Geophysical Research Letters最新文献

The large annual carbon source over northern tropical Africa (NTA), inferred from satellite CO₂, remains highly debated. Using observing system simulation experiments with Orbiting Carbon Observatory-2 (OCO-2) sampling, we show that seasonally dependent sampling can lead to overestimated annual fluxes. These biases arise when prior flux seasonal cycle differs from the assumed truth. Since OCO-2 provides more observations during the non-growing season, posterior fluxes are more constrained in that period. When prior fluxes underestimate the seasonal amplitude, the posterior carbon sink during the growing season is underestimated, leading to a net positive bias. This effect is supported by real OCO-2 data, where we hypothesize that underestimating fire emissions during non-growing season and weaker seasonality of prior fluxes may contribute to overestimated annual fluxes. Our results highlight the need to improve prior flux estimates and expand observational coverage during the growing season to reduce biases in regional carbon budget assessments over NTA.

Coronae, which are weak electrical discharges, have long been hypothesized to form on trees under thunderstorms, though never directly observed, characterized, or quantified. Using a newly developed instrument that measures ultraviolet emissions from coronae, the first direct observations and quantifications of coronae are presented for two trees under a thunderstorm in North Carolina. Coronae moved sporadically among leaves on every tree branch in a narrow field of view while the thunderstorm was directly overhead. Coronae emitted ∼10¹¹ photons at 260 nm, corresponding to electrical currents of ∼1 μA, derived from unique measurements relating corona intensity to tree electrical current. Similar results across four additional storm intercepts from Florida to Pennsylvania give rise to a vision of swaths of scintillating corona glow as thunderstorms pass over forests. Such widespread coronae have implications for the removal of hydrocarbons emitted by trees, subtle tree leaf damage, and limited thunderstorm electrification.

Employing the multi-station Thunderstorm Energetic Radiation Observation System, we detected X-ray bursts during two rocket-triggered lightning events in 2024. By innovatively integrating optical imaging with three-dimensional lightning channel reconstruction based on Distributed Acoustic Sensing (DAS), we analyzed the X-ray emission characteristics from these events. During the Tl_20240812 event, lateral deflection of a descending negative leader resulted in X-rays being detected exclusively by a distal sensor. This clear spatial correlation provides direct and conclusive geometric evidence that the radiation is emitted in a beam-like pattern along the leader propagation path. Furthermore, based on the Tl_20240801 event, this study achieved the first quantitative estimation of the X-ray photon beam half-angle width, determined to be between 40° and 46°. This angular range aligns with the predicted structure of the leader tip electric field, thereby providing robust support for the hypothesis that X-rays originate from the leader tip high-field runaway electron mechanism.

Ice shelves fringing the Antarctic continent experience low or high basal melt rates depending on local shelf conditions, ocean circulation and intensity of ice-sea-air exchanges. Recent studies have uncovered potential cold-to-warm transitions in specific ice-shelf cavities, which could lead to a dramatic increase in sea-level rise. Here we demonstrate that brine rejection in coastal polynyas promotes bistable dynamics in ice-shelf cavities, which would be otherwise monostable, for a broad diversity of Circumpolar Deep Water temperatures. We develop a generic low-dimensional box model featuring warm and cold circulation modes and apply it to nine ice-shelf cavities. We find that most ice-shelf cavities are in a bistable regime and are therefore susceptible to irreversible abrupt transitions for a realistic range of sea-ice formation rates. Bistability is robust to changes in cavity parameterization. However, the vertical mixing scheme at the ice-shelf front can be tuned to make the transitions reversible.

This study analyzes laboratory data of beach response to sea-level rise (SLR), isolating shoreline changes driven by passive flooding (PF) of the beach and consequent wave-driven processes. The disequilibrium concept relates shoreline change to instantaneous and equilibrium beach states. While PF shifts the shoreline geometrically, SLR induces disequilibrium that produces wave-driven changes due to apparent profile changes. For the first time, 24 experiments from wave flumes of different scale (including new high-low energy cyclic waves experiments) are gathered into a dimensionless data set through a scaling technique to investigate SLR-induced processes. The data indicate trends (possibly linear) between relative wave power and wave-driven shoreline changes for a given SLR, highlighting the effects of changing background wave energy. Cyclic wave experiments best represent Bruun model's behavior. Wave-energy dissipation emerges as a key variable for quantifying SLR-induced disequilibrium, offering new pathways for future improvements of equilibrium shoreline models under SLR and wave-climate change.

Fracture connectivity is a key parameter controlling fluid flow throughout the Earth's crust. While some theoretical and numerical studies suggest that seismic waves are sensitive to fracture connectivity, an experimental validation of this critically important phenomenon was so far unavailable. In this study, we present a novel methodology for fabricating synthetic analogs of rock samples containing connected and unconnected fluid-saturated fractures with well-constrained geometric characteristics. Using a low-frequency forced-oscillation apparatus, we show that the P-wave velocities are higher in samples with unconnected fractures than in those with connected ones. Complementary numerical simulations corroborate these findings and indicate that the dominant mechanism behind the observed differences is wave-induced fluid pressure diffusion within connected fractures. Our results provide direct experimental evidence that, for otherwise identical fracture networks, the presence of interconnectivity produces a measurable reduction in P-wave velocity at seismic frequencies, which is consistent with that previously predicted by corresponding numerical models. This, in turn, opens new and important perspectives for the seismo-hydraulic characterization of fractured rocks.

The gradient drift instability (GDI) commonly occurs in the high-latitude ionosphere and is widely recognized for producing elongated striation structures. While previous studies have established the linear growth and primary nonlinear development of striations, the formation of secondary structures remains not fully understood. Using two-dimensional numerical simulations, we show that smaller branch structures evolve asymmetrically on the sides of striations when either the background electric field or the wave vector has a component along the density gradient. Our results indicate that in the linear stage, the electric field in the direction of density gradient modifies the effective growth rate by altering the wave vector orientation. In the nonlinear stage, electric field and wave vector direction coupling govern the emergence of branch structure, with electric field dominating when its effect opposes that of the wave vector. These results highlight the critical roles of electric field and wave vector orientation in generating secondary GDI structures.

Rivers self-organize to convey water and sediment, giving rise to robust downstream scaling between channel geometry and drainage area, underpinning landscape evolution models. However, these relations rely on limited observations per watershed. We quantify downstream changes in channel slope and bankfull width for six gravel rivers. We develop a novel method to automatically extract bankfull width and determine high-resolution (10-m), catchment-specific width-area scaling, revealing new insights on the covariation between slope and width hidden in large data compilations. We identify a threshold slope, below which average width is slope-independent. Notably, slope and width deviations display contrasting patterns depending on the channel's elevation profile. Deviations are anticorrelated when knickpoints are present and correlated when they are absent. High-resolution, catchment-specific scaling laws capture systematic, interpretable deviations reflecting underlying controls on channel adjustment and fluvial erosive power. With growing availability of high-resolution topography, our approach provides new insights into river process and form.

Extreme open-ocean phytoplankton events can influence marine ecosystems, yet their global occurrence, drivers, and consequences remain poorly understood. Most large-scale studies rely on satellite chlorophyll, which provides only a surface view, is affected by physiological variability, and is often missing due to clouds and low sunlight. Here, we use an Earth system model with a satellite chlorophyll simulator to test when and where vertically integrated phytoplankton biomass extremes align with satellite-detected chlorophyll extremes. Globally, about 10% of low and 19% of high phytoplankton biomass extremes are detected. The detection rate is the result of the combined impacts of missing data and extreme misalignment: only 34% of low and 56% of high detected chlorophyll extremes correspond with true biomass extremes, with the largest discrepancies occurring in the subtropical gyres. These findings highlight the need for caution when interpreting satellite chlorophyll as a proxy for phytoplankton biomass extremes.