Most of the world’s population faces freshwater scarcity threats, and reservoirs, built both for ensuring water supply during prolonged droughts and reducing downstream flood risks, are critical infrastructure for water sustainability. Historical inflow data and water demand were used to estimate reservoir storage allocation and operation policies when designing and building reservoirs, 50 to 100 years ago. This study assesses historical reservoir operations in 16 Southeastern reservoirs and evaluates the potential for utilizing existing flood control storage for alternative purposes without increasing downstream flood risk. Using a reservoir simulation model, we evaluate the resulting storage under four initial storage conditions for observed and synthetic seasonal maximum six-day flood pulses. For most reservoirs, we find conservation storage is depleting and did not exceed the flood storage capacity in their historical operation. The simulation model resulted in most of the reservoirs’ storage levels staying within the flood control pool for all scenarios (for observed and synthetic floods). Additional flood risk was lowest for initial storage condition 1 (flood control pool empty) and highest with condition 2 (50% of the flood control pool full). Flood risk increased the most for reservoirs with small ratios of flood control to conservation pool storage. Our study shows the potential for reallocation and utilization of flood control storage to meet the increasing demand. As limited opportunities for new reservoirs exist, utilizing current reservoir storage without introducing additional downstream risk may be an effective management strategy to mitigate flood and drought risk under climate change and population growth.

We present observations from two field deployments of a calibrated tiltmeter that we name the Self-Calibrating Tilt Accelerometer (SCTA). The tiltmeter is based upon a triaxial quartz crystal accelerometer; the horizontal channels measure tilt and are periodically rotated into the vertical to obtain a measurement of the acceleration of gravity. Changes in the measured total acceleration are ascribed to drift in the vertical channel and used as calibrations for removing that same drift from the tilt time series observed between calibrations. Changes in the span (sensitivity) of the accelerometer channels can also be measured by calibrating them pointing up and down. A 3-year test on the seafloor at Axial Seamount show that the calibrations are consistent with a linear-exponential model of drift to ~0.5 μg (μrad). The calibrated tilt time series was impacted by platform settling for the first 2 years, but after repositioning the tiltmeter, the calibrated observations were consistent for the final year with the tilt observed on a nearby LILY tiltmeter. A separate 15-month test in a stable vault at Piñon Flat Observatory was complicated by seasonal temperature variations of >5°C; the calibrations are consistent with a linear-exponential model of drift to ~2 μg when temperature and temperature time-derivative dependence is included. Similarly, the calibrated tilt time series was impacted by thermal deformation of the SCTA assembly. A future test in a thermally and tectonically stable borehole will be required to assess the accuracy of the SCTA.

We analyzed the temporal trends (1998-2022) of surface phytoplankton Chlorophyll (Chl) concentration in the Arctic at the local, regional, and pan-Arctic scales. We used four empirically-derived Chl satellite ocean color products: three calibrated at the global scale, and one  algorithm calibrated to Arctic waters. Depending on the months considered, between 10-40\% of the area with valid pixels showed statistically significant Chl trends, with $\sim2/3$ of those pixels showing Chl increases, and the other third indicating a decrease. At the pan-Arctic level, three of the Chl products suggest median Chl has increased by 12-16\% since 1998. However, the product calibrated to the Arctic did not show a statistically significant trend. Our findings suggest that Chl trends in the Arctic are heterogeneous, and that magnitude and significance of trends can vary depending on the satellite product used.

Extreme El Niño events (e.g. 1982-83, 1997-98) are characterized by strong, eastward-shifted warm Sea Surface Temperature anomalies, and a southward migration of the eastern Pacific Intertropical convergence zone (ITCZ) to the equator. Using an ensemble simulation with a single AMIP6 model, Beniche et al. (2024) suggested that such events uniquely yield an eastward shift of the Pacific North America (PNA) teleconnection pattern, with specific impacts over North America. Here, we first examine the robustness of these results in 135 ensemble members from 23 AMIP6 models. The specific, eastward-shifted extreme El Niño teleconnection pattern is robust in all models but one. It is also highly reproducible across years and ensemble members, due to a stronger signal than the internal noise. This yields specific, predictable impacts (defined as > 0.5 std) such as warm conditions over Northeast America (69% chances), and wet anomalies over California (77%) and Florida (97%). We then show that 26 out of the 42 CMIP6 models we examined reproduce extreme El Niño events, defined as large eastern Pacific rainfall anomalies. These models tend to have a weaker cold tongue bias than the rest of CMIP6. Despite a degradation in performance from AMIP6 to CMIP6, 18 out of the 26 selected models capture the specific extreme El Niño teleconnections, albeit with some underestimation of wet anomalies over California and Florida. We end by discussing implications for future north American climate projectionsbased on CMIP6.

Seepage boundary conditions are commonly used in groundwater simulations to allow groundwater to discharge at the upper surface of the model when groundwater head exceeds atmospheric pressure. However, the extent and transient behavior of the seepage zone is often unknown a priori and is difficult to predict. A mathematical description of the boundary condition is straightforward, such that head is equivalent to elevation only when groundwater flow indicates a seepage condition, which is a mixed conditional Dirichlet and Neumann boundary condition. This standard representation of the boundary condition has been successfully implemented and applied in a real-world context by most groundwater models. However, it is rarely reported that convergence is only guaranteed when both the efflux and zero pressure conditions are simultaneously satisfied, often requiring unnecessarily small timestep sizes, which results in low computational efficiency. This study suggests a continuous differentiable equation as an alternative to model the seepage boundary. The new formulation is derived by analogy to the first-order exchange equation, which is commonly used to represent the interactions between surface water and groundwater flow in integrated hydrologic simulations. The results of this study suggest that mixed Dirichlet and Neumann boundary conditions can be effectively converted into a Robin boundary condition, which is a head-dependent flux condition that incorporates appropriate physical considerations. This new approach has the potential to significantly improve the accuracy and efficiency of groundwater flow simulations and can help to advance the understanding of subsurface hydrology.

Authors: Lee E. Brown1*, Taylor Maavara1, Xiaohui Chen2, Jiangwei Zhang 2, Megan Klaar1, Felicia Orah Moshe3, Elad Ben-Zur4, Shaked Stein4, Richard Grayson1, Laura Carter1, Elad Levintal5, Gideon Gal5, Pazit Ziv1, Frank Tarkowski6, Devanshi Pathak7, Kieran Khamis8, José Barquín9, Hemma Philamore10, Misael Sebastián Gradilla-Hernández11 & Shai Arnon5.Affiliations1. School of Geography and water@leeds, University of Leeds, Leeds. UK2. School of Civil Engineering, University of Leeds, Leeds. UK3. Department of Soil Conservation, Soil Erosion Research Station, Ministry of Agriculture, Bet Dagan, Israel4. Kinneret Limnological Laboratory, Israel Oceanographic & Limnological Research, 14950000 Migdal, Israel5. Zuckerberg Institute for Water Research, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Israel6. Yorkshire Water Services Ltd, Bradford, UK.7. Department of Aquatic Ecosystem Analysis and Management, Helmholtz Centre for Environmental Research-UFZ, Magdeburg, Germany8. School of Geography, Earth & Environmental Science, University of Birmingham, Birmingham, UK9. IHCantabria – Instituto de Hidráulica Ambiental de la Universidad de Cantabria, Santander, Spain10. School of Engineering, Mathematics and Technology, University of Bristol, Bristol, UK11. Tecnologico de Monterrey, Escuela de Ingenieria y Ciencias, Nuevo Mexico, Jalisco, Mexico* Author for correspondence: [email protected]

The northern shallow seas of Australia exhibit significant interannual mass variation characteristics. The sources and physical mechanisms underlying these variations are not fully understood and warrant further investigation. To this end, we utilized satellite gravity, satellite altimetry, and Global Navigation Satellite System (GNSS) to study and analyze sea level changes and their loading effects in this region from 2003 to 2022. The results indicate that sea levels in the northwestern sea and the Gulf of Carpentaria (GOC) have been continuously rising, primarily due to the increase in ocean water mass (0.37±0.05 cm/a and 0.43±0.07 cm/a, respectively). The contribution of oceanbottom deformation caused by mass changes to sea level rise accounted for 4% and 5%, respectively. However, using reanalysis products to study the steric sea level results in significant errors, leading to an underestimation of the annual amplitude by 63%. Monsoons and monsoon rainfall drive the increase in water mass in GOC, while ocean currents outside the GOC also significantly influence the mass changes, with an annual total outflow flux of 72.09 Gt. We also found that a single GNSS station on island within the GOC can effectively capture the seasonal and interannual water mass variations in GOC. For seasonal changes, the correlation between vertical displacement and gulf mass variation reached 0.90, and the station was able to recover 72.7% of the interannual amplitude of long-term ocean mass changes in the GOC.

The Antarctic continental shelf (ACS) hosts processes that impact the climate system globally, which has motivated ongoing efforts to characterize its state, circulation and variability. However, the nature and consequences of eddies over the ACS, and their contributions to the budgets of heat and freshwater, remain systematically understudied. This study uses hydrographic measurements collected from instrumented seals, supported by a high-resolution model of the southern Weddell Sea, to characterize eddies and their role in vertical heat transport around the entire ACS. A key finding is that eddies are ubiquitous, and exhibit frequent (10-50% of hydrographic casts) occurrences of O(1) Richardson numbers, indicative of submesoscale variability. However, along-track density power spectra exhibit wavenumber dependences of k-3, consistent with quasigeostrophic turbulence. Approximately 1% of the points in the surface mixed layer satisfy conditions favorable for symmetric instability, though its prevalence is likely higher than this due to the relatively coarse resolution of the seal tracks. Vertical heat transports, estimated from a regional model-calibrated parameterization of submesoscale restratification, are largest in dense shelf regions, which have previously been identified as key sites of warm water intrusions onto the ACS. These regions also exhibit the largest seasonal cycles, with elevated winter eddy activity and heat fluxes accompanying the formation of high salinity shelf waters. These findings indicate that eddies may contribute substantially to ACS heat and tracer budgets, and motivate further study of their role in determining the pathways and fate of heat that intrudes onto the ACS.

The meteorology-dependent multiscale behavior of snow depth over the Tibetan Plateau was examined via moderate-resolution datasets. By analyzing spatial variance and power spectra at specific scales (5 km, 10 km, 20 km, and 50 km) over 28 intraseasonal continuous snow cover regions, key meteorological factors affecting snow depth across different seasons were identified based upon the Kullback-Leibler distance. In spring and winter, the homogeneous power spectra highlight the significant roles of precipitation, atmospheric moisture, and temperature in snow accumulation and melt processes. Summer exhibits the highest snow depth spatial variance driven by radiation and wind dynamics. Autumn with the lowest spatial heterogeneity underscores the impact of windspeed on snow distribution through sublimation and redistribution. The alignment between spatial variance maps and spectra across these scales implies the future downscaling or upscaling without substantial loss of spatial information. These findings are essential for improving snow cover modeling and forecasting, particularly in the context of climate change, as well as for effective water resource management and climate adaptation strategies in this strategically important plateau.

• A general form of the Two-Stream Equations better lends itself to physical interpretation than the standard form. • Numerically-optimized coupling coefficients reproduce shortwave reflectance and transmittance accurate to within 10 −4. • These optimized coefficients explain the accuracy regimes of existing two-stream closures.

Radiative heating of clouds, particularly those in the upper troposphere, alters temperature gradients in the atmosphere, affecting circulation and precipitation in today's and future climates. However, the response of cloud radiative heating to global warming remains largely unknown. We study changes to high cloud radiative heating in a warmer climate, identify physical mechanisms responsible for these changes, and develop a theory based on well-understood physics to predict them. Our approach involves a stepwise procedure that builds upon a simple hypothesis of an upward shift in cloud radiative heating at constant temperature and gradually incorporates additional physics. We find that cloud radiative heating intensifies as clouds move upward, suggesting that the role of high clouds in controlling atmospheric circulations increases in a warmer climate.

This paper introduces a modernized application of the falling spheres technique for measurement of neutral winds and densities in the mesosphere and lower thermosphere (MLT) region regardless of time of day or tropospheric conditions, using an Observing System Simulation Experiment (OSSE) of falling spheres equipped with commercial-off-the-shelf (COTS) Global Navigation Satellite Systems (GNSS) receivers. The modernization of this technique is crucial since current techniques to measure the neutral winds in the MLT are often tied to the location of a certain instrument or heavily dependent on clear nighttime skies. We show how state-of-the-art COTS inertial measurement units (IMUs) and GNSS receivers enable precise retrieval of neutral wind and density profiles under various atmospheric conditions represented both by wind profiles measured during the Super Soaker and Auroral Jets sounding rocket missions and by wind profiles simulated via the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X). Even under highly conservative conditions (e.g., relatively extreme position uncertainty of $\sigma =\sim$100 m, a low sampling rate of 100~Hz, and strong vertical winds of 25 m/s) the estimated neutral densities exhibit errors of less than 1\%, while estimated neutral wind errors typically do not exceed 2 m/s. The latter errors are largest where the shears maximize in the lower thermosphere. The significance of this work lies in its potential to enhance our understanding of the dynamics within the MLT region, including in situ processes and the interaction with the lower atmosphere.

Implicit time-stepping for advection is applied locally in space and time where Courant numbers are large, but standard explicit time-stepping is used for the remaining solution which is typically the majority. This adaptively implicit advection scheme facilitates efficient and robust integrations with long time-steps while having negligible impact on the overall accuracy, and achieving monotonicity and local conservation on general meshes. A novel and important aspect for the efficiency of the approach is that only one linear solver iteration is needed for each advection solve. The implementation in this paper uses a second-order Runge-Kutta implicit/explicit time-stepping in combination with a second/third-order finite volume spatial discretisation. We demonstrate the adaptively implicit advection in the context of deformational flow advection on the sphere and a fully compressible model for atmospheric flows. Tracers are advected over the poles of latitude-longitude grids with very large Courant numbers and through hexagonal and cubed-sphere meshes with the same algorithm. Buoyant flow simulations with strong local updrafts also benefit from adaptively implicit advection. Stably stratified flow simulations require a stable combination of implicit treatment of gravity and acoustic waves as well as advection in order to achieve long stable time-steps.

Greenland ice cores reveal an abrupt cooling of up to 3.3°C 8.2 kyr ago (8.2 ka), lasting for roughly 160 years. The event was likely caused by a weakening of the Atlantic Meridional Overturning Circulation (AMOC) due to freshwater drainage into the North Atlantic. It was associated with a global-scale climate change but is recorded in very few high-resolution paleoclimatic time series from the Southern Hemisphere (SH). In this study, we investigate the 8.2 ka event in the SH, particularly the Australian climate response to a weakened AMOC. Five North Atlantic meltwater experiments are conducted with the Australian Earth System Model, ACCESS-ESM1.5, to evaluate the sensitivity of AMOC responses to freshwater perturbations under early Holocene conditions as well as their climate impact. Our results suggest a 100-year freshwater pulse reproduces a global climate change that best matches existing proxy records for the 8.2 ka event. Australian surface air temperatures show significant cooler conditions in the northern half of the continent but warmer anomalies in the south in response to a weakened AMOC. Australian hydroclimate displays a more complex response at 8.2 ka. Northern Australian precipitation is influenced by a southward shift in the mean position of the Intertropical Convergence Zone and a strengthened Indo-Australian summer monsoon, while the southern part of the continent is more sensitive to weakening of the winter westerly winds. These results highlight the importance of understanding the Australian climate response to a weakened AMOC under different background climate in order to better predict potential future impacts.

Observations of far-ultraviolet (FUV) dayglow by the Global-scale Observations of Limb and Disk (GOLD) mission provide an opportunity for quantifying the global-scale response of the thermosphere to solar extreme-ultraviolet (EUV) variability and geomagnetic activity. Relative temperature changes can be measured by monitoring changes in the rotational structure observed in molecular nitrogen Lyman-Birge-Hopfield (LBH) band emissions. We present a new technique for deriving neutral temperatures from GOLD FUV observations using optimal estimation fits to spectra containing LBH band emissions. We provide an overview of the theoretical basis for the GOLD Level 2 neutral temperature algorithm. Effects on derived neutral temperatures from instrument artifacts and particle background are reviewed. We also discuss GOLD Level 1C DAY and Level 2 TDISK data products and present representative examples of each. We show that neutral temperatures vary with local time, exhibit a strong seasonal dependence, and correlate strongly with geomagnetic and solar activity. Finally, we present results from a preliminary data product validation that show good agreement with coincident GOLD exospheric temperatures and predictions from a global reference atmospheric model.

Future carbon neutral or low carbon economies will inevitably restore past climate regulating parameters.  The physics of past climates appears to be an ongoing research subject.  Past surface temperature variations were induced by changes in the size of green matter and amplified by the ocean.  Future climates may be managed by photosynthesis.

Land reclamations influence the morphodynamic evolution of estuaries and tidal basins, because an altered planform changes tidal dynamics and associated residual sediment transport. The morphodynamic response time to land reclamation is long, impacting the system for decades to centuries. Other human interventions (e.g., deepening of fairways or port construction) will add more morphodynamic adaptation timescales. Our understanding of the cumulative effects of anthropogenic interference with estuaries is limited because observations usually do not cover the complete morphological adaptation period. We aim to assess the impact of land reclamation works and other human interventions on an estuarine system by means of digital reconstructions of historical morphologies of the Ems Estuary over the past 500 years. Our analysis demonstrates that the intertidal-subtidal area ratio altered due to land reclamation works and that the ratio partly restored after land reclamation ended. The land reclamation works have led to the degeneration of an ebb and flood channel system, transitioning the estuary from a multichannel to a single-channel system. We infer that the 20th-century intensification of channel dredging and re-alignment works accelerated rather than caused this development. The centennial-scale observations show that the Ems estuary evolution corresponds to a land reclamation response following tidal asymmetry-based stability theory as it moves towards a new equilibrium configuration with modified tidal flats and channels. Considering the long history of land reclamation in the Ems Estuary, it provides an analogy for expected developments in comparable tidal systems where land reclamations were recently carried out

The results of prior work indicated that the Southeast United States (SEUS) region contains extreme Quantitative Precipitation Forecast (QPF) skill that is lower than other regions in the U.S. (e.g., the Western and Northeast US). A hypothesis is that this skill is influenced by a diversity of storm types such as Atmospheric Rivers (ARs), Mesoscale Convective Systems (MCS) and Tropical Cyclones (TCs) occurring within distinct synoptic patterns, which have resulted in significant floods such as in Nashville (2010) and Waverly (2021) Tennessee. Similarly, previous investigations have identified that synoptic patterns with higher integrated vapor transport (IVT) potentially have greater QPF skill than those with lower IVT. There is further opportunity to investigate pattern-wise contribution of storm types and QPF skill in SEUS. This study identified six synoptic patterns associated with heavy precipitation in Tennessee. These patterns exhibited distinct seasonality, with three patterns occurring in the cool season, two in the warm season, and one in the transition season. Approximately, 66 % of heavy precipitation in cool season and 47 % in transition season is associated with coincident ARs and MCS. Pattern-wise QPF skill derived from the GEFS Reforecast dataset illustrated that the cool season pattern with the highest IVT and largest fraction of ARs has better skill, whereas the warm season pattern with the highest CAPE and integrated water vapor has worse skill at multiple lead times. These results provide insights into the dynamical characteristics and predictability of heavy precipitation by storm type over the SEUS.