Assessing Global Water Storage Variability from GRACE: Trends,Seasonal Cycle,Subseasonal Anomalies and Extremes

Throughout the past decade, the Gravity Recovery and Climate Experiment (GRACE) has given an unprecedented view on global variations in terrestrial water storage. While an increasing number of case studies have provided a rich overview on regional analyses, a global assessment on the dominant features of GRACE variability is still lacking. To address this, we survey key features of temporal variability in the GRACE record by decomposing gridded time series of monthly equivalent water height into linear trends, inter-annual, seasonal, and subseasonal (intra-annual) components. We provide an overview of the relative importance and spatial distribution of these components globally. A correlation analysis with precipitation and temperature reveals that both the inter-annual and subseasonal anomalies are tightly related to fluctuations in the atmospheric forcing. As a novelty, we show that for large regions of the world high-frequency anomalies in the monthly GRACE signal, which have been partly interpreted as noise, can be statistically reconstructed from daily precipitation once an adequate averaging filter is applied. This filter integrates the temporally decaying contribution of precipitation to the storage changes in any given month, including earlier precipitation. Finally, we also survey extreme dry anomalies in the GRACE record and relate them to documented drought events. This global assessment sets regional studies in a broader context and reveals phenomena that had not been documented so far.     

Coastal upwelling along the north coast of Papua New Guinea and El Niño events during 1981–2005

We investigate the relationship between sea surface temperature (SST) cooling and upwelling along Papua New Guinea’s (PNG) north coast before the onset of El Niño events using a hindcast experiment with a high-resolution ocean general circulation model. Coastal upwelling and related SST cooling appear along PNG north coast during the boreal winter before the onsets of six El Niño events occurring during 1981–2005. Relatively cool SSTs appear along PNG north coast during that time, when anomalous northwesterly surface wind stress, which can cause coastal upwelling by offshore Ekman transport appearing over the region. In addition, anomalous cooling tendencies of SST are observed, accompanying anomalous upward velocities at the base of the mixed layer and shallow anomalies of 27°C isotherm depth. It is also shown that entrainment cooling plays an important role in the cooling of the mixed layer temperature in this region.     

Laboratory measurements of seismic attenuation and Young's modulus dispersion in a partially and fully water‐saturated porous sample made of sintered borosilicate glass

We measured the extensional‐mode attenuation and Young's modulus in a porous sample made of sintered borosilicate glass at microseismic to seismic frequencies (0.05–50 Hz) using the forced oscillation method. Partial saturation was achieved by water imbibition, varying the water saturation from an initial dry state up to ～99%, and by gas exsolution from an initially fully water‐saturated state down to ～99%. During forced oscillations of the sample effective stresses up to 10 MPa were applied. We observe frequency‐dependent attenuation, with a peak at 1–5 Hz, for ～99% water saturation achieved both by imbibition and by gas exsolution. The magnitude of this attenuation peak is consistently reduced with increasing fluid pressure and is largely insensitive to changes in effective stress. Similar observations have recently been attributed to wave‐induced gas exsolution–dissolution. At full water saturation, the left‐hand side of an attenuation curve, with a peak beyond the highest measured frequency, is observed at 3 MPa effective stress, while at 10 MPa effective stress the measured attenuation is negligible. This observation is consistent with wave‐induced fluid flow associated with mesoscopic compressibility contrasts in the sample's frame. These variations in compressibility could be due to fractures and/or compaction bands that formed between separate sets of forced‐oscillation experiments in response to the applied stresses. The agreement of the measured frequency‐dependent attenuation and Young's modulus with the Kramers–Kronig relations and additional data analyses indicate the good quality of the measurements. Our observations point to the complex interplay between structural and fluid heterogeneities on the measured seismic attenuation and they illustrate how these heterogeneities can facilitate the dominance of one attenuation mechanism over another.     

Oxygen isotope disequilibrium during serpentinite dehydration

Talc + olivine in metaperidotites result from the serpentinite breakdown due to increasing temperature in the Bergell contact aureole. Jack‐straw olivine textures are present in close proximity to the serpentine breakdown reaction. As the intrusion is approached, the number of olivine crystals increases while the size of the crystals decreases; this feature documents increased overstepping with increased heating rates. Talc veinlets are observed in the outer parts of the talc–olivine zone and are interpreted to be pathways of fluid produced during devolatilization of serpentine. None of the talc–olivine oxygen isotope pairs analysed are in isotopic equilibrium with respect to the peak contact temperature. This implies that escaping fluids cannot be in equilibrium with both phases. Hence, the fluid produced by serpentine reaction does not directly reflect the protolith composition, and attention must be given to the reaction mechanism before interpreting fluid isotope composition.     

Topographic controls on plateau icefield recession: insights from the Younger Dryas Monadhliath Icefield,Scotland

Plateau icefields are a common form of mountain ice mass, frequently found in mid‐latitude to high‐arctic regions and increasingly recognized in the Quaternary record. Their top‐heavy hypsometry makes them highly sensitive to changes in climate when the equilibriaum line altitude (ELA) lies above the plateau edge, allowing ice to expand significantly as regional ELAs decrease, and causing rapid recession as climate warms. With respect to future climate warming, it is important to understand the controls on plateau icefield response to climate change in order to better predict recession rates, with implications for water resources and sea‐level rise. Improving knowledge of the controls on glacier recession may also enable further palaeoclimatic information to be extracted from the Quaternary glacial record. We use the distribution of moraines to examine topographic controls on Younger Dryas icefield recession in Scotland. We find that overall valley morphology influences the style of recession, through microclimatic and geometric controls, with bed gradient affecting moraine spacing. Ice mass reconfiguration may occur as recession progresses because ice divide migration could alter the expected response based on hypsometric distribution. These results add to a growing body of research examining controls on glacier recession and offer a step towards unravelling non‐linear ice mass behaviour. Copyright © 2019 John Wiley & Sons, Ltd.     

Aspects of CO2 advection measurements

Observations of vegetation–atmosphere exchange of carbon dioxide (CO₂) by the eddy covariance (EC) technique are limited by difficult conditions such as nighttime and heterogeneous terrain. Thus, advective flux components are included into the net ecosystem exchange (NEE) budget. However, advection measurements are experimentally challenging and do not always help to solve the night flux problem of the EC technique. This study investigates alternative methods for the observation of horizontal advection, in particular horizontal concentration gradients, as well as different approaches to coordinate rotation and vertical advection. Continuous high-frequency measurements of the horizontal CO₂ concentration field are employed and compared to the often used discontinuous sequential sampling. Significant differences were found in the case of 30-min mean concentration values between the conventional discontinuous sampling approach and the complete observation of the time series by continuous sampling. Estimates of vertical advection rely on accurate estimates of vertical wind velocity ( $\emph{w}$ ). Therefore, different approaches to the planar fit coordinate rotation have been investigated. Sector-wise rotation was able to eliminate directional dependencies of mean $\emph{w}$ . Furthermore, the effect of the data set length used for rotation (window length) was investigated and was found to have significant impact on estimates of vertical advection, with larger window lengths yielding about 50% larger vertical advection. A sequential planar fit with controlled window length is proposed to give reproducible results. The different approaches to the measurement and calculation of horizontal and vertical advection presented are applied to data obtained during the exchange processes in mountainous region experiment at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay). Estimates of NEE including advection are compared to NEE from turbulent and storage flux alone without advection. NEE including vertical advection with sector-wise planar fit rotation and controlled window length and including horizontal advection from continuous gradient measurements, which were comprehensively bias corrected by a new approach, did compare well with the expected night flux error, with meteorological drivers of the fluxes and with soil chamber measurements. Unrealistically large and noisy values of horizontal advection from the conventional discontinuous sampling approach, which lead to unrealistic values of NEE, could be eliminated by the alternative approaches presented. We therefore suggest the further testing of those approaches at other sites in order to improve the accuracy of advection measurements and, subsequently, estimates of NEE.     

Large calcite and bulk-rock volume loss in metacarbonate xenoliths from the Quérigut massif (French Pyrenees)

Chemical mass transfer was quantified in a metacarbonate xenolith enclosed within the granodiorite of the Quérigut massif (Pyrenees, France). Mass balance calculations suggest a strong decrease of CaO, SrO and CO₂ contents (up to −90%), correlated with a decrease of modal calcite content as the contact is approached. Most other chemical elements behave immobile during metasomatism. They are therefore passively enriched. Only a small increase of SiO₂, Al₂O₃ and Fe₂O₃ contents occurs in the immediate vicinity of the contact. Hence, in this study, skarn formation is characterized by the lack of large chemical element influx from the granitoid protolith. A large decrease of the initial carbonate volume (up to −86%) resulted from a combination of decarbonation reactions and loss of CaO and CO₂. The resulting volume change has potentially important consequences for the interpretation of stable isotope profiles: the isotope alteration could have occured over greater distances than those observed today.     

Effects of Bubbles and Sea Spray on Air–Sea Exchange in Hurricane Conditions

The lower limit on the drag coefficient under hurricane force winds is determined by the break-up of the air–sea interface due to Kelvin–Helmholtz instability and formation of the two-phase transition layer consisting of sea spray and air bubbles. As a consequence, a regime of marginal stability develops. In this regime, the air–sea drag coefficient is determined by the turbulence characteristics of the two-phase transition layer. The upper limit on the drag coefficient is determined by the Charnock-type wave resistance. Most of the observational estimates of the drag coefficient obtained in hurricane conditions and in laboratory experiments appear to lie between the two extreme regimes: wave resistance and marginal stability.     

Grid convergence of variable-density flow simulations in discretely-fractured porous media

Grid convergence in space and time of variable-density flow in fractured-porous rock is systematically assessed. Convergence of the flow simulation is attained using both uniform and adaptive time-stepping. This contrasts to variable-density flow in unfractured porous media where grid convergence variable-density flow problems is almost never achieved. At high discretization levels, the number of fingers in fractured-porous rock is no longer influenced by spatial-temporal grid discretization, which is not the case in unfractured porous media. However, similar to unfractured porous media, the number of fingers in fractured-porous media varies at low discretization levels. Simulated convective pattern and penetration depth of the dense plume in fractured rock depend more on spatial discretization than on temporal discretization. The appropriate spatial-temporal grid is then used to examine some aspects of mixed convection in fractured-porous rock, characterized by the mixed convection number M. The critical mixed convection number M_c = 46 represents the transition between forced and free convection in fractured porous media, which is much higher than M_c = 1 in unfractured porous media. Thus, for mixed convective flow problems, the value of M_c is not a sufficient indicator to predict the convective mode (free convection-forced convection), and the presence of vertical fractures must be included in the prediction of convective flow modes.