A framework is presented for the study of hydrodynamics in semienclosed basins. The framework is anchored on a generalized momentum balance in which the drivers of momentum are tides, density gradients and winds. The drivers are balanced by friction and Earth’s rotation. The relative influence of either drivers or balancing agents may be cast in terms of their ratio, represented by nondimensional numbers. Thus, the relative influence between three different pairs of forcings is expressed as follows. Comparisons between tidal forcing and density gradients are cast in terms of a densimetric tidal Froude number. Similarly, the effect of wind forcing versus density gradients is contained in a Wedderburn number, and the impact of wind forcing compared to tidal forcing is described by a ‘Stress number’. Comparisons of balancing agents are contained in the Ekman number, representing the dynamic depth of a semienclosed basin – high Ekman numbers indicate dynamically shallow basins. The hydrodynamic interplay among drivers and balancing agents can thus be drawn on a triangular prism. Each face of the prism represents a parametric space that characterizes the relative dominance of any pair of forcing agents. The height of the prism denotes the dynamic depth. Every semienclosed basin, except any dominated by wind-wave forcing, can be represented on a face of this triangular prism or inside the prism, depending on the dominant hydrodynamics. Thus, any semienclosed basin can be described by a cloud of points inside and on the prism faces, representing the basin’s temporal and spatial variability in its hydrodynamics.  

How to cite: Valle-Levinson, A.: Triangular-prism framework for the study of hydrodynamics in semienclosed basins, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8682, https://doi.org/10.5194/egusphere-egu23-8682, 2023.

In estuarine networks, channel junctions control the division and dispersion of salt between network branches. Whereas dominant width-averaged salt transport processes in single channels are relatively well understood in an idealized sense (e.g. Hansen and Rattray (1965)), the intrinsic three-dimensional geometry and bathymetry in channel junctions, which in urbanized regions are often heavily influenced by human engineering, complicates similar idealized analyses. Expanding our knowledge of salt transport processes around channel junctions is needed to understand salt distribution in estuarine networks and develop efficient one-dimensional salt intrusion models.

As a first step in resolving salt transport processes around junctions, we construct a three dimensional subtidal idealized model for water motion and salinity in partially stratified estuaries. It provides an extremely fast and numerically accurate way of computing salinity distributions in general geometries and analyzing the dominant salt transport processes. The model extends the width-averaged approaches of Hansen & Rattray (1965) and MacCready (2004) to general 3D geometries. Following these authors, the vertical dimension is solved analytically. The solutions for the horizontal dimensions is extended to a numerical finite element method with flexible grid size. The resulting coupled system of nonlinear partial differential equations is solved iteratively. The idealized model is limited to well-mixed and partially stratified conditions and will be compared to high-complexity numerical models to test its validity.

As a proof of concept using the newly derived model, we investigate the sensitivity of dominant salt transport processes and salt intrusion with respect to channel junction geometries, such as cross-sectional shapes and angles between the branches. Systematic exploration of these sensitivities is expected to lead to improved salt dispersion coefficients and, eventually, nodal point relations between junction branches.

References

Hansen, D. V., & Rattray, M. (1965). Gravitational circulation in straits and estuaries. Journal of Marine Research, 23(2), 104–122.

MacCready, P. (2004). Toward a unified theory of tidally-averaged estuarine salinity structure. Estuaries, 27(4), 561–570. https://doi.org/10.1007/BF02907644

How to cite: Jongbloed, H., Schuttelaars, H., Dijkstra, Y., and Hoitink, T.: Influence of channel junction geometry on subtidal salt transport processes and salt intrusion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15417, https://doi.org/10.5194/egusphere-egu23-15417, 2023.

Around the world, estuaries have been partially or completely closed-off from the sea and their number may increase with rising sea levels. Concurrently, there is a trend to reintroduce seawater inflow into enclosed former estuaries for ecosystem improvement. This is also the case in the Haringvliet, a former estuary in the Rhine-Meuse Delta, closed-off in 1970 with floodgates blocking seawater inflow and regulating outflow. As the reintroduced salt water inflow may threaten fresh water intake from the basin, the dispersion of salt through the system needs to be well understood and carefully managed.

To study the circulation, mixing and salt transport in the Haringvliet, we developed a high-resolution 3D numerical model using the unstructured hydrostatic modelling software DFlow-FM. The model has a horizontal grid with typical cell side lengths of 60 m and a combination of z- and σ-layers in the vertical with a typical thickness of 0.125 m. In agreement with observational data, the model results show that the incoming salt water reaches the deeper parts of the system, induces a strong stratification and is only flushed out of the system after multiple events of large outward floodgate discharges. When the floodgates are closed during the low river discharge season and salt is still present in the system, wind becomes the dominant forcing of mixing and transport. For axial winds, the model results show a considerable horizontal circulation, with downwind currents over the shallow parts and significant upwind currents over the deep parts of the system. These upwind currents are an important mechanism for inland transport of salt after upward mixing, and increased salinity values are found at landward locations for seaward wind. Using the model to explore the mixing mechanisms, we found that the current-related shear is generally not strong enough to induce interfacial mixing directly above the deep parts. Mixing mostly occurs when salt water reaches less deep areas after tilting of the pycnocline. We will explore how to relate this competition of mixing processes to non-dimensional parameters like the Wedderburn number.

With scenario analyses, we study the dynamics for a range of wind conditions and determine which condition provides most risk for fresh water intake. We also investigate for what rates of seawater inflow and outward floodgate discharges dynamic equilibria can be reached in which the incoming salt mass equals the mass flushed out during the subsequent ebb. The insights in circulation, mixing and salt transport due to forcing by floodgate discharges and wind are relevant for other semi-enclosed former estuaries.

How to cite: Kranenburg, W., Bom, S., Tiessen, M., and van Leeuwen, B.: Circulation, mixing and salt transport in a former estuary after reintroduction of seawater inflow: a 3D modelling study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17378, https://doi.org/10.5194/egusphere-egu23-17378, 2023.

Salt intrusion occurring in estuarine environments can be aggravated by the presence of scour holes in the bed. Saltwater has a higher density than freshwater. Therefore saltwater accumulates in scour holes which may exacerbate the salinization of freshwater once it is entrained by wind or other forcings. Wind can contribute to estuarine circulation and stratification through three main mechanisms: direct wind mixing, wind straining and wind-driven lateral circulation. Previous studies (Csanady, 1982; Winant, 2004), suggest that horizontal wind circulation mostly comes into play in estuaries with laterally varying bathymetry, which is the case for our study site. The Haringvliet estuary in the Rhine-Meuse delta is a former tidal basin in the western part of the Netherlands; it varies in bathymetry and has been closed off by floodgates. The gates are only opened during ebb tide to discharge river into the sea, and also for a short period of time during flood tide for ecological purposes. The complex geomorphology of the estuary is composed of shoals and deep scour holes. An extensive field campaign was carried out for over six months in the Haringvliet, at the locations of the scour holes, in which we measured flow velocity, salinity, discharge, and wind speed and direction. Results indicate that, under an axial wind over the estuary, a horizontal circulation forms by downwind flow over shoals and upwind flow in the deep channels. Based on the collected dataset, a change from a down-estuary to an up-estuary wind direction occurred while the floodgates were closed. As a result of the wind influence, the flow direction in the stratified deep channel changed quickly, which provided sufficient shear and turbulence in the whole water column for vertical mixing. The sharp drop in the salinity concentration corresponding to the mixing and flushing in the scour hole occurred due to the wind-induced lateral circulation without having high river discharge. This research shows that, in a semi-closed estuary like the Haringvliet, lateral currents and the momentum transfer corresponded to that can exert a predominant control on estuarine circulation and stratification.

Acknowledgments: This research was funded by the Netherlands Organisation for Scientific Research (NWO), research program SALTISolutions with project number P18-32. Rijkswaterstaat, the Dutch Ministry of   Infrastructure and Water Management, is thanked for providing extensive field data for this research.

References:

Csanady, G.T., 1981. Circulation in the coastal ocean. In Advances in geophysics (Vol. 23, pp. 101-183). Elsevier.

Winant, C.D., 2004. Three-dimensional wind-driven flow in an elongated, rotating basin. Journal of Physical Oceanography, 34(2), pp.462-476.

How to cite: Ebrahimi Erami, F., Kitsikoudis, V., Vermeulen, B., and Hulscher, S.: Observational study on mixing in a stratified scour hole due to the wind-driven lateral circulation in a semi-closed estuary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5369, https://doi.org/10.5194/egusphere-egu23-5369, 2023.

Under the influence of climate change, estuaries around the world are increasingly exposed to more extreme weather conditions. In recent years droughts specifically have been occurring more frequently and for prolonged periods. During a period of drought, salt intrusion is exacerbated, impacting the availability and quality of water resources and the estuarine ecosystem. As the impacts of droughts can be severe, assessment of droughts and their influence on the estuarine system is of great importance.

Using a high-resolution 3D coupled ocean-delta model we investigate the influence of the record-breaking European drought of the summer of 2022 on the Rhine-Meuse Delta and compare this to the estuarine response under average discharge conditions, putting the drought’s influence into perspective. Spatial patterns of stratification, mixing, and straining and their evolution throughout the drought period are studied by a salinity variance analysis. The progression of the salt wedge and retreat of the tidal plume fronts are examined and related to the changing strength of the individual estuarine processes influencing stratification. We show that as the tidal plume fronts retreat during the drought, we see a corresponding change in the structure of the salt wedge, demonstrating the importance of the coupling between the tidal plume fronts and the estuarine dynamics.

How to cite: Geraeds, M., Pietrzak, J., Verlaan, M., and Katsman, C.: The influence of extreme drought conditions on spatial patterns of stratification and mixing in a dynamic salt wedge estuary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9557, https://doi.org/10.5194/egusphere-egu23-9557, 2023.

Salt intrusion is part of natural estuary dynamics, where saline water enters inland by tides and exchange flow, and is diluted by fresh river water. These processes and feedbacks are complicated and inherently stochastic; and it is not yet well understood how they determine the statistical behavior of the salt intrusion length. More importantly, there are large uncertainties regarding future changes of the forcing of the salt intrusion in a warming climate. In this presentation, we will introduce a new stochastic model that computes temporal changes of the salt balance equation with the decompositions of river discharge and salt intrusion length into deterministic and stochastic components. The developed framework is applied to field observations in the San Francisco Bay (USA) and shown to well reproduce general statistics of salt intrusion length. Next, the model is applied to estuaries in Europe under projected river discharge distributions up to 2100 using two large ensembles of the Community Earth System Model. The key assumption in the model is that the changes in the river discharge are the main driver that induces variability and changes in salt intrusion length in the coming decades. Our results show that there will be significant increase of salt intrusion during dry periods in many European estuaries, especially those at low latitudes. The analysis stresses that, for adequate water management, great attention is needed in monitoring and predicting salt intrusion lengths in the future.

How to cite: Lee, J., Biemond, B., de Swart, H., and Dijkstra, H.: Stochastic properties and statistics of salt intrusion in estuaries in a warming climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4696, https://doi.org/10.5194/egusphere-egu23-4696, 2023.

Suspended particulate matter (SPM) plays an important role in both physical and biogeochemical processes in the estuarine system, and the variation of SPM concentrations have multiple environmental and societal implications. Previous research has shown that gravitational circulation, tidal trapping, sediment resuspension and deposition, and runoff of rivers are the primary controlling factors for the formation of the estuarine turbidity maximum (ETM) in the Pearl River estuary (PRE). However, the mechanistic connection between surface riverine sediment plumes and the ETM, and the spatial and temporal variation of the ETM caused by human-induced morphological change and climate- and land subsidence-induced sea level change remain largely unknown. In our study, Landsat data from the 1970s to 2010s were analyzed to identify the variation of the surface sediment concentration. A 3-Dimensional hydrodynamics-sediment transport model (SCHISM) were used to investigate the impacts of decadal-changes (1970s-2010s) of morphology, riverine sediment discharge and sea level on the spatial and temporal variation of ETM. Sediment trapping mechanisms (e.g. topographic and tidal trapping, estuarine fronts) were investigated for their influence on the variation of the ETM to clarify the nonlinear relationships between various sediment trapping processes and the ETM in the PRE. With this study, we aim to better understand the response of the ETM to past and future environment changes caused by both climate and human activities.

How to cite: Ma, M., Zhang, W., Porz, L., and Schrum, C.: Response of the estuarine turbidity maximum to a changing hydrodynamic environment in the Pearl River estuary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9400, https://doi.org/10.5194/egusphere-egu23-9400, 2023.

Residual exchange flows in estuaries can be either laterally sheared, vertically sheared, or a mix of both, depending on the competition between baroclinic and barotropic forcings. Observations of the residual flow structure at subtropical, semi-arid and temperate estuaries indicate that the dominant forcing (horizontal density gradient or tidal stress) may vary at a fortnightly to seasonal timescale. This work aims at characterizing the variability of residual flow drivers in estuaries based on process-oriented numerical simulations with simplified geometry. The hydrodynamic model (Delft3D) is constituted by a straight estuarine channel of 80 km in length and a Gaussian-shape cross-section of 1 km in width, forced by a semi-diurnal (M2) tide. The computational grid includes 240 and 15 nodes along and across the channel, respectively (about 333 m x 67 m resolution) and 20 uniform vertical layers. A total of 20 runs of 6 months were performed to explore the effects of various channel depths, river discharges and tidal amplitudes on the spatial structure of residual flows. The results show distinct cross-channel structures for each experiment and suggest a switch of the dominant driver (between tidally-driven and density-driven) along the channel in some cases. The spatial variability of residual flows is first examined against both the horizontal density gradient and the tidal velocity amplitude to characterize the competition between barotropic and baroclinic forcings. The dynamics of residual flow is then approximated by the tidally averaged momentum equation, applied at every grid point of the model domain, to derive dimensionless parameters (such as the densimetric tidal Froude number) for prediction of the dominant residual flow drivers both temporally and spatially (i.e., along the channel).

How to cite: Garel, E., Khosravi, M., Fortunato, A., Lopez-Ruiz, A., and Valle-Levinson, A.: Variability of residual flow drivers in estuaries: numerical investigations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16158, https://doi.org/10.5194/egusphere-egu23-16158, 2023.

Estuaries on the Indian subcontinent are influenced by the Asian Summer Monsoon and show strong seasonality. As a result, these estuaries are often referred to as monsoonal estuaries (Vijith et al., 2009).  The seasonal cycle is superposed with intra-seasonal oscillations (ISO) with periods ranging from 10 to 90 days. Due to the lack of high-resolution data in Indian estuaries, even while the seasonal cycle of monsoonal estuaries is comparatively well understood, the intra-seasonal variability has not yet been examined. The active-break monsoon spells drive quasi-biweekly oscillations (10--20 days) within the ISO, and oscillations of 30–60 days are present due to northward-moving cloud bands.  In this study, we use high-resolution salinity, temperature and sea level measurements from the Cochin estuary, located on the southwestern coastal plain of India, to investigate the ISO-related variability. The spectral analysis of the sub-tidal signals of salinity, temperature and sea level shows that ISO exists year round (December 2019 to May 2021). During the dry season (December to April), the salinity was on an average of 30 PSU, and the amplitude of ISO was from 3 to 5 PSU. In the wet monsoon season, the amplitude of ISO varies around 5 to 10 PSU. The temperature, sea level, and precipitation spectrum also exhibited similar patterns in the wet season. The coherence of salinity with sea level and precipitation is also quantified. 

Reference

Vijith, V., Sundar, D., Shetye, S.R., 2009. Time-dependence of salinity in monsoonal estuaries. Estuar. Coast. Shelf Sci. 85, 601-608. . http://dx.doi.org/10.1016/j.ecss.2009.10.003

How to cite: Safin, I. P. P., Vijith, V., Sajeev, R., Anup, N., Vasudevan, S., Shehsin, N. M. I., Saji, P. K., Anoop, K. A., and Shameem, K.: Seasonal and intraseasonal oscillations in a monsoonal estuary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7586, https://doi.org/10.5194/egusphere-egu23-7586, 2023.

A 10-year time series of surface salinity values along an estuary-river transition documents their spatial structure and their temporal variability. Decomposition of detided time series into empirical modes indicates that the first mode, explaining almost 95% of the overall variance, has the typical sigmoid spatial structure found in estuaries. The temporal variability of mode 1, referred to as salinity index, displays annual and semiannual signals that are modulated from year to year. A fit of the salinity index to harmonics associated with luni-solar orbital motions and solar activity explain more than 70% of the index variance. This suggests that a) gravitational forcing from moon and sun, and b) the thermodynamic influence of solar activity, both impact salinity intrusions into rivers. The mechanistic linkage is unknown, but it is likely that it develops through atmospheric pressure, winds, air temperature, and water level. These variables are influenced by gravitational forcing and solar activity as observed elsewhere. The main finding is that the salinity variability at an estuary-river transition is influenced by gravitational forces and solar activity, i.e., saltwater intrusion could be determined by the modulation of astronomic influences. Similar variability may be found for mixing processes in coastal regions.

How to cite: Valle-Levinson, A. and Laurel-Castillo, J. A.: Astronomic influences on seasonal to interannual variability in saltwater intrusion in a subtropical estuary, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16854, https://doi.org/10.5194/egusphere-egu23-16854, 2023.

Tides in shallow water environments near estuaries and tidal inlets are subjectto nonlinear interactions, or asymmetries, that produce higher frequency harmonics known as overtides. The tidal asymmetries produced due to the generation of overtides can contribute to long term material transport in coastal regions. The primary mechanisms generating overtides are well known and arise from varying bathymetry, friction, and water depth. However, in systemsdominated by wind waves and barotropic tides other mechanisms may exist. This study utilizes the COAWST modeling system to run process-oriented tide-wavesimulations of an idealized barrier island and tidal inlet system to investigate new mechanisms of overtide generation. The simulations investigate variations in tidal amplitude from micro to meso-tidal coupled with several combinations of wave heights, wave periods, and wave directions. Analysis of overtide amplitudes extracted from 92 numerical simulations indicates that an increase in wave height amplifies overtide current velocities inthe along- and cross-shore directions. Variations in wave period and direction proved to have a lessereffect on elevating overtide amplitudes. A maximum increase of ∼140% in overtide current velocity magnitudes was observed from a tide only to a coupled tide-wave simulation. A decomposition of the depth-averagedmomentum balance of the tide only and tide-wave simulations were used to determine mechanisms responsible for elevating overtide amplitudes. Results identified bottom stress, pressure gradient, andbottom wave streaming as the dominant mechanisms enhancing overtide generation. Accelerations from wave-enhanced pressure gradientsand bottom wave streaming produced a shoreward near-bottom current and an offshore directed surface return flow thatinteracted with tidal currents and enhanced tidal asymmetries.

How to cite: Ross, L., Rickerich, S., and Valle-Levinson, A.: Wave Enhanced Overtides in an Idealized Tidal Inlet System, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17407, https://doi.org/10.5194/egusphere-egu23-17407, 2023.

How to cite: Wegman, T., Pietrzak, J., Horner-Devine, A., Dijkstra, H., Ralston, D., and Kranenburg, W.: Unique salt intrusion observations during the severe drought of 2022, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9412, https://doi.org/10.5194/egusphere-egu23-9412, 2023.

In the face of climate change and changing sea levels, it is important to understand the different drivers of sea-level variability. Tides and atmospheric forcing are the major drivers of sea-level variability, while steric contributions are often neglected in shallow coastal and shelf seas. Here, we investigate the impact of the Rhine River plume on the sea surface height in the North Sea.

Because the river plume is modulated by the tides, steric changes coincide with tidal signals. Therefore, they are hard to observe directly using tide gauge measurements or satellite altimetry. Here, we use a 3D hydrodynamic model, allowing us to quantify the steric contribution. We find steric contributions up to 15 cm in the estuary, 5 cm in the near-field plume and 1 cm in the far-field plume.

The exact height will depend on the river discharge, wind conditions and the tides, which strongly affect the river plume, and thus also the sea surface height. During the summer of 2022, the discharge of the Rhine River was historically low. We investigate the consequences for the river plume, and its impact on the sea surface height. We find that river plumes can induce significant steric changes in sea-surface height, however these are often neglected. Climate change will affect the different drivers of sea level variability in our coastal zones and shelf seas, including river discharges. Understanding their impact is therefore crucial to assess changing sea levels.

How to cite: Keyzer, L., Pietrzak, J., Snellen, M., Katsman, C., Zijl, F., Afrasteh, Y., Guarneri, H., Verlaan, M., Klees, R., and Slobbe, C.: The impact of river plumes on the sea-level variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11395, https://doi.org/10.5194/egusphere-egu23-11395, 2023.

Analysis of satellite imagery of the Carolinas continental shelf (the US East Coast) shows frequent occurrences of cross-shelf buoyant plumes under upwelling winds. Forcing conditions for fifteen representative events spanning 2017 through early 2020 are analyzed. The buoyancy forcing is represented as an estuarine Richardson number accounting for freshwater discharge and its tidal mixing. The wind forcing is represented as the low-passed alongshore wind stress component, the wind stress magnitude and its standard deviation. Forcing elements are averaged over three days preceding the event. Three cross-shelf plume patterns emerge: the separated plume, when a single streak of buoyant water spreads offshore (an archetypical cross-shelf plume structure), the curving-back plume turning against the wind at some offshore distance, and the multi-lobe plume partially trapped by the coast, with more than one streaks protruding offshore. The latter two regimes represent a low-wind and a strong-wind limit of cross-shelf plumes, respectively. 

How to cite: Yankovsky, A., Dykstra, S., and Ricche, G.: Forcing of cross-shelf plumes on a wide continental shelf, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9000, https://doi.org/10.5194/egusphere-egu23-9000, 2023.

The spatio-temporal variability of transport in coastal ecosystems is of primary importance for many biological processes. This is critical in highly heterogeneous regions, like systems of intertidal basins, where transport can exhibit a strong anisotropic response to different forcing mechanisms like winds. To understand the local and temporal variability of transport and exchange of water and freshwater content within a system and with its surroundings, Lagrangian transport time scales (LTTS), such as the residence and exposure times, are commonly employed. They are also used as a proxy to understand ecological processes (e.g., eutrophication) and as a first order estimation of the capacity of a system to expel pollutants. Additional information about the preferential pathways in multiple-inlet systems can be obtained by determining the capture areas of the inlets. Tracers deployed in a capture area have the largest probability of exiting the system through the associated inlet. In the present research, we study the spatial and temporal variations of the LTTS and the capture areas of inlets in a multiple-inlet coastal system and their relation to the dominant forcing mechanisms. The results are based on a realistic simulation of the Dutch Wadden Sea (DWS), a wind-dominated estuarine multiple-inlet system, in the period 1980-2015. We found that most of the spatio-temporal variability of the LTTS is explained by winds from the most dominant and energetic sectors (the southwesterly quadrant), which are aligned with the topographical orientation of the system. The LTTS are strongly anti-correlated with these wind directions in most of the domain, except near the inlets. Periods with easterly winds trigger a dipole-like response on the spatial structure of the LTTS with a decrease of their values in the western DWS and an increase in the eastern DWS. This is explained by easterly winds favoring the export from the western DWS towards the North Sea trough the closest inlets. On the eastern side, particles travel towards the western DWS (and hence longer distances before leaving the system), which increases the LTTS. North-northwesterly winds trigger a more complex spatial structure in the system, but in comparison to the other wind directions they only explain little variability. We found a strong influence of the wind seasonality, which is characterized by stronger wind conditions during autumn-winter than spring-summer, on the size of the capture areas of inlets. The monthly variability of these capture areas can be predicted by the wind energy, especially during the stormy season (autumn-winter). During this season, winds from the southwesterly quadrant push particles towards the eastern part of the DWS, thereby reducing the capture areas of the western inlets and triggering an expansion of the areas on the eastern ones. Other wind directions seem to play a negligible role in this variability.

How to cite: Duran-Matute, M., Fajardo-Urbina, J. M., Gräwe, U., Clercx, H. J. H., and Gerkema, T.: Spatio-temporal Variability of the Lagrangian Transport  in a System of Intertidal Basins, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6761, https://doi.org/10.5194/egusphere-egu23-6761, 2023.

The Dutch Wadden Sea (DWS) is an event-driven aquatic system between the Wadden Islands and the Dutch mainland. The dynamics of the system is mainly modulated by the collective influence of seasonally varying wind, the meso-tidal forcing and the freshwater release from two sluices. A three-dimensional hydrodynamic model (GETM- General Estuarine Transport Model) was applied to this region to study the hydrodynamics of the system for over a period of 11 years. In the present study, we have extracted the depth-averaged current from each inlet that connects the DWS with the North Sea. Furthermore, Simpson-Hunter parameter, which is the ratio between the depth and the cubic power of the current speed, was estimated at each inlet. This empirical formula is adopted to determine the mixing capacity of the DWS inlets on a seasonal and inter-annual scale. The initial results manifested that the Texel and the Vlie inlet undergo well-mixed nature in general; however, sporadic stratification events occur in the low wind condition during slack tide. On a contrary, stratification events occur quite often in the other three inlets (i.e., Eierlandse Gat, Borndiep and Friesche+Pinkegat). On a seasonal scale, the frequency of stratification events in the minimum during autumn and maximum during spring. The present results bear a resemblance to the earlier ones’ where low to intermediate magnitudes of Simpson number (quantification of the density-driven stratification) were obtained in the Texel inlet which is indicative of low stratification. Also, the density-driven stratification was mainly controlled by the wind climatology. On a nutshell, the present research emphasizes the influence of wind on the mixing capacity of the DWS. In addition, further simulations are continued considering the scale of 40 years.

How to cite: Mitra, A., Donatelli, C., Duran-Matute, M., Gräwe, U., and Gerkema, T.: Mixing characteristics of the Dutch Wadden Sea: Elucidation based on 11 years of simulation results, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9055, https://doi.org/10.5194/egusphere-egu23-9055, 2023.

Modeling the ocean from the open sea to the coasts requires an adequate choice of the vertical coordinate system.

A category of generalized vertical coordinates, the so-called z^∗ coordinates ([Stacey et al 1995; Adcroft and Campin 2004]), has been implemented into an unstructured grid, finite elements and layer integrated model which was previously z−geopotential coordinate based.

Idealized and realistic test cases are shown to assess the vertical coordinate impact on key physical processes, i.e. the internal tide generation at a continental slope, a river plume shape and intensity and the salt water intrusion.

To our knowledge this is the first work that uses the z∗ coordinates in an estuary and river plume with an unstructured grid model. Other studies have used the z∗ coordinates with unstructured grid models covering the global ocean ([Ringler et al 2013, Scholz et al 2019] as examples), but without assessing their performance for the small scale dynamics of estuaries and river plumes. Coastal ocean models are currently mostly based on the terrain following coordinates, used within optimised or hybrid approaches ([Zhang  et al 2015; Bruciaferri et al 2018, Fofonova et al 2021, Wise et al 2022] as examples).

The results show that the z^∗ coordinate model produces, with respect to z−geopotential coordinates, a stronger water column stratification. An idealized twin experiment configuration of the model to simulate the production of internal tides at the continental slope confirms that the z^∗ coordinate model reproduces the expected internal tide fields while the z−geopotential model fails. A set of realistic experiments in the Po river delta coastal region, demonstrates that the representation of the dynamical processes at the coastal scales benefits from a very fine resolution of a few tens of centimeters at the surface (four layers of 25 cm are considered in the upper water column), that is only allowed by the z^∗ formulation.

How to cite: Verri, G., Barletta, I., Pinardi, N., Federico, I., Alessandri, J., and Coppini, G.: Shelf slope, estuarine dynamics and river plumes in a generalized vertical coordinate, unstructured grid model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13015, https://doi.org/10.5194/egusphere-egu23-13015, 2023.

Despite the small area that shelf seas occupy, these regions play a significant role in the global carbon cycle. Through the so-called continental shelf pump mechanism they support more than 20% of the global marine uptake of atmospheric CO₂. However, these regions are highly dynamic and the complex interactions within them make it difficult to identify and estimate the contribution of the different processes involved and changes therein. The North Sea, a shelf sea situated on the north-western European shelf is an iconic example of such a shelf sea, making it ideal to study these processes, connections and their variability. The North Sea is connected with the tidal basin of the Wadden Sea via channels that allow not only exchange of water but also dissolved and suspended materials. Through these connections, the Wadden Sea supplies large amount of dissolved inorganic carbon (DIC) but also total alkalinity (TA) generated from processes such as organic matter reduction and as a conduit from riverine inputs. In this study, pH, DIC and TA were monitored on discrete samples allowing the highest possible accuracy and precision at the NIOZ monitoring platform at the edge of the Marsdiep channel, through which most of the water exchange occurs between Wadden Sea and the North Sea. The flow direction reverses with the tides, so both North Sea and Wadden Sea waters are dominant at different points through a tidal cycle. The platform is also equipped with salinity, temperature and pH sensors, which measure every 10 minutes. Here, we use these data to assess the impact of the Wadden Sea on the North Sea, quantifying the TA and DIC supply to the North Sea and the role of the Wadden Sea in regional CO₂ uptake. Through cross calibrating these parameters we aim to unravel the relative impacts of the different processes involved in mixing in a tidal inlet. The permanent observatory moreover allows us to investigate the drivers of variability in the TA and DIC exchange from diurnal to seasonal and even multi-annual timescales. 

How to cite: Ourradi, Y., Ossebaar, S., Reichart, G.-J., and Humphreys, M.: Impact of the Wadden Sea-North Sea interactions on the sea water carbonate system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12567, https://doi.org/10.5194/egusphere-egu23-12567, 2023.

Queen Charlotte Sound (QCS) is a broad shelf region off Canada's west coast that is highly biologically productive and hosts several Marine Protected Areas. However, ecosystems in QCS are becoming increasingly susceptible to climate change stressors such as marine heatwaves, ocean acidification, and deoxygenation.  In this system, stratification plays an important role in setting the physical and chemical environment, thus impacting how climate change affects the region, including its biogeochemical cycles and ecosystems. Here, one year of near-continuous underwater glider observations are used to investigate how variability in stratification and exchanges between the coast and open ocean influence the physical and biogeochemical properties in QCS. Specifically, we document how varying relative contributions of temperature and salinity to density stratification set up distinct stratification regimes: a salinity-dominated beta regime, a temperature-dominated alpha regime and a transitional regime, whose presence and spatial extent vary seasonally across the shelf. We then use this stratification regime characterisation to 1) map where and when these regimes manifest and consider the drivers of variability in regime spatial and temporal extent ; 2) quantify the stratification strength as a function of regime and understand regime impacts on the vertical structure of the upper ocean; and 3) demonstrate the usefulness of this regime characterisation to present the systematic differences in chlorophyll and oxygen concentrations and their vertical distribution in the alpha and beta regimes. In addition, we make a comparison of the upper ocean stratification from glider observations with a 1-D mixed layer model driven by meteorological data to test the sensitivity of each regime to atmospheric and lateral exchange processes. Lastly, we will discuss what these findings inform us about stratification in QCS in the future in the context of climate change with increased riverine inputs, melting glaciers, increased precipitation and warmer waters.

How to cite: Jhugroo, K., Waterman, S., Jackson, J., Klymak, J., Ross, T., and Hannah, C.: Distinctive stratification regimes and their biochemical implications across Queen Charlottte Sound, a highly-productive shelf in the Northeast Pacific, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9599, https://doi.org/10.5194/egusphere-egu23-9599, 2023.

The circulation within marginal seas subject to periodic winds, and their exchange with the open ocean, are explored using idealized numerical models and theory. This is motivated by the strong seasonal cycle in winds over the Nordic Seas and the exchange with the subpolar North Atlantic Ocean through the Denmark Strait and Faroe Bank Channel, although the analysis is general in nature and relevant to other marginal seas. Two distinct regimes are identified: an interior with closed 𝑓 /ℎ contours and a shallow shelf region that connects to the open ocean. The interior develops a strong oscillating along-topography circulation with weaker ageostrophic radial flows. The relative importance of the bottom Ekman layer and interior ageostrophic flows depends only on 𝜔ℎ/𝐶_d , where 𝜔 is the forcing frequency, ℎ is the bottom depth, and 𝐶_d is a linear bottom drag coefficient. The dynamics on the shelf are controlled by the frictional decay of coastal waves over an along-shelf scale 𝐿 = 𝑓₀ 𝐿_s 𝐻_s /𝐶_d , where 𝑓₀ is the Coriolis parameter, and 𝐿_s and 𝐻_s are the shelf width and depth. For 𝐿 much less than the perimeter of the basin, the surface Ekman transport is provided primarily by overturning within the marginal sea and there is little exchange with the open ocean. For 𝐿 on the order of the basin perimeter or larger, most of the Ekman transport is provided from outside the marginal sea. There is also an opposite exchange through the deep part of the strait, as required to conserve mass within the marginal sea. This demonstrates a direct connection between the dynamics of coastal waves on the shelf and the exchange of deep waters through the strait, some of which is derived from below sill depth.

How to cite: Spall, M. A.: Wind-forced seasonal exchange between marginal seas and the open ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2794, https://doi.org/10.5194/egusphere-egu23-2794, 2023.

Internal gravity waves (IGW) are ubiquitous in the open and coastal ocean. IGWs are refracted by their changing environment and may gain energy from or loose energy to the geostrophically balanced mean flow with vertical (wave drag) or horizontal shear (wave capture). Refraction, wave-wave interaction, or scattering at mean flow, topography, or turbulence can generate wave energy fluxes towards smaller wavelengths, where IGW break and mix density, which can drive in turn large scale mean flow. Two case studies of interaction of waves and mean-flow and their consequences for mixing are presented: Using a novel numerical model (IWEM) of the spectral energy balance of IGWs, we simulate the interaction of IGWs with an observed coherent meso-scale eddy in the coastal upwelling region off Mauretania. Using ray traycing we study the interaction of low mode IGWs at tidal frequencies (internal tides) with a geostrophic mean flow given by a realistic meso-scale eddy field.

How to cite: Eden, C., Sebastia Saez, P., Chouksey, M., and Dennert, P.: Internal gravity wave and tide interaction with geostrophic mean flow, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17498, https://doi.org/10.5194/egusphere-egu23-17498, 2023.

The classical Ekman (1905)'s theory of wind-driven currents at the surface boundary layer is a well-known mathematical model that describes transport phenomena in coastal processes, (e.g.) upwellings and downwellings, and represents an essential part of modern oceanography. Understanding the Ekman layer is important to quantify deviations to observed magnitudes from the classical behavior. In this theoretical work, the Ekman layer is revisited through Noether's theorem. This theorem plays a central role in theoretical physics and Lie group theory showing a direct connection between symmetries and conservation laws. Therefore, the goal of the work is to determine what quantities are conserved in the Ekman layer.

How to cite: Llorente, V. J., Padilla, E. M., and Díez-Minguito, M.: Conserved quantities in the Ekman layer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8914, https://doi.org/10.5194/egusphere-egu23-8914, 2023.

Oxygen is a vital resource in the ocean, particularly for the high oxygen demand consumers such as fish. In temperature shelf seas, the bottom water oxygen is frequently seen to decrease during the stratified period as a natural consequence of organic matter being remineralised and the seasonal thermocline preventing the replenishment of oxygen from the atmosphere. However, the subsurface chlorophyll maximum (SCM) is a generator of oxygen in the base of the thermocline. Mixing across the thermocline by episodic strong wind events could supply oxygen from the SCM into the bottom water and so offset some of the oxygen reduction arising from organic matter degradation. To explore this possibility, we set up a simple 1-D numerical model to simulate the seasonal cycle of stratification, phytoplankton, nutrients and oxygen in a temperate shelf sea. By adding strong wind mixing, the oxygen concentration in the bottom water becomes lower by the end of autumn than in the case with no wind events. This paradoxical result occurs because the wind mixing also brings organic matter from the SCM into the bottom water, which increases respiration and degradation. A warmer climate will lead to lower oxygen concentrations simply as a result of the reduction in oxygen solubility in seawater; our results also suggest that any climate-driven increases in wind mixing could further worsen bottom water oxygen conditions in temperate shelf seas.

How to cite: Meng, X., Sharples, J., and Mahaffey, C.: Increased summer storms will reduce bottom water oxygen concentrations in a temperate shelf sea., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8010, https://doi.org/10.5194/egusphere-egu23-8010, 2023.

Eddies, fronts, and filaments of varying scales populate the upper ocean and are particularly important in coastal regions. These features play a vital role on biogeochemical and mixing processes as well as in the energy budget. To capture their high spatial variability, it is desirable to simultaneously resolve the horizontal and vertical gradients of hydrographic properties on scales from O(10) m to O(100) km. We present an improved towed CTD chain for rapid quasi-synoptic in situ measurements of submesoscale oceanographic features to fill this observational gap. The advanced towed CTD chain is robust, lighter and scientifically more useful than previous versions. Added flexibility in terms of freely adaptable chain and sensor setup enables tailor-made surveys for a variety of research questions. The advanced towed CTD chain collects data at a very high horizontal resolution in O(1) m with a vertical resolution between 1 to 10 m, depending on CTD probe count and spacing. Individual CTD probes used within the chain are self-contained instruments equipped with temperature, conductivity, pressure and either fast response dissolved oxygen or fluorescence sensors placed at multiple depths enabling simultaneous hydrographic and biogeochemical studies at high resolution. With the flexible probe hardware it is possible to collect data either with real-time data visualisation for adaptive sampling missions or - in a much simpler and lighter setup - log data internally for offline evaluation. Together with the towed CTD chain a set of software tools and techniques for processing CTD chain data has been developed to provide an easy-to-use and complete system. Data examples collected in various areas like the Amazonas river plume and Cape Verde Island wake highlight the advanced CTD chains robustness, flexibility and scientific capabilities.

How to cite: Kock, T., Calil, P., Wobbe, F., Seidel, G., Riethmüller, R., Deschner, S., Heineke, M., and Baschek, B.: An advanced towed CTD chain for high resolution physical-biological in situ measurements in the upper ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6048, https://doi.org/10.5194/egusphere-egu23-6048, 2023.

The Gulf of Cadiz is located between the North Atlantic Ocean and the Mediterranean Sea. The majority of studies have focused on the presence of the Mediterranean Outflow water (so-called, MOW) in depth. Nevertheless, there are not so many studies that analyse the surface circulation in this area.

To improve the knowledge of the former circulation, an experiment was conducted with Lagrangian drifters during the autumn of 2022 off the western sector of the northern margin of the Gulf of Cadiz. A total of 8 drifters (SouthTEK®, tethered to a 0.5 m drogue) were deployed in two different areas: (1) 4-offshore drifters (GPS tracking and Iridium coverage) over the shelf slope (200 m isobath) off Cape San Vicente (24 September, 36.837ºN 8.893ºW) sending position every 2 hours; and (2) 4-coastal drifters sending GPS positions every hour, launched in pairs on the inner shelf eastward of Cape Santa Maria (12 October): two at 25 m water depth (37.080ºN 7.476ºW) and two at 100 m water depth (36.993ºN 7.423ºW). The resulting position time-series was between 8 and 18 days for the coastal drifters and ~1-3 months for the offshore ones.

The 4 offshore drifters were displaced together eastward for more than one month after their deployment. They completed an anticyclonic gyre centered around ~36.500ºN 7.500ºW, in agreement with the geostrophic field for these days. All drifters reach the Strait of Gibraltar, but not at the same time (and only two of them entered the Alboran Sea). One drifter was able to enter the Strait when it arrived at the easternmost Gulf of Cadiz (1-month after the deployment). The other drifters reached the Strait later (1.5 and 2.5-months after the deployment) and were conditioned by the Easterly wind blowing over the Strait at that time, describing different trajectories before entering the Strait that are typical of e.g., Coastal Counter Current and sub-mesoscale structures along the Moroccan coast. All the coastal drifters described a similar cyclonic gyre extending up to Cape Santa Maria (~7 to 8ºW). This structure is in agreement with the presence (previously studied by other authors) of a Coastal Counter Current. Moreover, a drop in the Sea Level Anomaly and a signature of cold temperature (Sea Surface Temperature) are observed in this area by the days of the study.

Drifter trajectories have revealed new insights into the surface circulation of the Gulf of Cadiz (i.e. meso- and sub-mesoscale processes). These trajectories have shown the connection between the surface water of the westernmost Gulf of Cadiz with the Alboran Sea. Moreover, these results could help to understand the transport patterns of floating litter or larvae (among others) in the area.

How to cite: Bolado-Penagos, M., de Oliveira Júnior, L., Vázquez, Á., Relvas, P., Garel, E., and Bruno, M.: Description of the Gulf of Cadiz surface circulation from drifters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11698, https://doi.org/10.5194/egusphere-egu23-11698, 2023.

Very little is known on mesoscale dynamics in the northern Gulf of Guinea, off West Africa. The purpose of our work is to quantify these mesoscale eddies dynamics in this region (0°N-7°N, 10°W-10°E) and their impact on the near-surface ocean and particularly in the coastal upwelling along the northern coast between 2°W and 2°E. We used a regional simulation of the NEMO model at 1/36° resolution of the year 2016 with daily outputs, validated with in situ and satellite data. On average, four cyclonic and four anticyclonic eddies were detected per day with a mean radius of 75 km and 72 km, respectively. Their lifetime is of the order of few days to a month with associated sea level anomaly from 0.5 cm to more than 1cm. The largest eddies with a relatively long life span are located between 2°N and 4°N, east of Cape Palmas (Ivory Coast) and Cape Three Points (Ghana). We then focused on the July-August-September upwelling period, during which we detected a cyclonic eddy east of the Cape of Three Points, from mid-July to mid-August 2016 with an average radius of 75 km. This cyclone is quasi-stationary and is located in the core of coastal upwelling.

Using a heat budget, we show that this eddy has an influence on sea surface temperature (SST) with a double effect. It expands offshore the upwelled cold and salty waters from July 14 to 24, then from July 25 until the dissipation of the cyclone, it weakens this upwelling by advection of warm offshore waters towards the coast, which mix with the upwelling cold waters and warm them.
A lagrangian study shows that the eddy waters come from the coastal upwelling, then mix with warmer offshore waters and later are transported eastward by the Guinea Current.
In conclusion, this study demonstrates the key role of eddies in SST intra-seasonal variability in the northern Gulf of Guinea.
Keywords : Gulf of Guinea, Modeling, Eddy, Coastal upwelling, Lagrangian simulation.

How to cite: Thiam, A. K., Alory, G., Dadou, I., Morel, Y., Napolitano, D., Cardot, C., Aguedjou, M., Morvan, G., and Jouano, J.: Mesoscale dynamics and its influence on coastal upwelling in the northern Gulf of Guinea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3661, https://doi.org/10.5194/egusphere-egu23-3661, 2023.

Benthic oxygen dynamics and the exchange of oxygen and other solutes across the sediment-water interface play a key role for the oxygen budget of many limnic and shallow marine systems. The sediment-water fluxes are largely determined by two factors: sediment biogeochemistry and the thickness of the diffusive boundary layer that is determined by near-bottom turbulence. Here, we present a fully coupled benthic-pelagic modeling system that takes these processes and their interaction into account, focusing especially on the modulation of the sediment-water fluxes by the effects of near-bottom turbulence and stratification. We discuss the special numerical methods required to guarantee positivity and mass conservation across the sediment-water interface in the presence of rapid element transformation, and apply this modeling system to a number of idealized scenarios. Our process-oriented
simulations show that near-bottom turbulence provides a crucial control on the sediment-water fluxes, the oxygen penetration depth, and the re-oxidation of reduced compounds diffusing upward from the deeper benthic layers especially on time scales of a few days, characterizing oceanic tides,internal seiching motions in lakes, and mesoscale atmospheric variability. Our results also show that the response of benthic-pelagic fluxes to rapidchanges in the forcing conditions (e.g., storm events) can only be understood with a fully coupled modeling approach.

How to cite: Umlauf, L., Klingbeil, K., Radtke, H., Schwefel, R., Bruggeman, J., and Holtermann, P.: Hydrodynamic control of sediment-water fluxes: Consistent parameterization and impact in coupled benthic-pelagic models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11151, https://doi.org/10.5194/egusphere-egu23-11151, 2023.

After detonations on the 26th September, methane gas leaked at four locations from the damaged Nord Stream pipelines into the Bornholm Basin of the Baltic Sea and into the atmosphere. A permanent Ocean Glider Observatory is located about 20 km east of the leaks, providing continuous data from prior, during and after the incident. We responded quickly by deploying an additional glider with a methane sensor close to the northern leak sites. The glider sampled the methane concentrations for the following three months. During the first week of deployment, we observe an saturation of the methane sensor in the downstream direction of the leaks, indicating methane levels above 1 μM. Outside of the projected downstream advection zone, the methane concentrations are about 100 times lower initially (~10 nM), but quickly increasing as the methane polluted area grows by advection and mixing. A week later the methane is spread along our observation trajectory. About four weeks after the onset of the leaks, the methane levels dropped significantly below 1 μM and after 6 weeks, they stabilize at the initially observed background levels in most places, but occasional patches of increased concentrations, especially in direct vicinity of the leaks remain. Using an advection-dilution scheme in combination with a high resolution Baltic Sea analysis model, we predict the temporal and spatial evolution of methane levels in different basins of the Baltic Sea. Comparing the modelled methane levels with our observations, we estimate rates of atmospheric outgassing and bacterial breakdown.

How to cite: Mohrmann, M., Queste, B., and Biddle, L.: Continuous high-resolution glider observations of methane following the Nord Stream leaks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14649, https://doi.org/10.5194/egusphere-egu23-14649, 2023.