ALBERT

Description: Abstract Energetic positive and negative cloud‐to‐ground (CG) flashes are both capable of producing sprites. Negative CGs typically outnumber the positive ones by 10 to 1. However, 〉99.9 % of reported sprites were found to be initiated by positive CGs—thus the polarity paradox. Here, sprites recorded by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) mission were analyzed along with extremely low‐frequency band magnetic field data to resolve this paradox. Approximately twenty‐five percent of the sprites are found to be associated with negative CG lightning. “Negative” sprites mainly congregate in the latitudinal regions below 20°, while positive sprites scatter up to 50°. The ISUAL negative sprites are evidently beyond the observable ranges of the ground sites reported in previous studies. Hence, the sprite polarity paradox is likely a selection effect of the middle‐ to high‐altitudinal observation sites. The charge moment changes and accompanying transient luminous events of sprites were also examined and found to be polarity dependent.

Description: Abstract Three transient National Center for Atmospheric Research Community Climate System Model, version 3 model simulations were analyzed to study the responses of El Niño–Southern Oscillation (ENSO) and the equatorial Pacific annual cycle (AC) to external forcings over the last 300,000 years. The time‐varying boundary conditions of insolation, greenhouse gases, and continental ice sheets, accelerated by a factor of 100, were sequentially added in these simulations. The simulated ENSO and AC amplitudes change in phase, and both have pronounced precession band variance (~21,000 years). The precession‐modulated slow (orbital time scales) ENSO evolution is dominated linearly by the change of the coupled ocean‐atmosphere instability, notably the Ekman upwelling feedback and thermocline feedback. In contrast, the greenhouse gases and ice sheet forcings (~100,000‐year cycles) are opposed to each other as they influence ENSO variability through changes in AC amplitude via a common nonlinear frequency entrainment mechanism. The acceleration technique could dampen and delay the precession signals below the surface ocean associated with ENSO intensity.

Description: Abstract We present first observations of OH and (HO2 + RO2) carried out in Antarctica outside the summer season. Measurements were made over 23 days in spring at the coastal Antarctic station Halley. Increases in concentrations were evident during the measurement period due to rapidly increasing solar irradiance, and clear diurnal cycles were present throughout. There were also notable differences in air mass composition depending on wind direction. Air masses that had traversed the sea‐ice‐zone had both higher concentrations of OH and a larger OH:(HO2 + RO2) ratio. We use steady‐state kinetic arguments and a 0‐D box model to probe the chemical drivers. We find that differences in bromine chemistry, previously measured at Halley, are sufficient to account for the observed differences in OH concentration as well as the ratio. There is some evidence also that chlorine chemistry is influencing concentrations of RO2.

Description: ABSTRACT Post‐seismic debris flows are an important hazard following large earthquakes, propagating destruction downstream from hillslopes where co‐seismic landslides occur and extending damage for years after shaking stops. Datasets of post‐seismic debris flows are necessary to predict initiation and runout characteristics, but are presently scarce. We used satellite imagery supplemented by field observations to compile an inventory of 〉1000 debris flows associated with the 2015 Gorkha Earthquake in Nepal. We identified two distinct debris flow types: 1) material from a co‐seismic landslide was remobilized in a steep channel during a later monsoon; and 2) a new post‐seismic hillslope failure occurred in saturated conditions and became fluidized and channelized. Runout distance was constrained by channel confluences and may be related to confluence geometry. Unstable landslide debris was largely flushed from steep channels during the first monsoon following the earthquake, and the rate of new hillslope failures tailed off over a few years.

Description: Abstract Sand cays are valuable paleo‐archives that can significantly increase our understanding of Holocene tropical cyclone variability. Here we conducted detailed sedimentological and chronological analyses from a 195‐cm‐depth pit excavated on Guangjin Island (northern South China Sea), a cay influenced by frequent tropical cyclones. Radiometric dating of multiple deposits revealed that foraminifera, soft coral spicules, and gastropod shells yielded variable age distributions, while U/Th ages of pristine Acropora branches provided a clear record of deposition and cay formation. Based on this robust chronostratigraphy, the proportions of 〉2‐mm grain size fraction within the deposits corresponded with the frequency of paleotyphoons recorded by historical records in recent centuries. U/Th ages (CE 1687 ± 12, CE 1735 ± 6, and CE 1813 ± 5) of Acropora branches from the deposits matched with three known historical typhoon events. Our results highlight the potential of cyclone‐deposited sand cays as new archives for recording paleocyclones.

Description: Abstract New broadband seismic data from Botswana and South Africa have been combined with existing data from the region to develop improved P and S wave velocity models for investigating the upper mantle structure of southern Africa. Higher craton‐like velocities are imaged beneath the Rehoboth Province and parts of the northern Okwa Terrane and the Magondi Belt, indicating that the northern edge of the greater Kalahari Craton lithosphere lies along the northern boundary of these terranes. Lower off‐craton velocities are imaged beneath the Damara‐Ghanzi‐Chobe Belt, and may in part reflect thinning of the lithosphere beneath the incipient Okavango Rift. Lower velocities are also imaged to the north and northwest of the Bushveld Complex beneath parts of the Okwa Terrane, Magondi Belt, and Limpopo Belt, indicating that cratonic upper mantle in some areas beneath these terranes may have been modified by the 2.05‐Ga Bushveld and/or 1.1‐Ga Umkondo magmatic events.

Description: Abstract Since near‐term predictions of present‐day climate are controlled by both initial condition (IC) predictability and boundary condition predictability, initialized prediction experiments aim to augment the external‐forcing related predictability realized in uninitialized projections with IC‐related predictability by appropriate observation‐based initialization. However, and notwithstanding the considerable effort expended in finding such "good" initial states, a striking feature of current, state‐of‐the‐art, initialized decadal hindcasts is their tendency to quickly drift away from the initialized state, with attendant loss of skill. We derive a dynamical model for such drift and after validating it, we show that including a recalibrated version of the model in a post‐processing framework is able to significantly augment skill of initialized predictions beyond that achieved by a use of current techniques of post‐processing alone. We also show that the new methodology provides further crucial insights into issues related to initialized predictions.

Description: Abstract Advancing the leading time for onset prediction of the Indian summer monsoon (ISM) onset is an imperative task; however, it has remained a challenging subject. In particular, the land‐atmosphere coupling associated with monsoon onset prediction is poorly understood. Here, we attempt to investigate the land factor as the ISM onset precursor through studying the internal mechanism of atmospheric heating, which is distinguished by monsoon onset. The low (high) soil moisture in the Iranian desert during March and April advances (delays) ISM onset by enhancing (disturbing) the vertical easterly wind shear. In addition, mid‐tropospheric heating is affected by soil moisture in the Iranian desert. By investigating the internal atmospheric heating process and suggesting the relationship between low soil moisture and ISM onset, these findings clarify the monsoon onset mechanism in terms of the vertical atmospheric profile and land–atmosphere interaction, which eventually extend the lead‐time for the onset prediction.

Description: Abstract Strain partitioning related to oblique plate convergence has long been debated in Northern Lesser Antilles. Geophysical data acquired during the ANTITHESIS cruises highlight that the sinistral strike‐slip Bunce Fault develops along the vertical, long and linear discontinuity between the sedimentary wedge and a more rigid backstop. The narrowness of the 20‐30‐km‐wide accretionary wedge and its continuity over ~850 km is remarkable. The Bunce Fault extends as far south as 18.5°N where it anastomoses within the accretionary prism where the sharp increase in convergence obliquity possibly acts as a mechanical threshold. Surface traces related to subducting seamounts suggest that 80% of the lateral component of the convergent motion is taken up by internal deformation within the accretionary prism and by the Bunce Fault. The absence of crustal‐scale, long‐term tectonic system south of the Anegada Passage casts doubt upon the degree of strain partitioning in the Northern Lesser Antilles.

Description: Abstract We measured shear wave velocities in the shallow subsurface by applying seismic interferometry to earthquake records from eight vertical borehole arrays in eastern Hokkaido, Japan. We detected an increase of several percent in the seismic velocity during January to March due to seasonal frost dynamics. The velocity changes associated with seasonal frozen soil are affected by the frost depth and the extent of freezing, while the frost depth and the extent of freezing are mainly controlled by the cumulative temperature and the current temperature, respectively. Thus, a weighted cumulative freezing degree day is proposed to consider these two factors and used for stage division of the annual freeze‐thaw cycle. Based on the results of observation, we present an empirical model to relate the velocity changes with the weighted cumulative freezing degree days, which allows us to estimate the influence of seasonal frozen soil on near‐surface seismic velocity.

Description: Abstract At‐many‐stations hydraulic geometry (AMHG), while useful for estimating river discharge from satellite data, remains empirical and has yet to be reconciled with the at‐a‐station hydraulic geometry (AHG) from which it was originally derived. Here we present evidence, using United States Geological Survey field measurements of channel hydraulics for 155 rivers, that AMHG can be hydraulically and geomorphically reconciled with AHG. Our results indicate that AMHG is rightly understood as an expression of a river‐wide model of hydraulics driven by changes in slope imposed upon AHG physics. The explanatory power of AHG and this river‐wide model combine to determine whether AMHG exists: if both AHG and the river‐wide model adequately describe hydraulics, then we show that AMHG is a necessary mathematical consequence of these two phenomena. We also orient these findings in the context of river discharge estimation and other applications.

Description: Abstract Fast dropout of relativistic and ultrarelativistic electrons at both high and low L* regions were observed during the intense coronal mass ejection driven storm in June 2015. An improved radial diffusion model, using an event‐specific last closed drift shell and newly available radial diffusion coefficients (DLL), is implemented to simulate the magnetopause shadowing loss of electrons. The model captures the fast shadowing loss of electrons well at high L* regions after both interplanetary shocks, and reproduces the initial adiabatic loss of the high‐energy storage ring at low L* regions after the second strong shock. We show for the first time that using the event‐specific and K‐dependent last closed drift shell and improved DLL is critical to reproduce the observed dropout features, including the timing, location, and the butterfly electron pitch angle distribution. Future inclusion of the electromagnetic ion cyclotron wave scattering process is needed to model the observed further depletion of the storage ring.

Description: Abstract High‐quality single‐crystals of (Al,Fe)‐bearing bridgmanite, Mg0.88 Fe3+0.065Fe2+0.035Al0.14Si0.90O3, of hundreds of micrometer size were synthesized at 24 GPa and 1800 °C in a Kawai‐type apparatus from the starting hydrous melt containing ~6.7 wt% water. Analyses of synthesized bridgmanite using petrographic microscopy, scanning electron microscopy, and transmission electron microscopy show that the crystals are chemically homogeneous and inclusion‐free in micrometer‐ to nanometer‐spatial resolutions. Nano‐secondary ion mass spectrometry (NanoSIMS) analyses on selected platelets show ~1020(±70) ppm wt water (hydrogen). The high water concentration in the structure of bridgmanite was further confirmed using polarized and unpolarized Fourier‐transform infrared (FTIR) analyses with two pronounced OH‐stretching bands at ~3230 and ~3460 cm‐1. Our results indicate that lower‐mantle bridgmanite can accommodate relatively high amount of water. Therefore, dehydration melting at the topmost lower mantle by downward flow of transition zone materials would require water contents exceeding ~0.1 wt%.

Description: Abstract Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth is found to be nearly impossible to achieve. We show that seismic noise generated by vehicle traffic, and especially heavy freight trains, can be turned into a powerful repetitive seismic source to continuously probe the Earth's crust at a few kilometers depth. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train‐generated seismic signals allow daily reconstruction of direct P body waves probing the San Jacinto Fault down to 4‐km depth. This new approach may facilitate monitoring most of the San Andreas Fault system using the railway and highway network of California.

Description: Abstract Resilience of soil moisture regimes (SMRs) describes the stability of a particular SMR and its ability to withstand disturbances. This study analyzes the resilience of SMRs with quantifiable ecological (ECO‐) and engineering (ENG‐) metrics for a stochastic dynamic soil moisture system. The SMR is defined by the stationary state, described by a stationary probability distribution function (pdf), of the soil moisture dynamical system, and further classified into arid, semi‐arid, semi‐wet and wet classes. Applying the stationary pdf of soil moisture dynamics derived by Rodriguez‐Iturbe et al. [1999] and Laio et al. [2001a], the ENG‐ and ECO‐ resilience metrics of the various SMRs are quantified. We show that the recovery rate of soil moisture is a convex function of the expected soil moisture at the stationary state — the recovery rate reaches a minimum value at some intermediate soil moisture status. We also show that the maximum acceptable changes in the infiltration condition indicate the capacity of a system to avoid possible regime shifts. SMR shifts are characterized by phenomena of stagnation and hysteresis, which suggest two distinct thresholds for SMR shifts and their reversion. In particular, the semi‐wet SMR that is favorable to agriculture requires stricter infiltration conditions than other SMRs. This resilience analysis provides better understanding of how natural hydrological conditions control soil moisture, which helps provide guidance on maintaining SMRs suitable for agricultural activities and desertification prevention.

Description: Abstract A fundamental understanding of the fluid movement and dynamic partitioning process at fracture intersections is important to accurately predict water infiltration and contaminant transport in networks of fractures. We present an experimental study on the flow‐splitting behavior at a T‐shaped intersection. Different combinations of apertures of the vertical (bv) and horizontal (bh) fractures are considered. Experimental results confirm that the gravity‐driven flow in the vertical fracture transitions from droplet to rivulet mode as the flow rate increases. We quantify the flow dynamics through the intersection and especially focus on the partitioning efficiency (η) defined as the percentage of flow partitioned into the horizontal fracture. We identify three regimes of flow partitioning at the intersection for the case of bv 〈 bh: total partitioning (η → 1), splitting or partial bypass (0 〈 η 〈 1), and total bypass (η → 0). The total bypass regime is associated with the rivulet mode with a flow rate higher than ~1.5 ml/min. We find a simple relationship between η and the flow rate Q for droplet flow, η = min(1, ChQ−1), where Ch is a threshold flow rate below which droplets almost completely imbibe into the horizontal fracture, leading to η → 1. A force balance analysis links Ch to a critical droplet length for the transition from complete partitioning to path splitting. The obtained relationship is further supported by numerical simulations of droplet flow through intersections. The results and analysis from this study may provide insights and physical constraints on construction of reduced order unsaturated flow models based on simplified discrete fracture networks.

Description: Abstract The lowest winter‐maximum areal sea‐ice coverage on record (1980–2019) in the Bering Sea occurred in the winter of 2017/2018. Sea ice arrived late due to warm southerly winds in November. More typical northerly winds (albeit warm) in December and January advanced the ice, but strong, warm southerlies in February and March forced the ice to retreat. The cold pool (shelf region with bottom water 〈 2 °C) was the smallest on record, because of two related mechanisms: (1) lack of direct cooling in winter by melting sea ice and (2) weaker vertical stratification (no ice melt reduced the vertical salinity gradient) allowing surface heating to penetrate into the near bottom water during summer. February 2019 exhibited another outbreak of warm southerly winds forcing ice to retreat. The number of 〉31‐day outbreaks of southerly winds in winter has increased since 2016.

Description: Abstract At 0735 UT on December 13 2015,the Rocket Experiment for Neutral Upwelling‐2 (RENU2) experiment launched north towards the auroral cusp region from Andoya, Norway. The instrumented rocket included an electron spectrometer, photometers that measured the auroral redline and greenline, and an instrument that measured ionospheric thermal electron temperature. On the down leg, just south of Svalbard, the rocket entered a region of poleward moving auroral forms (PMAFs) that were characterized by narrow structures due to a combination of spatial and temporal variations. A noticeable feature was that the redline to greenline brightness ratio was much smaller than expected. A model is developed that shows that these emissions can be used to estimate the lifetimes of bursty electron precipitation. This model is shown to be consistent with some PMAF lifetimes being on the order of 100 ms. The correlation between the precipitation and temperature bursts suggest that some transport occurred.

Description: Abstract We present the electron density (ne) altitude profiles of Saturn's ionosphere at near‐equatorial latitudes from all 23 orbits of Cassini's Grand Finale. The data are collected by the Langmuir probe part of the Radio and Plasma Wave Science investigation. A high degree of variability in the electron density profiles is observed. However, organizing them by consecutive altitude ranges revealed clear differences between the southern and northern hemispheres. The ne profiles are shown to be more variable and connected to the D‐ring below 5,000 km in the southern hemisphere compared to the northern hemisphere. This observed variability is explained to be a consequence of an electrodynamic interaction with the D‐ring. Moreover, a density altitude profile is constructed for the northern hemisphere indicating the presence of three different ionospheric layers. Similar properties were observed during Cassini's final plunge, where the main ionospheric peak is crossed at ∼1,550‐km altitude.

Description: Abstract The Rocket Experiment for Neutral Upwelling 2 (RENU2) rocket was launched on Dec 13, 2015 at 07:34 UT. The payload transited the cusp region during a neutral upwelling event, supported by a comprehensive set of onboard and ground‐based instrumentation. RENU2 data highlight two important processes. One is that a proper understanding of neutral upwelling by Poleward Moving Auroral Forms (PMAFs) requires a treatment that mimics the quasi‐periodic passage of a sequence of PMAFs. As a PMAF reaches a fluxtube, its physical consequences must be determined including the residual history of effects from previous passages, implying that understanding such a process requires an accounting of the system hysteresis. Second, RENU2 observations suggest that neutral density enhancements driven by precipitation and/or Joule heating can be highly structured in altitude and latitude. In addition, timescales involving neutral dynamics suggest that the structuring must be slowly‐changing, e.g., over the course of 10 to 10s of minutes.

Description: Abstract The Rocket Experiment for Neutral Upwelling 2 (RENU2) sounding rocket launched from the Andøya Space Center on 13 December 2015 into the dayside polar cusp. An ultraviolet photomultiplier tube (UV PMT) on the RENU2 payload was oriented to look up along the spin axis for emissions of neutral atomic oxygen above the payload. Data from the UV PMT has been compared to predicted auroral emissions calculated by the Global Airglow (GLOW) model. The comparison between GLOW calculations driven by RENU2 electron precipitation measurements and UV PMT data suggest enhanced neutral density in the cusp at altitudes above the RENU2 trajectory.

Description: Abstract The polar orbit of Juno at Jupiter provides a unique opportunity to observe high latitude energetic particle injections. We measure energy‐dispersed impulsive injections of protons and electrons. Ion injection signatures are just as prevalent as electron signatures, contrary to previous equatorial observations. Included are previously unreported observations of high energy banded structures believed to be remnants of much earlier injections, where the particles have had time to disperse around Jupiter. A model fit of the injections used to estimate timing fits the shape of the proton signatures better than it does the electron shapes, suggesting that electrons and protons are different in their abilities to escape the injection region. We present UV observations of Jupiter's aurora and discuss the relationship between auroral injection features and in situ injection events. We find, unexpectedly, that the presence of in situ particle injections does not necessarily result in auroral injection signatures.

Description: Abstract During the 2018 Multidisciplinary Arctic Program– Last Ice in the Lincoln Sea we sampled 45 multiyear (MYI) and 34 first‐year ice (FYI) cores, combined with snow depth, ice thickness and transmittance surveys from adjacent level‐FYI and undeformed‐MYI. FYI sites show a decoupling between bottom‐ice chlorophyll a (chl a) and snow depth, however, MYI showed a significant correlation between ice‐algal chl a biomass and snow depth. Topographic control of the snow cover resulted in greater spatio‐temporal variability of the snow over the level FYI, and consequently transmittance, compared to MYI with an undulating surface. The coupled patterns of snow depth, transmittance, and chl a indicate that MYI provides an environment with more stable light conditions for ice algal growth. The importance of sea ice surface topography for ice algal habitat underpins the potential ecological changes associated with projected increased ice dynamics and deformation.

Description: Abstract A magnetosphere controls a planet's evolution by suppressing or enhancing atmospheric loss to space. In situ measurements of Uranus' magnetosphere from the Voyager 2 flyby in 1986 provide the only direct evidence of magnetospheric transport processes responsible for this atmospheric escape at Uranus. Analysis of high‐resolution Voyager 2 magnetic field data in Uranus' magnetotail reveals the presence of a loop‐like plasmoid filled with planetary plasma traveling away from the planet. This first plasmoid observation in an Ice Giant magnetosphere elucidates that: (1) both internal and external forces play a role in Uranus' magnetospheric dynamics; (2) magnetic reconnection contributes to the circulation of plasma and magnetic flux at Uranus; and (3) plasmoids may be a dominant transport mechanism for mass loss through Uranus' magnetotail.

Description: Abstract Streamflow simulation of the headwater catchment of the Yellow River basin (HCYRB) in China is important for water resources management of the Yellow River basin. A statistical‐dynamical model, combining regular vine copulas with an optimization method for structure estimation, is presented with an application for simulating the monthly streamflow with local climate drivers at HCYRB. Local climate drivers for streamflow in every month are analyzed using rank‐based correlation. Precipitation, evaporation, and temperature generally show strong associations with streamflow. Winter streamflows relate to total precipitation of the wet season, and total evaporation of Oct and Nov, while unfrozen‐month streamflows are correlated with evaporation and precipitation of current and previous one months in the wet season. Both canonical vine and D‐vine copulas are applied to develop different conditional quantile functions for streamflows in different months with their dynamical covariates. The covariates are selected from historical streamflows and climate drivers with appropriate lags using partial correlations. The optimal vine trees are selected using the sequential maximum spanning tree algorithm with the weight based on both dependence and goodness of fit. The model demonstrates higher skill than existing vine‐based models and the seasonal autoregressive integrated moving average model. The enhanced skill of the hybrid statistical‐dynamical model comes from an improved capability of capturing nonlinear correlation and tail dependence of streamflow and climate drivers with the optimization of vine structure selection. The model provides an effective advance to enhance water resources planning and management for HCYRB and the whole basin.

Description: Abstract Seasonally flooded forests along tropical rivers cover extensive areas, yet the processes driving air‐water exchanges of radiatively active gases are uncertain. To quantify the controls on gas transfer velocities, we combined measurements of water‐column temperature, meteorology in the forest and adjacent open water, turbulence with an acoustic Doppler velocimeter, gas concentrations, and fluxes with floating chambers. Under cooling, measured turbulence, quantified as the rate of dissipation of turbulent kinetic energy (ε), was similar to buoyancy flux computed from the surface energy budget, indicating convection dominated turbulence production. Under heating, turbulence was suppressed unless winds in the adjacent open water exceeded 1 m/s. Gas transfer velocities obtained from chamber measurements ranged from 1 to 5 cm/hr and were similar to or slightly less than predicted using a turbulence‐based surface renewal model computed with measured ε and ε predicted from wind and cooling.

Description: Abstract This study describes a model of Phillips' Λ(c) distribution, which is the expected length of breaking fronts (per unit surface area) moving with velocity c to c+dc, providing a framework for coupled atmosphere‐wave‐ocean models to explicitly account for wave breaking related air‐sea fluxes. The model of Λ depends on the spectral saturation, based on the statistics of the lengths of crest exceeding wave slope criteria, including long‐wave short‐wave modulation. A wave breaking dissipation function based on Λ was implemented in the model WaveWatchIII. The wave solutions are consistent with the observations, including several metrics of the spectrum and Λ(c) distributions. The whitecap coverage derived from Λ reproduces recent parameterizations saturating at high winds. The wave breaking variability due to wave‐current interaction is significant at submesoscales (order 1 km or smaller). The wave breaking model can be further developed to model gas transfer coefficients and aerosol production.

Description: Abstract We model mudstone permeability during consolidation and grain rotation, and during fluid injection by simulating porous media flow using the lattice Boltzmann method. We define the mudstone structure using clay platelet thickness, aspect ratio, orientation, and pore widths. Over the representative range of clay platelet lengths (0.1–3 μm), aspect ratios (length/thickness = 20–50), and porosities (ϕ = 0.07–0.80) our permeability results match mudstone datasets well. Homogenous kaolinite and smectite models document a log linear decline in vertical permeability from 8.31 × 10−15–6.84 × 10−17 m2 at ϕ = 0.76–0.80 to 6.33 × 10−19–1.30 × 10−23 m2 at ϕ = 0.14–0.16, showing good correlation with experimental data (R2 = 0.42 and 0.56).We employ our methodology to predict the permeability of two natural mudstone samples composed of smectite, illite, and chlorite grains. Over ϕ = 0.32–0.58, the permeability trends of two models replicating the mineralogical composition of the natural mudstone samples match experimental datasets well (R2 = 0.78 and 0.74). We extend our methodology to evaluate how vertical permeability might evolve during microfracture network growth or macrofracture propagation upon fluid injection in compacted mudstone. Fluid injection results in a permeability increase from 1.02 × 10−20 m2 at ϕ = 0.07 to 2.07 × 10−16 m2 at ϕ = 0.29 for growth of a microfracture network, and from 1.02 × 10−20 m2 at ϕ = 0.07 to 1.23 × 10−16 m2 at ϕ = 0.32 for macrofracture propagation. Our results suggest that a distributed microfracture network results in greater permeability during fluid injection in compacted mudstones (ϕ = 0.07–0.32) in comparison to a wide macrofracture. Our modeling approach provides a simple means to estimate permeability during burial and compaction or fluid injection based on knowledge of porosity and mineralogy.

Description: Abstract Phase five of the Coupled Model Intercomparison Project (CMIP5) enabled a range of decadal modelling experiments where climate models were initialised with observations and allowed to evolve freely for 10‐30 years. However, climate models struggle to realistically simulate rainfall and the skill of rainfall prediction in decadal experiments is poor. Here, we examine how predictions of sea surface temperature anomaly (SSTA) indices from CMIP5 decadal experiments can provide skilful rainfall forecasts at interannual timescales for Australia. Forecasts of commonly used SSTA indices relevant to Australian seasonal rainfall are derived from decadal hindcasts of six different climate models and corrected for model drift. The corrected indices are then combined to form a multi‐model ensemble. The resultant forecasts are used as predictors in a statistical rainfall model developed in this study. As SSTA forecasts lose skill with increasing lead time, a new methodology for predicting interannual rainfall is proposed. We allow our statistical prediction model to evolve with lead time while accounting for the loss of skill in SSTA forecasts instead of using one statistical model for all lead times. Results in this pilot study across two of the largest climate zones in Australia show that SSTA outputs from the decadal experiments provide enhanced skill in rainfall prediction over using the conventional model (based purely on lagged observed indices) up to a maximum of three years ahead. This methodology could be used more broadly for other regions around the world where rainfall variability is known to have strong links to ocean temperatures.

Description: Abstract The equilibrium climate sensitivity, that is, the global‐mean surface‐air temperature change in response to a doubling of the carbon dioxide concentration is a widely used metric in climate change studies. Its exact value is rarely known because its estimation requires a long integration time of several thousand years. We propose a method to estimate an accurate value of the equilibrium response from fully coupled climate models at a reasonable computational cost. Using this method, our state‐of‐the‐art climate model CNRM‐CM6‐1 reaches a stationary state after only few hundred of years of integration. This “Fast‐Forward” method consists of an optimal two‐step forcing pathway designed using the framework of a two‐layer energy balance model. It can be applied easily to any coupled climate model.

Description: Abstract Probabilistic modelling of streamflow in ephemeral catchments, where streamflow is frequently zero or negligible, is a major scientific and operational challenge. This paper evaluates the benefits of an explicit treatment of zero flows in the residual error models used for hydrological model calibration and prediction. In this approach, the lower bound of zero for streamflow is implemented using a censoring approach. The explicit approach is compared to a simpler pragmatic approach, which imposes the zero streamflow bound in prediction but not in calibration. Following a theoretical exposition, empirical comparisons are reported using a daily rainfall‐runoff model (GR4J), four residual error schemes (based on log, log‐sinh and Box‐Cox (BC) transformations with λ = 0.2 and 0.5), 74 Australian catchments with diverse hydroclimatology, and five performance metrics (reliability, precision, bias, proportion of zero flow days and CRPS skill score). The key findings are: (1) in “mid‐ephemeral” catchments (5‐50% zero flows) the explicit approach improves predictive performance, especially reliability, through better characterization of residual errors; (2) BC0.2 and BC0.5 schemes are Pareto optimal in mid‐ephemeral catchments (when the explicit approach is used): BC0.2 achieves better reliability and is recommended for probabilistic prediction, whereas BC0.5 attains lower volumetric bias; (3) in “low‐ephemeral” catchments (〈5% zero flows) the pragmatic approach is sufficient; (4) in “high‐ephemeral” catchments (〉50% zero flows) theoretical limitations result in poor performance of these particular explicit and pragmatic approaches, and further development is needed. The findings provide guidance on improving probabilistic streamflow predictions in ephemeral catchments.

Description: Abstract We examine the role of ions and electrons in reconnection using the highest resolution observations from the MMS mission on kinetic ion and electron scales. We report magnetic field and plasma observations from several approaches to the electron diffusion region in the current sheet in 2018. Besides magnetic field reversals, changes in the direction of flow velocity, ion and electron heating, MMS observed large fluctuations in the electron flow speeds in the magnetotail. We have verified that when the field lines and plasma become decoupleda large reconnecting electric field related to the Hall current (1‐10 mV/m) is responsible for the fast reconnection in the ion diffusion region. Although inertial acceleration forces remain moderate (1‐2 mV/m), the electric fields resulting from the electron pressure tensor provide the main contribution to the generalized Ohm's law at the neutral sheet (as large as 200 mV/m). This illustrates that when ions decouple electron physics dominates.

Description: Abstract It is assumed that the potential intensity of tropical cyclones (TC) will increase with rising global temperature. The western North Pacific is one of the three principal TC centers, but TC records from the region are scarce and sometimes controversial. Here we present grain‐size distributions and element contents of sediment cores from the East China Sea, in the western North Pacific. We interpret changes in the mean grain size of the coarse fraction as a proxy for TC intensity, and we infer a linkage of TC intensity to temperature changes over the last two millennia. Supported by model simulations, our results show that TC intensity increased (decreased) during relatively warm (cool) periods, confirming the control of temperature on TC intensity on a multicentennial scale. Our results suggest that long‐term TC intensity in the western North Pacific may increase with continued global warming.

Description: Abstract Surface evaporation in arid regions determines the fraction of rainfall that remains to support vegetation and recharge. The surface evaporation capacitor approach was used to estimate rainfall partitioning to surface evaporation and leakage into deeper layers. The surface evaporation capacitor estimates a soil‐specific surface evaporation depth and critical storage capacitance that defines rainfall events that exceed local capacitance and result in leakage into deeper layers protected from surface evaporation. A decade‐long record of hydrologic observations in deep and barren lysimeters near Las Vegas revealed the dominance of a few large rainfall events in generating leakage and increasing interannual soil water storage. The surface evaporation capacitor was used to estimate mean annual surface evaporation and leakage protected from surface evaporation in all arid regions globally. About 13% of arid region rainfall contributes to soil water storage (in the absence of vegetation), similar to 11% found in the lysimeter study.

Description: Abstract Global climate models generally overestimate recent tropospheric warming trends. While a number of explanations have been suggested, their relative impacts have not been quantified. In particular, interannual and long‐term variability of tropospheric temperatures (TTT) is known to be strongly constrained by near‐surface conditions in ocean regions of deep convection. Here, we analyze the role played by tropical sea surface temperature (SST) variability in recent decades in setting TTT. We find that Coupled Model Intercomparison Project Phase 5 models and observations agree on the interannual relationship between SSTs in regions of deep, tropical convection and TTT. Over the 1979–2018 period, most of the difference between model and satellite‐based TTT trends can be explained by respective differences in SST warming trends in regions of deep convection. While large multidecadal patterns of SST variability certainly play a role, notably in the Pacific Ocean, other mechanisms may also contribute to the overestimation of recent SST warming in climate models.

Description: Abstract The low‐altitude, high‐velocity trajectory of the Juno spacecraft enables the Jovian Auroral Distributions Experiment to make the first in situ observations of the high‐latitude ionospheric plasma. Ions are observed to energies below 1 eV. The high‐latitude ionospheric ions are observed simultaneously with a loss cone in the magnetospheric ions, suggesting precipitating magnetospheric ions contribute to the heating of the upper ionosphere, raising the scale height, and pushing ionospheric ions to altitudes of 0.5 RJ above the planet where they are observed by Jovian Auroral Distributions Experiment. The source of the magnetospheric ions is tied to the Io torus and plasma sheet, indicated by the cutoff seen in both the magnetospheric and ionospheric plasma at the Io M‐shells. Equatorward of the Io M‐shell boundary, the ionospheric ions are not observed, indicating a drop in the scale height of the ionospheric ions at those latitudes.

Description: Abstract Lakes support globally important food webs through algal productivity and contribute significantly to the global carbon cycle. However, predictions of how broad‐scale lake carbon flux and productivity may respond to future climate are extremely limited. Here, we used an integrated modeling framework to project changes in lake‐specific and regional primary productivity and carbon fluxes under 21st century climate for thousands of lakes. We observed high uncertainty in whether lakes collectively were to increase or decrease lake CO2 emissions and carbon burial in our modeled region owing to divergence in projected regional water balance among climate models. Variation in projected air temperature influenced projected changes in lake primary productivity (but not CO2 emissions or carbon burial) as warmer air temperatures decreased productivity through reduced lake water volume. Cross‐scale interactions between regional drivers and local characteristics dictated the magnitude and direction of lake‐specific carbon flux and productivity responses to future climate.

Description: Abstract Iceberg discharge is estimated to account for up to 50% of the freshwater flux delivered to glacial fjords. The amount, timing, and location of iceberg melting impacts fjord‐water circulation and heat budget, with implications for glacier dynamics, nutrient cycling, and fjord productivity. We use Sentinel‐2 imagery to examine seasonal variations in freshwater flux from open‐water icebergs in Sermilik Fjord, Greenland during summer and fall of 2017–2018. Using iceberg velocities derived from visual‐tracking and changes in total iceberg volume with distance down‐fjord from Helheim Glacier, we estimate maximum average two‐month full‐fjord iceberg‐derived freshwater fluxes of ~1,060 ± 615, 1,270 ± 735, 1,200 ± 700, 3,410 ± 1,975, and 1,150 ± 670 m3/s for May–June, June–July, July–August, August–September, and September–November, respectively. Fluxes decrease with distance down‐fjord, and on average, 86–91% of iceberg volume is lost before reaching the fjord mouth. This method provides a simple, invaluable tool for monitoring seasonal and interannual iceberg freshwater fluxes across a range of Greenlandic fjords.

Description: Abstract Density of silicate melt dictates melt migration and establishes the gross structure of Earth's interior. However, due to technical challenges, the melt density of relevant compositions is poorly known at deep mantle conditions. Particularly, water may be dissolved in such melts in large amounts and can potentially affect their density at extreme pressure and temperature conditions. Here we perform first‐principle molecular dynamics simulations to evaluate the density of Fe‐rich, eutectic‐like silicate melt (E melt) with varying water content up to about 12 wt %. Our results show that water mixes nearly ideally the with nonvolatile component in silicate melt and can decrease the melt density significantly. They also suggest that hydrous melts can be gravitationally stable in the lowermost mantle given its likely high iron content, providing a mechanism to explain seismically slow and dense layers near the core‐mantle boundary.

Description: Abstract The dust generated in East Asia influences the western North America (WNA) through its trans‐Pacific transport. This study investigates the distinct contribution between two main dust sources in East Asia on this remote influence based on Modern‐Era Retrospective analysis for Research and Applications version 2 data set. Results show that the dust generated in Gobi desert (GD) exerts a larger influence on the WNA compared to those in Taklimakan desert (TD). This difference is attributed to the different terrain and background winds in GD and TD. The GD is relatively flat and dominated by westerlies throughout the troposphere, which facilitates the trans‐Pacific transport of dust to WNA. However, the TD is located in the Tarim Basin and dominated by easterly wind in the lower troposphere. The uplifted dust is largely redeposited in TD. Moreover, the influence of GD on dust in WNA experiences decadal change around 1999, which is related to intrinsic change of dust loading in GD.

Description: Abstract Antarctic sea ice cover is projected to significantly decrease by the end of the twenty‐first century if greenhouse gas concentrations continue to rise, with potential consequences for Southern Hemisphere weather and climate. Here we examine the atmospheric response to projected Antarctic sea ice loss at quadrupled CO2, inferred from 11 Coupled Model Intercomparison Project phase 5 models. Our study is the first multimodel analysis of the atmospheric response to Antarctic sea ice loss. Projected sea ice loss enhances the negative phase of the Southern Annular Mode, which slightly damps the positive Southern Annular Mode response to increased CO2, particularly in spring. The negative Southern Annular Mode response largely reflects a weakening of the eddy‐driven jet, and to a lesser extent, an equatorward shift of the jet. Sea ice loss induces near‐surface warming over the high‐latitude Southern Ocean, but warming does not penetrate over the Antarctic continent. In spring, we find multimodel evidence for a weakened polar stratospheric vortex in response to sea ice loss.

Description: Abstract Microbially induced carbonate precipitation (MICP) is a promising technique that could be used for soil stabilization, for permeability control in porous and fractured media, for sealing leaky hydrocarbon wells, and for immobilizing contaminants. Many further field trials are required before optimum treatment strategies can be established. These field trials will be costly and time consuming to \carry out and are currently a barrier to transitioning MICP from a lab‐scale process to a practical field‐scale deployable technology. To narrow down the range of potential treatment options into a manageable number, we present a field‐scale reactive transport model of MICP that captures the key processes of bacteria transport and attachment, urea hydrolysis, tractable CaCO3 precipitation, and modification to the porous media in terms of porosity and permeability. The model, named biogroutFoam, is implemented in OpenFOAM, and results are presented for MICP treatment in a planar fracture, three‐dimensional sand media at pore scale, and at continuum scale for an array of nine injection/abstraction wells. Results indicate that it is necessary to model bacterial attachment, that bacterial attachment should be a function of fluid velocity, and that phased injection strategies may lead to the most uniform precipitation in a porous media.

Description: No abstract is available for this article.

Description: Abstract Wildfires are extreme events associated with weather, climate, and environment and have been increasing globally in frequency, burn season length, and burned area. It is of great interest to understand the impacts of wildfires on severe convective storms through releasing heat and aerosols into the atmosphere. We have developed a model capability that can account for the impact of sensible heat fluxes from wildfires on thermodynamics and is computationally efficient. The pyrocumulonimbus clouds associated with the Texas Mallard Fire on 11–12 May 2018 are well simulated by accounting for both heat and aerosols emitted from the wildfire. Both heat and aerosol effects increase low‐level temperatures and midlevel buoyancy and enhance convective intensity. Intensified convection along with more supercooled liquid condensate due to stronger vertical transport results in larger hailstones and enhanced lightning. The effects of heat flux on the convective extremes are more significant than those of aerosol emissions.

Description: Abstract Atmospheric boundary layer depths (BLDs) over continental sites have long been meticulously characterized. However, a downwind‐footprint concept for BLDs over plains under the impact of seasonally and spatially changing horizontal advection of BLDs off elevated terrains has remained unexplored. For the first time, we provide observational evidence of the impact of mountains on regional BLDs using 25‐years (1991–2015) of rawinsonde‐retrieved afternoon BLDs over 22 sites located in the mountains' (Rockies and Appalachians) downstream. Results suggest that mountain‐advected air mass, elevated terrains, and wind play a significant role in modulating BLD variability “miles away” from terrains. We found significant BLD contrasts over the plains (400–1,500 m) under mountain‐advected versus flatland‐advected flows pertaining to elevated mixed layers off the mountain ranges. The BLD contrasts were higher in the downwind of Rockies than the Appalachians, and higher BLD contrasts were observed in spring and summer (900–1,500 m) than in fall and winter (100–500 m). These findings will help build advanced parameterizations in models where BLD simulations around complex terrain still remain a hurdle.

Description: Abstract Tsunami earthquakes produce some of the most devastating tsunamis. These rare events have comparatively modest magnitudes but rupture the shallowest portion of a subduction zone megathrust with exceptionally large seafloor displacements. Previous teleseismic observations found that they radiate seismic waves weakly. They should therefore not be strongly felt in the near field, but to date no near‐source seismic recordings of these events exist that confirm this. Here we analyze near‐field records of a tsunami earthquake, the 2010 M7.7 Mentawai, Indonesia event, which show remarkably weak shaking. This is strong evidence that this earthquake does indeed have a weakly radiating or inefficient source process, in spite of its large slip. Finally, we find that, when combined with near‐source Global Navigation Satellite System displacement recordings it is possible to correctly characterize tsunami earthquakes in real‐time and to provide local tsunami warning which is currently out of reach today for monitoring agencies.

Description: Abstract Evidence from proxy records indicates that millennial‐scale abrupt climate shifts, called Dansgaard‐Oeschger events, happened during past glacial cycles. Various studies have been conducted to uncover the physical mechanism behind them, based on the assumption that climate mean state determines the variability. However, our study shows that the Dansgaard‐Oeschger events can regulate the mean state of the Northern Hemisphere ice sheets. Sensitivity experiments show that the simulated mean state is influenced by the amplitude of the climatic noise. The most likely cause of this phenomenon is the nonlinear response of the surface mass balance to temperature. It could also cause the retreat processes to be faster than the buildup processes within a glacial cycle. We propose that the climate variability hindered ice sheet development and prevented the Earth system from entering a full glacial state from Marine Isotope Stage 4 to Marine Isotope Stage 3 about 60,000 years ago.

Description: Abstract We analyzed the single‐grain mineralogical composition of aeolian dust transported to central East Antarctica for provenance purposes. Comparison with data from the last glacial period shows for the first time disappearance of carbonates during the Holocene related to sea level rise and suppressed deflation from the Argentinean continental shelf, exposed during Marine Isotope Stage (MIS) 2. Zeolites, related to alteration of volcanic glass in the subglacial/periglacial environment of Patagonia, show a similar behavior. The remaining minerals, remarkably similar between the two climatic periods, are compatible with a Pampean and Patagonian provenance, but Holocene data show a more pronounced volcanic and metamorphic imprint, and presence of minerals related to warm climate weathering environments compatible with an additional contribution from subtropical latitudes of South America. These results do not imply a major large‐scale reorganization of atmospheric circulation after the last climatic transition.

Description: Abstract Tropopause temperature (TPT) is a useful indicator and a key component of climate change. Well simulating its value and seasonal‐to‐decadal variability by climate models is still challenging. How the vertical resolution influences the representation of TPT and its response to a climate forcing is largely unknown. This study investigates TPT responses to sea surface temperatures using a series of model simulations in various vertical resolution. With high vertical resolution (HV‐Res), the model gives a better representation of tropical TPTs in absolute values and seasonal variations. The corresponding changes in TPTs associated with sea surface temperature anomalies (El Niño–Southern Oscillation and Pacific Decadal Oscillation) are 30% stronger and more realistic in the HV‐Res model. Such improvements may get benefits from better representations of equatorial waves with more realistic structure and stronger interannual variations. A proper vertical resolution is therefore essential to well simulate the stratosphere‐troposphere coupling and should be used in climate change assessment.

Description: Abstract A Bayesian model that uses the spatial dependence induced by the river network topology, and the leading principal components of regional tree‐ring chronologies for paleo‐streamflow reconstruction is presented. In any river basin, a convergent, dendritic network of tributaries comes together to form the main stem of a river. Consequently, it is natural to think of a spatial Markov process that recognizes this topological structure to develop a spatially consistent basin‐scale streamflow reconstruction model that uses the information in streamflow and tree‐ring chronology data to inform the reconstructed flows, while maintaining the space‐time correlation structure of flows that is critical for water resource assessments and management. Given historical data from multiple streamflow gauges along a river, their tributaries in a watershed, and regional tree‐ring chronologies, the model is fit and used to simultaneously reconstruct the full network of paleo‐streamflow at all gauges in the basin progressing upstream to downstream along the river. Our application to eighteen streamflow gauges in the Upper Missouri River Basin shows that the mean adjusted‐R2 for the basin is approximately 0.5 with good overall cross‐validated skill as measured by five different skill metrics. The spatial network structure produced a substantial reduction in the uncertainty associated with paleo‐streamflow as one proceeds downstream in the network aggregating information from upstream gauges and tree‐ring chronologies. Uncertainty was reduced by more than 50% at six gauges, between 6 and 50% at one gauge, and by less than 5% at the remaining eleven gauges when compared with the traditional PCR reconstruction model.

Description: Abstract The partitioning of carbon between the core and mantle during the formation of terrestrial planets may have controlled the distribution of carbon in terrestrial planets. However, the abundance of carbon in the Earth's mantle is higher than a prediction based on previous metal‐silicate partitioning experiments of carbon at carbon‐saturated conditions by more than an order of magnitude. Here, we report new metal‐silicate partitioning experiments of carbon at carbon contents of 0.25–0.5 wt%. We show that the metal‐silicate partition coefficient of carbon ( ) strongly correlates with nonbridging oxygen per tetrahedral cations (nbo/t) of silicate melts at conditions where C‐H species are stable. Moreover, the results suggest that at carbon‐undersaturated conditions may be lower than that at carbon‐saturated condition. Thus, at low carbon concentrations is essentially important to investigate the distribution of carbon in the Earth during the core formation.

Description: Abstract The moisture‐entrainment‐convection (MEC) feedback posits that a moist environment favors deep convection, which further moistens the atmosphere through its associated circulation and detrainment. The MEC feedback has been proposed to be crucial to spontaneous convective aggregation. Here we test this hypothesis by performing minimal cloud‐resolving simulations, without the buoyancy effect due to water vapor, evaporation of rain, or radiative and surface‐flux feedbacks. Convection can self‐aggregate in this minimal simulation, in which the MEC feedback is active. We then switch off this feedback by relaxing moisture to its horizontal mean over a timescale of three hours. Convection still self‐aggregates in this mechanism‐denial experiment, suggesting that the MEC feedback is not essential to self‐aggregation. We further show thatconvective heating coincides with positive temperature anomalies, generating available potential energy. Therefore, we propose that this convective heating – overturning circulation (CHOC) feedback can lead to spontaneous development of large‐scale circulations.

Description: Abstract Interannual variability of mountain snowpack has important consequences for ecological and socioeconomic systems, yet changes in variability have not been widely examined under future climates. Physically based snowpack simulations for historical (1970–1999) and high‐emission scenario (RCP 8.5) mid‐21st century (2050–2079) periods were used to assess changes in the variability of annual maximum snow water equivalent (SWEmax) and SWEmax timing across the western United States. Models show robust declines in the interannual variability of SWEmax in regions where precipitation is projected to increasingly fall as rain. The average frequency of consecutive snow drought years (SWEmax 〈 historical 25th percentile) is projected to increase from 6.6% to 42.2% of years. Models also project increases in the variability of SWEmax timing, suggesting reduced reliability of when SWEmax occurs. Differences in physiography and regional climate create distinct spatial patterns of change in snowpack variability that will require adaptive strategies for environmental resource management.

Description: Abstract In the South Atlantic, a reorganization of the Mid‐Atlantic Ridge began before anomaly C34n (83.6 Ma) and ended before anomaly C30n (66.4 Ma), complicating tectonics of Rio Grande Rise and older Walvis Ridge (WR), which formed together at the Mid‐Atlantic Ridge. This reorganization is poorly understood because magnetic anomalies C30n‐C34n are poorly defined near WR. We interpreted these anomalies along western WR to improve knowledge of Rio Grande Rise‐WR tectonic development. Anomaly trends indicate that Valdivia Bank has an E‐W age progression, perpendicular to that predicted by hot spot models. Anomaly spacing and width is irregular and anomalous near WR, implying a series of ridge jumps and possibly a microplate between anomalies C34n and C32n. Eastward ridge jumps transferred microplate lithosphere to the South American plate. This study shows that Late Cretaceous tectonic evolution of the Rio Grande Rise‐WR large igneous provinces was more complex than previously understood.

Description: Abstract The National Aeronautics and Space Administration Global‐scale Observations of the Limb and Disk ultraviolet spectrograph has been imaging the equatorial ionization anomaly (EIA), regions of the ionosphere with enhanced electron density north and south of the magnetic equator, since October 2018. The initial 3 months of observations was during solar minimum conditions, and they included observations in December solstice of unanticipated variability and depleted regions. Depletions are seen on most nights, in contrast to expectations from previous space‐based observations. The variety of scales and morphologies also pose challenges to understanding of the EIA. Abrupt changes in the EIA location, which could be related to in situ measurements of large‐scale depletion regions, are observed on some nights. Such synoptic‐scale disruptions have not been previously identified.

Description: Abstract Tropical average shortwave cloud radiative effect (SWCRE) anomalies observed by CERES/EBAF v4 are explained by observed average sea surface temperature ( ) and the difference between the warmest 30% where deep convection occurs and ). Observed tropospheric temperatures show variations in boundary layer capping strength over time consistent with the evolution of SST#. The CERES/EBAF v4 data confirm that associated cloud fraction changes over the colder waters dominate SWCRE. This observational evidence for the “pattern effect” noted in General Circulation Model simulations suggests that SST# captures much of this effect. The observed sensitivities (dSWCRE/d W·m−2·K−1, dSWCRE/dSST#≈−4.8W·m−2·K−1) largely reflect El Niño–Southern Oscillation. As El Niño develops, increases and SST# decreases (both increasing SWCRE). Only after the El Niño peak, SST# increases and SWCRE decreases. SST# is also relevant for the tropical temperature trend profile controversy and the discrepancy between observed and modeled equatorial Pacific SST trends. Causality and implications for future climates are discussed.

Description: Abstract The interleaving of impermeable and permeable surfaces along a runoff flow path controls the hillslope hydrograph, the spatial pattern of infiltration, and the distribution of flow velocities in landscapes dominated by overland flow. Predictions of the relationship between the pattern of (im)permeable surfaces and hydrological outcomes tend to fall into two categories: (i) generalized metrics of landscape pattern, often referred to as connectivity metrics, and (ii) direct simulation of specific hillslopes. Unfortunately, the success of using connectivity metrics for prediction is mixed, while direct simulation approaches are computationally expensive and hard to generalize. Here we present a new approach for prediction based on emulation of a coupled Saint Venant equation‐Richards equation model with random forest regression. The emulation model predicts infiltration and peak flow velocities for every location on a hillslope with an arbitrary spatial pattern of impermeable and permeable surfaces but fixed soil, slope, and storm properties. It provides excellent fidelity to the physically based model predictions and is generalizable to novel spatial patterns. The spatial pattern features that explain most of the hydrological variability are not stable across different soils, slopes, and storms, potentially explaining some of the difficulties associated with direct use of spatial metrics for predicting landscape function. Although the current emulator relies on strong assumptions, including smooth topography, binary permeability fields, and only a small collection of soils, slope, and storm scenarios, it offers a promising way forward for applications in dryland and urban settings and in supporting the development of potential connectivity indices.

Description: Abstract Northern line‐of‐sight extinction within Gale Crater during the 2018 global dust storm was monitored daily using Mars Science Laboratory's Navcam. Additional observations with Mastcam (north) and Navcam (all directions) were obtained at a lower cadence. Using feature identification and georeferencing, extinction was estimated in all possible directions. Peak extinction of 〉1.1 km−1 was measured between sols 2086 and 2090, an order of magnitude higher than previous maxima. Northern and western directions show an initial decrease, followed by a secondary peak in extinction, not seen in column opacity measurements. Due to foreground topography, eastern direction results are provided only as limits, and southern results were indeterminable. Mastcam red and green filter results agree well, but blue filter results show higher extinctions, likely due to low signal‐to‐noise. Morning results are systematically higher than afternoon results, potentially indicative of atmospheric mixing.

Description: Abstract Many general circulation models fail to capture the observed frequency of atmospheric blocking events in the Northern Hemisphere; however, few studies have examined models in the Southern Hemisphere and those studies that have, have often been based on only a few models. To provide a comprehensive view of how the current generation of coupled general circulation models performs in the Southern Hemisphere and how blocking frequency changes under enhanced greenhouse gas forcing, we examine the output of 23 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We find that models have differing biases during winter, when blocking occurrence is highest, though models underestimate blocking frequency south of Australia during summer. We show that models generally have a reduction in blocking frequency with future anthropogenic forcing, particularly in the Australia‐New Zealand sector with the number of winter blocked days reduced by about one third by the end of the 21st century.

Description: Abstract Using the latest daily Moderate Resolution Imaging Spectroradiometer satellite snow cover data, the present study reveals distinctly different sources of 10‐ to 30‐day intraseasonal snow cover variations over the western and eastern Tibetan Plateau (TP) during September–December. The intraseasonal snow variation over the western TP is related to a midlatitude wave train associated with the Arctic Oscillation and that over the eastern TP is related to a subtropical wave train triggered by the North Atlantic Oscillation. The Rossby wave train in both cases leads to anomalous water vapor convergence and ascending motion, which contributes to snow accumulation and positive snow cover anomalies. For the western TP snow events, the moisture comes from the Caspian Sea. During the eastern TP snow events, the moisture originates from the Bay of Bengal.

Description: Abstract Through the analysis of propagation times of infragravity wave packets along ray paths, reanalysis data, and our field measurements in the East Mediterranean, we find evidence of deep water infragravity wave generation by offshore storms. We confirmed the results also using deep water pressure cell measurements in the Pacific. The known nearshore generation mechanism showed large discrepancies with the observed infragravity energy near Aogashima, Japan, during winter. A new model of deep water infragravity wave generation is developed, based on nonlinear interactions of wind wave triads with submesoscale wind oscillations. The observed underprediction of infragravity waves is resolved using this new gustiness‐based model. The new source term is found to be of importance during strong storms in the open ocean and underlines the importance of accounting for submesoscale wind oscillations in wind wave models.

Description: Abstract Solar geoengineering has been suggested as a potential means to counteract anthropogenic warming. Major volcanic eruptions have been used as natural analogues to large‐scale deployments of stratospheric aerosol geoengineering, yet difference in climate responses to these forcings remains unclear. Using the National Center for Atmospheric Research Community Earth System Model, we compare climate responses to two highly idealized stratospheric aerosol forcings that have different durations: a short‐term pulse representative of volcanic eruptions and a long‐term sustained forcing representative of geoengineering. For the same amount of global mean cooling, decreases in land temperature, precipitation, and runoff in the pulse case are much larger than that in the sustained case. The spatial pattern changes differ substantially between these two cases. Thus, direct extrapolations from volcanic eruption observations provide limited insight into impacts of potential stratospheric aerosol geoengineering. However, simulations of volcanic eruptions can be useful to test process representations in models that are used to simulate geoengineering deployments.

Description: Abstract The physical parameterization of key processes in land surface models (LSMs) remains uncertain, and new techniques are required to evaluate LSMs accuracy over large spatial scales. Given the role of soil moisture in the partitioning of surface water fluxes (between infiltration, runoff, and evapotranspiration), surface soil moisture (SSM) estimates represent an important observational benchmark for such evaluations. Here, we apply SSM estimates from the NASA Soil Moisture Active Passive Level‐4 product (SMAP_L4) to diagnose bias in the correlation between SSM and surface runoff for multiple Noah‐Multiple Physics (Noah‐MP) LSM parameterization cases. Results demonstrate that Noah‐MP surface runoff parameterizations often underestimate the correlation between prestorm SSM and the event‐scale runoff coefficient (RC; defined as the ratio between event‐scale streamflow and precipitation volumes). This bias can be quantified against an observational benchmark calculated using streamflow observations and SMAP_L4 SSM and applied to explain a substantial fraction of the observed basin‐to‐basin (and case‐to‐case) variability in the skill of event‐scale RC estimates from Noah‐MP. Most notably, a low bias in LSM‐predicted SSM/RC correlation squanders RC information contained in prestorm SSM and reduces LSM RC estimation skill. Based on this concept, a novel case selection strategy for ungauged basins is introduced and demonstrated to successfully identify poorly performing Noah‐MP parameterization cases.

Description: Abstract We demonstrate a simple, cheap method for pore size characterization of porous media that generates a distribution of pore radii for improved flow and transport modeling. The new method for pore structure characterization utilizes recent theoretical developments in non‐Newtonian fluids. Numerical evaluations and validations with synthetic porous media showed potential for obtaining a distribution of effective pore radii and their contribution to total flow only by complementing water with non‐Newtonian fluids in saturated infiltration experiments. To demonstrate this ability on real sands, a series of one‐dimensional column experiments was conducted with varying porous medium packings, including Accusands and a polydisperse sand/glass bead mixture. For each packing, distilled water and varying concentrations of guar and xanthan gum were injected over a range of flow rates and pressure gradients. The model‐generated pore radii were compared with pore radius distributions measured by X‐ray microcomputed tomography (μCT), with results demonstrating good agreement between the model and μCT data. Simulations of saturated water flow and drainage curves using model‐generated pore radii compared favorably to experimental data, with errors typically between 2% and 10% for single‐phase flow and approaching the error of the μCT measured radius distributions for the drainage curves.

Description: Abstract Reported groundwater recovery in South India has been attributed to both increasing rainfall and political interventions. Findings of increasing groundwater levels, however, are at odds with reports of well failure and decreases in the land area irrigated from shallow wells. We argue that recently reported results are skewed by the problem of survivor bias, with dry or defunct wells being systematically excluded from trend analyses due to missing data. We hypothesize that these dry wells carry critical information about groundwater stress that is missed when data are filtered. Indeed, we find strong correlations between missing well data and metrics related to climate stress and groundwater development, indicative of a systemic bias. Using two alternative metrics, which take into account information from dry and defunct wells, our results demonstrate increasing groundwater stress in South India. Our refined approach for identifying groundwater depletion hot spots is critical for policy interventions and resource allocation.

Description: Abstract Because of the possibility of getting the right answers for the wrong reasons, the predictive performance of a complex systems model is not by itself a reliable indicator of hypothesis quality for the purposes of scientific learning about processes. The predictive performance of a structurally adequate model should be an emergent property of its functional performance. In this context, any Pareto trade‐off between measures of predictive performance versus functional performance indicates process‐level error in the model; this trade‐off, if it exists, indicates that the model's predictions are right for the wrong functional reasons. This paper demonstrates a novel concept based on information theory that is capable of attributing observed errors to specific processes. To demonstrate that the concept and method hold true for models and observations of real systems, we employ a minimal single‐parameter‐variation sensitivity analysis using a sophisticated ecohydrology model, MLCan, for a well‐monitored field site (Bondville IL Ameriflux Soybean). We identify both functional and predictive error in MLCan, and also evidence of the hypothesized tradeoffs between the two. This trade‐off indicates structural error within MLCan. For example, the sensible heat flux process can be calibrated to achieve good predictive performance at the cost of poor functional performance. In contrast, we find little structural error for processes driven by solar radiation, which appear “right for the right reasons.” This method could be applied broadly to pinpoint process error and structural error in a wide range of system models, beyond the ecohydrological scope demonstrated here.

Description: Abstract Monthly evapotranspiration (ET) rates for 1979–2015 were estimated by the latest, calibration‐free version of the complementary relationship (CR) of evaporation over the conterminous United States. The results were compared to similar estimates of three land surface models (Noah, VIC, Mosaic), two reanalysis products (National Centers of Environmental Protection Reanalysis II, ERA‐Interim), two remote‐sensing‐based (Global Land Evaporation Amsterdam Model, Penman‐Monteith‐Leuning) algorithms, and the spatially upscaled eddy‐covariance ET measurements of FLUXNET‐MTE. Model validations were performed via simplified water‐balance derived ET rates employing Parameter‐Elevation Regressions on Independent Slopes Model precipitation, United States Geological Survey two‐ and six‐digit Hydrologic Unit Code (HUC2 and HUC6) discharge, and terrestrial water storage anomalies from Gravity Recovery and Climate Experiment, the latter for 2003–2015. The CR outperforms all other multiyear mean annual HUC2‐averaged ET estimates with root‐mean‐square error = 51 mm/year, R = 0.98, relative bias of −1%, and Nash‐Sutcliffe efficiency = 0.94, respectively. Inclusion of the Gravity Recovery and Climate Experiment data into the annual water balances for the shorter 2003–2015 period does not have much effect on model performance. Similarly, the CR outperforms all other models for the linear trend of the annual ET rates over the HUC2 basins. Over the significantly smaller HUC6 basins where the water‐balance validation is more uncertain, the CR still outperforms all other models except FLUXNET‐MTE, which has the advantage of possible local ET measurements, a benefit that clearly diminishes at the HUC2 scale. As the employed CR is calibration‐free and requires only very few meteorological inputs, yet it yields superior ET performance at the regional scale, it may serve as a diagnostic and benchmarking tool for more complex and data intensive models of terrestrial evapotranspiration rates.

Description: Abstract Recent advances in machine learning open new opportunities to gain deeper insight into hydrological systems, where some relevant system quantities remain difficult to measure. We use deep learning methods trained on numerical simulations of the physical processes to explore the possibilities to close the information gap of missing system quantities. As an illustrative example we study the estimation of velocity fields in numerical and laboratory experiments of density‐driven solute transport. Using high resolution observations of the solute concentration distribution, we demonstrate the capability of the method to structurally incorporate the representation of the physical processes. Velocity field estimation for synthetic data for both, variable and uniform concentration boundary conditions, showed equal results. This capability is remarkable because only the latter was employed for training the network. Applying the method to measured concentration distributions of density‐driven solute transport in a Hele‐Shaw cell makes the velocity field assessable in the experiment. This assessability of the velocity field even holds for regions with negligible solute concentration between the density fingers, where the velocity field is otherwise inaccessible.

Description: Abstract In view of the rapid proliferation of water infrastructures worldwide, balancing human and ecosystem needs for water resources is a critical environmental challenge of global significance. While there is abundant literature on the environmental impacts of individual water infrastructures, little attention has been paid to their cumulative effects in river networks, which may have basin‐to‐global impacts on freshwater ecology. Here we developed a methodological framework based on Pareto frontier analysis for optimizing trade‐offs between water withdrawal and ecological indicators. We applied this framework to a mountainous Ecuadorian headwater river network that is part of a continental water transfer for supply and demand management to optimize ecological conditions and the operation of 11 water intake structures used to provide potable water to the city of Quito. We found that the current water intake configuration has an important effect on the total length of fifth‐order stream sections (65% reduction compared to premanaged condition) and isolates 70.9% of the headwater stream length. The Pareto frontier analysis identified water intake portfolios (i.e., different combinations of intake sites) that decreased ecological impacts by 7.8% points (pp) and 13.0 pp for connectivity and stream order change, respectively, while meeting Quito's water demands. Additional portfolios accounting for monthly variability in water demand and resources further decrease the ecological impact up to 9.6 pp in connectivity and 13.4 pp in stream order. These eco‐friendly portfolios suggest that adaptive management at basin level may help optimize water withdrawal to fulfill urban demands while preserving ecological integrity.

Description: Abstract With CO2 concentrations similar to today (410 ppm), the Pliocene Epoch offers insights into climate changes under a moderately warmer world. Previous work suggested a low zonal sea surface temperature (SST) gradient in the tropical Pacific during the Pliocene, the so‐called “permanent El Niño.” Here, we recalculate SSTs using the alkenone proxy and find moderate reductions in both the zonal and meridional SST gradients during the mid‐Piacenzian warm period. These reductions are captured by coupled climate model simulations of the Pliocene, especially those that simulate weaker Walker circulation. We also produce a spatial reconstruction of mid‐Piacenzian warm period Pacific SSTs that closely resembles both Pliocene and future, low‐emissions simulations, a pattern that is, to a first order, diagnostic of weaker Walker circulation. Therefore, Pliocene warmth does not require drastic changes in the climate system—rather, it supports the expectation that the Walker circulation will weaken in the future under higher CO2.

Description: Abstract This study explores the “tug of war” between the effects of Arctic amplification (“AA”) and upper‐troposphere tropical warming (“UTW”) on the response of the future North Atlantic atmospheric circulation. The late 21st century AA and UTW temperature anomalies are imposed in a high‐top atmospheric model by nudging the temperature. Two sets of experiments are performed, with and without feedback of the polar stratosphere to highlight its role in the response to UTW, AA, and both combined. With interactive polar stratosphere, UTW forces an equatorward shift of the eddy‐driven jet that reinforces the response to AA. However, when the polar stratosphere feedback is suppressed, the response to UTW is opposite and reflects the previously identified tug of war between the effects of UTW and AA in midlatitudes. This study highlights that the polar stratosphere is a key component for future changes in the North Atlantic atmospheric circulation and that it must be accurately represented in climate change scenarios.

Description: Abstract Gravity waves impacting ice shelves illicit a suite of responses that can affect ice shelf integrity. Broadband seismometers deployed on the Ross Ice Shelf, complemented by a near‐icefront seafloor hydrophone, establish the association of strong icequake activity with ocean gravity wave amplitudes (AG) below 0.04 Hz. The Ross Ice Shelf‐front seismic vertical displacement amplitudes (ASV) are well correlated with AG, allowing estimating the frequency‐dependent transfer function from gravity wave amplitude to icefront vertical displacement amplitude (TGSV(f)). TGSV(f) is 0.6–0.7 at 0.001–0.01 Hz but decreases rapidly at higher frequencies. Seismicity of strong icequakes exhibits spatial and seasonal associations with different gravity wave frequency bands, with the strongest icequakes observed at the icefront primarily during the austral summer when sea ice is minimal and swell impacts are strongest.

Description: Abstract Downstream of Cape Hatteras, the Gulf Stream (GS) is bounded to the north by a sharp temperature front known as the North Wall (NW). Previous studies have generally assumed that variations of the NW and GS are equivalent. Using satellite sea surface height to identify the GS and the 15 °C isotherm at 200‐m depth to represent the NW, this paper examines their similarities and differences during 1993–2016. The NW and GS are geographically close and vary similarly only to the west of 71°W. Downstream of that, they rapidly diverge—and the variances of their latitudes increase by more than a factor of 2—as the GS flows past the New England Seamounts. Evidence is presented to show that the difference in properties of the NW and the GS is related to the presence of mesoscale eddies in the region separating them.

Description: Abstract Accurately representing the properties and impact of tropical convection in climate models requires an understanding of the relationships between the state of a convective cloud ensemble and the environment it is embedded in. We investigate this relationship using 13 years of radar observations in the tropics. Specifically, we focus on convective cell number and size and quantify their relationship to atmospheric stability, midtropospheric vertical motion and humidity. We find several key convective states embedded in their own unique environments. The most area‐averaged rainfall occurs with a moderate number of moderate size convective cell in an environment of high humidity, strong vertical ascent, and moderate convective available potential energy (CAPE) and convective inhibition (CIN). The strongest rainfall intensities are found with few large cells. Those exist in a dry and subsiding environment with both high CAPE and CIN. Large numbers of convective cells are associated with small CAPE and CIN, weak ascent, and a moist midtroposphere.

Description: Abstract Past studies of noble gas concentrations in the deep ocean have revealed widespread, several percent undersaturation of Ar, Kr, and Xe. However, the physical explanation for these disequilibria remains unclear. To gain insight into undersaturation set by deep‐water formation, we measured heavy noble gas isotope and elemental ratios from the deep North Pacific using a new analytical technique. To our knowledge, these are the first high‐precision seawater profiles of 38Ar/36Ar and Kr and Xe isotope ratios. To interpret isotopic disequilibria, we carried out a suite of laboratory experiments to measure solubility fractionation factors in seawater. In the deep North Pacific, we find undersaturation of heavy‐to‐light Ar and Kr isotope ratios, suggesting an important role for rapid cooling‐driven, diffusive air‐to‐sea gas transport in setting the deep‐ocean undersaturation of heavy noble gases. These isotope ratios represent promising new constraints for quantifying physical air‐sea gas exchange processes, complementing noble gas concentration measurements.

Description: Abstract Hydroelectric dams often create highly dynamic downstream flows that promote surface water‐groundwater (SW‐GW) interactions including bank storage, the temporary storage of river water in the riverbank. Previous research on SW‐GW exchanges in dammed rivers have been local studies conducted within the bed or the bank, limiting the understanding of these exchanges which occur over potentially hundreds of kilometers. This study evaluates how dam releases affect SW‐GW exchange continuously over a 100 km distance. This is accomplished by longitudinally routing water releases through a synthetic river and modeling bed and bank fluid and solute exchange across transverse transects spaced along the reach. Peak and square dam release hydrograph shapes with three magnitudes (0.5, 1.0, and 1.5 m) were considered. The effect of four ambient groundwater flow conditions (very slightly losing, neutral, and two gaining from the perspective of the river) were evaluated for each dam release scenario. Both types of dam release shapes cause SW‐GW interaction over the entire 100 km distance, and our results show square type releases cause bank storage exchange well beyond this distance. Strongly gaining conditions reduce the amount of exchange and allow flushing of river‐sourced solute out of the bank after the dam pulse has passed. Both neutral and losing conditions have larger fluid and solute flux into the bank and limit the amount of solute that returns to the river. Our results support that river corridors downstream of dams have increased river‐aquifer connectivity, and that this enhanced connectivity can extend at least 100 km downstream.

Description: Abstract Watershed studies often rely on the assumption that interannual storage changes are negligible in the hydrologic balance of a watershed. The assumption can be useful and is sometimes necessary, but it is widely acknowledged as unrealistic. Identifying and understanding systematic deviations from hydrologic steady state has important implications for both hydrologic research and water management. To that end, we evaluated the magnitude of interannual changes in storage for nearly 1000 watersheds in the conterminous US (CONUS) for the ten‐year period 2002 to 2011 using ground‐based and remotely sensed data. We evaluated relationships between changes in storage (i.e., deviations from hydrologic steady state), vegetation cover, and hydroclimatic variables. Analysis of results using a Budyko framework revealed that, in general, greater evaporative partitioning led to smaller deviations from hydrologic steady state. Additional analysis using gradient boosted regression tree modeling identified an inverse relationship between forest cover and the magnitude of deviations from hydrologic steady state. In fact, modeling showed forest cover to be a stronger driver of variability in deviations from steady state than any hydroclimatic variable. We discuss ecohydrological feedbacks capable of contributing to steady state conditions in forested watersheds, and we discuss implications of these results for the co‐evolution of watersheds, vegetation, and climate.

Description: Abstract Flood damage processes are complex and vary between events and regions. State‐of‐the‐art flood loss models are often developed on basis of empirical damage data from specific case studies and do not perform well when spatially and temporally transferred. This is due to the fact, that such localized models often cover only a small set of possible damage processes from one event and a region. On the other hand, a single generalized model covering multiple events and different regions ignores the variability in damage processes across regions and events due to variables that are not explicitly accounted for individual households. We implement a Hierarchical Bayesian approach to parameterize widely used depth‐damage functions resulting in a Hierarchical (multi‐level) Bayesian Model (HBM) for flood loss estimation that accounts for spatio‐temporal heterogeneity in damage processes. We test and prove the hypothesis that, in transfer scenarios, HBMs are superior compared to generalized and localized regression models. In order to improve loss predictions for regions and events for which no empirical damage data is available, we use variables pertaining to specific region‐ and event‐characteristics representing commonly available expert knowledge as group‐level predictors within the HBM.

Description: Abstract “Eddy saturation” is the regime in which the total time‐mean volume transport of an oceanic current is relatively insensitive to the wind stress forcing and is often invoked as a dynamical description of Southern Ocean circulation. We revisit the problem of eddy saturation using a primitive‐equations model in an idealized channel setup with bathymetry. We apply only mechanical wind stress forcing; there is no diapycnal mixing or surface buoyancy forcing. Our main aim is to assess the relative importance of two mechanisms for producing eddy saturated states: (i) the commonly invoked baroclinic mechanism that involves the competition of sloping isopycnals and restratification by production of baroclinic eddies, and (ii) the barotropic mechanism, that involves production of eddies through lateral shear instabilities or through the interaction of the barotropic current with bathymetric features. Our results suggest that the barotropic flow‐component plays a crucial role in determining the total volume transport.

Description: Abstract Climate change, in particular the increase in air temperature, has been shown to influence lake thermal dynamics, with climatic warming resulting in higher surface temperatures, stronger stratification, and altered mixing regimes. Less‐studied is the influence on lake thermal dynamics of atmospheric stilling, the decrease in near‐surface wind speed observed in recent decades. Here we use a lake model to assess the influence of atmospheric stilling, on lake thermal dynamics across the Northern Hemisphere. From 1980‐2016, lake thermal responses to warming have accelerated as a result of atmospheric stilling. Lake surface temperatures and thermal stability have changed at respective rates of 0.33 and 0.38°C decade‐1, with atmospheric stilling contributing 15 and 27% of the calculated changes, respectively. Atmospheric stilling also resulted in a lengthening of stratification, contributing 23% of the calculated changes. Our results demonstrate that atmospheric stilling has influenced lake thermal responses to warming.

Description: Abstract Most existing numerical research on tide‐induced groundwater dynamics assume a constant surface water salinity on the seaward boundary (constant salinity case). Few studies have investigated the influence of tidally‐varying salinity on shallow groundwater dynamics in coastal aquifers (tidal salinity case). We compiled field observations of tidally‐varying salinity in multiple estuaries across the eastern coast of China and a tidal creek in North inlet‐Winyah Bay, the USA. Numerical simulations were then conducted to explore the effect of tidally‐varying salinity on groundwater flow and salt transport in an idealized creek‐marsh aquifer. Results showed that the upper saline plume and classical saltwater wedge appeared in all cases, but the salinity in the saltwater wedge was diluted in the tidal salinity cases. Notably, groundwater transit times were shorter in the tidal salinity case than in the constant salinity case, especially under the creek bottom. Quantitative analyses indicated that tidally‐varying salinity significantly enhanced surface water‐groundwater exchange, increasing submarine groundwater discharge by 10% and the total inflow of surface water across the water‐sediment interface by 7%. As the density of groundwater differs from that of the overlying surface water, fingered saltwater flow formed in sediments under the creek bottom, leading to some small local water circulation cells. These small cells reduced groundwater transit times, and almost doubled the water exchange rate. Coupling the density‐dependent flow to a simplified nitrogen reaction network revealed that the tidally‐varying salinity may have the potential to influence nitrogen biogeochemical transformations that modify nitrogen loads prior to discharge.

Description: Abstract A high‐resolution mooring record from the Changjiang River plume (45 m depth) is used to investigate how air‐sea CO2 flux responds to typhoon in the productive plume. With strong wind, surface partial pressure of carbon dioxide (pCO2) increased sharply from 369 to 606 μatm due to entrainment of high‐CO2 subsurface water. Though it was followed by pCO2 decrease of 250 μatm and Chl a increase days after the typhoon, the typhoon caused a net CO2 efflux overall. The maximum CO2 efflux (+111.6 mmol m−2 d−1) is much greater than that under non‐typhoon condition (−2.3 to −11.7 mmol m−2 d−1). Based on historical typhoon records, we estimate typhoon‐induced CO2 efflux to be +0.27 Tg C a−1, which can cancel 18% of summer CO2 influx in the East China Sea shelf. It may likely occur in other coastal waters. Ignoring such contribution may induce large bias in estimating regional air‐sea CO2 flux.

Description: Abstract The Jovian polar regions produce X‐rays that are characteristic of very energetic oxygen and sulfur that become highly charged on precipitating into Jupiter's upper atmosphere. Juno has traversed the polar regions above where these energetic ions are expected to be precipitating revealing a complex composition and energy structure. Energetic ions are likely to drive the characteristic X‐rays observed at Jupiter (Haggerty et al., 2017; Houston et al., 2018; Kharchenko et al., 2006). Motivated by the science of X‐ray generation, we describe here Juno JEDI measurements of ions above 1 MeV, and demonstrate the capability of measuring oxygen and sulfur ions with energies up to 100 MeV. We detail the process of retrieving ion fluxes from pulse width data on instruments like JEDI (called “puck's”; Clark et al., 2016; Mauk et al., 2013) as well as details on retrieving very energetic particles (〉20 MeV) above which the pulse width also saturates.

Description: Abstract Much theoretical and observational work has been devoted to studying the occurrence of F‐region polar cap patches in the Northern hemisphere; considerably less work has been applied to the Southern hemisphere. In recent years, the Madrigal database of mappings of total electron content (TEC) has improved in southern hemisphere coverage, to the point that we can now carry out a study of patch frequency and occurrence. We find that southern hemisphere patch occurrence is very similar to that of the northern hemisphere with a half‐year offset, plus an offset in universal time of approximately 12 hours. This is further supported by running an ionospheric model for both hemispheres and applying the same patch‐to‐background technique. Further, we present a simple physical mechanism involving a sunlit dayside plasma source concurrent with a dark polar cap, which yields a patch‐to‐background pattern very much like that seen in the TEC mappings for both hemispheres.

Description: Abstract Rosetta observations of 67P/Churyumov‐Gerasimenko (67P) reveal that most changes occur in the fallback‐generated smooth terrains, vast deposits of granular material blanketing the comet's northern hemisphere. These changes express themselves both morphologically and spectrally across the nucleus, yet we lack a model that describes their formation and evolution. Here we present a self‐consistent model that thoroughly explains the activity and mass loss from Hapi's smooth terrains. Our model predicts the removal of dust via re‐radiated solar insolation localized within depression scarps that are substantially more ice‐rich than previously expected. We couple our model with numerous Rosetta observations to thoroughly capture the seasonal erosion of Hapi's smooth terrains, where local scarp retreat gradually removes the uppermost dusty mantle. As sublimation‐regolith interactions occur on rocky planets, comets, icy moons and KBOs, our coupled model and observations provide a foundation for future understanding of the myriad of sublimation‐carved worlds.

Description: Abstract Recent observations of significant enhancements in exospheric hydrogen (H) emission in response to geomagnetic storms have been difficult to interpret in terms of the evolution of the underlying global, 3‐D exospheric structure. In this letter, we report the first measurement of the timescales and spatial gradients associated with the exospheric response to a geomagnetic storm, which we derive from a novel, time‐dependent tomographic analysis of H emission data. We find that global H density at 3 RE begins to rise promptly, by ~15%, after storm onset, and that this perturbation appears to propagate outward with an effective speed of ~60 m/s, a response which may be associated with enhanced thermospheric temperature and vertical neutral wind. This effective upwelling has significant implications for atmospheric escape as well as for charge exchange reaction rates, which drive important space weather effects such as plasmaspheric refilling and ring current decay.

Description: Abstract The extratropical teleconnection from the tropical Pacific in boreal summer exhibits a significant shift over the past 70 years. Cyclonic circulation anomalies over the North Atlantic and Eurasia associated with El Niño in the later period (1978‐2014) are absent in the earlier period (1948‐1977). An initialised atmospheric model ensemble, performed with prescribed sea surface temperature (SST) boundary conditions, replicates some key features of the shift in the teleconnection, providing clear evidence that this shift is not simply due to internal atmospheric variability or random sampling. Additional ensemble simulations, one with detrended tropical SSTs and another with constant external forcing are analysed. In the model, the teleconnection shift is associated with climatological atmospheric circulation changes, which are substantially reduced in the simulation with detrended tropical SSTs. These results demonstrate that the climatological atmospheric circulation and associated teleconnection changes are largely forced by tropical SST trends.

Description: Abstract Despite substantial progress made in the theoretical understanding and practical prediction of the El Niño‐Southern Oscillation (ENSO), accurate predictions of particular ENSO characteristics (e.g., evolution, intensity, spatial pattern) remain challenging. Using two models from the North American Multimodel Ensemble (NMME) Phase‐II hindcasts, we find that the austral winter atmospheric internal variability is a key determinant of how the South Pacific atmospheric circulation responds to concurrent tropical Pacific sea surface temperature anomalies. While this internal variability may not trigger ENSO onsets, it regulates the southeasterly trades and contributes to thermodynamic feedbacks that grow into an ENSO‐like structure during the following austral summer. The difference in the simulation of South Pacific atmospheric variability amongst ensemble members appears to be a significant source of the inter‐member spread in ENSO predictions. Monitoring South Pacific atmospheric variability provides an opportunity to improve the prediction of ENSO intensity and flavor with about a 2‐season lead time.

Description: Abstract The relation between seismic moment and earthquake duration for slow rupture follows a different power law exponent than sub‐shear rupture. The origin of this difference in exponents remains unclear. Here, we introduce a minimal one‐dimensional Burridge‐Knopoff model which contains slow, sub‐shear and super‐shear rupture, and demonstrate that different power law exponents occur because the rupture speed of slow events contains long‐lived transients. Our findings suggest that there exists a continuum of slip modes between the slow and fast slip end‐members, but that the natural selection of stress on faults can cause less frequent events in the intermediate range. We find that slow events on one‐dimenional faults follow with transition to for longer systems or larger prestress, while the sub‐shear events follow . The model also predicts a super‐shear scaling relation . Under the assumption of radial symmetry, the generalization to two‐dimensional fault planes compares well with observations.

Description: Abstract In this work we revisit an event observed by Mars Atmosphere and Volatile Evolution MissioN (MAVEN) in the solar wind where the Interplanetary Magnetic Field (IMF) rotates around the Mars‐Sun axis during approximately 6 minutes. Based on a time‐dependent LATMOS Hybrid Simulation, we determine recovery timescales of the dayside Martian magnetosphere normalized by the IMF variability timescale. Particularly, we find that such recovery timescales range between 8 s and 11 min for an ~90° IMF rotation that lasted 50 s (observed as part of the 6 minutes time interval), depending on the considered magnetospheric region. We also find that the O+ plume recovery timescales range between 40 s and 120 s, taking greater values for further downstream distances (at least up to 1 RM downstream from the terminator plane). This range is on the order of the magnetosheath O+ gyroperiod, showing the kinetic nature of the plume recovery process.

Description: Abstract Hydraulic fracturing enables oil and gas extraction from low‐permeability reservoirs, but there remains a need to reduce the environmental footprint. Resource use, contaminant‐bearing flowback water, and potential for induced seismicity are all scaled by the volume of injected fluid. Furthermore, the greenhouse gas emissions associated with each extracted unit of energy can be decreased by improving resource recovery. To minimize fluid use while maximizing recovery, a rapidly‐computing model is developed and validated to enable the thousands of simulations needed to identify opportunities for optimization. Lower pumping pressure approaches that minimize pressure loss through the wellbore perforations combined with non‐uniform spacing are shown to be capable of substantially reducing fluid consumption and/or increasing created fracture surface area when the stress variation is mainly from fracture interaction instead of in‐situ stress. When in‐situ stress variation is dominant, “limited entry” methods promote more uniform growth but with higher pumping pressures and energy consumption.

Description: Abstract Separating recoverable (elastic) from permanent (inelastic) deformation of aquifer systems is critical to develop sustainable pumping practices but remains a challenge because the preconsolidation stress is unknown. Previous works often assume that inelastic deformation occurs over years while elastic deformation is seasonal. This assumption may not hold for systems where groundwater extraction controls drawdowns and recharge because elastic deformation may not be exclusively seasonal (e.g. drought and recovery periods). Here we present an Independent Component Analysis (ICA)‐based method for extracting elastic aquifer properties without assumptions. We applied this method to 2015‐2019 InSAR measurements of deformation in the San Joaquin Valley and show that elastic deformation is always present and captured by IC2 with both seasonal patterns and post drought‐recovery longer‐term deformation. ICA is the first method which enables isolating elastic deformation and properties even through periods that do not exhibit a clear seasonal signal.

Description: Abstract Extensional plate boundaries are segmented by offsets that transfer extension between the ends of adjacent portions of the rift by linkage zones ranging in width from a few tens of km to several hundreds of km. However, the kinematics of linkage zones is poorly constrained as direct observations are difficult to make. Here we combine InSAR, seismicity and structural geology data from the Afar rift to show that an active linkage zone currently connects the two offset Erta Ale and Tat Ali segments. The overall right‐lateral shear between the segments is accommodated primarily by oblique left‐lateral slip along faults sub‐parallel to the rift segments but an active conjugate fault system with right‐lateral slip is also present. Our results provide the first direct observational evidence that offset rift segments during continental breakup can be linked by a shear zone composed of a conjugate set of oblique slip faults.

Description: Abstract Along with the intracrustal heat source, crustal permeability is considered as the controlling factor for hydrothermal circulation within zero‐age oceanic crust. To obtain fine‐scale, 2‐D models of upper crustal permeability along the East Pacific Rise 9°50′N, known for prolific hydrothermal activity, we use recently‐derived high‐resolution seismic velocity and examine a number of the existing velocity‐permeability relationships. To constrain our preferred permeability model, we compare thus derived permeability models with collocated permeability estimates from poroelastic response to tidal loading at L‐vent. Furthermore, using the preferred permeability result, we model hydrothermal convection in 2‐D and find that the distribution of recharge and discharge zones are in good agreement with seafloor observations, including locations of the vent fields. Our results suggest that seismic velocities can be used as a tool for deriving spatial variation of permeability, which must be considered in modeling of hydrothermal flow.

Description: Abstract We analyze trends of compound flooding resulting from High Coastal Water Levels (HCWL) and peak river discharge over northwestern Europe during 1901‐2014. Compound peak discharge associated with 37 stream gauges with at least 70 years of record availability near the North and Baltic Sea coasts is used. Compound flooding is assessed using a newly developed index, Compound Hazard Ratio (CHR), that compares the severity of river flooding associated with HCWL with the at‐site, T‐year (a flood with 1/T chance of being exceeded in any given year) fluvial peak discharge. Our findings suggest a spatially coherent pattern in the dependence between HCWL and river peaks and in compound flood magnitudes and frequency. For higher return levels, we find upward trends in CHR frequency at mid‐latitudes (gauges from 47‐60°N) and downward trends along the high latitude (〉 60°N) regions of northwestern Europe.

Description: Abstract Heavy PM2.5 pollution and urban heat island (UHI) pose increasing threats to human health and living environment in populated cities. However, how PM2.5 pollution affects the UHI intensity (UHII) has not been fully understood. The impacts of PM2.5 on the wintertime UHII in the Beijing‐Tianjin‐Hebei (BTH) megalopolis of China are explored during 2013‐2017. The results show that the UHII at the time of daily maximum/minimum temperature (UHIImax/UHIImin) exhibits a decreasing/increasing tendency as PM2.5 concentration increases, causing a continuous decrease in the diurnal temperature range (DTR). These effects are mediated via aerosol‐radiation interaction (aerosol‐cloud interaction) under clear‐sky (cloudy) condition. The changes in PM2.5 concentration further cause different relative trends of UHIImax/UHIImin/DTR across different cities in the BTH, which are likely related to the differences in both the PM2.5 composition and city size. This study provides insights on how air pollution affects urban climate and would help to design effective mitigation strategies.

Description: Abstract The variability of the Atlantic Meridional Overturning Circulation (AMOC) and its governing processes during the Last Glacial Maximum (LGM) is investigated in the Kiel Climate Model (KCM). Under LGM conditions, multidecadal AMOC variability is mainly forced by the surface heat flux variability linked to the East Atlantic pattern (EAP). In contrast, the multidecadal AMOC variability under preindustrial conditions is mainly driven by the surface heat flux variability associated with the North Atlantic Oscillation (NAO). Stand‐alone atmosphere model experiments show that relative to preindustrial conditions, the change in AMOC forcing under LGM conditions is tightly linked to the differences in topography.

Description: Abstract In this study, we report the phenomenon of fast changes in aerosol hygroscopicity between clean and polluted periods observed frequently in urban Beijing during winter using a hygroscopicity tandem mobility analyzer (H‐TDMA). The cause of this phenomenon and the formation process of particles in different modes are discussed. During clean periods, ultrafine‐mode particles (i.e., nucleation and Aitken modes) stem mainly from nucleation events with subsequent growth. During heavily polluted periods, ultrafine‐mode particles originate chiefly from primary emissions. Larger‐mode particles like accumulation mode particles are mainly from primary emissions during clean periods and aqueous reactions during polluted periods. This finding based on H‐TDMA measurements can make up the deficiency of mass‐dependent instruments in analyzing sources and chemical processes of ultrafine‐mode particles.

Description: Abstract Nowadays, national and international requirements and laws emphasize the “natural” development of river‐floodplain systems. One goal is to increase the connectivity between the river and its floodplains and thus reactivate floodplains as flooding areas, which potentially increases the mobility of fine sediments. The objective of this study is to analyze the long‐term effects of reactivated floodplains on the mobility of floodplain deposits of small rivers based on two river restoration scenarios: elevating the riverbed or lowering the floodplains. Past channel fixation and degradation as well as the subsequent increase in the floodplain elevation led to the decoupling of the channel and floodplain morphodynamics associated with the reduction of the habitat connectivity. Here, the floodplain sedimentation rates were determined using a numerical model based on the Delft3D software. The novelty of this numerical investigations is the morphological long‐term analysis over time scales of decades, which is not comparable to other short‐term hydro‐ and morphodynamic studies for small meandering lowland rivers. The results of 11 river restoration scenarios show that lowering the floodplain and raising the riverbed elevation both lead to an increase in the fine sediment deposition on the floodplain. However, lowering the floodplain elevation is generally more effective. Based on the numerical model results and the assumption of a fixed river channel, only anthropogenic activity might have increased the amount of fine sediments deposited on floodplains and has accelerated the decoupling of the floodplains from the riverbed in the past centuries.

Description: Abstract Crowd‐sourcing incorporates common citizens as rich sources of data, and is promising for environmental monitoring. In this paper, we propose and test the idea of incorporating incentives to crowd‐sourcing management for rainfall monitoring. Specifically, we model the allocation of incentives (quantitatively measurable and limited rewards) among crowd‐sourcing participants for a theoretical rainfall monitoring case. For this purpose, we develop an integrated model comprising a reward allocation component to represent the decision‐making process of a central manager, an agent‐based model to simulate the interactions between the manager and participants, and a rainfall simulation model to evaluate the effectiveness of various reward allocation policies. We simulate six reward allocation policies of varying levels of administrative cost, and consideration of participant and rainfall spatial heterogeneities. The results suggest the performance of each policy to improve with the reward budget and their spatial uniformity. Among the six policies tested, we find that the participant density weighted maximum participation policy (MDPP) yields the most accurate estimation of rainfall intensity due to its more explicit consideration of the spatial distribution of participants; however, this policy associates with a high administrative cost. This highlights the tradeoff between performance and cost in designing effective reward allocation policies. This paper provides a physical and behavior simulation modeling tool to study the feasibility and complexity of reward‐based participant management for crowd‐sourcing rainfall monitoring. The proposed crowd‐sourcing method is beneficial for a wide range of applications that require rainfall data with fine resolution, such as stormwater management and water availability and biomass assessment for food and energy crops.