Geophysical Research Letters最新文献

Kinetic energy (KE) transfer between spatial scales contributes to the ocean's energy budget by linking scales of KE supply and KE dissipation. Numerical simulations have indicated that for scales smaller than the baroclinic deformation radius, cross-scale KE transfer has complex spatial and temporal variability, modulated by mixed layer properties, fronts, and eddies. Here, over a decade of upper-ocean surface velocity data, collected from high-frequency radar within the Santa Barbara Channel, are used to estimate cross-scale KE transfer. The transfer of KE across 7 km has strong seasonal and interannual variations linked to energy exchange with the atmosphere. This study observationally confirms (a) the importance of the surface divergence field in determining the direction of the KE transfer and (b) the equi-partitioning of KE transfer between divergent and straining motions. The temporal variability in KE transfer suggests that surface forcing influences the long-term redistribution of energy between scales.

At Mars, the MAVEN spacecraft has made observations of Hot Flow Anomalies (HFAs) in the foreshock. Due to the bow shock's proximity to the planet, it is theorized that HFAs contribute to atmospheric escape at Mars through the excavation of ionospheric ions. A case study investigates one HFA observation, with parameters suggesting a novel mechanism for planetary ion extraction. The event is further characterized by elevated number densities of $���O^{+}��$ and $���O_2^{+}��$ ions observed prior to the current sheet crossing. Statistical study is conducted on a set of 91 events, facilitating the estimation of HFA frequency at Mars to be 1/day. The estimated ion escape of the event is approximately 9$���\%��$ of the typical ion escape rate under nominal conditions at Mars. Calculation of further events reveals escape rates between 1 and 9$���\%��$ of the nominal conditions. This represents a modest contribution to the overall escape, highlighting a potentially underexplored pathway.

The radiative effect of aerosol on cloud albedo via altering cloud droplet effective radius (r_e) is a major uncertainty in the Earth's climate system. Remote sensing studies have reported either negative or positive relationships between r_e and aerosol number concentration (N_a) or other aerosol proxies. However, there are much fewer in situ observational evidences and physical explanation remains elusive for the contrasting N_a-r_e relationships. Here we quantify the N_a-r_e relationship by using in situ aircraft measurements, together with a r_e decomposition method. Our analysis reveals that the cloud-planetary boundary layer (PBL) coupling plays a pivotal role on the N_a-r_e relationship. Quantitative r_e decomposition indicates that the contrasting N_a-r_e relationships in two cloud-PBL coupling regimes result from different balances of four distinct aspects. The widely recognized number effect may be outweighed by the joint effects of the remaining three that have been rarely investigated and largely ignored in N_a-r_e parameterizations.

Slow, aseismic fault slip has emerged as a significant contributor to the seismic cycle. However, whether slow and fast slip arise from similar physical processes remains unresolved, due to detection biases affecting noisy surface measurements and the analysis of the source properties of slow slip. Using daily geodetic time series denoised with a deep learning model, we invert for 15 years of slow slip evolution on the Cascadia subduction with unprecedented temporal resolution. Our observations show that an upper bound for slow-slip moment rates exists, and that scaling laws are strongly influenced by the chosen detection threshold and the signal-to-noise ratio. Moment rate functions evolve with magnitude: slow slip nucleates as a two-dimensional expanding crack, propagating laterally when encountering the along-dip limits of the transition zone. Our findings highlight a continuum of slow slip events of various sizes controlled by subduction interface geometrical constraints.

Bursty bulk flows (BBFs) play a significant role in transporting plasma earthward in the magnetotail. While their properties have been extensively studied, their behavior during geomagnetic storms needs further understanding. In this study, we investigate the stormtime characteristics of BBFs, and compare them to non-stormtime, by performing a superposed epoch analysis using data from ISAS/NASA's Geotail mission. Our results show that the properties of BBFs during stormtime and non-stormtime remain largely consistent relative to the background plasma sheet conditions. The convection electric field is higher for stormtime BBFs which is primarily associated with an elevated magnetic field in the plasma sheet during storms. Moreover, stormtime plasma sheet conditions, such as an enhanced magnetic field and an elevated ion temperature, are reflected in the properties of BBFs indicating the strong influence of the background plasma environment on BBF dynamics.

Using satellite observations, ice water path (IWP), liquid water path (LWP), and surface precipitation across warm frontal regions are examined in the Northern (NH) and Southern (SH) Hemispheres, accounting for the life stages and characteristics of extratropical cyclones (ETCs). Focusing only on oceanic ETCs over a 4-year period, composite transects of the observations reveal that most hemispheric differences in warm frontal IWP, LWP, and precipitation align with variations in precipitable water, cyclone strength, and storm maturity. However, for similar cyclone strength and environmental moisture, NH warm fronts during early development contain more ice but are less efficient at precipitating than those in the SH. Higher dust concentrations in NH might explain the greater ice amounts, while higher sea-salt concentrations in SH might explain the greater precipitation efficiency in their respective warm frontal regions.

Exchanges between continental shelves and open ocean basins regulate the transport of heat, salt, and nutrients. In the Southwestern Atlantic, the western boundary current known as the Malvinas Current (MC) fertilizes the outer shelf through recurrent slope-water intrusions. Here we analyze the 2003–2024 interannual variability of satellite chlorophyll-a around 41°S, where the inflection of the 100-m isobath promotes these incursions. The first Empirical Orthogonal Function mode explains 43% of the variance and exhibits a spatial pattern consistent with the MC intrusion zone. Backward Lagrangian simulations reveal that low-chlorophyll periods correspond to waters advected by the onshore MC jet, whereas high-chlorophyll years are linked to offshore-origin parcels likely richer in nutrients. Sea-level anomaly composites indicate that mesoscale eddies near 40°S can block or deflect the MC, favoring intrusions onto the shelf. These results provide new quantitative evidence that variability in boundary-current pathways strongly modulates interannual changes in chlorophyll-a over continental shelves.

Carbon mobilization and sulfur transformation play a significant role in deep carbon and sulfur cycling. However, sulfur biogeochemistry and its coupling with carbon and iron cycling remain poorly constrained in hydrothermal sediments. We investigated the effect of temperature on carbon-sulfur-iron diagenesis in subsurface sediments (≤370 m depth) of the Guaymas Basin, Gulf of California. Sediments have a low average carbon-to-sulfur ratio (∼1.6), especially at hydrothermal active sites (1.3–0.67). These values are well below the typical value of 2.8 for marine sediment and could reflect carbon release and relative sulfur enrichment. Elevated temperatures accelerate the thermal breakdown of sedimentary organic matter, leading to hydrocarbon expulsion, increased dissolved organic carbon release, and ultimately carbon loss. Elemental sulfur content correlates positively with reactive iron, indicating iron oxides facilitate elemental sulfur accumulation. These results highlight the temperature effect on carbon storage and iron redox chemistry control on sulfur dynamics in hydrothermal systems.

Earthquakes can grow either monotonically from a single, stressed patch or through linking multiple stressed regions. The distinction has implications for magnitude predictability with single ruptures requiring knowledge of the local stress state, while linked ruptures require knowing the global stress and energy distribution. Here, we use a laboratory fault that allows direct observation of slip to determine which fault conditions promote linked ruptures, and how to observationally evaluate their likelihood. Higher concentrations of normal stress due to increased normal force, applied stress asperities, or larger heterogeneity between the samples, all lead to significant increases in linked rupture likelihood. The mean radiated energy enhancement factor (REEF) of large events is an excellent proxy for linked event likelihood, and a combination of REEF and duration can identify the rupture style of some but not all events. The results imply that globally observed variations of REEF can be interpreted as variations in stress concentration.

Seismological observations in the lunar mantle, in conjunction with experimental knowledge on the elastic wave velocities and density of lunar mantle minerals, provide important constraints on the composition and mineralogy of the lunar mantle. Here we report elastic wave velocities and density of a lunar orthopyroxene (Mg_0.84Fe_0.13Ca_0.03SiO₃) up to 5.5 GPa and 1,273 K. The result shows that the bulk and shear moduli of orthopyroxene decrease with increasing iron content. Based on the mineral elasticity data, we modeled the P- and S-wave velocities and density of petrologically suggested lunar upper mantle rock composition. The petrological lunar upper mantle rock model shows consistent seismic wave velocities with those observed in the lunar upper mantle whereas markedly lower density. Our modeling suggests an iron-rich (Fe/(Mg + Fe) = 0.20) lunar upper mantle to explain P- and S-wave velocities and density of the lunar upper mantle at 40–740 km depth.