GRL

Different meteor radars at low latitudes observed abnormally strong westward mesospheric winds around the March Equinox of 2023, that is, during the first phase of the Mesospheric Semiannual Oscillation. This event was the strongest of at least the last decade (2014–2023). The westward winds reached −80 m/s at 82 km of altitude in late March, and decreased with increasing altitude and latitude. A considerable increase in the diurnal tide amplitude was also observed. The Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension constrained to meteorological reanalysis up to ∼50 km does not capture the observed low-latitude behavior. Additionally, these strong mesospheric winds developed during the westerly phase of the Quasi-Biennial Oscillation, in accordance with the filtering mechanism of gravity waves in the stratosphere proposed in previous works. Finally, analysis of SABER temperatures strongly suggests that the breaking of the migrating diurnal tide may be the main driver of these strong winds.

Anti-dipolarization fronts (ADFs), characterized by the rapid increase of the negative magnetic field Bz component, are typically formed at the leading edge of the tailward reconnection jets in the Earth's magnetotail. To date, the electron-scale current structures, which govern the energy conversion at ADFs, are still barely understood due to the lack of high-resolution measurements. Here, using Magnetospheric Multiscale mission, we for the first time report a tailward ADF associated with strong field-aligned current (FAC). The FAC appears at the leading part of the ADF and its densities can reach about 200 nA/m2, which is significantly larger than those reported before. Such current is primarily contributed by the electron flow, which also forms electron beam distribution in the anti-parallel direction. Significant energy conversion (E⋅ J, E is electric field and J is current density) is also observed at the ADF, which is mainly contributed by the FAC and the fluctuating electric fields. This study makes essential steps toward understanding the current system and the energy conversion at the ADF in the Earth's magnetotail.

Silver is a highly toxic element for marine organisms. However, its controlling factor in marine sediments remains largely unknown, limiting our understanding of its biogeochemical cycling. Based on a sediment core from the Vietnam upwelling area in the South China Sea, it is found that Ag is significantly enriched in sediments of this area (as high as 0.39 μg/g), and it shows a very similar geochemical behavior to Ca and Sr. Our study supports the theory that Ag could be a marine paleo-productivity indicator. Burial of Ag over the past 3,200 years shows an abrupt increase at around 1850 CE, in concordance with the global atmospheric CO2 record. It is hypothesized that elevated CO2 and global warming enhance marine productivity in the Vietnam coastal upwelling area, favoring the burial of Ag. Human-induced global warming thus significantly impacts its biogeochemical cycling.

Dry lightning is a prevalent episodic natural ignition source for wildfires, particularly in remote regions where such fires can escalate into uncontrollable events, burning extensive areas. In this study, we aimed to understand the interplay of environmental, fuel, and geographical factors in evaluating the probability of fire initiation following dry lightning strikes in Tasmania, Australia. We integrated dry lightning, active fire records, and gridded data on fire weather, fuel, and topography into a binary classification framework for both fire-initiating and non-fire-causing lightning strikes. Employing statistical and machine learning techniques, we quantified the likelihood of fire initiation due to dry lightning, with the resampled Random Forest model exhibiting notable performance with an ROC-AUC value of 0.98. Our findings highlight how fuel characteristics and moisture content associated with particular vegetation types influence fire initiation and provide an objective approach for identifying susceptible regions of dry lightning ignitions, informing associated fire management responses.

Idealized general circulation models (GCMs) suggest global-mean precipitation ceases to increase with warming in hot climates because evaporation is limited by the available solar radiation at the surface. We investigate the extent to which this generalizes in comprehensive GCMs. We find that in the Community Atmosphere Model, global-mean precipitation increases approximately linearly with global-mean surface temperatures up to about 330 K, where it peaks at 5 mm day−1. Beyond 330 K, global-mean precipitation decreases substantially despite increasing surface temperatures because of increased atmospheric shortwave absorption by water vapor, which decreases the shortwave radiation available for evaporation at the surface. Precipitation decreases in the tropics and subtropics but continues to increase in the extratropics because of continuously strengthening poleward moisture transport. Precipitable water increases everywhere, resulting in longer water-vapor residence times and implying more episodic precipitation. Other GCMs indicate global-mean precipitation might exhibit a smaller maximum rate and begin to decrease at lower surface temperatures.

As a key factor influencing the cloud life cycle and radiative properties, liquid-ice mass partitioning remains a major source of uncertainties in modeling mixed-phase clouds. One of the unresolved problems is that liquid-ice mixing is highly inhomogeneous, but it has not been well understood and quantified for parameterization. In this study, the liquid-ice mixing homogeneity (χ) is quantified using the information-theoretic entropy based on airborne measurements. It is demonstrated that χ is positively correlated with the condensed water content (CWC). Cloud regions with low χ are consist of liquid and ice clusters. With the increase in χ, the size and frequency of continuous mixed-phase clusters increase. For a given CWC, χ is lower at relatively warm temperatures as sedimentation of large ice crystals can enhance the inhomogeneity. The strong positive relationship between CWC and χ indicates CWC should be considered when parameterizing the liquid-ice mixing in models.

Regional floods result from various flood generation mechanisms. Traditional analyses mainly link flooding to extreme rainfall, with limited input from soil moisture. Total water storage (TWS) is a holistic measure of basin wetness, including additional storage components from surface water, snow, and groundwater. Utilizing a new 5-day Gravity Recovery and Climate Experiment and its Follow On (GRACE(-FO)) data set, we investigated the linkage between short-term TWS anomaly (TWSA) and regional flooding. The 5-day TWSA solutions revealed flood signals missed by monthly TWSA solutions. Global basins exhibit distinct storage-discharge co-evolution patterns, offering new insights into flood mechanisms and propensity. Our bivariate event analyses show the annual maximum river discharges co-occur more often with the TWSA maxima than with precipitation in many basins. Further analyses revealed TWSA's time-lagged effect on river discharge, particularly in basins susceptible to floods triggered by saturation-excess runoff. The 5-day TWSA provides a new source of information for enhancing global flood preparedness.

No abstract is available for this article.

Fault normal stress (σ n ) changes dynamically during earthquakes. However, the impact of these changes on fault strength is poorly understood. We explore the effects of rapidly varying σ n by conducting rotary-shear experiments on simulated fault gouges at 1 μm/s, under well-drained, hydrothermal conditions. Our results show both elastic and anelastic (time-dependent but recoverable) changes in gouge layer thickness in response to step changes and sinusoidal oscillations in σ n . In particular, we observe dilation associated with marked weakening during ongoing σ n -oscillations at frequencies >0.1 Hz. Moreover, recovery of shear stress after such oscillations is accompanied by transient (anelastic) compaction. We propose a microphysically based friction model that explains most of the observations made, including the effects of temperature and step versus sinusoidal perturbation modes. Our results highlight that σ n -oscillations above a specific frequency threshold, controlled by the loading regime and frictional properties of the fault, may enhance seismic hazards.

Saturn Kilometric Radiation (SKR), being the dominant radio emission at Saturn, has been extensively investigated. The low-frequency extension of SKR is of particular interest due to its strong association with Saturn's magnetospheric dynamics. However, the highly anisotropic beaming of SKR poses challenges for observations. In most cases, the propagation of SKR is assumed to follow straight-line paths. We explore the propagation characteristics of SKR across different frequencies in this study. An extended equatorial shadow region for low-frequency SKR is identified, resulting from the merging of the Enceladus plasma torus and the previously known equatorial shadow zone. Ray-tracing simulations reveal that low-frequency (≲ $\lesssim $100 kHz) SKR is unable to enter the shadow region and is instead reflected toward high latitudes. In contrast, high-frequency SKR (≳ $\gtrsim $100 kHz) generally propagates without hindrance. Observations suggest that some low-frequency SKR can enter the shadow region through reflection by the magnetosheath or leakage from the plasma torus.

The mechanisms by which clouds impact the variability of the mid-latitude atmosphere are poorly understood. We use an idealized, dry atmospheric model to investigate the relationship between Atmospheric Cloud Radiative Effects (ACRE) and annular mode persistence. We force the model with time-varying diabatic heating that mimics the observed ACRE response to the Southern Annular Mode (SAM). Realistic ACRE forcing reduces annular mode persistence by 5 days (−16%), which we attribute to a weakening of low-frequency eddy forcing via modified low-level temperature gradients, though this effect is partly compensated by reduced frictional damping due to zonal wind anomalies becoming more top-heavy. The persistence changes are nonlinear with respect to the amplitude of ACRE forcing, reflecting nonlinearities in the response of the eddy forcing. These results highlight the ACRE's impact on low-frequency eddy forcing as the dominant cause of changes in annular mode persistence.

Projections of sea-level rise from ice-sheet shrinkage in a warming world have large uncertainties, linked to limited knowledge of changes at the ocean-ice sheet interface. This interface most typically is modeled as a grounding line, across which still-connected ice flows into the ocean to float as an ice shelf, or where icebergs calve from a cliff before the ice begins to float. But, extensive and rapidly increasing evidence shows that this is really a grounding zone, and that processes in this grounding zone omitted from many models could exert major controls on sea-level rise.

In July and August 2022, the Yangtze River basin (YRB) experienced its hottest summer since 1961. The SINTEX-F2 seasonal prediction system initialized in early May predicted the hotter-than-normal summer due to its successful prediction of central Pacific La Niña, negative Indian Ocean Dipole and the resultant warming in the tropical West Pacific-East Indian Ocean (TWP_EIO). The common SST forcing explains only about 26% to the heatwave strength, while the internal variations in the anomalous warming in the TWP_EIO and Europe, surplus precipitation in Pakistan, and local land-air interaction account for approximately 65%, based on the analysis of 108 ensemble members. These factors have collectively increased the maximum temperature over the YRB through the enhancement and westward expansion of western North Pacific subtropical high. Our findings quantify the relative contributions of external forcing and internal variations to the unprecedented hot event, offering insights into its forming mechanism and potential predictability.

Currently, there is no clear understanding of the comprehensive set of variables that controls fluxes of relativistic electrons within the outer radiation belt. Herein, the methodology based on causal inference is applied for identification of factors that control fluxes of relativistic electrons in the outer belt. The patterns of interactions between the solar wind, geomagnetic activity and belt electrons have been investigated. We found a significant information transfer from solar wind, geomagnetic activity and fluxes of very low energy electrons (54 keV), into fluxes of relativistic (470 keV) and ultra-relativistic (2.23 MeV) electrons. We present evidence of a direct causal relationship from relativistic into ultra-relativistic electrons, which points to a local acceleration mechanism for electrons energization. It is demonstrated that the observed information transfer from low energy electrons at 54 keV into energetic electrons at 470 keV is due to the presence of common external drivers such as substorm activity.

Using NASA's Global-scale Observations of the Limb and Disk (GOLD) imager, we report nightside ionospheric changes during the G5 super geomagnetic storm of 10 and 11 May 2024. Specifically, the nightside southern crest of the Equatorial Ionization Anomaly (EIA) was observed to merge with the aurora near the southern tip of South America. During the storm, the EIA southern crest was seen moving poleward as fast as 450 m/s. Furthermore, the aurora extended to mid-latitudes reaching the southern tips of Africa and South America. The poleward shift of the equatorial ionospheric structure and equatorward motion of the aurora means there was no mid-latitude ionosphere in this region. These observations offer unique insights into the ionospheric response to extreme geomagnetic disturbances, highlighting the complex interplay between solar activity and Earth's upper atmosphere.

The earliest Asian summer monsoon onset (SMO) occurs in the Bay of Bengal (BoB), heralding the coming of the rainy season. In late April or early May, the strong sea surface temperature (SST) diurnal variation accompanied by ocean surface warming triggers the SMO. However, this observed diurnal cycle intensity cannot be reasonably simulated by state-of-the-art climate models, resulting in a spurious delayed SMO. To address this issue, the SST diurnal cycle parameterized by a diagnostic sublayer scheme was incorporated into a climate model named FIO-ESM v2.0. The large diurnal amplitude of SST contributes to surface warming and changes atmospheric circulation. Consequently, the high-pressure anomaly at high levels and an inverted trough at low levels promote more convective activity, triggering an earlier SMO. Our findings improve the ability of climate models in simulating the evolution of the Asian monsoon system.

Powerful lightning strikes generate broadband electromagnetic signals. At Extremely Low Frequencies (ELF), the signal partly leaks into the ionosphere and produces whistlers that can be detected by satellites. Indeed, the satellites of the European Space Agency (ESA) Swarm Earth Explorer mission can detect those signals during 250 Hz burst-mode acquisition campaigns of their Absolute Scalar Magnetometers (ASM). The dispersion of these whistlers depends on their propagation path and the distribution of ionization in the ionosphere crossed along that path. In this paper, we introduce a technique to derive a new measure of ionosphere electron content, the Total square-Root Electron Content (TREC), using the arrival times of two frequencies of the whistler signal. We validate this approach by using data from ionosondes and from in situ measurements of the electron density at Swarm location. This technique brings new opportunities for sounding the ionosphere in regions poorly observed by other techniques.

The tropical arm of Atlantic Multidecadal Variability (AMV) influences climate worldwide, yet the mechanisms generating it remain unclear. Here, we examine experiments with sea surface temperature (SST)-restoring in the extratropical North Atlantic in multiple models and use mixed-layer heat budgets to elucidate the important mechanisms. Our results demonstrate that the tropical AMV is driven by wind-mixed-layer-SST feedback. The evolution has two phases with tropical AMV SST anomalies growing from April to October and decaying from November to March. The amplitude of the growth phase surpasses that of the decay phase, resulting in overall tropical Atlantic warming during positive AMV phases. During summer, positive SST anomalies in the extratropics weaken the trade winds, resulting in a shallower mixed-layer with reduced heat capacity. Subsequent absorption of climatological shortwave radiation in this shallower mixed-layer then causes SSTs to warm, generating the tropical AMV. Importantly, anomalous surface heat-fluxes make modest contributions to tropical AMV in these experiments.

The closure of an ancient ocean basin via oceanic arc-continent collision has two subduction styles with opposite polarities, which may proceed via subduction polarity reversal (SPR) or a subduction zone jump (SZJ). Interpreting the geometry or kinematic evolution of ancient collisional zones, especially the original subduction polarity, can be challenging. Here we used 2D thermo-mechanical modeling to investigate the dynamic evolution process of SPR versus SZJ. Our modeling predicts different structural, topographic, magmatic, and basin histories for SPR and SZJ, which can be compared against, and help interpret, the geologic record past sites of oceanic closure during collisional orogens. Our results match geologic observations of past collisions in Kamchatka, eastern Russia, and the Banda Arc, eastern Indonesia, and thus our results can help effectively decode the evolutionary history of past arc-continent collisions.

Earth experienced the strongest geomagnetic storm in 20 years over 10–13 May 2024. The Ap and Dst geomagnetic indices were 273 and −291.94 nT on 11 May. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite observed significant enhancement in thermospheric infrared emission at 15, 5.3, 4.3, and 1.27 μm. On 11 May the daily global power radiated by nitric oxide (NO) at 5.3 μm was 1.41 TW and by carbon dioxide (CO2) at 15 μm was 1.35 TW. These are the largest single day power values observed by SABER in 22 years and the first time the daily power radiated by NO exceeded that of CO2. The total infrared power (above background) radiated during the storm was 2.64 TW (2.28 × 1017 J). Significant enhancement in limb radiance observed at 4.3 μm (to 250 km tangent height) is likely indicative of NO + formation during the storm.