Freshwater Inflow Research Articles

First insights into the ectoparasitic helminth fauna of freshwater fish on an Adriatic island.

Surface flow of freshwater on Adriatic islands is rare due to the extreme permeability of the karst terrain. Hence, most helminthological studies of freshwater fishes in the Adriatic drainage have focused on mainland freshwater systems, while data from islands are scarce. We collected minnow, Phoxinus lumaireul (Schinz, 1840), specimens in the Suha Ričina stream on Krk Island and screened them for helminth ectoparasites. Identification of metacercariae cysts (black spots) was carried out by sequencing part of the 28S rDNA gene, and a single monogenean worm was identified by sequencing part of the internal transcribed spacer 2 (ITS2). To estimate the level of infection, the number of black spots was counted to calculate prevalence, mean abundance, and mean intensity. Phylogenetic inference based on maximum likelihood and Bayesian inference revealed a rare black spot disease taxon, genus Uvulifer, which represents one of the first records in Europe on fish (second intermediate host), a first record from a Phoxinus host, a first record for the Adriatic drainage and Croatia, and a first record for Southern Europe in general. Furthermore, the monogenean was identified as Gyrodactylus sp., potentially representing a new species.

Habitat of early stegocephalians (Chordata, Vertebrata, Sarcopterygii): a little saltier than most paleontologists like?

A controversy on the degree of marine influence in the paleoenvironments represented by many Paleozoic stegocephalian-bearing fossiliferous localities has persisted for decades. Many authors have equated the absence of a typical stenohaline marine fauna with freshwater environments, but this ignores continental salt lakes and the many transitional environments (deltaic, estuarine, lagoonal, and some epicontinental seas that receive much freshwater influx, like the Baltic Sea) that separate typical marine environments from freshwater environments. This is problematic because it seems plausible that many of the late Paleozoic sediments that have been preserved were deposited on coasts in deltas and estuaries. The author had compiled a dataset of paleoenvironmental interpretations of Devonian to Early Permian stegocephalian (“tetrapod”)-bearing fossiliferous localities in 2010. How have these interpretations withstood the test of time, especially in the face of new results from different kinds of evidence? An updated dataset and a new literature review show that the case for a marine origin of stegocephalians has strengthened, especially through additional discoveries or reinterpretations of fossils that suggest marine influence in various classical vertebrate-bearing Permo-Carboniferous localities traditionally interpreted as freshwater, and a recent analysis of stable isotopes in Late Devonian localities.

ПОТОК ПЛАВУЧЕСТИ КАК МЕТРИКА ДЛЯ АНАЛИЗА ВЗАИМОДЕЙСТВИЯ ОКЕАНА И АТМОСФЕРЫ НА РАЗЛИЧНЫХ ВРЕМЕННЫХ МАСШТАБАХ

The article provides a comprehensive review of publications, both in journals and monographs, devoted to the metric of ocean–atmosphere interaction – the buoyancy flux. This method of assessing the tendency to vertical mixing in the ocean, derived theoretically in the second half of the XX century, is currently used. The buoyancy flux allows not only a qualitative but also a quantitative assessment of ocean-atmosphere interaction through heat, moisture, and radiative fluxes on different time scales. It can be used to account for the contribution of different components of the ocean-atmosphere interaction to the dynamics of the upper mixed layer of the ocean: radiative fluxes, heat fluxes, and freshwater fluxes. To assess the ability of land bodies to absorb or emit CO2 as a function of synoptic atmospheric dynamics, and even to estimate the variability of upwelling that affects the socio-economic situation in coastal areas. The review summarises scientific papers using buoyancy flux for various purposes, presents the first paper to introduce the metric, and summarises methodologies that offer promise in this line of research based on current ocean and atmospheric data. Forty-two English-language papers and 5 publications by Russian researchers have been analysed.

Upper Devonian сalcareous algae of the Northern part of the Lviv Paleozoic Trough (systematic composition, facial timing and stratigraphic significance)

As a result of the study of core material in the volume of 211 thin sections from five boreholes of the Northern part of the Lviv Paleozoic Trough, a complex of fossil green, red, and charophyte algae was revealed, which consists of 20 species of 13 genera of green, red, and charophyte algae, among which five species of five genera are identified for the first time for the Devonian deposits of the Lviv Paleozoic Trough. The carbonate deposits belong to the Frasnian and Famennian stages and are represented mainly by sandstones, argillites, siltstones, limestones, dolomites, marls, and white chalk. Carbonate deposits are represented mainly by pelitomorphic wackestones, packstones, wackestone-packstones, grainstones, grainstone-packstones, micrite, bioclastic wackestones, and microbioclastic pelitomorphic wackestones. Limestones often have a pelitomorphic texture, enriched with sand material or dolomitized. The algae are confined primarily to pelitomorphic wackestones of the Waulsortian type, which indicates the intermediate facies of the shelf margins, which were formed by the open circulation of sea waters. To a lesser extent, algae occur in packstones, grainstones, wack-packstones, and are found predominantly along the remains of foraminifera, crinoids, bryozoans, ostracods, and sponge spicules. It is revealed that the algal assemblage of the Famennian stage is depleted and represented mainly by the green tubular algae of the families Palaeoberesellaceae Mamet et Roux and Dasycladaceae Kützing. Higher up the section, a few charophyte algae of the genus Planoumbella Platonov are found. The assemblage of the Famennian stage is significantly enriched, both numerous and systematic. Numerous charophyte and red algae appear, especially in the upper part of the section, which corresponds to the stratigraphic level of the zone C1ta of Donbass. These deposits were considered for a long time to be Tournaisian until at the All-Union Congress in 1986 the Devonian-Carboniferous boundary at the base of the Gattendorfia (Acutimoceras) goniatite zone and the Siphonodella sulcata conodont zone was drawn, which represents the top of the C1ta zone of Donbass. At this level, the richest and most varied algae assemblage is revealed. Above this level, representatives of the genus Parachaetetes Deninger and charophyte algae are not found. The prevalence of the green siphoncladal algae in the Frasnian association indicates a calm hydrodynamic regime below the level of the tide, as well as the depth of the basin, which does not exceed 3-5 m, slight fluctuation in the depth of the sea basin, good illumination, and warm water. The significant distribution of charophyte algae in the Famennian time indicates a decrease in salinity in the paleobasin due to the flow of fresh water from the Kovel uplift.

A Predictive Analysis of Beach Susceptibility to Jellyfish Arrivals in Costa del Sol

This study investigates the susceptibility of beaches to jellyfish arrivals, focusing on the summer seasons from 2015 to 2020. The objective was to develop a predictive model that identifies the characteristics of beaches prone to higher jellyfish presence. This research utilized data from the Infomedusa application, with a focus on key structural and circumstantial variables, such as beach orientation, coastal currents, and morphology. Binomial logistic regression was applied to two models to assess the influence of these variables on jellyfish occurrence. The results showed that beaches oriented toward the east and south, with protection from natural or artificial barriers, and those with limited open sea exposure are more likely to experience jellyfish arrivals. Conversely, beaches facing southwest, with opposing currents and freshwater inflows, tend to have lower risks. Although the models’ predictive capacity was moderate, with a 76% validation rate against empirical data, they provided valuable insights for coastal management and risk prevention. The findings highlight the importance of beach-specific characteristics in forecasting jellyfish presence, contributing to more effective coastal protection strategies.

Surface buoyancy control of millennial-scale variations in the Atlantic meridional ocean circulation

Abstract. Dansgaard–Oeschger (DO) events are a pervasive feature of glacial climates. It is widely accepted that the associated changes in climate, which are most pronounced in the North Atlantic region, are caused by abrupt changes in the strength and/or northward extent of the Atlantic meridional overturning circulation (AMOC), possibly originating from spontaneous transitions in the ocean–sea-ice–atmosphere system. Here we use an Earth system model that produces DO-like events to show that the climate conditions under which millennial-scale AMOC variations occur are controlled by the surface ocean buoyancy flux. In particular, we find that the present-day-like convection pattern with deep-water formation in the Labrador and Nordic seas becomes unstable when the buoyancy flux integrated over the northern North Atlantic turns from negative to positive. It is in the proximity of this point that the model produces transitions between different convection patterns associated with strong and weak AMOC states. The buoyancy flux depends on the surface freshwater and heat fluxes and on sea surface temperature through the temperature dependence of the thermal expansion coefficient of seawater. We find that larger ice sheets tend to stabilize convection by decreasing the net freshwater flux, while CO2-induced cooling decreases buoyancy loss and destabilizes convection. These results help to explain the conditions under which DO events appear and are a step towards an improved understanding of the mechanisms of abrupt climate changes.

Lifetime carryover of early partial migration behaviors in an estuarine-dependent fish under climate change.

Spatial structure within populations promotes population stability and resilience through asynchronous responses among population sub-components (i.e., portfolio effect). In fishes, spatial structure frequently develops via early-life partial migration, leading to diversified nursery use. However, the portfolio effect depends on how adult recruitment from different nurseries exhibits asynchronous dynamics in response to climate variables, and whether nursery experiences carry over to adult demographics. For a three-decade span, we tested adult nursery recruitment and carryover effects associated with early-life partial migration in Hudson River striped bass. Early-life partial migration led to structured utilization of freshwater, brackish, and coastal nurseries, all of which recruited to the adult population. Adult recruitment from brackish nurseries increased with freshwater flow, while coastal nurseries produced more adults during severe winter years. First-year nursery experiences carried over to influence adult sex, but not growth. Most adult females utilized brackish nurseries in their first year, while adult males recruited from freshwater and brackish nurseries. Early-life partial migration led to diversified nursery use and influenced adult demographics, which buffered populations against perturbations.

Zonally Asymmetric Increase in Southern Ocean Heat Content

Abstract A zonally symmetric perspective has proven to be very useful in studying the key role of the Southern Ocean for global ocean heat uptake. Despite this, reconstructions of changes in Southern Ocean heat content ΔOHC over the past few decades have revealed substantial deviations from symmetry. Here, we investigate the zonal asymmetry of ΔOHC and the processes driving it for the period 1979–2019 using an eddy-permitting ocean model forced by atmospheric reanalysis (ERA5). We find that the model successfully reproduces the observed zonal asymmetry in ΔOHC and that it is primarily due to zonal asymmetries in changing surface fluxes. Zonal asymmetries in mean ocean circulation play an important secondary role. Factorial simulations are used to attribute the asymmetry in ΔOHC to the different surface flux components (wind stress, heat, and freshwater fluxes), revealing strong regional differences. North of the Antarctic Circumpolar Current (ACC), we find roughly equal contributions from changes in wind stress and surface heat flux. Within the ACC, all three forcings play an important role, while south of it only wind stress and freshwater flux changes are important. Our findings argue for full consideration of the three-dimensional circulation to understand heat uptake and storage in the Southern Ocean, especially when considering multidecadal time scales where natural variations are as important as the long-term warming trend.

Contrasting Southern Ocean Sea Level Responses to Surface Flux Changes in Eddy-Rich and Eddy-Parameterized Climate Model Configurations

Abstract The impact of ocean model resolution on sea level projections in the Southern Ocean is investigated using eddy-rich (ER) and eddy-parameterized configurations of the Max Planck Institute Earth System Model under the Shared Socioeconomic Pathway (SSP) 5-8.5 scenario. We employ the Flux-Anomaly Forced Model Intercomparison Project (FAFMIP) experiment—heat, stress, and freshwater perturbations—at both resolutions to pinpoint the sources of these differences. South of 55°S, we found that the changes in thermosteric and halosteric sea levels vary substantially between resolutions due to different responses to freshwater perturbations. In the eddy-parameterized model, the resulting increase in stratification suppresses the mixing of salt and heat from the Circumpolar Deep Water with surface layers. These cause differences in the response of surface fluxes and meridional transports yielding an increase in thermosteric sea levels and a decrease in halosteric sea levels. In the eddy-rich configuration, the main driver of eddy-induced warming and salinification between 40° and 44°S is wind stress perturbations. The efficiency of direct eddy effects in ER is restricted to small areas such as the Agulhas Retroflection, the Brazil–Malvinas confluence zone, the Tasman Sea, and, to some extent, the Antarctic Circumpolar Current (ACC). Contrary to expectations, ACC transport increases in the eddy-rich model while decreasing in the eddy-parameterized model under the SSP5-8.5 scenario. FAFMIP results reveal that this decrease is a result of the overcompensation of wind-induced changes by freshwater flux forcing. These results underscore the critical importance of high-resolution models for capturing the processes in sea level projections in the Southern Ocean and beyond. Significance Statement We studied how ocean model resolution affects sea level projections in the Southern Ocean using Max Planck Institute Earth System Model simulations. Higher-resolution models provide a more accurate representation of ocean circulation and its response to changing forcings. We examined how surface heat, momentum, and water fluxes, both separately and combined, shape ocean dynamics. In a strong global warming scenario, significant differences in steric sea level change were observed south of 55°S between the model that simulates eddies and the one that has their effects parameterized. The response to surface freshwater forcing is the primary cause of these differences. Our findings emphasize the critical role of ocean model resolution in accurately understanding and predicting future sea level changes, which is essential for effectively addressing our needs for adaptation.

Investigation of the Origin of Elevated Amounts of Iron and Manganese in a Dam Reservoir

On the outskirts of the Pinios dam reservoir (Ilia Regional Unit, Greece), a water treatment plant serves the water supply needs of the surrounding municipalities, in which high concentrations of Fe and Mn, before treatment, have been observed. The main purpose of this research was to investigate the mechanism of increased iron (Fe) and manganese (Mn) levels in the reservoir of the Pinios dam, which impacts its water treatment plant operation. A wide range of hydrochemical and sedimentological analyses were conducted over a hydrological year, focusing on the spatial and temporal distribution of Fe and Mn in both water and sediment samples across the established research monitoring stations. Sediment samples from the reservoir’s bottom revealed predominantly fine-grained material, rich in total organic carbon, with elevated Mn and Fe oxide levels. Significant seasonal variations in Fe and Mn levels were also discovered, with higher Mn levels observed in the anoxic bottom waters of the reservoir during the dry season, attributed to the reduced conditions favoring Mn oxide dissolution over Fe. Conversely, during the wet season, a homogenization of metal concentrations throughout the water column was observed due to increased oxygenation and freshwater inflow. These outcomes were confirmed by the hydrochemical analysis, indicating that the redox conditions, pH, and temperature, as well as the presence of organic matter, significantly influence the mobility and bioavailability of these metals in the reservoir. The findings of this study clarify that the high concentration of Fe and Mn can be linked to the mineral composition of the upstream Neogene and flysch formations in the study area. As these formations are affected by geological weathering, they tend to enrich the streams, through soil erosion and runoff processes, with metals like Fe and Mn, which are eventually transported into the dam reservoir. This study highlights the critical influence of lithological, sedimentological, and hydrological factors on the redox conditions and seasonal stratification that govern the behavior of Fe and Mn concentrations and mobility in dam reservoirs. These findings are critical for informing water resource management practices and dam infrastructure operators and developing effective environmental conservation strategies in similar cases.

Future Freshwater Fluxes From the Antarctic Ice Sheet

AbstractSurface freshening of the Southern Ocean driven by meltwater discharge from the Antarctic ice sheet has been shown to influence global climate dynamics. However, most climate models fail to account for spatially and temporally varying freshwater inputs from ice sheets, introducing significant uncertainty into climate projections. We present the first historically calibrated projections of Antarctic freshwater fluxes (sub‐shelf melting, calving, and surface meltwater runoff) to 2300 that can be used to force climate models lacking interactive ice sheets. Our findings indicate substantial changes in the magnitude and partitioning of Antarctic freshwater discharge over the coming decades and centuries, particularly under very‐high warming scenarios, driven by the progressive collapse of the West Antarctic ice shelves. We project a shift in the form and location of Antarctic freshwater sources, as liquid sub‐shelf melting increases under the two climate scenarios considered, and surface meltwater runoff could potentially become a dominant contributor under extreme atmospheric warming.

Benthic communities restructure quickly under shifting salinities: a reciprocal transplant experiment

Salinity regimes in estuarine habitats will shift pending sediment diversions, which will reconnect Louisiana, USA, coastal wetlands to the Mississippi River. The river has been hydrologically disconnected from fringing estuaries for over a century, but high local rates of relative sea-level rise have contributed to land loss, and sediment diversions will be built to prevent future loss. A reciprocal transplant experiment of benthic cores was used to determine how macrofaunal communities may be altered pending salinity regime shifts. A total of 147 benthic cores (3142 cm3) were excavated and transplanted within 3 salinity regimes in Barataria Bay, LA, for 2 or 5 mo. We observed a significant effect of salinity treatment on benthic macroinfaunal community composition in every transplant, regardless of origin or date sampled. Results show that communities recolonized in as little as 2 mo, and reveal the species assemblages responsible for the restructure. Species assemblage changes included dominance of generalist species (spionid and capitellid polychaetes) and shifts between deposit feeders and suspension feeders as freshwater flow varied. Lower-salinity treatment decreased species richness, while higher-salinity treatment increased both species richness and abundance. Benthic communities will likely restructure quickly under the sediment diversion scenarios and leave few founder effects in response to changes in salinity regimes across the estuary. Pulsed diversion operations may yield different results, but only springtime operations are likely to impact benthic recruitment. Comprehensive knowledge of how benthic communities restructure post-restoration will enable both appropriate regional planning as well as future predictions in coastal estuaries undergoing hydrologic alteration.

Wind speed effects on rainfall-induced salinity and temperature anomalies at the sea surface microlayer at mid-latitudes

Sea surface salinity (SSS) and temperature (SST) can serve as proxies to detect freshwater fluxes at the sea surface during precipitation. Widely unknown, however, is the impact of precipitation (droplet sizes and velocities) and wind speed on the sea surface microlayer (SML), the <1 mm boundary layer between atmosphere and ocean. We used the autonomous surface vehicle HALOBATES in the German Bight, North Sea, to collect SSS and SST data with high vertical resolution at 7 depths in the top meter, spaced 10–30 cm apart, including the SML. During two rain events with similar maximum rainfall intensity, the magnitude and persistence of the resulting salinity anomaly (between SML and 100 cm depth) was dependent on wind speed. The maximum rate of change of salinity in the SML was 4 times faster with high wind speeds: 0.009 g kg min−1 at low wind speeds, 0.037 g kg min−1 at high wind speeds. A threshold of about 5 m s−1 determined whether stratification occurred in the near-surface layer or freshwater mixed with the underlying layers during precipitation. Strong winds still caused salinity changes in the near-surface layer and SML, but the water mixed rapidly with the underlying water masses. The SML salinity was affected by droplet distributions with smaller droplets, as small droplets stay at the surface. Salinity and temperature changes in the SML were >9 times higher than at 100 cm depth (−0.037 g kg min−1 vs. −0.004 g kg min−1) and still detectable during very high wind speeds. Overall, these results contribute to a better understanding of the vertical distribution of freshwater in the surface ocean and its dependence on rain intensity, wind speed, and droplet properties, helping to improve understanding of the fate of freshwater in a changing ocean due to climate warming.

Long‐Term Changes in Salinity in the South China Sea Due To Anthropogenic Forcing

AbstractChanges in ocean salinity are essential for the stratification of the upper ocean and the regional marine ecosystem. In this study, 10 sets of large ensemble experiments and multi‐model ensembles from the Coupled Model Intercomparison Project Phase 6 (CMIP6) are used to investigate the effect of anthropogenic forcing on upper ocean salinity in the South China Sea (SCS). In most models, surface salinity increases during the historical period due to external forcing. Using the salinity budget, we find that a decrease in freshwater flux, particularly precipitation, is responsible for the increase in salinity, although horizontal advection also contributes to the change. Single forcing experiments reveal that the change in salinity in the SCS is mainly attributed to anthropogenic forcing, while the influence of natural forcing is relatively small. Anthropogenic aerosols (AAs) can decrease the dynamic and thermal components of precipitation, resulting in a considerable increase in salinity. In contrast, anthropogenic greenhouse gases (GHGs) have less effect on long‐term trend in SCS salinity because the GHG forcing leads to an increased thermal response of precipitation but a decreased dynamic response. Additionally, we use the Community Earth System Model version 1 (CESM1) to evaluate the role of different aerosol emission sources in modulating the salinity change in the SCS. The experimental results show that aerosol emissions from Asia dominate the salinity change in the SCS by changing the local Hadley circulation. In contrast, the contribution of aerosol emissions from North America and Europe (NAEU) is much smaller.

HALOBATES: An Autonomous Surface Vehicle for High-Resolution Mapping of the Sea Surface Microlayer and Near-Surface Layer on Essential Climate Variables

Abstract Essential climate variables (ECVs) are physical, chemical, or biological parameters supporting the characterization of the climate system. The oceanic ECVs temperature and salinity have been defined for the sea surface (upper 1 mm) and bulk water (upper 2 m). Direct observation and sampling of the sea surface from ships and buoys are limited due to the potential loss of integrity of the sea surface and contamination by the research platform. The design, development, and operation of a research vehicle with autonomous capabilities based on commercially available components are described in this study. The autonomous operation and fully automated sampling enable high-resolution mapping of the sea surface microlayer (SML), i.e., the upper < 1-mm layer and the near-surface layer (<1.2 m). Sampling the SML is based on a rotating glass disk sampler. The autonomous surface vehicle HALOBATES has been equipped with multiple conductivity–temperature–depth (CTD) sensors in a flow-through system to measure both sea surface and bulk temperature and salinity. The performance of the autopilot under various scenarios is demonstrated. Typical anomalies of ECVs within the SML and near-surface layer are evident and forced by both atmospheric and oceanic processes. Data analyses reveal HALOBATES’ ability to resolve thermohaline structure on different spatial and temporal scales, for example, small-scale freshwater lenses and thermohaline changes across oceanic fronts. The integration of an acoustic Doppler current profiler supports the interpretation of thermohaline structures. HALOBATES is a platform with cutting-edge technology for understanding climate-related processes between the ocean and atmosphere and providing high-resolution sea surface data for validating satellite products of ECVs. Significance Statement An autonomous surface vehicle was developed to address the challenges in the in situ measurement of essential climate variables at the sea surface (upper 1 mm) and near-surface layer (upper 1 m) of the ocean. Autonomous operation for up to 12 h and fully automated sampling enable high-resolution mapping of the sea surface and near-surface layers. The ability of the vehicle to resolve thermohaline structures at various spatial and temporal scales is demonstrated, providing valuable data on freshwater fluxes (evaporation minus precipitation) between the ocean and the atmosphere. This cutting-edge technology enhances the understanding of climate-related processes, offering crucial “sea surface data” for validating satellite products of essential climate variables.

Impact of water residence time and stratification on water quality improvement of an artificial brackish waterway

Study regionThe Ara Waterway, an artificial inland brackish waterway in South Korea. Study focusThis study investigates how water residence time and stratification affect water quality in an artificial brackish water system, which has different characteristics from natural environments and is under-researched, using numerical simulations. Various hydrodynamic conditions, including freshwater discharge and travel distances, were analyzed to understand their impact on residence times and stratification, offering insights for optimizing water quality management. New hydrological insights for the regionThis study demonstrates that stratification significantly impacts water quality more than residence time in artificial brackish waterways. In contrast to lakes or reservoirs, increasing freshwater discharge in artificial brackish waterways can enhance stratification, reducing vertical mixing and degrading water quality, especially in the bottom layers. In the F0 scenario, excluding freshwater inflow increased summer maximum dissolved oxygen by 1.37 times compared to the validation case. This suggests that breaking stratification, such as through underwater aeration systems, may be more effective than focusing on residence time. In the L15 scenario, with the longest travel distance, upstream total nitrogen and total phosphorus increased by 18.37 % and 26.12 %, while downstream levels decreased by 3.35 % and 10.52 %. Additionally, poorer quality freshwater can degrade overall water quality, even with shorter residence times. These findings emphasize the need for careful management of inflowing water to maintain water quality in artificial brackish waterways.

Influence of river runoff and precipitation on the seasonal and interannual variability of sea surface salinity in the eastern North Tropical Atlantic

Abstract. In tropical regions, the freshwater flux entering the ocean originates primarily from precipitation and, to a lesser extent when considering basin-scale averages, from continental rivers. Nevertheless, at the regional scale, river flows can have a significant impact on the surface ocean dynamics. Riverine freshwater modifies salinity and, therefore, density, stratification, and circulation. With its particular coastline and high cumulative river discharge, as well as its being in the vicinity of the intertropical convergence zone (ITCZ), the eastern part of the North Tropical Atlantic (e-NTA) region off northwestern Africa is a particularly interesting location to study the linkage between precipitation, river outflow, and sea surface salinity (SSS). Here, we focus on the regional e-NTA SSS seasonal cycle and interannual variability and on the impact of using various river runoff and precipitation forcing data sets to simulate SSS with a regional model. The simulated SSS values are compared with the Climate Change Initiative (CCI) satellite SSS values; in situ SSS values from Argo floats, ships, and a coastal mooring; and the GLORYS reanalysis SSS values. An analysis of the mixed-layer salinity budget is then conducted. Overall, the simulations reproduce the seasonal cycle and interannual variability well despite a positive mean model bias north of 15° N. The seasonal cycle is impacted by the phasing of the different runoff products. The mixed-layer SSS decrease during the rainy season is mainly driven by precipitation followed by runoff by means of horizontal advection and is partly compensated for by vertical mixing. In terms of interannual anomalies, river runoff has a more direct impact on SSS than precipitation. This study highlights the importance of properly constraining river runoff and precipitation to simulate realistic SSS values and the importance of observing SSS in coastal regions to validate such constraints.

Quantifying Global Hydrological Sensitivity to CO2 Physiological and Radiative Forcings Under Large CO2 Increases

AbstractPrediction of surface freshwater flux (precipitation or evaporation) in a CO2‐enriched climate is highly uncertain, primarily depending on the hydrological responses to physiological and radiative forcings of CO2 increase. Using the 1pctCO2 (a 1% per year CO2 increase scenario) experiments of 12 CMIP6 models, we first decouple and quantify the magnitude of global hydrological sensitivity to CO2 physiological and radiative forcings. Results show that the direct global hydrological sensitivity (for land plus ocean precipitation) to CO2 increase only is −0.09 ± 0.07% (100 ppm) −1 and to CO2‐induced warming alone is 1.54 ± 0.24% K−1. The latter is about 10% larger than the global apparent hydrological sensitivity (i.e., including all effects, not only direct responses to warming, = 1.39 ± 0.22% K−1). These hydrological sensitivities are relatively stable over transient 2× to 4 × CO2 scenario. The intensification of the global water cycle are dominated by the CO2 radiative effect (79 ± 12%) with a smaller positive contribution from the interaction between the two effects (6 ± 12%), but are reduced by the CO2 physiological effect (−10 ± 8%). This finding underlines the importance of CO2 vegetation physiology in global water cycle projections under a CO2‐enriched and warming climate.

Revisiting climate impacts of an AMOC slowdown: dependence on freshwater locations in the North Atlantic.

The key locations of freshwater input driving Atlantic Meridional Overturning Circulation (AMOC) slowdown and their climate responses remain inconclusive. Using a state-of-the-art global climate model, we conduct freshwater hosing experiments to reexamine AMOC sensitivity and its climate impacts. The Irminger basin emerges as the most effective region for additional freshwater fluxes, causing the greatest AMOC weakening. While global temperature and precipitation responses are relatively homogeneous, subcontinental responses-especially in the northern mid-latitudes-are heterogeneous. At high latitudes, sea ice responses to freshwater fluxes and associated ice-albedo feedbacks determine temperature changes. In tropical and extratropical regions, temperature dynamics are shaped by atmospheric circulation and oceanic heat transport. Precipitation shows seasonal and regional variability due to altered surface turbulent heat flux and the southward movement of the Intertropical Convergence Zone (ITCZ). The widespread heterogeneity in climate extremes underscores the need to monitor freshwater release regions linked to AMOC slowdown. These findings hold vital implications for understanding paleoclimate and future AMOC impacts.

Spatiotemporal changes in riverine input into the Eocene North Sea revealed by strontium isotope and barium analysis of bivalve shells

Reconstructing hydrological conditions of past warm periods, such as the Eocene ‘hot house’ provides empirical data to compare to state of the art climate models. However, reconstructing these changes in deep time is challenging, for example, given the complex interplay between evapotranspiration, precipitation and runoff. As a proxy for past changes in these hydrological systems, the dynamics of fresh water input into marginal seas can be used to identify the spatiotemporal distribution of riverine runoff. Elemental barium (Ba) and radiogenic strontium (87Sr) are, depending on the amount of runoff and the background geology of the catchment area, typically enriched in river waters in comparison to seawater and can thus be utilized to determine changes in riverine fresh water discharge. Here, we use barium to calcium ratios (Ba/Ca) and radiogenic strontium isotopes (87Sr/86Sr) measured in fossil bivalve shells to reconstruct patterns of fresh water input into the paleo North Sea during the early to middle Eocene. Our reconstruction shows the potential of Ba/Ca and 87Sr/86Sr to serve as proxies for riverine runoff and highlights the spatiotemporal complexity of Eocene hydrological conditions in western Europe. In particular, our results enable changes in riverine input along geological to perennial time scales for different coastal regions to be determined, revealing a steady influx of fresh water, but with distinct spatiotemporal differences.