Geophysical Research Letters最新文献

Electrostatic Cyclotron Harmonic (ECH) waves have been considered a potential cause of pitch angle scattering of electrons in the energy range from a few hundred eV to tens of keV. Theoretical studies have suggested that scattering by ECH waves is enhanced at lower pitch angles near the loss cone. Due to the insufficient angular resolution of particle detectors, it has been a great challenge to reveal ECH-driven scattering based on electron measurements. This study reports on variations in electron pitch angle distributions associated with ECH wave activity observed by the Arase satellite. The variation is characterized by a decrease in fluxes near the loss cone, and energy and pitch angle dependence of the flux decrease is consistent with the region of enhanced pitch angle scattering rates predicted by the quasi-linear diffusion theory. This study provides direct evidence for energy-pitch angle dependence of pitch angle scattering driven by ECH waves.

The decadal variability of sea surface salinity (SSS) is crucial for the global water cycle and the climate system. Previous studies have indicated that the SSS in the western tropical Pacific (WTP) Ocean exhibits significant decadal variability, which plays an important role in shaping tropical climate. Here we find that the amplitude of SSS decadal variability in the WTP has been intensified significantly since the early 1950s. A comprehensive analysis of the salinity budget indicates that the amplifying of SSS decadal variability in the WTP is primarily attributed to changes in both the freshwater flux and ocean dynamics. During the past decades, surface currents in the WTP get stronger and result in enhanced salinity advection. In addition, the amplitude of decadal variability of freshwater flux is increased as well due to anomalous atmospheric circulation associated with the Victoria mode.

Uncrewed aircraft systems (UAS) demonstrate significant potential for filling data gaps in the atmospheric boundary layer. However, the extent to which UAS observations—typically vertical profiles taken over 15 min—are representative of the boundary layer as a whole remains poorly characterized. Using large eddy simulations (LES) of the daytime convective boundary layer (CBL), we quantify random errors in UAS measurements that occur due to insufficient statistical convergence of the time average to the true ensemble mean. Random errors in first-order moments increase as the CBL becomes increasingly unstable, and are largest near the surface for most quantities. Errors are on the order of 2–6 m $��s^{�-�1�}�$ for wind speed, 15–60$��°�$ for wind direction, 0.2–3 K for potential temperature, and 0.1–1 g $��{\text{kg}}^{�-�1�}�$ for specific humidity, with errors in turbulent fluxes on the order of 50%–100%. Sampling strategies that mitigate random errors are discussed in light of our results.

Dansgaard-Oeschger (DO) events are a dominant mode of millennial-scale climate variability during the last glacial period with most pronounced impacts in the North Atlantic region. In Antarctica, they manifest primarily as a muted and phase-shifted temperature signal, but recent studies suggest an additional in-phase component. Here, we analyze the Southern Hemisphere (SH) response to spontaneous DO-type oscillations in a general circulation model. The dominant Antarctic temperature mode is phase-shifted compared to Greenland temperature variations and consistent with the oceanic pathway described by the bipolar seesaw model. However, the leading SH atmospheric circulation mode varies synchronously with Greenland temperatures. A westward-shifted Walker circulation and strengthened Hadley cell during Greenland temperature maxima cause zonally heterogeneous jet stream anomalies differing from the Southern Annular Mode pattern. Comparison of simulated $��{\delta }^{18}�$O with speleothems and ice cores indicates a good agreement in the tropics and SH mid-latitudes but deviations in Antarctica warrant further research.

Sustainable management of groundwater resources requires a comprehensive understanding of groundwater recharge; including when and under what conditions groundwater recharge occurs. However, recharge is one of the least understood hydrologic processes. Here we show how event-scale rainfall recharge thresholds vary over a year using a novel network of subterranean drip loggers installed in caves, mines, and tunnels to observe groundwater recharge events. These cannot be used to directly estimate groundwater recharge volumes, but instead detail temporal aspects, such as the rainfall amount required to trigger recharge. We describe how these thresholds vary over time and space from a range of geological, environmental, and climatic conditions. At our Southeast Australian sites, median rainfall recharge thresholds of 10–20 mm in 48 hr were needed to activate recharge. Rainfall events of this magnitude are infrequent, they are expected to change with climate change, and they are fundamentally important for informing groundwater recharge.

Changes in the properties of rainfall distributions at sub-daily scales are key to assessing soil erosion rates under climate transition. However, such changes are difficult to detect and model, especially over landscape evolution timescales. In this contribution, we validate a new catchment-scale landscape evolution model against event-scale runoff and sediment records. Through multi-century numerical experiments, we also show that changes in the sub-daily rainfall distribution, like those observed under modern climate change, can increase soil erosion rates by $��\sim �$40% but cannot be accurately inferred from changes in the average event properties and total rainfall. We quantify erosion and topographic trajectories associated with plausible changes in the sub-daily rainfall distribution, highlighting scenarios in which shifting tail properties impact landscape evolution, at times, contrary to expectations based on changes in total rainfall.

Cold oxygen ions escaping from the ionosphere and temporarily trapped near the plasmapause form the oxygen torus. Mass-loading by these oxygen ions significantly affects magnetospheric plasma processes. However, due to the technical challenges in measuring cold oxygen ions and the limited spatial and temporal coverage of space missions within the magnetosphere-ionosphere coupling system, the generation mechanism of oxygen torus remains unclear. Here, we propose a novel approach to determine the ion abundances from the observable polarization and propagation characteristics of electromagnetic ion cyclotron (EMIC) waves and identify a narrow oxygen torus near the noonside plasmapause during a geomagnetically quiet period. Our data and theoretical calculations suggest that the formation of this oxygen torus involved excitation of EMIC waves by ring current protons, wave-driven Landau heating of plasmaspheric electrons, heat conduction from the magnetosphere to the ionosphere, and upwelling of ionospheric oxygen ions into the magnetosphere.

Despite advances in our understanding of changes to severe weather events due to climate change, uncertainty regarding rare extreme events persists. Atmospheric rivers (ARs), which are directly responsible for the majority of precipitation extremes on the US West Coast, are projected to intensify in a warming world. In this study, we utilize two unique large-ensemble climate models to examine rare extreme AR events under various warming scenarios. By quantifying changes to rare extremes, we can gain some insight into the potential for these destructive unprecedented events to occur in the future. Additionally, the abundance of data used in this study enables changes to both seasonal extreme AR occurrences and changes to extremes during various synoptic-scale flow patterns to be explored. From this analysis, we find substantial changes to AR extremes under even mild warming scenarios with disproportionately large changes during weather regimes that are conducive to AR activity.

We investigate the contribution of the Hall term on the generalized Ohm's law in magnetospheric plasmas. In particular, we focus on its role in processes that lead to the formation of substorm perturbations deep inside the magnetosphere. Using data from the THEMIS mission, we calculate the average Hall length $��\left(L_{\text{Hall}}\right)�$ and its spatial distribution near the equatorial plane. Our findings reveal that $��L_{\text{Hall}}�$ significantly exceeds the ion inertial length, which suggests that the Hall term's contribution to generalized Ohm's law is significantly greater than the convective term. In this case, the magnetic field lines are able to slip through the plasma, something that conventional magnetohydrodynamic models cannot adequately describe. We explore how such slippage facilitates the development of substorm perturbations that do not require changes in magnetic field topology. These perturbations include dipolarization of magnetic field lines, particle acceleration, electrojet formation, and other phenomena typically associated with substorms.

Microphysical characteristics of tropical cyclones (TCs) over the open ocean remain elusive due to observation constraints. In this study, dual-polarization observations derived from a ship-borne C-band polarimetric radar are utilized to investigate the microphysical characteristics of an outer rainband associated with TC Choiwan during its evolution over the South China Sea from 0500 to 1300 UTC on 3 June 2021. Based on TC and convection intensity, the eight-hour period is categorized into three stages with distinct microphysical features: pre-mature, mature and post-mature. During the mature stage, both ice-phase and warm rain processes are active, resulting in a distribution pattern where most convective cells (CCs) contain high concentrations of small raindrops with a few exhibiting extremely large raindrops, and making raindrop size distributions (DSDs) fall between “continental-like” and “maritime-like”. In the pre-mature and post-mature stages, ice-phase processes predominantly govern the growth of raindrops, with CCs displaying “continental-like” DSD characteristics.