bluuebunny/crossref_metadata_2025 huggingface dataset:
On the strongly negative cloud feedback over the satellite period implied by observational SST reconstructions
Authors: Fueglistaler Stephan , Silvers Levi
Published: March 2021

Clouds strongly modulate Earth's radiative budget, and uncertainties in numerical model simulations of the global cloud field contribute substantially to uncertainties in future warming. In coupled atmosphere-ocean General Circulation Model (GCM) simulations, the global cloud field and its radiative effect are well correlated with global average surface temperature. However, GCM simulations with prescribed Sea Surface Temperatures (SSTs) from observational SST reconstructions over the historical period show time-varying relationships between the cloud field and average surface temperature (known as the "pattern effect"). We show that CERES/EBAF observational data confirms the presence of a second mode (in addition to mean SST) in particular in low cloud amount (and correspondingly SWCRE) that is consistent with variations in tropical atmospheric stability in ERA-Interim reanalysis data. This second mode in observations is tied to ENSO, and evolves in quadrature to ENSO indexes. It arises from differences in surface temperature change between regions of tropical deep convection and the tropical (or global) average. In contrast to the multidecadal trends over the full historical period, trends in this second mode since the year 2000 are small. The PCMDI/AMIPII SSTs recommended for CMIP6 stand out as having the largest trend over the full historical period. Different SST reconstructions agree on a trend over the satellite period - specifically the 1980s-90s - that is much larger than what coupled GCM simulations show: In forced coupled GCM simulations the regions of deep convection warm order 10% more than the tropical average, whereas over the satellite period the amplification is order +50%  in the AMIP simulations and in estimates using rainfall observations to identify regions of deep convection.
The mid- to high-latitude migrating semidiurnal tide: Results from a mechanistic tide model and SuperDARN observations
Authors: van Caspel Willem , Espy Patrick
Published: March 2021

Simulations of the solar thermal migrating semidiurnal (SW2) tide in the mesosphere-lower-thermosphere (MLT) are compared against meteor wind observations from a longitudinal chain of high-latitude SuperDARN radars. The simulations span two full years and are performed using a 3D non-linear mechanistic primitive equation model. In our model, the background Middle Atmosphere is specified to daily mean zonal mean winds and temperatures from the Navy Global Environmental Model - High Altitude (NAVGEM-HA) meteorological analysis system. Thermal tides are forced from the surface to the thermosphere using 3-hourly temperature tendency fields from the Specified Dynamics Whole Atmosphere Community Climate Model With Thermosphere and Ionosphere Extension (SD-WACCMX). Our model accurately reproduces the observed seasonal cycle in the SW2 amplitude and phase, with the exception of summertime amplitudes being overestimated. Sensitivity studies reveal the impact of the seasonal variations in the background atmosphere and tidal forcing. The tropospheric forcing response is found to be highly sensitive to the seasonal variations in the background atmosphere, leading to strong amplification during the summer and mid-winter months. In contrast, the stratospheric forcing response is found to be much less sensitive to the background atmosphere, while being similar in magnitude to the tropospheric forcing response. Based on simulations using a zero-wind atmosphere, the impact of seasonal variations in the tidal forcing is found to be very small for both the tropospheric and stratospheric forcing response. Furthermore, the inclusion of an idealized surface friction profile is found to delay the phase of the tropospheric forcing response, which can strongly impact the simulated tide at MLT altitudes. Both the tropospheric forcing response and the surface friction specification are identified as being possible factors contributing to summertime amplitudes being overestimated.
Vespagram-based approach for microbarom radiation and propagation model assessment using infrasound recordings
Authors: Vorobeva Ekaterina , De Carlo Marine , Espy Patrick , Näsholm Sven Peter
Published: March 2021

This study investigates a vespagram-based approach as a tool for multi-direction comparison between simulated microbarom soundscapes and infrasound data recorded at ground-based stations. The used microbarom radiation model takes into consideration both finite ocean-depth and the source radiation dependence on elevation and azimuth angles, while the effects of the atmospheric ducting from the source regions to the station are estimated using a semi-empirical attenuation law. The infrasound data recorded at the IS37 station in northern Norway during 2014-2019 are processed in the framework of the velocity spectrum analysis to generate vespagrams presenting signal power depending on time and back-azimuth direction. The analysis is performed for five frequency bands distributed between 0.1 and 0.6 Hz. The processed infrasound data and the modelled microbarom soundscapes are compared in three different aspects: i) azimuthal distribution of dominating signal, ii) signal amplitude and iii) ability to track atmospheric changes during extreme events such as sudden stratospheric warmings (SSW). The back-azimuth resolution between the vespagrams and the microbarom model output is harmonized by smoothing the modelled soundscapes along the back-azimuth axis with a kernel corresponding to the frequency-dependent array resolution. The time-dependent similarity between the model output and the processed infrasound data is estimated using the image processing approach of mean-square difference. The results reveal good agreement between the model and the infrasound data and demonstrate the ability of vespagrams to monitor the time-dependent microbaroms azimuth distribution, amplitude, and frequency on a seasonal scale, as well as changes during SSWs. The presented vespagram-based approach is computationally low-cost and can uncover microbarom source variability. There is also a potential for near-real-time diagnostics of atmospheric model products and microbarom radiation models, especially when applied to multiple stations, e.g. exploiting the CTBTO International Monitoring System network.
ICMEs and low plasma density in the solar wind observed at L1
Authors: Schmieder Brigitte , Verbeke Christine , Chané Emmanuel , Démoulin Pascal , Poedts Stefaan , Grison Benjamin
Published: March 2021

Different regimes of the solar wind have been observed at L1 during and after the passage of ICMEs, particularly anomalies with very low plasma density. From the observations at L1 (ACE) we identified different possible cases. A first case was explained by the evacuation of the plasma due over expansion of the ICME (May 2002). The second case on July 2002 is intriguing.In July 2002, three halo fast speed ICMEs, with their origin in the central part of the Sun, have surprisingly a poor impact on the magnetosphere (Dst > -28 nT).   Analyzing the characteristics of the first ICME at L1, we conclude that the spacecraft crosses the ICME with a large impact (Bx component in GSE coordinates is dominant). The plasma density is low, just behind this first ICME. Next, we explore the generic conditions of low density formation in the EUHFORIA simulations.The very low density plasma after the sheath could be explained by the spacecraft crossing, on the side of the flux rope, while behind the front shock. We investigate two possible interpretations. The shock was able to compress and accelerate so much the plasma that a lower density is left behind. This can also be due to an effect of the sheath magnetic field which extends the flux rope  effect on the sides of it,    so a decrease of plasma density could occur like behind a moving object (here the sheath field). The following ICME, with also a low density, could be an intrinsic case with the formation in the corona of a cavity. Finally, we present some runs of EUHFORIA which fit approximately these data and argue in favor of the possible interpretations detailed above.
Large-scale calibration of conceptual rainfall-runoff models for two-stage probabilistic hydrological post-processing
Authors: Papacharalampous Georgia , Tyralis Hristos , Koutsoyiannis Demetris , Montanari Alberto
Published: March 2021

Probabilistic hydrological modelling methodologies often comprise two-stage post-processing schemes, thereby allowing the exploitation of the information provided by conceptual or physically-based rainfall-runoff models. They might also require issuing an ensemble of rainfall-runoff model simulations by using the rainfall-runoff model with different input data and/or different parameters. For obtaining a large number of rainfall-runoff model parameters in this regard, Bayesian schemes can be adopted; however, such schemes are accompanied by computational limitations (that are well-recognized in the literature). Therefore, in this work, we investigate the replacement of Bayesian rainfall-runoff model calibration schemes by computationally convenient non-Bayesian schemes within probabilistic hydrological modelling methodologies of the above-defined family. For our experiments, we use a methodology of this same family that is additionally characterized by the following distinguishing features: It (a) is in accordance with a theoretically consistent blueprint, (b) allows the exploitation of quantile regression algorithms (which offer larger flexibility than parametric models), and (c) has been empirically proven to harness the “wisdom of the crowd” in terms of average interval score. We also use a parsimonious conceptual rainfall-runoff model and 50-year-long monthly time series observed in 270 catchments in the United States to apply and compare 12 variants of the selected methodology. Six of these variants simulate the posterior distribution of the rainfall-runoff model parameters (conditional on the observations of a calibration period) within a Bayesian Markov chain Monte Carlo framework (first category of variants), while the other six variants use a simple computationally efficient approach instead (second category of variants). Six indicative combinations of the remaining components of the probabilistic hydrological modelling methodology (i.e., its post-processing scheme and its error model) are examined, each being used in one variant from each of the above-defined categories. In this specific context, the two large-scale calibration schemes (each representing a different “modelling culture” in our tests) are compared using proper scores and large-scale benchmarking. Overall, our findings suggest that the compared “modelling cultures” can lead to mostly equally good probabilistic predictions.
Physical storylines of future European drought events like 2018 based on ensemble climate modelling
Authors: van der Wiel Karin , Lenderink Geert , de Vries Hylke
Published: March 2021

The summer of 2018 was exceptionally warm and dry in western Europe. In the aftermath of such extreme weather events, questions arise on the role of climate change in the event and what future events might look like. We present physical storylines of similar future events to answer some of these questions. A storyline approach, focusing on physical processes and plausibility rather than probability, improves risk awareness through its relation with our memory of the observed event and contributes to decision making processes through their user focus. We select analogue events from large ensemble climate model simulations representing 2 °C and 3 °C global warming scenarios, and discuss how event severity, event drivers and physical processes are influenced by climate change. We show that future Rhine basin summer droughts like 2018 will be more severe. Decreased precipitation and increased potential evapotranspiration, caused by higher temperatures and increased incoming solar radiation, lead to higher precipitation deficits and lower plant available soil moisture. Possibly, changes in atmospheric circulation contribute to increased spring drought, amplifying the most severe summer drought events. The spatial extent of the most severe drought impacts increases substantially. The noted changes can partly be explained by changes in mean climate, but for many variables changes in the relative event severity on top of these mean changes contribute as well. Furthermore, the newly developed method is tested for robustness. It showcases that a balance, or compromise, is needed between analogue composite size and analogue extremity. Having a sufficiently large ensemble, such that robust analogues can be created for the observed event under consideration, is essential to provide reliable and robust climate change information.
Distribution of iron porphyrin like complexes along the land sea continuum of the Iroise Sea
Authors: Laes Agathe , Dulaquais Gabriel , Hemery Alexandre , Waeles Matthieu , Davy Romain , Devesa Jeremy , Riso Ricardo , Lalonde Stefan
Published: March 2021

The aim of FeLINE project (Fer Ligands In the aulNe Estuary) was to determine the distribution of iron and associated ligands concentrations along the land sea continuum of the Iroise Sea (Bay of Brest, France). Iron porphyrin like ligands (Fe-Py) such as heme and hemoproteins are relevant complexes in iron biogeochemical cycling as they can persist in seawater and on marine particulates. This work reveals for the first time the distribution of Fe-Py concentrations (dissolved plus reactive particulate) along a temperate macrotidal estuary. Unfiltered samples were collected in October 2019 across a transect of the Aulne river and estuary / Rade of Brest / Iroise Sea during low tidal coefficient (39). Fe-Py concentrations were determined using flow injection analysis with chemiluminescence detection adapted from Vong et al. (2007). Various interferences (organic, metallic, pH and salinity) were tested. The detection limit attained was 11 pmol.l-1 and the time of analysis 1min30s per sample. The Fe-Py concentrations varied from 0.007 ±0.002 nmol.l-1 for S=33.98 and 1.177 ±0.007 nmol.l-1 for S = 0.92. The Fe-Py concentrations clearly showed a non-conservative behavior due to various processes other than simple mixing of natural and seawater. The highest values revealing a Fe-Py enrichment were observed in the Estuarine Turbidity Maximum (ETM) for which concentrations varied between 1.177 ±0.007, S = 5.2 and 0.738 ±0.004 nmol.l-1 S = 8.59. This positive anomaly of Fe-Py concentrations (40%) also corresponded to the lowest pH values (pH =7.27-7.32). The distal part of the transect displayed a negative anomaly for salinities comprised between 15 and 25 (loss of 37%). The four last points geographically corresponding to the Bay of Brest (S>35) exhibited low and stable Fe-Py concentrations of 0.007±0.002 and 0.024 ± 0.003 nmol.l-1. The supply and removal fluxes were respectively estimated at 2.4±0.2g/d and 8.1 ± 0.8g/d, revealing an average Fe-Py removal of 39.8% that is probably due to particle flocculation.
The role of the Microbial Conveyor Belt on Earth´s system resilience
Authors: Mestre Mireia , Höfer Juan
Published: March 2021

Despite being major players on the global biogeochemical cycles, microorganisms are generally not included in holistic views of Earth’s system. The Microbial Conveyor Belt is a conceptual framework that represents a recurrent and cyclical flux of microorganisms across the globe, connecting distant ecosystems and Earth compartments. This long-range dispersion of microorganisms directly influences the microbial biogeography, the global cycling of inorganic and organic matter, and thus the Earth system’s functioning and long-term resilience. Planetary-scale human impacts disrupting the natural flux of microorganisms pose a major threat to the Microbial Conveyor Belt, thus compromising microbial ecosystem services. Perturbations that modify the natural dispersion of microorganisms are, for example, the modification of the intensity/direction of air fluxes and ocean currents due to climate change, the vanishing of certain dispersion vectors (e.g., species extinction or drying rivers) or the introduction of new ones (e.g., microplastics, wildfires). Transdisciplinary approaches are needed to disentangle the Microbial Conveyor Belt, its major threats and their consequences for Earth´s system resilience.
Enhancement of soil organic carbon storage and aggregation following cropland afforestation
Authors: Cheng Chih-Hsin , Lee Pei-Chen , Fang Xiao-Yi
Published: March 2021

The cropland afforestation policy was initiated in 2002 in Taiwan and had been approaching the 20-year term. From the scientific perspective, it is a critical issue to understand the public welfare role and ecosystem services provided by the cropland afforestation. In this study, we investigated the changes of soil organic carbon (SOC) on plantations after 14 years conversion from the sugarcane fields. Soil samples were collected at 0-10 and 10-20 cm depth. Soil organic C concentration, bulk density, soil aggregation, and the stable isotopic 13C of the SOC and aggregates were determined. The results indicated the SOC stocks on the afforested plots were between 1000 and 1500 g m-2 significantly higher than those under the sugarcane plots (p < 0.05). The analyses of stable 13C indicated that the net increases in SOC stocks on the afforested plots were mainly attributed to the inputs of the forest-derived SOC that outweighed the loss of sugarcane-derived SOC. The afforestation also enhanced the aggregation with higher stability and SOC concentration. The comparatively depleted 13C values in the stable macroaggregates further suggested the ecological function from this new SOC source. Combining with the stand development and aboveground biomass accumulation, we expected the cropland afforestation would provide ecosystem services and functions.
Multidimensional risk in a nonstationary climate: changes in joint probability of extreme conditions in space and time
Authors: Diffenbaugh Noah
Published: March 2021

As has been made acutely clear in recent years, many natural and human systems are particularly prone to the co-occurrence of extremes like severe heat, heavy rainfall, storm-surge flooding, severe drought, and extreme wildfire conditions. The co-occurrence of these conditions, both simultaneously (or in rapid succession) in a given location or in different parts of the world, is critical for a broad suite of climate-sensitive concerns, including agricultural markets, food security, poverty vulnerability, supply chains, weather-related insurance and reinsurance, and disaster preparedness and recovery - particularly when those conditions are sufficiently extreme to fall outside of historical experience. This seminar will summarize recent work quantifying changes in the frequency of unprecedented events without consideration for joint probability probability, and then present a framework for quantifying the spatial and temporal co-occurrence of climate stresses in a nonstationary climate. This framework shows that, globally, anthropogenic climate forcing has doubled the joint probability of years that are both warm and dry in the same location (relative to the 1961–1990 baseline). In addition, the joint probability that key crop and pasture regions simultaneously experience severely warm conditions in conjunction with dry years has also increased, including high statistical confidence that human influence has increased the probability of previously unprecedented co-occurring combinations. The potential for this methodology to lend insight for other sectors that are accustomed to deploying resources based on historical probabilities, such as wildfire risk management, will also be discussed.
Impact of Thiomargarita on the rates of N, S and P turnover in mudbelt sediments from the Benguela Upwelling System: a model study
Authors: Chuang Pei-Chuan , Zabel Matthias , Sommer Stefan , Scholz Florian , Vosteen Paul , Dale Andrew W.
Published: March 2021

The availability of major nutrients, nitrogen (N) and phosphorus (P), largely controls primary productivity in eastern boundary upwelling systems. The oxygen minimum zone (OMZ) on the Namibian shelf is characterized by high productivity and extraordinarily high particulate organic carbon (POC) contents (up to 19 % dry weight) in the surface sediments. The anaerobic degradation of POC by bacterial sulfate reduction leads to the production of hydrogen sulfide (H2S) that supports extensive communities of large sulfur bacteria Thiomargarita namibiensis in surface shelf sediments. These bacteria oxidize sulfide by reducing nitrate (NO3-) to either ammonium (NH4+) or dinitrogen (N2). Thiomargarita also affect phosphorus cycling by intracellular incorporation of polyphosphates and extracellular formation of hydroxyapatites. In order to understand and quantify the complexity of the coupled benthic cycles of C, N, P, S, Fe in the Benguela Upwelling System, a reaction-transport model (RTM) was used to simulate sediment biogeochemical data collected from the RV Meteor cruise (M157, August 4th-September 16th 2019) off Namibia. This allowed deeper insights into the role of sulfur-oxidizing bacteria on P and N fluxes across the sediment surface. Results are presented that point toward potentially strong feedbacks by Thiomargarita on primary production in response to ongoing global warming and ocean deoxygenation.
Spatio-temporal Modelling of Significant Wave Height and Wave Energy Potential Estimation in the Aegean and Ionian Sea
Authors: Kozyrakis Georgios , Spanoudaki Katerina , Varouchakis Emmanouil
Published: March 2021

The continuous search for affordable and renewable energy resources is a topic of interest for decades. Many large-scale measuring campaigns have been conducted and various different tools have been developed over the years (both numerical and statistical in nature), in order to locate regions with high wind, wave and solar energy potential. Depending on the energy resource, not all regions are performing equally, as expected. To pinpoint regions with high energy gain requires state-of-the-art tools and unremitting research efforts.

The objective of the current research effort is the spatio-temporal wave data analysis, originated from satellite data, and sensor buoy data scattered in the Aegean and Ionian Sea, with the use of geostatistical and dynamic downscaling methods, for estimating the wave energy potential for the Hellenic region. The main areas of interest are the Aegean and Ionian islands, with unsustainable energy production.

WRF model is used to  dynamically downscale coarse global climate model output to provide the regional wind forcing for a 40-year hindcast period on a 3 x 3 km grid over the Aegean and Ionian Seas. The calculated wind forcing is used as a driver for the WAVEWATCH-III wave model to calculate the significant wave height and period in the region and subsequently achieve a high-resolution estimation of the wave energy potential spatial distribution and temporal evolution. Model results have been validated with mooring time series of wave parameters in the Aegean Sea and satellite-based along track Significant Wave Height data available through CMEMS Wave Thematic Assembly Center (CMEMS WAVE TAC). To strengthen the results outcome, a spatio-temporal geostatistical methodology has been introduced to validate the computational results and provide a fast and robust estimation of the wave and energy fields. The results between the two different approaches are compared in order to establish either spatial or temporal correlation patterns.

 Acknowledgements

This project has received funding from the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under grant agreement No [1237].
The Sensitivity of Power System Expansion Models on Climate Scenarios
Authors: Schyska Bruno , Kies Alexander , Schlott Markus , von Bremen Lueder , Medjroubi Wided
Published: March 2021

Power system expansion models are a widely used tool for planning power systems, especially considering the integration of renewable resources. Studies using these models form the basis for far-reaching political decisions. The backbone of power system models is an optimization problem, which depends on a number of economic and technical parameters. Although these parameters contain significant uncertainties, a consistent way to quantify the sensitivity to these uncertainties does not yet exist. Here, we analyze and quantify the sensitivity of a power system expansion model to the meteorological parameter time series based on a novel misallocation metric. We find that the sensitivity to the weather data is in the same order of magnitude as the sensitivity to the definition of cost. By comparing different climatic periods both from a meteorological perspective and with respect to the impacts on the optimal power system design we can, additionally, identify representative weather years and periods which should rather not be used for expansion planning.
On the correction for polar motion in gravimetry and in 3-D positioning
Authors: Mäkinen Jaakko
Published: March 2021

In the correction for polar motion, terrestrial gravimetry and 3-D positioning follow different conventions. The 3-D positions were first corrected to refer to the "mean pole" (IERS Conventions up to 2010) and now to the "secular pole" (IERS Conventions update since 2018). In any case, the pole reference evolves in time and describes the track of secular or low-frequency polar wander. However, since 1988 terrestrial gravimetry follows the IAGBN (International Gravity Basestation Network) Processing Standards where the gravity values are corrected to refer to the IERS Reference Pole, a fixed quantity. This may lead to discrepancies when for instance gravity change rates from absolute gravity measurements are interpreted together with vertical velocities from GNSS. I discuss the size and geographical distribution of the possible discrepancies and how to account for them in geodynamical problems. The fixed reference of the IAGBN Processing Standards has served the gravity community well, by providing a stable system for terrestrial gravity for the last 30 years during which time the pole reference in the IERS Conventions has been revised several times. In fact, the recently proposed conventions for the International Gravity Reference System (IGRS) and the International Gravity Reference Frame (IGRF) maintain the IAGBN principle. However, it appears that with the adoption of the “secular pole” the reference in 3-D positioning may have become predictable for the foreseeable future. Therefore, it could be suggested that now is the time to harmonize terrestrial gravity with 3-D, by adopting the time-dependent secular pole as a reference for it as well, especially as this is already happening with satellite gravity. I argue that at present the practical drawbacks from such a change of reference would outweigh any theoretical advantages, and the harmonization, where necessary, can be simply performed a-posteriori to published gravity trends/values.
Understanding Magmatic System of Unzen Volcano (Nagasaki, Southwest Japan) Inferred from Broad-band Magnetotelluric Observation
Authors: Triahadini Agnis , Aizawa Koki , Hashimoto Tasuku , Uchida Kazunari , Yamamoto Yuto , Chiba Keita , Muramatsu Dan , Miyano Kanta , Kawamura Yuta , Satoru Aniya
Published: March 2021

Unzen Volcano is located in Shimabara Peninsula, Nagasaki, Japan. After 198 years of dormancy, the volcano erupted throughout 1990-1995 and resulted the emergence of new lava dome called Heisei-Shinzan. Following the eruption, numerous studies have been intensively conducted in Unzen volcano to assess the eruption mechanism and the magma plumbing system. Regarding to the magmatic system, the most preferred model is that the primary supply of magma is stored beneath Chijiwa bay. This magma chamber is located about 15 km west of the active dome at vertical depth approximately 15 km, and followed by subordinate shallower magma chambers beneath the volcano (e.g. Nakamura 1995; Kohno et al 2008). Upon the eruption, the magma ascended obliquely towards the summit in east direction (e.g. Umakoshi et al 2001). However, how main magma chamber  and shallower chambers are connected to the summit via oblique pathway is poorly imaged in terms of structure.
As widely known, Magnetotelluric method is highly sensitive to low resistivity zone caused by interconnected fluids. Low resistivity zone detected in the volcanic area usually can be interpreted as hydrothermal/magmatic fluid and or magma chamber containing partial melt (e.g. Aizawa et al 2014; Hill et al 2015). Thus, by using broadband Magnetotelluric method, we aim to investigate resistivity structure of Unzen volcano associated with magmatic system and its controlling structure (e.g. pathway and faults).
Although the shallow structures around Unzen volcano are estimated by the 2017-2019 campaigns (Triahadini et al 2019; Hashimoto et al 2020), we are unable to image deeper structure around the proposed location of magma chambers and magma pathway. To achieve our goals, during November-December 2020, we installed 35 new sites to cover whole area in Shimabara Peninsula. In total, deployed 99 Magnetotelluric stations covering Unzen volcano and Shimabara Peninsula. On this meeting, we would like to present our resistivity structure derived from all dataset.
Seasonal variation in water and sediment fluxes of the Yangtze River under precipitation change and human interference
Authors: Yue Yao , Chai Yuanfang , Xu Shitian , Zhang Xiaofeng
Published: March 2021

Seasonal change of water and sediment fluxes is an important issue in flood/drought control and ecosystem protection. Based on trend analysis in dry and flood seasons during 1960–2014 at six major gauging stations on the Yangtze River, the largest river in China, significant homogenization of intra-year water discharge was found, while sharp decrease of sediment load in both seasons was tested. By reconstructing water and sediment series without the human interference, contributions of precipitation change, large dam constructions on the mainstream, and other human activities in each of the sub-basins of the Yangtze River were separated and quantified. It shows that precipitation change attributed for 9.5–23.6% to discharge homogenization in the middle and lower reaches of the Yangtze River, and increased sediment yield by 1.9–25.5% in both dry and flood seasons. Being the largest hydraulic project in the world, the Three Gorges Dam only explains 17.5–27.2% of the downstream homogenization in water flux, and 3.2–23.9% of sediment reduction in both seasons. Relatively small but massive human interference in the sub-basins was recognized as the primary factor, contributing over 60% to discharge homogenization and over 70% to seasonal sediment reduction, most notably in the Hanjiang sub-basin for water flux and in the Jianglingjiang sub-basin for sediment load.
­­Mean Atlantic Subtropical Cells in the CMIP6 models
Authors: Fu Yao , Brandt Peter , Tuchen Franz Philip , Lübbecke Joke F. , Wang Chunzai
Published: March 2021

The Atlantic Subtropical Cells (STCs) consist primarily of poleward Ekman divergence in the surface layer, subduction in the subtropics, and equatorward convergence in the thermocline that largely compensates the surface Ekman divergence through equatorial upwelling. As a result, the STCs play an important role in connecting the tropical and subtropical Atlantic Ocean, in terms of heat, freshwater, oxygen, and nutrients transports. However, their representation in state-of-the-art coupled models has not been systematically evaluated so far. In this study, we investigate the performance of the Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating the Atlantic STCs. Comparing model results with observations, we first present the simulated mean state with respect to ensembles of the key components participating in the STC loop, i.e., the meridional Ekman and geostrophic flow at 10°N and 10°S, and the Equatorial Undercurrent (EUC) at 23°W. We then examine the inter-model spread and the relationships between these key components. We find that there is a general weak bias in the Southern Hemispheric ensemble Ekman transports and mixed-layer geostrophic transports in comparison to the observations. The inter-model spread of mean EUC strengths are primarily associated with the intensity of the mean wind stress in the tropical South Atlantic among the models. Since the poleward Ekman transports induced by the trade winds are regarded as the driver of the STC loop, our results point out the necessity to improve skills of coupled models to simulate the Southern Hemisphere atmospheric forcing in driving the Atlantic STCs.
Ionospheric F-layer scintillation observations using COSMIC and COSMIC2 GPS/GNSS radio occultation data
Authors: Tsai Lung-Chih , Su Shin-Yi , Liu Chao-Han
Published: March 2021

The FormoSat-3/ Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) has been proven a successful mission on performing active limb sounding of the ionosphere using the GPS radio occultation (RO) technique. The follow-on program called FS7/COSMIC2 is in progress with satellite launched on 25 June of 2019 and includes six low-Earth-orbit (LEO) satellites at 24°-inclination and ~720-km orbits to receive multi-channel (1.5GHz and 1.2GHz) GPS and GLONASS satellite signals. The FS7/COSMIC2 can provide about 5,000 GNSS RO observations per day which are increased by a factor of about 5 comparing to FS3/COSMIC and within the region from the geographic equator to the latitude at 40°. We process 1-Hz amplitude data and obtain complete limb-viewing profiles of the undersampling-S4 scintillation index to study global F-layer irregularity morphology. There are a few percent of FS3/COSMIC and FS7/COSMIC2 GPS/GNSS RO observations having >0.09 undersampling S4max values on average. However, seven identified areas Central Pacific Area, South American Area, African Area, European Area, Japan Sea Area, Arctic Area and Antarctic Area have been designated to have a much higher percentage of strong limb-viewing L-band scintillations. Generally, the F-layer scintillation climatology, namely, its variations with each identified zone, altitude, season, and local time have been documented. The large dataset from the FS3/COSMIC and FS7/COSMIC2 programs enable statistical studies on equatorial and low-latitude ionospheric irregularity and their models.
Physical mechanism of winter precipitation variations in southern arid Central Asia
Authors: Xie Tingting , Huang Wei , Zheng Fei , Chen Jianhui , Chen Fahu
Published: March 2021

The climate of southern arid central Asia (SACA, 35.25°-45°N，46.25°-80°E) is controlled by the subtropical high and it exhibits typical Mediterranean characteristics. Given the lack of systematic research on the physical mechanism of precipitation variations in the major precipitation seasons of SACA, we analyzed the physical mechanism of winter precipitation variations during 1979-2017. The results suggest that two water vapor pathways influence the winter precipitation in SACA and that they are closely related to a low-latitude high-pressure anomaly and the mid-high latitude North Atlantic Oscillation (NAO). Specifically, at low latitudes, the northern Indian Ocean heated by El Nino  causes the anomalous intensification of the subtropical high throughout the low latitude region, especially over the Indian subcontinent, resulting in increased water vapor transport from the northern Indian Ocean to SACA. At middle and high latitudes, the negative NAO phase leads to the southward movement of the water vapor pathway, which causes it to pass over a greater number of upwind water bodies, resulting in the transport of more westerly-associated water vapor to SACA. Further analysis showed that there is a northwest-southeast teleconnection wave train, from the North Atlantic to Central Asia and to the Indian subcontinent, which allows wave fluxes originating in the North Atlantic and the northern Indian Ocean to propagate from high and low latitudes, respectively, to the study area. At the same time, high-latitude cold air advection, brought by the low-pressure system in Central Asia, converges with the flow of warm water vapor from the low-latitude northern Indian Ocean, generating an ascending motion and reducing atmospheric static stability, which results in increased precipitation in SACA. Therefore, the key to determining the origin of precipitation variations in SACA is understanding the interaction of large-scale circulation systems at low and mid-high latitudes. In the future, with continued global warming, strong El Nino events will occur more frequently, and the subtropical high is like to intensify and move northward. As a result, there will be a tendency for the NAO and the Arctic Oscillation (AO) to remain in a negative phase; consequently the Arctic vortex in the eastern hemisphere is likely to provide favorable water vapor and dynamic conditions promoting increased winter precipitation in SACA. Overall, our findings are valuable for understanding the regional response of precipitation in arid areas against the background of ongoing global warming.
Propagation of gravity waves and its effects on pseudomomentum flux in a sudden stratospheric warming event
Authors: Song In-Sun , Lee Changsup , Chun Hye-Yeong , Kim Jeong-Han , Jee Geonhwa , Song Byeong-Gwon , Bacmeister Julio
Published: March 2021

Effects of realistic propagation of gravity waves (GWs) on distribution of GW pseudomomentum fluxes are explored using a global ray-tracing model for the 2009 sudden stratospheric warming (SSW) event. Four-dimensional (4D; x–z and t) and two-dimensional (2D; z and t) results are compared for various parameterized pseudomomentum fluxes. In ray-tracing equations, refraction due to horizontal wind shear and curvature effects are found important and comparable to one another in magnitude. In the 4D, westward pseudomomentum fluxes are enhanced in the upper troposphere and northern stratosphere due to refraction and curvature effects around fluctuating jet flows. In the northern polar upper mesosphere and lower thermosphere, eastward pseudomomentum fluxes are increased in the 4D. GWs are found to propagate more to the upper atmosphere in the 4D, since horizontal propagation and change in wave numbers due to refraction and curvature effects can make it more possible that GWs elude critical level filtering and saturation in the lower atmosphere. GW focusing effects occur around jet cores, and ray-tube effects appear where the polar stratospheric jets vary substantially in space and time. Enhancement of the structure of zonal wavenumber 2 in pseudomomentum fluxes in the middle stratosphere begins from the early stage of the SSW evolution. An increase in pseudomomentum fluxes in the upper atmosphere is present even after the onset in the 4D. Significantly enhanced pseudomomentum fluxes, when the polar vortex is disturbed, are related to GWs with small intrinsic group velocity (wave capture), and they would change nonlocally nearby large-scale vortex structures without substantially changing local mean flows.
Spatial Distribution of Slip and Stress Changes in Contained Laboratory-Generated Earthquakes with Heterogeneous Initial Stress
Authors: Ke Chun-Yu , McLaskey Gregory , Kammer David
Published: March 2021

Earthquake ruptures arrest due to either encountering a barrier with high fracture energy or entering unfavorable stress conditions. Our large-scale laboratory earthquake experiments use heterogeneity in initial stress to confine the rupture within a 3-m long saw-cut granite fault. All earthquake processes, i.e., initiation, propagation, and arrest, were spontaneous and contained within the simulated fault. We proposed an analytical crack model to fit our experimental measurements and to better constrain the features in the spatial distribution of both slip and stress changes. Similar to natural earthquakes, laboratory measurements show coseismic slip that gradually tapers near the rupture tips. Measured stress changes show roughly constant stress drop in the center of the ruptured region, a maximum stress increase near the rupture tips, and a smooth transition in between, in a region we describe as the earthquake arrest zone. In our experiments, the earthquake arrest zone is more than one order of magnitude wider than the cohesive zone described by fracture mechanics. We propose that the transition in stress changes and the corresponding linear taper observed in the slip distribution are the result of rupture termination conditions primarily controlled by the initial stress distribution and are not related to the fault strength evolution. We also performed dynamic rupture simulations that confirm how arrest conditions can affect slip distribution and static stress changes, especially near the tip of an arrested rupture. If applicable to larger natural earthquakes, this distinction between the earthquake arrest zone resulted from heterogeneous initial stress and a cohesive zone that depends primarily on strength evolution has important implications for how seismic observations of earthquake fracture energy should be interpreted.
Climate change impacts on thermal comfort in one of the most popular tourist destinations in the world, Santorini Island, Greece
Authors: Katavoutas George , Founda Dimitra , Kitsara Gianna , Giannakopoulos Christos
Published: March 2021

To date, climate change has caused serious problems both in human societies and in various ecosystems. Worldwide, the observed climate change hazards include increased droughts and floods, extreme heat waves, sea-level rise, storms and changes in natural land cover. Tourism, as an important pillar of the economy, is expected to be further affected until the end of the century by climate change hazards. An important factor in the selection of a tourist destination is the climatic conditions of the location.

This research aims to investigate the observed and projected heat stress conditions in a top world tourist destination, the island of Santorini, in Greece. The Mediterranean has been identified as a vulnerable region regarding the heat related risk. Simulations by Regional Climate Models downscaled over the island of Santorini were performed for the 1982–2005 control period, the near future period 2035–2058 and the distant future period 2075–2098. The data for the future simulations are under the RCP4.5 and RCP8.5 future emissions scenarios. Thermal stress conditions were evaluated employing the Universal Thermal Climate Index (UTCI), which has a thermo-physiological basis and derived from the heat exchange theory between the thermal environment and the human body.

The analysis reveals that the thermal conditions in Santorini that cause moderate heat stress and strong heat stress are expected to increase in both RCPs scenarios in near and distant future. In particular, the exposure time under at least strong heat stress reaches 1.8% in the control period (1982–2005), increasing to 5.3% in the near future (2035–2058) and to 7.8% in the distant future (2075–2098) under the RCP4.5 scenario. In the distant future (2075–2098), under the RCP8.5 scenario, the exposure time under these conditions will exceed 12%.

The increasing heat related risk in one of the most popular tourist destinations in the world could be a wake-up call to the policy makers urging them to take prevention measures.
Anthropogenic ocean acidification below the surface: does organic matter cycling result in enhanced vulnerability?
Authors: Lauvset Siv K , Goris Nadine
Published: March 2021

Ocean acidification is a process driven by the ocean uptake of anthropogenic CO2 emissions. Because this uptake happens at the ocean-atmosphere interphase, ocean acidification is presently foremost a surface ocean phenomenon. A recent study (Lauvset et al., 2020) shows that away from the surface ocean pH changes primarily due to organic matter remineralization, and in ocean depths between 500–1500 m this process enhances ocean acidification by on average 28 ± 15%. Presently, this signal is very weak, and not detectable outside calculation uncertainties. However, as time passes the ocean overturning circulation will transport all carbon chemistry perturbations on and near the surface into the interior ocean, which can already be seen in the deep North Atlantic. Our hypothesis is that if CO2 emissions, and thus ocean acidification, continue in the future then this remineralization enhancement will become significant and lead to some regions and habitats being more vulnerable to continued ocean acidification than others. Here we evaluate this enhancement over the 21st century using the Norwegian Earth System Model (NorESM), to assess which oceanic regions are made more vulnerable to future ocean acidification from this enhancement, and at what timescales the enhancement becomes important.
Roles of Hydrothermal Circulations on Heat Flow in the Fore-arc, Northeast Japan Subduction Zone, A Numerical Modeling Study.
Authors: Han Dongwoo , Lee Changyeol
Published: March 2021

Heat flow in the fore-arc, Northeast Japan shows characteristic highs and lows in the seaward and landward regions of the trench axis, respectively, compared to 50 mW/m2 that is constrained from the corresponding half-space cooling model (135 Ma). For example, the high average of 70 mW/m2 at the 150-km seaward region from the trench was observed while the low average of 30 mW/m2 at the 50-km landward region was. To explain the differences between the constraints and observations of the heat flow, previous studies suggested that the high heat flow in the seaward region results from the reactivated hydrothermal circulations in the oceanic crust of the Pacific plate along the developed fractures by the flexural bending prior to subduction. The low heat flow is thought to result from thermal blanket effect of the accretionary prism that overlies the cooled subducting slab by the hydrothermal circulations. To understand heat transfer in the landward region of the trench, a series of two-dimensional numerical models are constructed by considering hydrothermal circulations in the kinematically thickening accretionary prism that overlies the converging oceanic crust of the Pacific plate where hydrothermal circulations developed prior to subduction. The model calculations demonstrate no meaningful hydrothermal circulations when the reasonable bulk permeability of the accretionary prism(<10-14m2) is used; the thermal blanket effect significantly hinders the heat transfer, yielding only the heat flow of 10 mW/m2 in the landward region, much lower than the average of 30 mW/m2. This indicates that other mechanisms such as the expelled pore fluid by compaction of the accretionary prism play important roles in the heat transfer across the accretionary prism.