Deeper Water Layers Research Articles

Dense Water Production in Storfjorden, Svalbard, From a 1‐Year Time Series of Observations and a Simple Model: Are Polynyas in a Warming Arctic Exporting Heat to the Deep Ocean?

AbstractThe formation of dense Brine‐enriched Shelf Water (BSW) in Storfjorden is analyzed during Winter 2016–2017 from mooring observations, a polynya model nudged to satellite observations, and an original BSW production model. The ice season was two months shorter than average, yet 44.2 of sea ice were formed, in line with estimates for the period preceding the atlantification of the Barents Sea in the mid‐2000s: A thinner, more fragile ice may favor polynya openings and frazil ice production. A saline specimen of BSW was produced in large volumes, corresponding to an annual mean transport of 0.042 Sv, larger than previous estimates. The important production is due to the preconditioning of the polynya with a more saline source water, exceeding the pre‐2005 values by 0.37. The BSW overflow was observed on the West Spitsbergen shelf slope from hydrographic sections down to 750 m, thus entering the Norwegian Sea Deep Water layer. Its core temperature was about C warmer than the pre‐2005 values owing to the entrainment of a warmer water in Storfjordrenna, suggesting that a part of the excess surface heat of the Barents Sea could be exported into the deep ocean. Overall our results suggest that dense water formation in the Storfjorden polynya may not, at least for now, be hampered by the atlantification of the Barents Sea, and perhaps even temporarily favored by the more saline source water. Anomalous atmospheric warming during the Winter‐Spring may however disrupt the production, as was observed 1 year before.

Floating active carpets drive transport and aggregation in aquatic ecosystems

Communities of swimming microorganisms often thrive near liquid–air interfaces. We study how such ‘active carpets’ shape their aquatic environment by driving biogenic transport in the water column beneath them. The hydrodynamic stirring that active carpets generate leads to diffusive upward fluxes of nutrients from deeper water layers, and downward fluxes of oxygen and carbon. Combining analytical theory and simulations, we examine the biogenic transport by studying fundamental metrics, including the single and pair diffusivity, the first passage time for particle pair encounters and the rate of particle aggregation. Our findings reveal that the hydrodynamic fluctuations driven by active carpets have a region of influence that reaches orders of magnitude further in distance than the size of the organisms. These non-equilibrium fluctuations lead to a strongly enhanced diffusion of particles, which is anisotropic and space dependent. Fluctuations also facilitate encounters of particle pairs, which we quantify by analysing their velocity pair correlation functions as a function of distance between the particles. We found that the size of the particles plays a crucial role in their encounter rates, with larger particles situated near the active carpet being more favourable for aggregation. Overall, this research broadens our comprehension of aquatic systems out of equilibrium and how biologically driven fluctuations contribute to the transport of fundamental elements in biogeochemical cycles.

Revisiting Thorpe scale analysis and diapycnal diffusivities in Drake Passage

Ship heave effects in the swell-prevalent Southern Ocean induce frequent false overturns in density profiles, and conventional Thorpe scale analysis leads to unrealistically large diapycnal diffusivities. Another critical factor causing large positive biases in Thorpe scale analysis concerns the Ozmidov to Thorpe scales ratio α often considered constant and equal to 0.8. A revised Thorpe scale analysis is proposed with an innovative segment-by-segment approach to circumvent heave-induced false overturns together with best-fitting α functions to Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) data, which yields realistic diffusivities in the Drake Passage region. The revised Thorpe scale method applied to a finely-resolved CTD section across the Drake Passage yields typical diffusivities of O (10-5-10-3 m2 s-1), with the largest values being preferentially concentrated close to the bottom at major circumpolar fronts, consistent with nearby DIMES microstructure observations. In the southern Drake Passage the method highlights a middepth (500-1500 m) mixing maximum of 10-4-10-3 m2 s-1 in the Lower Circumpolar Deep Water (LCDW) layer due to large intrusions of Warm Deep Water (a cold/fresh variety of LCDW) of Weddell Gyre origin. The inferred middepth mixing maximum in intrusive regions, which is associated with isopycnal eddy stirring rather than caused by internal waves or double diffusive mixing, supports the short-circuiting paradigm of meridional overturning circulation in the southwestern Scotia Sea region and has important climatic implications in a warming climate.

Status and Migration Activity of Lead, Cobalt and Nickel in Water and in Bottom Sediments of Lake Markakol, Kazakhstan

Lake Markakol is located in a metal-rich mountain area of Kazakhstan. Metal input into the lake water and in the bottom sediments can be expected. Lead, cobalt and nickel monitoring in both near-surface and deep-water layers and in bottom sediments was carried out using flame atomic absorption spectrometric analyses. Lead contamination of surface water ranging from 2.6 to 6.8 µg/L occurs in all water samples with the exception of the surface water layer. In the deep-water section concentrations reach up to 13.0–16.2 µg/L. Cobalt concentrations range from 36.8 to 67.5 µg/L in the surface layer and from 25.5 to 69.2 µg/L in the deep-water layer. High values of nickel were found in the surface and bottom layers of the water, ranging from 13.5 to 49.0 and 17.2 to 49.0 µg/L, respectively. High concentrations of lead, cobalt and nickel were identified in all samples of the bottom sediments. The lead content in bottom sediments reaches 11.3, cobalt reaches 10.3–18.0 and nickel reaches 15.0 mg kg−1. The results and their assessment can serve as a basis for future monitoring and measures to reduce pollution, restore the lake ecosystem and ensure the safety of fishery products for humans.

Modelled Water and Phosphorus Transports in the Archipelago Sea and through the Åland Sea in the Northern Baltic Sea and Their Links to Water Quality

The Baltic Sea is currently classified as ‘affected by eutrophication’. In this study, water exchanges and net phosphorus flows in the Archipelago Sea and through the Åland Sea from the Baltic proper was estimated with the aid of a 3-D hydrodynamic model for the years 2000–2021. The modelling configuration is based on the Copernicus regional reanalysis data. Water flowed from the Baltic proper to the Bothnian Sea at 669 km3/a and out from there at 879 km3/a. The inflow occurred in the deep-water layer (over 40 m), while the outflow occurred in the surface layer (0–40 m). With the inflow, 14,500 tons/a of phosphorus were transported during the years 2000–2014, but the amount increased to 20,300 tons per year from 2015 to 2021. At the same time, the winter-time concentrations of DIP in the Bothnian Sea almost doubled. In the Archipelago Sea, the main flow direction of water was from south to north from 2000 to 2014. From 2015 to 2021, the net flow direction reversed, and water flowed from the Archipelago Sea to the Baltic proper in the surface layer at a rate of 140 km3/a. At the same time, the background loading of phosphorus entering the Archipelago Sea with the flows decreased significantly and the chlorophyll-a concentration decreased below the threshold for a good ecological status. The U-turn in surface currents in the Archipelago Sea since 2015 may be related to variations in upwellings caused by climate change.

Repeatability of Hydroacoustic Results versus Uncertainty in Assessing Changes in Ecological Status Based on Fish: A Case Study of Lake Widryńskie (Poland)

Assessments of changes in the ecological state of aquatic ecosystems are always burdened with uncertainty, which results from environmental reasons and poor repeatability of measurement results of elements enabling the assessment. This study determines the uncertainty related to the elements of the assessment of the hydroacoustic structure of fish communities’ (1) vertical target strength distribution (TS) in two-meter layers of water and (2) changes in the area where fish were recorded (which was determined on the basis of maps of their distribution in 2 m deep water layers). The object of this study was a lake (depth: 27 m) in which at the end of June 2016 the O2 concentration was <1.4 mg L−1 below 8 m depth, which resulted in the accumulation of fish to a depth of 6 m. Hydroacoustic acquisition was carried out along transects arranged in the east–west (WE), north–south (NS), and zigzag (ZZ) directions in three repetitions. It was shown that the empirical probability of obtaining statistically different results was 2/9 when (1) Kendall’s τ coefficient, used to determine the similarity of the TS distribution, was less than 0.7—moderate correlation—and (2) fish occurrence areas in two cases (WE and ZZ on the third day of research) in layers 2–4 m and 4–6 m differed statistically significantly from the average area for all repetitions by 10–14% and 56–66% (p < 0.05), respectively. The obtained results indicate quite good repeatability of acoustic measurements; however, in order to reduce the uncertainty, it is recommended that tests be conducted in this type of lake in three series of measurements.

Calibration of horizontal seismic design spectra of offshore earthquake ground motions by applying improved differential evolution method

Design Response Spectrum (DRS) serves as a crucial cornerstone for seismic design, encapsulating the characteristic parameters of the design spectrum. To investigate the offshore design spectrum required for seismic design of offshore structures, improved differential evolution (DE) methods are first used to determine the characteristic parameters of the horizontal design spectra of offshore ground motions. In assessing the stability and precision of the chosen improved DE methods, we scrutinize the applicability of various renowned DE methodologies in the calibration process. Our findings indicate that differential evolution with global and local neighborhoods (GLDE) exhibits superior stability and accuracy compared to other DE methods, rendering it suitable for calibrating the design spectrum to delineate optimal characteristic parameters of the DRS. Subsequently, the properties of the offshore horizontal design spectra are investigated by applying the recommended GLDE to 6756 seismic data collected from three offshore regions. The site conditions of water depth and sedimentary thickness of the ocean-bottom stations were investigated by the finely divided seismic data with classified earthquake types. The ocean-bottom stations are classified as A-D by the water depth and sedimentary thickness, and the offshore DRS for four site classes with different earthquake types are presented in this study. According to the observed offshore response spectra with different water depth groups, it is found that the water depth should be considered in the offshore design spectra mainly due to the differences between buried and unburied seafloor stations, and the spectral values with deep water layer are considerably larger than those with shallow water. Offshore DRSs are compared with onshore DRSs of seismic codes and different peak ground acceleration (PGA) groups are presented in this study.

Carbon dioxide sequestration in brackish water: Principles, techniques and environmental benefits

Carbon dioxide storage is a technology to separate carbon dioxide gas from the atmosphere and inject it into the brackish water layer to achieve the purpose of carbon dioxide emission reduction. In this way, carbon dioxide is dissolved in salt water and then sequestered in deep underground water layers, preventing it from being released into the atmosphere and causing a greenhouse effect. This technology can effectively reduce carbon dioxide emissions, but also can use geological structures for storage, has great potential application prospects. It is important to note that further research and practice is needed to address possible environmental risks and costs.

Variability of Inflowing Current Into the Dotson Ice Shelf and Its Cause in the Amundsen Sea

AbstractThe inflow of warm and salty Circumpolar Deep Water affects the melting of the ice shelf on the Amundsen Sea, a significant contributor to global sea level rise. Multi‐year mooring data (2014–2016 and 2018–2020) from the front of the Dotson Ice Shelf show the modified Circumpolar Deep Water layer was thicker during 2018–2020 than during 2014–2016. During 2014–2016, Ocean surface stress curl influenced the barotropic process and strengthened southward velocity, while during 2018–2020, it caused lift and downwelling of thermocline depth, increasing the impact of the baroclinic process in ocean circulation. The heat transport to the ice shelf during 2018–2020 (57.42 MW m−1) was half as much as it was during 2014–2016 (111.06 MW m−1) due to a weaker lower layer current. The difference in ocean circulation between two periods, caused by a difference in warm layer thickness, ultimately impacts the heat transport entering the ice shelf cavity.

Flow measurement in the southern coast of the Caspian Sea

Oceanographic monitoring was conducted in the southern basin of the Caspian Sea to evaluate the physical structure of seawater and sea currents. This monitoring aimed to gather data and analyze the characteristics of the seawater, including temperature, salinity, density, and other relevant parameters. Additionally, the monitoring also focused on studying the patterns and dynamics of sea currents in the region. The collected data and analysis from this monitoring period provided valuable insights into the oceanographic conditions of the southern basin of the Caspian Sea. During storm events, a high level of correlation between the current and wind data was observed in the southern basin of the Caspian Sea during the 2017-2018 monitoring period. The measured current data indicated that the predominant directions were north (N) and northwest (NW). It was observed that strong gusts predominantly originated from the north (N) and northwest (NW) directions. Additionally, a smaller portion of the strong gusts was observed to come from the South-Southeast (S-SE) direction. These findings indicate that the prevailing wind patterns during the monitoring period were primarily from the north and northwest, with a lesser contribution from the South-Southeast direction. The current profiles observed during the monitoring period in the southern basin of the Caspian Sea were primarily influenced by the general circulation pattern of the region. This circulation pattern played a significant role in shaping the current profiles observed during the measurements. In terms of the surface current speed, the maximum recorded value was approximately 200 cm/s, which occurred in January. This indicates that there were instances of relatively high-speed currents in the southern basin of the Caspian Sea during that time. The evaluation of inter-annual variability in vertical structure and seawater temperature profiles during the monitoring period confirmed the presence of seasonal stratification in the water column of the southern basin of the Caspian Sea. This stratification was observed to consist of different layers, including the surface mixed layer, thermocline, and deep-water layers. The formation of these layers is indicative of the seasonal variations in temperature and density within the water column. The vertical changes in the water’s physical parameters during the monitoring period revealed the formation of stratification in the southern basin of the Caspian Sea. In March, it was observed that the water temperature decreased from 11.5°C at the surface to 8.5°C at a depth of 100 m, indicating the presence of a temperature gradient. As the monitoring progressed into May, the stratification became stronger, with the surface water temperature reaching around 23°C. By August, the surface layer of the sea water experienced a significant increase in temperature, reaching 29°C. These observations highlight the development of stratification and the seasonal variations in water temperature during the monitoring period in spring and summer seasons. The water temperature beneath the thermocline layer, specifically at a depth of 100 meters, was recorded to be around 8-8.5 oC. Additionally, the water salinity in the water column exhibited fluctuations between 12-12.5 (psu). Monitoring and understanding physical properties variations are crucial for assessing the oceanographic conditions and their potential impact on marine ecosystems in the southern basin of the Caspian Sea.

An experimental study of the correlation between P-wave velocity and the physical properties of weakly cemented formations

Deep water and shallow layers mostly feature weakly cemented formations, with complex geological structures, geological looseness, susceptibility to collapse. In order to obtain information on weakly cemented formation materials, weakly cemented argillaceous siltstone is simulated as the research object and the focus is on analysing the influence of ultrasonic frequency, density, particle size (porosity), and compressive strength on P-wave velocity and establishing the correlation relationship between longitudinal wave velocity and each parameter through indoor simulation experiments. The results showed that there is a linear relationship between P-wave velocity and ultrasonic frequency in terms of positive correlation as well as compressive strength. The nonlinear relationship between P-wave velocity and particle size (porosity) is a negative correlation, while the nonlinear relationship between sound velocity and density is a positive correlation. In addition, the influence of core height on P-wave velocity is analysed; it is found that as the core height increases, the velocity slightly decreases, and each ultrasonic frequency has an ultimate height for sound wave penetration. Through the response relationship between ultrasound and the physical properties of weakly cemented formations, indirect acquisition can be achieved, which is of great significance for the development of oil and gas in weakly cemented formations.

The diversity of the antimicrobial resistome of lake Tanganyika increases with the water depth

The presence of antimicrobial resistance genes (ARGs) in the microbiome of freshwater communities is a consequence of thousands of years of evolution but also of the pressure exerted by anthropogenic activities, with potential negative impact on environmental and human health. In this study, we investigated the distribution of ARGs in Lake Tanganyika (LT)'s water column to define the resistome of this ancient lake. Additionally, we compared the resistome of LT with that of Lake Baikal (LB), the oldest known lake with different environmental characteristics and a lower anthropogenic pollution than LT. We found that richness and abundance of several antimicrobial resistance classes were higher in the deep water layers in both lakes. LT Kigoma region, known for its higher anthropogenic pollution, showed a greater richness and number of ARG positive MAGs compared to Mahale. Our results provide a comprehensive understanding of the antimicrobial resistome of LT and underscore its importance as reservoir of antimicrobial resistance. In particular, the deepest water layers of LT are the main repository of diverse ARGs, mirroring what was observed in LB and in other aquatic ecosystems. These findings suggest that the deep waters might play a crucial role in the preservation of ARGs in aquatic ecosystems.

Interactive effects of viral lysis and warming in a coastal ocean identified from an idealized ecosystem model

Marine viruses have been identified as key players in biogeochemical cycles and in the termination of phytoplankton blooms; however, most biogeochemical models have yet to resolve viral dynamics. Here, we incorporate a viral component into a 1D ecosystem model to explore the influence of viruses on ecosystem dynamics under current and future climatic conditions in a coastal ocean. Virus-phytoplankton interactions and zooplankton grazing were mechanistically described through size-based contact rates. The model demonstrated that the presence of viruses increased nutrient retention in the upper water column. This corresponded to a reduction in phytoplankton biomass, production of dead organic matter and transfer of biomass to higher trophic levels. Viral presence played a key role in deeper water layers, near the thermocline. While warming alone reversed these trends, the combination of warming and viral presence enhanced the effect of viruses, indicative of synergistic interaction. Our results highlight the need of incorporating viral dynamics in the existing marine ecosystem models to better predict ecosystem responses to climate change.

Mineralogy, geochemistry and genesis of the polymetallic crusts from the Chagos-Laccadive Ridge, Arabian Sea, India

Recent offshore exploration by the Geological Survey of India (GSI) located ferromanganese crusts (FeMn) along the Chagos-Laccadive Ridge (CLR) offshore western continental margin of India. CLR FeMn crusts grew on phosphorite substrate rock (phosphorite cemented sand/silt with moderate carbonate fluorapatite (CFA) cement and goethite). Phosphorite precipitation is the product of intense upwelling and phosphorus regeneration due to organic matter degradation and fish debris dissolution. CLR FeMn crusts are of hydrogenetic origin precipitated in an oxic environment. As a result of hydrogenetic processes under oxic conditions, Fe-vernadite dominated the mineral phase of FeMn crusts. CLR FeMn crusts formed when the North Indian deep-water layer mixed with oxygen-depleted waters from the Arabian Sea, Persian Gulf, and Red Sea. Three genetic layers are identified in the FeMn crusts; Type-I, Type-II, and Type-III developed in response to fluctuation in dissolved oxygen during periods of intensified suboxic bottom waters and monsoon maxima associated with past climatic events in the Arabian Sea. Type-I layers are columnar showing low Mn/Fe ratio (∼1), reflectivity, Mg (average 1.24%), and Ni (average 0.29%). Dendritic Type-II layers show high Mn/Fe ratios (average 12.29), reflectivity, Mg (average 3.51%) and Ni (average 1.69%). Type-III layers show very low Mn/Fe ratios (average 0.21), reflectivity, Mg (average 1.4%) and Ni (average 0.01%) and contain a substantial amount of terrigenous debris of aeolian origin. Elements of economic interest such as Mn, Co, Ni, Cu, Mo, Li, Te, and REY in CLR FeMn crusts are depleted owing to terrigenous contribution.

Biogeographic distribution patterns of the bacterial and archaeal communities in two seamounts in the Pacific Ocean

Seamounts are important components of seafloor topography and have a significant influence on the biogeographic distribution of marine microorganisms. However, current studies on the biogeographic distribution patterns of microorganisms in the ocean around seamounts are still inadequate. This study investigated the bacterial and archaeal communities present in the water column at various depths around the Kocebu and M5 seamounts located in the western Pacific Ocean using 16S rRNA gene high-throughput sequencing. The analyses showed that microbial communities had different alpha diversities and species compositions in samples from the same depth, and disruption of vertical stratification of microbes was observed in deep water layers at both seamounts. The stochastic processes dominated the microbial community assembly around two seamounts, for the Kocebu Seamount, drift accounted for 34.99% and 64.85% in the bacterial and archaeal community assembly processes, respectively; while the corresponding values for the M5 Seamount were 14.07% and 58.65%. Despite the low explanatory power of environmental variables for microbial communities, dissolved inorganic nitrogen was the most significant factor influencing the microbial community structure in seawater around seamounts. Overall, our results revealed that the presence of Kocebu and M5 seamounts enhance the vertical mixing of microbial communities in the surrounding seawater, and stochastic processes dominate microbial community assembly. Considering the numerous seamounts in the global ocean, the impacts of seamounts on marine ecosystems and biogeochemical cycles may have been underestimated.

Photobleaching-induced changes in the optical and photochemical properties of algal organic matter.

Algal organic matter (AOM), a significant source of endogenous dissolved organic matter (DOM) is released in high concentrations during cyanobacterial blooms, along with cyanotoxins. Subsequent photobleaching of AOM is an important phenomenon to investigate. In this study, intracellular organic matter (IOM) and extracellular organic matter (EOM) were extracted from cultured cyanobacteria taken from Taihu Lake in China. The formation of photochemically produced reactive intermediates in different stages of IOM and EOM photobleaching was compared to Suwannee River DOM (SRDOM, reference standard DOM). Results revealed notable differences influenced by the pigment component among IOM, EOM, and SRDOM. The pigment in IOM contributed to a triplet state pool with strong energy-transfer but limited electron-transfer capabilities. Notably, IOM exhibited the highest triplets state quantum yield value in the visible region, suggesting its potential significance in pollutant degradation in deeper water layers. For EOM, one of the pools exhibits photolability and remarkable electron-transfer capability, indicating it as a high-energy triplet state component. Moreover, three cyanotoxins (MC-LR, ACA, and ATX-a) were detected in the extracted AOM, and their photodegradation was monitored during the AOM photobleaching process. This highlights the potential role of AOM as a photosensitizer in the natural self-cleaning mechanisms of water bodies, facilitating the degradation of organic pollutants through photochemical reactions. The findings of this study contribute to understanding the dynamic nature of AOM and its implications in environmental processes.

Исследование оптическими методами явлений генерации брызг по типу "парашют" в природных и технических системах

This paper discusses the results of a detailed study of bag breakup spray fragmentation process obtained using various panoramic optical methods: shadow imaging and laser-induced fluorescence (LIF) with high-speed video recording. The experiments were carried out in two fundamentally different configurations. When blowing the air flow above a deep-water layer, with the presence of large-scale waves on the surface, and vice versa in conditions of thin layers of liquid. Comparison of the results of observations demonstrated not only the general similarity of the evolution scenario, but also the dependences of the spatiotemporal scales of the process of fragmentation and the formation of droplets on the characteristic flow velocities according to the bag breakup events. This indicates the universal nature of the phenomena leading to the fragmentation and separation of drops from the surface of a liquid blown by a gas flow in natural and technical hydrodynamic systems of a wide class with scales varying over a wide range.

Linkages between fluorescent dissolved organic matter and lignin phenols in the Canada Basin, Arctic Ocean

Lignin is one of the more refractory components of dissolved organic matter (DOM) and its concentration and composition are used to trace terrigenous DOM in the ocean. In this study, lignin oxidation products (LOP), and chromophoric and fluorescent DOM were measured in Polar Mixed Layer (PML), Atlantic water (AW), and Canada Basin Deep Water (CBDW) layers in the Canada Basin to elucidate the composition and source of terrestrial DOM. Significant differences in total lignin phenol concentration (Σ11) and cinnamyl to vanillyl ratio (C/V) measured with CuO oxidation in PML, AW, and CBDW samples were found. The fluorescence intensities of humic-like C1 were correlated with Σ11 in PML, congruent with the humic-like terrestrial origin. However, no significant correlation was found when all layers are considered (p > 0.05), suggesting a change in the molecular composition of C1. The protein-like C3 and C5 intensities were strongly correlated with Σ11 (rs = 0.55–0.64) corroborating that lignins can contribute to the protein-like fluorescence in the water column. The principal component analysis (PCA) confirmed the origin and composition of DOM vary between the different water masses, especially in terms of C/V, DOC, and fluorescent components. Significant relationships between humic-like fluorescence intensities and C/V reveal the significant contribution of woody sources to humic material in the Canada Basin. Elevated S/V and humic-like C6 levels were associated with a cold eddy DOM sample but further studies are required to validate the use of these DOM characteristics to trace deep eddy.

Hydrological Structure and Water Dynamics in the Powell Basin in January–February 2022

In January–February 2022, a CTD/LADCP section across the Powell Basin in the Weddell Sea with hydrochemical observations was carried out in the Antarctic expedition of the R/V “Akademik Mstislav Keldysh”. Transect was located from the Antarctic Peninsula to the South Orkney Islands; thus, it crossed the Weddell Gyre in its northwestern part. As a result, we collected new data about hydrological structure and water dynamics in this area. In this study, we present new results on the data analysis related to the structure of the Weddell Gyre, Antarctic Bottom Water formation, as well as variability of hydrophysical and hydrochemical characteristics of waters in the Powell Basin. Only two of the three currents, Antarctic Coastal Current and Antarctic Slope Front, were identified in the structure of the Weddell Gyre. Velocities of these currents were about 10–15 cm/s. The structure of waters was typical for the Weddell Sea, but a change in the temperature maximum was recorded in the layer of Warm Deep Water.

Changes in Seasonality of Saltwater Inflows Caused Exceptional Warming Trends in the Western Baltic Sea

AbstractDuring the last decades, the Baltic Sea has been among the fastest warming seas in the world. The warming is mainly driven by increasing air temperatures but deeper water layers can also be warmed by lateral advection of heat. By analyzing a 159 years long (1850–2008) hindcast simulation of the Baltic Sea, we link the exceptionally strong bottom water warming in the western Baltic Sea to a shift in the seasonality of saltwater inflows from the North Sea to the Baltic Sea. Over the model period, warm summer and early autumn inflows have increased while cold winter inflows have decreased. Sensitivity experiments reveal that these changes were partly driven by a shift in river runoff seasonality. The strong warming could lead to faster oxygen depletion in the affected layers and thus have ecological consequences.