Quantification of the magnitude and long-term changes of ozone concentrations transported into the US is important for effective air quality policy development. We synthesize multiple published trend analyses of western US baseline ozone, and show that all results are consistent with an overall, non-linear change – rapid increase during the 1980s that slowed in the 1990s, maximized in the mid-2000s, and was followed by a slow decrease thereafter. This non-linear change accounts for ~2/3 of the variance in the published linear trend analyses; we attribute the other 1/3 to unquantified autocorrelation in the analyzed data sets. Recent systematic changes in baseline ozone at the US West Coast have been relatively small - the standard deviation of the 2-year means over the 1990-2017 period is 1.5 ppb. International efforts to reduce anthropogenic precursor emissions from all northern mid-latitude sources could possibly reduce baseline ozone concentrations, thereby improving US ozone air quality.

Overland flow self-organized into rills and eroded colluvium from steep hillslopes after a wildfire in the Santa Ynez Mountains. During 12-15 minutes of runoff, rill erosion generated a slurry that scoured bouldery alluvium from mountain canyons to form large debris flows that severely impacted the community of Montecito, CA. The timing, volume, and peak discharge of the debris flows, and their capacity to scour boulders from canyons, depended on the mechanism and generation rate of the granular fluid (slurry) responsible for the sediment transport. Field surveys and aerial imagery revealed dense networks of rills on bare, burned hillslopes, and levees of fine-grained sediment lining the rill margins indicated that flows were highly viscous and the water and sediment were already intimately mixed before entering stream channels. We mapped networks of rills and measured their cross-sectional geometry to quantify the influences of lithology, hillslope length, gradient, and planform on the sediment volumes released by the rills and their volumetric contribution to the canyon-scale debris flows in six watersheds. We interpreted the mechanism of slurry generation by rill erosion, as far as field evidence will allow, through the lens of field and laboratory experiments conducted by others under comparable conditions. We used the geometry of rills, and the duration of intense rainfall and modelled surface runoff, to develop an empirical model of the rate of slurry generation by rill erosion and to suggest a mechanism by which the necessary intensive mixing occurred.

The first observations of 3He2+ density extremely enhanced to thousand times of average solar wind by SWID/Chang’E-2 is reported in this paper. The two events occurred near the lunar and the Lagrange L2 point respectively. The synchronized energetic electrons increasing can induce the lunar surface charging to a large negative voltage and produce the electron-impact double ionization of helium-3 in the exosphere. 3He2+ will converge to the lunar surface by the negative potentials, and fully pick up via non-resonance stochastic heating process by Alfvén wave of the solar wind. It indicates 3He2+ in the events can originate from the lunar and transport to L2. Besides SEP (solar energetic events), the increasing of energetic electrons occurs frequently in quiet time, so the loss of the lunar exosphere will be recurrent, and the ionosphere ion void may be formed.

This work provides an updated set of 12 dominant geopotential height fields - or ‘regimes’ - over Aotearoa New Zealand. These regimes were initially produced by Kidson (2000) and have provided the basis for many other subsequent studies. These maps provide a guide to the prevailing weather due to the broad equivalence of 1000hPa geopotential height and mean-sea-level pressure. The results presented here are broadly in agreement with previous work but with some important differences. The most notable of these being the need to average two blocking regimes together to provide good agreement between this work and Kidson (2000). These differences are attributed to the software used, improvements to the underlying dataset itself and to the ‘mixing’ of statistically indistinguishable empirical orthogonal functions - EOFs - in different linear combinations. All data and code used in this work is publicly accessible and it is hoped that this will provide a catalyst for open discussions on this topic, particularly with relation to future perturbations to these regimes under climate change.

By filtering the incoming climate signal when producing streamflow, river basins can attenuate – or amplify – projected increases in rainfall variability. A common perception is that river systems dampen rainfall variability by averaging spatial and temporal variations in their watersheds. However, by analyzing 671 watersheds throughout the United States, we find that many catchments actually amplify the coefficient of variation of rainfall, and that these catchments also likely amplify changes in rainfall variability. Based on catchment-scale water balance principles, we relate that faculty to the interplay between two fundamental hydrological processes: water uptake by vegetation and the storage and subsequent release of water as discharge. By increasing plant water uptake, warmer temperatures might exacerbate the amplifying effect of catchments. More variable precipitations associated with a warmer climate are therefore expected to lead to even more variable river flows – a significant potential challenge for river transportation, ecosystem sustainability and water supply reliability.

We present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary shocks occurring within an interval of ~2 hours on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for ~30 minutes. Significant Landau damping by suprathermal electrons followed by their gradual removal by magnetospheric compression opposed the generation of hiss causing the disappearance. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler mode waves within the core frequency range of plasmaspheric hiss (0.1 - 0.5 kHz) during this interval making conditions unfavorable for the generation of the waves. The recovery began at ~16:45 UT which is attributed to an enhancement in local plasma instability initiated by the first shock-induced substorm and additional possible contribution from chorus waves. This time, the wave growth rate peaked within the core frequency range (~350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to ~19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock-induced poloidal Pc5 mode (periodicity ∼240 sec) ULF waves resulted in the excitation of hiss waves during this period. The hiss wave amplitudes were found to be additionally modulated by background plasma density and fluctuating plasmapause location. The investigation highlights the important roles of interplanetary shocks, substorms, ULF waves and background plasma density in the variability of plasmaspheric hiss.

Rivers and streams are an important component of the global carbon budget, emitting CO2 to the atmosphere. However, our ability to accurately predict carbon fluxes from stream systems remains uncertain due to small scales of pCO2 variability within streams (100-102 m), which make monitoring intractable. Here we incorporate CO2 input and output fluxes into a stream network advection-reaction model, representing the first process-based representation of stream CO2 dynamics at watershed scales. This model includes groundwater (GW) CO2 inputs, water column and benthic hyporheic zone (BZ) respiration, downstream advection, and atmospheric exchange. We evaluate this model against existing statistical methods including upscaling techniques and multiple linear regression models through comparisons to high-resolution stream pCO2 data collected across the East River Watershed in the Colorado Rocky Mountains. The stream network model accurately captures topography-driven pCO2 variability and significantly outperforms multiple linear regressions for predicting pCO2. Further, the model provides estimates of CO2 contributions from internal versus external sources and suggests that streams transition from GW- to BZ-dominated sources between 3rd and 4th Strahler orders, with GW and BZ accounting for 53 and 47% of CO2 fluxes from the watershed, respectively. Lastly, stream network model CO2 fluxes are 5-13x times smaller than upscaling technique predictions, largely due to inverse correlations between stream pCO2 and atmosphere exchange velocities. Taken together, the stream network model presented improves our ability to predict and monitor stream CO2 dynamics, and future applications to regional and global scales may result in a significant downward revision of global flux estimates.

Biomass burning (BB) is a major source of pollutants that impact local, regional, and global climate, air quality, and public health. However, the influence of burning conditions and fuel type on emission factors of pollutants is still not well understood. Here, we present the results from a laboratory study of emission factors of pollutants from seven different sub-Saharan African biomass fuels combusted under a wide range of burning conditions ranging from smoldering to flaming. We found that particulate matter (PM) and CO emission factors (EF; g (kg wood)-1) are highly sensitive to the burning conditions, with an order of magnitude variation between flaming and smoldering burning conditions. NO EF shows a fuel type dependence, with higher NO EF for fuels with larger nitrogen content. We observed a linear correlation between PM and CO EF and proposed that CO EF could be used as a proxy for estimating PM EF in cookstove emissions. The proposed relationship is valid regardless of the fuel type, moisture content, stove types, and origin of the fuel. Unlike total PM, EF of inorganic species do not show dependence on burning conditions. Finally, we showed that burning biomass fuels in a tube furnace would be an exceptional experimental approach to study BB emission under controlled burning conditions.

Reconstructing hydroclimate over the Common Era is essential for understanding the dominant mechanisms of precipitation change and improving climate model projections, which currently suggest Northeast Mexico will become drier in the future. Tree-ring reconstructions have suggested regional rainfall is primarily controlled by Pacific sea surface temperatures (SST). However, tree ring records tend to reflect winter-spring rainfall, and thus may not accurately record total annual precipitation. Using the first multiproxy speleothem record spanning the last millennium, combined with results from an atmospheric general circulation model, we demonstrate mean annual rainfall in Northeast Mexico is highly sensitive to Atlantic SST variability. Our findings suggest precipitation in Northeast Mexico may increase in the future in response to the relative warming of Tropical North Atlantic SSTs.

Previous experiments and modeling on the nucleation process of an earthquake reveal accelerating slip and development of a final patch of fixed or expanding length, but whether the nucleation phase is spatially large enough to be detected in real-Earth conditions with durations long enough to be helpful is unknown. This study performed a new round of simulation of the nucleation process based on the rate-and-state friction law. Our results reveal the development of a weakening-zone core where stress releases continuously, which expands first to a dimension less than the half fault length then shrinks to a final length before re-expanding, consistent with seismicity migration before large subduction earthquakes. Onset times of the weakening-zone core are weeks before large subduction earthquakes under high loading rate but years for continental faults with loading rate below 10mm/yr. The onset of weakening-zone core is marked by fault-length-dependent slip rates>3.6-4.8 times the loading rate.

Large igneous provinces (LIPs) are often associated with mass extinctions and are vital for life evolution on Earth. However, the precise relation between LIPs and their impacts on biodiversity is enigmatic as they can be asynchronous. If the environmental impacts are primarily related to sill emplacement, the structure of LIPs’ magma storage system becomes critical as it dictates the occurrence and timing of mass extinction. Here we use surface wave tomography to image the lithosphere under the Permian Emeishan Large Igneous Province (ELIP) in SW China. We find a NE-trending zone of high shear-wave velocity (Vs) and negative radial anisotropy (Vsv > Vsh) in the crust and lithosphere and interpret it as a mafic-ultramafic, dike-dominated magma storage system on the hidden hotspot track of the ELIP. An area of less-negative radial anisotropy, on the hotspot track but away from the eruption center, reflects an elevated proportion of sills emplaced at the incipient stage of the ELIP. Liberation of poisonous gases and mercury by the sills explains why the mid-Capitanian global biota crisis preceded the peak ELIP eruption by 2-3 million years.

Marine carbonates, including shallow-water carbonates and pelagic carbonates, precipitating from seawater and representing the largest sink of calcium (Ca) and carbon (C) at Earth’s surface, could record seawater chemistry. Therefore, marine carbonates help regulate atmospheric carbon dioxide (CO2) concentration and eventually Earth’s long-term climate change. Calcium stable isotope geochemistry is a powerful tool to reconstruct paleo-seawater Ca isotopic compositions (δ44/40Casw) and to constrain the global Ca and C cycles over geological history. Here, we present a Neogene record of δ44/40Ca from pure dolomites of the core XK-1 in the South China Sea. We propose that the dolomites formed in seawater-like fluid-buffered diagenetic environments near the seawater-sediment interface. We demonstrate that XK-1 dolomites display no Ca isotope fractionation from seawater, and hence may serve as good archives of contemporaneous seawater δ44/40Ca. Further, we quantify respective contributions of shallow-water and pelagic carbonates in sequestering carbon over the Neogene, using a Ca isotope mass balance box model. We find that more CO2 may have been sequestered by pelagic carbonate burial during global cooling. The enhanced continental weathering, global cooling, sea-level fall, seawater chemical changes, and pelagic carbonate burial are tightly linked.

Triple-oxygen isotope (δ18O and Δ17O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which can rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18α and 17α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18α data and no 17α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18α and 17α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate isomers—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion “water droplets”; using the B3LYP/6-31G+(d,p) method; we additionally determine higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) and anharmonic zero-point energy (ZPE) scaling factors using a suite of gaseous sulfoxy compounds and test their impact on resulting sulfoxyanion fractionation-factor estimates. When including redox state-specific CCSD/aug-cc-pVTZ and anharmonic ZPE scaling factors, our theoretical 18α predictions for protonated isomers closely agree with all existing experimental data, yielding root-mean-square errors of 1.8 ‰ for SO3(OH)-/H2O equilibrium (n = 18 experimental conditions), 2.2 ‰ for SO2(OH)-/H2O (n = 27), and 3.9 ‰ for S2O2(OH)-/H2O (n = 3). This result supports the idea that oxygen exchange occurs via isomers containing oxygen-bound protons. By combining 18α and 17α predictions, we additionally estimate that SO3(OH)-, SO2(OH)-, SO(OH)-, and S2O2(OH) exhibit Δ17O values as much as 0.167 ‰, 0.097 ‰, 0.049 ‰, and 0.153 ‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples.

Major gaps exist in our understanding of the pathways between internal wave generation and breaking in the Southern Ocean, with important implications for the distribution of internal wave-driven mixing, its sensitivity to change, and the necessary ingredients of mixing parameterizations. Here we assess the dominant processes in internal wave evolution by characterizing wave and mesoscale flow scales based on full-depth measurements in a Southern Ocean mixing hot spot and a ray tracing calculation. The exercise highlights the importance of Antarctic Circumpolar Current (ACC) jets as a dominant influence on internal wave life cycles through advection, the modification of wave characteristics via wave-mean flow interactions, and the set-up of critical layers for both upward- and downward-propagating waves. Our findings suggest that it is important to represent mesoscale flow impacts in parameterizations of internal wave-driven mixing in the Southern Ocean.

A devastating tsunami struck Palu Bay in the wake of the 28 September 2018 M$_{\mathrm{w}}=7.5$ Palu earthquake (Sulawesi, Indonesia). With a predominantly strike-slip mechanism, the question remains whether this unexpected tsunami was generated by the earthquake itself, or rather by earthquake-induced landslides. In this study we examine the tsunami potential of the co-seismic deformation. To this end, we present a novel geodetic dataset of GPS and multiple SAR-derived displacement fields to estimate a 3D co-seismic surface deformation field. The data reveal a number of fault bends, conforming to our interpretation of the tectonic setting as a transtensional basin. Using a Bayesian framework, we provide robust finite fault solutions of the co-seismic slip distribution, incorporating several scenarios of tectonically feasible fault orientations below the bay. These finite fault scenarios involve large co-seismic uplift (~2 m) below the bay due to thrusting on a restraining fault bend that connects the offshore continuation of two parallel onshore fault segments. With the co-seismic displacement estimates as input we simulate a number of tsunami cases. For most locations for which video-derived tsunami waveforms are available our models provide a qualitative fit to leading wave arrival times and polarity. The modeled tsunamis explain most of the observed runup. We conclude that co-seismic deformation was the main driver behind the tsunami that followed the Palu earthquake. Our unique geodetic dataset constrains vertical motions of the sea floor, and sheds new light on the tsunamigenesis of strike-slip faults in transtensional basins.

The fluid mechanics of hurricanes strongly depends on boundary layer energetics due to the warm-core nature of the system with peak velocities located at lower levels. One barrier that has inhibited a more complete characterization of energy transfer in the boundary layer is a lack of observations that resolve large, turbulent eddies. In particular, the occurrence and structure of upscale energy transfer (backscatter) in the hurricane boundary layer as well as the effects of backscatter on the vortex intensity are unknown. The analysis presented here of very high-resolution, three-dimensional wind observations from Hurricane Rita (2005) at peak intensity reveals large regions of organized backscatter in the boundary layer associated with coherent, turbulent eddies. Strong forwardscatter is also found next to the backscatter regions due to the interaction between adjacent eddies. Two components of the stress tensor are primarily responsible for this alternating scatter structure, as shown by large correlation coefficients between the fields: the radial–vertical component (τ13) and the azimuthal–vertical component (τ23) with average correlations of 79% and 49 %, respectively. The Leonard, Reynolds and cross-term stress components are also provided. The impact of the sub-filter-scale energy transfer is estimated by computing the kinetic energy budget for the resolved-scale and eddy-scale motions. The results show that the sub-filter-scale energy transfer term is of the same order as the other terms in the eddy-scale budgets, contributing between 16% and 40% to the local time tendency with an average contribution of approximately 30%. These results indicate that the coherent turbulent eddies can affect the vortex dynamics through wave–wave nonlinear interactions, which can subsequently influence the wave–mean flow interactions. This is the first study to examine the full sub-filter-scale energy transfer and its impact on the kinetic energy budget in the hurricane boundary layer. These findings emphasize the importance of coherent turbulence in the energy cascade and have the potential to improve turbulence closure schemes used in numerical simulations. Sroka, S. and S.R. Guimond, 2021. Organized kinetic energy backscatter in the hurricane boundary layer from radar measurements. Journal of Fluid Mechanics, 924, A21. doi:10.1017/jfm.2021.632

Titan is a prime target for astrobiological research. Organic materials from atmospheric chemistry precipitate on the surface and are subject to geological processes (e.g. eolian and fluvial erosion) that lead to the formation of dune fields, river networks and seas similar to their terrestrial counterparts. The analysis of the surface reflectance in the near-infrared (NIR) allows to constrain the surface composition, which is crucial to understand these atmosphere/surface interactions. However, Titan’s atmosphere prevents the surface from being probed in the NIR, except in 7 transmission windows where the methane absorption is sufficiently low (centered at 0.93, 1.08, 1.27, 1.59, 2.01, 2.7- 2.8 and 5 μm). We use an updated version of the Radiative Transfer (RT) model of Hirtzig et al. (2013), with updated gases and aerosols opacities, in order to better simulate atmospheric absorption and scattering and retrieve surface albedos in the 7 NIR transmission windows with an enhanced accuracy. Our RT model is based on the SHDOMPP and CDISORT (Evans, 2007 and Buras, 2011) solvers to solve the RT equations in plane-parallel and pseudo-spherical approximations respectively. We recently improved atmospheric inputs of the model with up-to-date gaseous CH4, CH3D, 13CH4, C2H2, HCN and CO abundances profiles and absorption coefficients (Vinatier et al. 2007, Niemann et al. 2010; Maltagliati et al. 2015; Serigano et al. 2016; Rey et al. 2018; Thelen et al. 2019; Gautier et al. 2021), and improved aerosol optical properties. In particular, optical properties of Titan’s aerosols are now computed from a fractal aggregate model (Rannou et al. 2003) constrained by measurements of the Huygens probe (Tomasko et al. 2008 and Doose et al. 2016). The new version of our RT model is benchmarked with the help of the most recent RT model for Titan (Coutelier et al. 2021) and validated using observations of the Descent Imager/Spectral Radiometer (DISR) onboard Huygens. Coupled with an efficient inversion scheme, our model can be apply to the Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) dataset to retrieve atmospheric opacity and surface albedos at regional and global scales. This will help to analyze future James Webb Space Telescope (JWST) observations of Titan (Nixon et al. 2016) and prepare the Dragonfly mission (Lorenz et al. 2018).

Magnitude and frequency are prominent features of river floods informing design of engineering structures, insurance premiums and adaptation strategies. Recent advances yielding a formal characterization of these variables from a joint description of soil moisture and daily runoff dynamics in river basins are here systematized to highlight their chief outcome: the PHysically-based Extreme Value (PHEV) distribution of river flows. This is a physically-based alternative to empirical estimates and purely statistical methods hitherto used to characterize extremes of hydro-meteorological variables. Capabilities of PHEV for predicting flood magnitude and frequency are benchmarked against a standard distribution and the latest statistical approach for extreme estimation in two ways. The methods are first applied to an extensive dataset to compare their skills for predicting observed flood quantiles in a wide range of case studies. Synthetic time series of streamflow, generated for select river basins from contrasting hydro-climatic regions, are later used to assess performances for rare events. Both analyses reveal fairly unbiased capabilities of PHEV to estimate flood magnitudes corresponding to return periods much longer than the sample size used for calibration. The results also emphasize reduced prediction uncertainty of PHEV for rare floods when the mechanistic hypotheses postulated by the method are fulfilled, notably if the flood magnitude-frequency curve displays an inflection point. These features, arising from the mechanistic understanding embedded in the novel distribution of the largest river flows, are key for a reliable assessment of the actual flooding hazard associated to poorly sampled rare events, especially when lacking long observational records.