study demonstrates that if the relative stratification parameter (i.e., 𝑆𝑟 = 𝜌 ̂𝐵 − 𝜌 ̂𝑆/𝜌 ̂𝐿 − 𝜌 ̂𝑆) is greater than unity (i.e., 𝑆𝑟 > 1), the head of the current would separate from the slope and intrude into the environment at the level of neutral buoyancy. Strong ambient stratification reduces the rate at which potential energy is converted to kinetic energy, while a higher particle settling velocity accelerates the rate at which kinetic energy is dissipated. We conclude that the velocity and fluid structure of gravity currents can be complicated by ambient stratification and present a theoretical model that accurately predicts the separation depth of turbidity currents in density-stratified environments.

Predicting water and gas movement through the vadose zone is of key importance for applications such as flow of liquids and gases in the subsurface in response to underground explosions. Understanding a rock’s fluid flow properties, such as porosity, permeability, and the capillary pressure curve are crucial to predict these water and gas movements in soil and rock. We use centrifugal drainage to track water loss in volcanic tuff core samples through time. For two tuff samples from Aqueduct Mesa, permeability, porosity, and van Genuchten model parameters were estimated by fitting observations of mass of water lost to PFLOTRAN predictions. The method works well for relatively high permeable samples; testing less permeable samples is still possible but took longer to drain and required higher centrifugal speeds. Drainage testing provides the most information about the portion of the capillary pressure curve near full liquid saturation, while imbibition tests constrain the behavior of samples near residual liquid saturation. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This is SAND2024-09706A.

This study investigates the effectiveness of ensemble singular vectors (EnSVs) for a mesoscale convective system over the East China Sea, a challenge due to its strong nonlinearity. Employing breeding ensembles with varying horizontal resolutions, we compare linear EnSV predictions with nonlinear perturbed forecasts. The results indicate that EnSVs consistently capture sensitivity to synoptic-scale features across all resolutions. However, the growth rate of the EnSVs decreases with increasing resolution. While nonlinear effects become more pronounced at higher resolutions, the nonlinearly developed perturbations from initial EnSVs by full models still demonstrate significant growth in the target region. The nonlinearity is likely associated with mesoscale error growth. These findings suggest that mesoscale EnSVs are capable of identifying key growing modes, but they have limitations in fully capturing the nonlinear amplification of errors inherent in mesoscale phenomena.

A document by Jagos Radovic. Click on the document to view its contents.

This study aimed to examine the variations in vorticity in the northern South China Sea (NSCS) resulting from the cutoff of the Kuroshio main path east of Taiwan Island. Absolute dynamic topography and satellite altimeter eddy tracking data from 1993 to 2021 were employed to analyze the momentum ratio of the Kuroshio and mesoscale eddies concerning Kuroshio cutoff events through their interaction. The results revealed 12 events where mesoscale cyclonic eddies east of Taiwan Island cut off the Kuroshio main path, resulting in intensified loop currents in the Luzon Strait and increased negative vorticity in the NSCS. Six of these events occurred between 1998 and 2013, coinciding with the negative Pacific Decadal Oscillation (PDO) regime. During this period, negative vorticity triggered by Kuroshio cutoff events rose by 79% in the NSCS, compared to 34% during the positive PDO regime. The Kuroshio interacted with mesoscale cyclonic eddies east of Taiwan for 28 days during the negative PDO regime and 41 days during the positive PDO regime. The Kuroshio velocity strengthened east of Luzon Island but weakened east of Taiwan Island during the negative PDO regime, elucidating the observed disparity. The average duration between the occurrence of mesoscale cyclonic eddy impinging on the Kuroshio east of Taiwan Island and the peak negative vorticity observed in the NSCS was approximately 28 days. This was comparable to data obtained using field measurements.

Analysis of conversions between compressional and shear waves is a workhorse method for constraining crustal and lithospheric structure on Earth; yet, such converted waves have not been unequivocally identified in seismic data from the largest events on the Moon, due to the highly scattered waveforms of shallow seismic events. We reanalyze the polarization attributes of waveforms recorded by the Apollo seismic network to identify signals with rectilinear particle motion below 1 Hz, arising from conversions across the crust-mantle boundary. Delay times of these converted waves are then inverted to estimate crustal thickness and wave speeds beneath the seismometers. Combined with gravimetric modeling, these new crustal thickness tie-points yield an updated lunar crustal model with an average thickness of 29-47 km. The model provides critical context for future lunar exploration and geophysical studies, predicting a 15-36 km thick crust at Schrödinger basin and 29-52 km at potential Artemis III landing locations.

The hydrological connectivity between channels and floodplains is modulated by vegetation and system geometry and plays a crucial role in determining the water transport timescale (WTT) in river deltas. It is well known that river deltas dynamically respond to many drivers including sea level rise, sediment composition, river discharge, and vegetation conditions, but less understood is how WTT changes throughout a delta's morphological evolution. In this study, a reduced-complexity model called pyDeltaRCM is used to examine the effects of sea level rise, vegetation, and sediment composition on system scale and local WTT throughout the evolution of a river delta. The model operates on stochastic parcel-based cellular routing systems for water, sediment, and phenomenological rules for sediment deposition and erosion. Due to the stochastic nature of pyDeltaRCM, ensemble results with or without vegetation were obtained for each set of boundary conditions. Absolute transport times are predicted to decrease with sea level rise, and WTT is anticipated to decrease as sand fraction rises. The impact of vegetation is influenced by the degree of hydrological connectivity with the delta floodplain. The findings of this study will have significant implications for enhancing our comprehension of the evolutionary patterns of hydrological transport in nonstationary coastal environments. Keywords: River delta, Water transport times, pyDeltaRCM, Vegetation, Sea level rise.

• Time-series forecasting methods, including a Lotka-Volterra inspired and ARIMA, effectively predict ensembles of five consecutive years over the Rio de la Plata Grasslands. • Increasingly available long-term annual LULC maps are fundamental to support short-term projections on threatened ecosystems.

The Beaufort Gyre mean state is theorized to be set by surface wind stress, local sea ice, and subsurface mesoscale eddy transport. The mesoscale eddies in this context are subject to an important restriction: they must be generated within the Beaufort Gyre region despite the presence of a heavy sea ice cover. This study considers how sea ice drag affects the characteristics of the Beaufort Gyre mesoscale eddy field. We use carefully constructed high-resolution (2km x 2km) MITgcm simulations forced by the Beaufort Gyre mean state to spin up a steady state eddy field. Each simulation is initialized with varying sea ice concentrations — a proxy for ice induced drag — and characterized by their steady state density transport and eddy kinetic energy. We find that simulations subject to high ($\geq80$\%) sea ice concentrations indeed generate a halocline intensified turbulent eddy field with max eddy energy near $2 \times 10^{-3}$ \unit{\metre\squared\per\second\squared}. In contrast, low sea ice concentration simulations exhibit surface intensified mixing with much higher maximum eddy energy $\order{10^{-2}\,\unit{\metre\squared\per\second\squared}}$. While high drag conditions largely isolate eddy transport to within the halocline, low drag conditions exhibit vertically coupled eddy transport extending from the surface and through the halocline to quiescent depths. This work both suggests the presence of locally generated halocline intensified eddies and provides insight into the future behavior of the Beaufort Gyre as we slowly transition to an ice free Arctic.

Understanding where and by how much sea-level rise (SLR) projections from ice-sheet models are affected by changes in remotely-sensed ice velocity observations used for calibration is essential for evaluating the accuracy and stability of these models, as well as identifying regions where improved observations can enhance model predictions, guiding future data collection efforts. Here, Automatic Differentiation (AD) and data assimilation provided an efficient method for this analysis via the generation of sensitivity maps that linked changes in the model forecasts to variations in model inputs (parameters and observations). Our results show that SLR projections are most sensitive to ice velocity variations at grounding zones and to changes in the basal drag coefficient in the same locations, while upstream velocities significantly influence long-term projections (>40 years). Sensitivity maps for Thwaites emphasise the importance of velocity observations in areas affected by tides, highlighting the need for targeted data collection in these regions.

We utilize seismograms recorded by dense linear and 2D arrays across the Banning Fault (BF) and Mission Creek Fault (MCF) strands of the Southern San Andreas Fault (SoSAF) in the Coachella to conduct a comparative analyses of seismic signals and fault structures in the study area. Continuous seismic waveforms are segmented and categorized into earthquake signals, traffic events and non-traffic noise. The seismic signal characteristics reveal differences in the dominant wave types present at the two faults. Beamforming is performed on various signals within 2D subarrays around the fault traces, retrieving apparent Rayleigh wave velocities in the frequency range of 5-10 Hz and various local propagation directions of earthquake phases from regional events. The results indicate prominent bimaterial interfaces in the structures of the BF and MCF, and shallow asymmetric damage zones across both faults with reduced seismic velocities to the northeast in the top ~100 m of the crust. These asymmetric damage zones produce local reversals of the velocity contrast with respect to the structure at depth of several kilometers, and are consistent with preferred propagation directions of earthquake ruptures on both faults to the northwest. The MCF has a more pronounced damaged fault zone, as suggested by scattered seismic wavefields and a larger reduction in subsurface velocities (35.9%) compared to the BF (13.7%). Our observations suggest that MCF is the primary strand of the SoSAF in the area, in agreement with previous studies, and the northeast branch of the BF likely connects to the main BF at depth.

Solar radiation management with stratospheric aerosol injection has been proposed as a potential mechanism to mitigate global warming and prevent it from reaching the tipping point. This study investigates the response of surface cloud radiative effects (CREs) to stratospheric aerosol injection (SAI) relative to Shared Socio-Economic Pathways (SSP2-4.5) across three regions: southern West Africa (SWA), Central Africa (CA) and the Sahara (SA). We utilize simulations from the Community Earth System Model version 2 (CESM2) with the Whole Atmosphere Community Climate Model version 6 (WACCM6) under SAI deployment, comparing the outputs to those from the SSP2.4-5 scenario, which aligns with current climate policy scenarios. The findings indicate that SAI has the potential to mitigate the decreasing trend of shortwave cloud cooling by -0.7W/m², -0.81W/m², -0.17W/m², while enhancing the longwave warming by +1.11W/m², +0.65W/m² and +0.31W/m², over CA, SWA, and SA, respectively. However, it is important to note that these observed changes may be attributable to natural variability rather than the direct effects of SAI, and should be taken with caution. An exception is the longwave cloud warming, which exhibits robust changes over CA. The results also reveal that changes in shortwave cloud cooling effect demonstrate high sensitivity to changes in liquid water path whereas changes in longwave cloud warming tend to exhibit greater sensitivity to variations in cloud fraction. It is noteworthy that current simulations involving SAI lack sufficient variables to facilitate comprehensive comparisons among different outputs.

In 2021, heavy rainfall from mid-August to late August caused severe floods in China’s Yangtze River Basin. Our analysis reveals that intraseasonal (30-80 day) oscillation contributed to more than half of the total rainfall anomalies. A concurrent dipolar northward propagating boreal summer intraseasonal oscillation (BSISO) episode coincided nicely with this extreme event, but it alone cannot explain its extremity. Through nonlinear K-means cluster analysis, we identify a new type of BSISO, termed Slanted Northward Propagating (SNP) BSISO. The 2021 event was categorized as one of eight cases of SNP BSISO characterized by a southwest-northeast orientated and westward-displaced convection structure over the northwestern Pacific and South China Sea. Additionally, the development of La Niña significantly influenced the SNP BSISO, generating specific wind patterns over the northwest Pacific and enhancing the moisture over the Maritime Continent, which facilitates the westward expansion of western Pacific subtropical high and occurrence of 2021 event.