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OS2.1

EDI
Open session on coastal and shelf seas

Contributions are invited on innovative observational, theoretical and modelling studies concerning physical processes in coastal and shelf seas. Processes can include hydrodynamics (e.g., waves, tides, river plumes, currents and Stokes drift, upwelling, eddies, density structures), transport of material (e.g., sediments, contaminants, litter, nutrients), and morphodynamics and sea-bed structure (e.g., evolution of bed forms, banks, Holocene-Antropogene strata or basin shape). Study areas are envisaged between the base of the shelf break and the seaward limit of the surf zone, including tidal basins. However, contributions on processes outside these geographical limits will be considered where they significantly influence processes within these limits. Equally, contributions on climate dynamics, biogeochemistry, and man-made structures will be considered where they significantly influence, or are significantly influenced by, the processes aimed at in this session. Special attention will be given to interactions between physics, biology and biogeochemistry and to global to local scaling of processes, their relative importance, and the representation of these transitions in models.

Convener: Alexander OsadchievECSECS | Co-conveners: Jonathan Tinker, Jian su
Presentations
| Fri, 27 May, 10:20–11:50 (CEST), 13:20–16:08 (CEST)
 
Room 1.85/86

Fri, 27 May, 10:20–11:50

Chairpersons: Alexander Osadchiev, Sheveenah Taukoor, David Ek

10:20–10:25
Introduction

10:25–10:26
Upwelling events

10:26–10:33
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EGU22-5053
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On-site presentation
Gordana Beg Paklar et al.

Extensive, but short oceanographic measurements were conducted in the spring seasons from 2017 to 2021 with an aim to investigate the middle Adriatic upwelling. The exception was cruise in 2020, when measurements were performed in August. As cruises have been conducted under different meteorological and hydrological conditions, high resolution CTD and shipborne ADCP measurements revealed strong variability in the upwelling strength and occurrence. The strongest upwelling, both in the coastal and open sea area, was recorded in May 2017 and it was related to strong NNW winds that had been blowing for several days before and during the 2017 cruise. Field measurements in June 2018, although conducted under upwelling-favourable winds from NW direction, revealed a rather flat pycnocline due to the low wind intensity, except in the first 5 km close to the coast where the pycnocline was rising onshore. Coastal upwelling was also recorded during the following cruises in June 2019, August 2020 and May 2021, whereas rising of thermocline through Ekman pumping in the open sea was detected only in May 2021. To overcome limitations of the measurements and to shed more light on the upwelling dynamics and its occurrence, realistic and idealised ROMS model simulations are conducted. Realistic simulations are performed by Adriatic scale ROMS model forced by surface momentum, heat and water fluxes calculated using results of operational numerical weather prediction model ALADIN-HR. In addition to atmospheric forcings, river discharges, tides and water mass exchange through the Strait of Otranto are also implemented in the realistic simulations. Reliable results of the realistic baseline experiments, assessed with available in situ and satellite data, allowed us to define sensitivity studies. Sensitivity experiments focus on the influence of both local and remote processes, particularly on the Adriatic river discharges and their parameterization in the ROMS simulations, as well as on the external dynamics and its parameterization at the model open boundary. Strength and character of the middle Adriatic upwelling simulated in the experiments with two available Adriatic river climatologies show no significant distinctions. Moreover, sensitivity of the model results to horizontal grid spacing of atmospheric forcing and oceanographic model grid is investigated. To further elucidate upwelling mechanism, additional idealised simulations with homogeneous upwelling-favourable wind for the eastern Adriatic coast from NW direction are set up and run.

How to cite: Beg Paklar, G., Medugorac, I., Mihanovic, H., Orlic, M., Pasaric, Z., and Stanesic, A.: Numerical analysis of the middle Adriatic upwelling , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5053, https://doi.org/10.5194/egusphere-egu22-5053, 2022.

10:33–10:40
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EGU22-4503
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ECS
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On-site presentation
Sheveenah Taukoor et al.

Port Alfred upwelling, located on the southeast African shelf, lies on the inshore edge of a western boundary current, the Agulhas Current. It is a semi-permanent upwelling covering a width of 15-100 km. The mechanisms responsible for these cold water events have not been studied in depth due to the poor sampling of Port Alfred. Previous studies have relied on in situ measurements and remote sensing datasets and suggested that this upwelling was current-driven while other studies claimed that it could be wind-driven. Recently, it was also argued that it could be due to a combination of the current and wind forcing.

 

In this study we aimed at identifying the dominant forcing drivers triggering this upwelling. Using the daily simulations of a CROCO model of a horizontal spatial resolution of ~2.5km, we explored its spatial and temporal characteristics. Building up from previous studies, we investigated several potential hypotheses from a modelling perspective. Some of these forcing mechanisms included the strength of the current, wind, the meandering of the current and the eddy. By performing a combined principal component analysis, we tested each hypothesis and found that a combination of current strength, meander and wind would lead to a stronger upwelling event, while some of the forcing mechanisms could trigger a weaker cold event individually.

 

This phenomenon is known to cause an increase of nutrients at the surface, setting the ideal conditions for primary production, a significant oceanographic process for biodiversity. Shedding more light on these cold water events and the main drivers allows oceanographers to focus more attention on Port Alfred in the future. It will also reinforce policymakers to consider Port Alfred as a marine protected zone.

How to cite: Taukoor, S., Penven, P., Ansorge, I., Mashifane, T., and Lamont, T.: Port Alfred upwelling: A numerical modelling approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4503, https://doi.org/10.5194/egusphere-egu22-4503, 2022.

10:40–10:47
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EGU22-13246
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Virtual presentation
Sadegh Yari and Volker Mohrholz

The Peruvian upwelling system (PUS) is the most productive marine ecosystem among the Eastern Boundary upwelling Systems (EBUS). The trade wind system of the South Pacific drives a nearly continuous upwelling which is subjected to variations on a wide range of times scales. The intensity and variability of upwelling control crucially the nutrient supply to the euphotic surface layer and thus, the overall productivity of the system.

Using long-term wind data (1950-2019) and SST data from ERA5, the upwelling components were analyzed to obtain information about decadal trends in the mean state and their variability. The HYCOM model data (GOFS 3.1 expt_53.X) for the period of 1994-2015 is also analyzed to estimate the Ekman transport in the coastal area and Ekman pumping due to wind stress curl.
Besides the strong annual cycle, the wind forcing is dominated by interannual and interdecadal oscillation. The interannual fluctuations with a period of 2-5 years are related to the known events of El Niño and La Niña. The wind anomaly shows a good correlation with Oceanic Niño Index (ONI). Interdecadal variation of wind depicts a period of 15-20 years with negative anomaly for 1950-1976, slightly positive for 1977-1982, negative for  1983-1996, and strong positive anomaly for 1996-2014. These long-term variations can be attributed to the Interdecadal Pacific Oscillations (IPO). The spatial distribution of wind stress along the Peruvian coast depicts a local maximum in the Lima-Marcona area (12º-15.4º S) decreasing sharply southward and gradually northward. The wind stress anomaly and SST anomaly are highly correlated in the coastal area. The alongshore wind stress reveals a positive trend in either negative or positive phases of IPO. Moreover, there are two sharp shifts during 1996 and 2015. These can be related somehow to climate warming and climate shifts which are already addressed by various authors. 

Additionally, the characteristics of the circulation in the coastal belt off Peru are derived from HYCOM outputs. Ekman transport and Ekman pumping from both wind data and model output are in good agreement. The water masses distribution shows annual as well as inter-annual variations mainly due to El Niño and La Niña events that apply to the mixed layer depth too.

How to cite: Yari, S. and Mohrholz, V.: Does impact climate change the Peruvian upwelling system?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13246, https://doi.org/10.5194/egusphere-egu22-13246, 2022.

10:47–10:54
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EGU22-8219
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ECS
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Virtual presentation
Luciano Júnior et al.

The norther margin of the Gulf of Cadiz (NMGC) at the southern limit of the Portuguese branch of the Canary upwelling system is often reported to be affected by local upwelling. However, the oceanic wind field over this region has not been fully documented yet in order to corroborate this effect. This study aims to describe the wind forcing to characterize the wind-driven upwelling over the NMGC and its spatio-temporal variability. The Upwelling Index (UI) and Ekman pumping (Ekp) are estimated using ERA5 reanalysis surface winds with 0.25° resolution. On average, the wind over the shelf is strongest at west and mainly orientated south-eastward. It weakens progressively towards the east and rotates counter-clockwise to eastward. Off the shelf, the wind is mainly south-eastward with a slightly less pronounced counter-clockwise rotation and is stronger than on the shelf.  This pattern result in a weak positive (upwelling favourable) mean UI along the coast with minor alongshore variability. By contrast, the mean Ekp is null at east but significant and positive (upwelling favourable) at west, due to sheltering effects induced by the presence of a cape (São Vicente). The highest positive Ekp values are observed near Cape São Vicente during spring summer. The largest range, due to variability between low positive and negative values are observed during autumn and winter. The seasonal mean maps suggest that enhanced upwelling due to Ekp develops in summer near Cape São Vicente, only. This pattern explains the recurrent signal of cold water and high chlorophyll concentrations often observed in the vicinity of the cape during this period. It is also suspected to be important for the development of a mesoscale cyclonic eddy observed when upwelling favourable winds relaxes. In this case, Ekp would promote the rising of the isopycnals slightly off the coast and the adjustment of the pressure field would promote such circulation pattern. UI patterns are consistent along the coast, being persistently positive and moderate during spring and summer and with largest range of variation in autumn and winter too. Thus, the seasonal mean is positive and stronger in spring and summer than in autumn and winter, even though some strong events may cause water to upwell in winter. These winter events do not have clear signal in the temperature variability but may be important in terms of nutrients supply. Overall, this study indicates the predominant seasons and locations for coastal upwelling along the NMGC and evaluates the distinct contributions of UI and Ekp. It confirms the effect of the wind in driving local upwelling as previously described for summer events but also indicate that upwelling in autumn and winter is a recurrent feature.

How to cite: Júnior, L., Garel, E., and Relvas, P.: Coastal upwelling variability along the Northern Margin of the Gulf of Cadiz., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8219, https://doi.org/10.5194/egusphere-egu22-8219, 2022.

10:54–10:55
Tides and sea level variability

10:55–11:02
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EGU22-2655
The influence of tides on the winter residual circulation of the North West European Shelf
(withdrawn)
Jonathan Tinker et al.
11:02–11:09
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EGU22-7294
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ECS
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On-site presentation
Håvard Espenes et al.

The Norwegian coastline is made up of a multitude of long and narrow fjords and straits. The near-shore flow here is severely restricted by the complex geometry, with subsequent consequences for the net transport and dispersion of biogeochemical material through the coastal zone.

One possibly important transport mechanism in such complex environments is non-linear pumping by vortex dipoles, generated by tidal currents through fjord and strait openings. Previous laboratory studies have demonstrated the transport capacity of such structures. And recent modelling studies have shown that the laboratory results should be relevant for coastal scale applications, also along the Norwegian coast. But there are still few observational studies of their existence and transport efficiency.

Recent mooring observations from Tromsø, Northern Norway, shows intermittent velocity maxima downstream of a tidally-dominated constriction. We reproduced similar signals using a high-resolution setup of the unstructured-grid ocean model FVCOM. The model shows that the velocity extrema occur when dipole vortices shed away from the constriction and propagate downstream to the mooring locations. We hypothesize that such dipoles develop when the constriction geometry allows for flow separation on both sides of the constriction, and that the dipoles leave the constriction when the dipole-generated sea surface depression is strong enough to break the adverse pressure gradient conditions required to achieve flow separation. The frequency of dipole formation - and velocity extrema - then depends on the forcing strength and constriction geometry.

How to cite: Espenes, H., Nøst, O. A., and Isachsen, P. E.: Observations and unstructured-grid simulations of tidally-generated eddies in a complex coastal environment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7294, https://doi.org/10.5194/egusphere-egu22-7294, 2022.

11:09–11:16
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EGU22-1562
Identification of extreme sea-level events in climate model data for the Swedish West Coast (Kattegatt)
(withdrawn)
Mehdi Pasha Karami et al.
11:16–11:23
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EGU22-324
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ECS
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On-site presentation
David Ek et al.

Global mean sea level is rising, however not uniformly. Regional deviations of sea surface height (SSH) are common due to local drivers, including surface winds, ocean density stratifications, vertical land- & crustal movements and more. The contribution of each background driver needs to be better understood to create reliable sea level rise projections, enable effective local policymaking and aid in urban planning decisions.

In this study, we assess region-specific historic sea levels along the western Swedish coastline (Kattegat, Skagerrak & South Baltic Sea).

We use monthly satellite altimetry observations spanning 26 years and daily observations spanning 6 years, as well as in situ tide gauge measurements to identify SSH covariance between sub-regions. We employed a number of manual statistical methods and found that SSH variability in the Skagerrak and Kattegat Seas behaves differently than areas south of the Danish Straits. While typically the correlation between SSH time series from different locations declines with distance, this is not seen at the entrance to the Baltic Sea due to the complexity of the region. To investigate this further and identify underlying primary forcings, we introduced re-analyzed ERA5 estimates of climatic drivers such as 10m-winds, sea surface temperature and sea level pressure, and tested these against principle components of the SSH variability signal within these regions. Zonal winds are most important for determining short-term sea level variability in areas north of the Danish Straits, while neither of these drivers successfully explain observed sea level variability south of them. As freshwater discharge from rivers and tributaries to the Baltic Sea is large, pressure- & density gradients may be more important as SSH regulators in this area.

Additionally, we used neural networks to try to capture non-linear dependencies between the sea level drivers and sea level that are not apparent from statistical analyses. By predicting sea level at selected locations from different combination of drivers, we can determine which drivers have the highest influence. While feedforward neural networks did successfully predict some variability, they prove rather limited as delays between signals are present. Future tests using recurrent neural networks with a long short-term memory architecture might prove more successful.

How to cite: Ek, D., Poropat, L., and Heuzé, C.: Different drivers of Sea Level Variability at the North – Baltic Sea transition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-324, https://doi.org/10.5194/egusphere-egu22-324, 2022.

11:23–11:30
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EGU22-1761
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ECS
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On-site presentation
Ezra Eisbrenner et al.

Understanding the impact of oceanic climate variability on shelf seas requires knowledge of their water-mass origins and associated pathways. Furthermore, identifying the dominant driving mechanisms responsible for water-mass variability is an important step towards better predictability of shelf sea properties. To determine the link between the large-scale circulation of the subpolar North Atlantic and the North Sea, we adopt a Lagrangian approach based on satellite altimetry. Utilizing the derived velocities, we are able to isolate the changes of water-mass pathways in response to decadal large-scale oceanic variability. In particular, during phases of a strong subpolar gyre we find that water-masses which are transported into the North Sea follow a comparatively direct path along the subpolar gyre, as well as being faster.  Subsequently, northern North Sea water-masses originate from further west in the southern and central subpolar North Atlantic. Supportive analysis on high resolution reanalysis data suggests that not only strong gyre regimes but also the transient phase into them are connected to pronounced along-shelf advection. In contrast, phases of a weak subpolar gyre lead to increased water-mass residence times in the north-eastern North Atlantic, prior to entering the North Sea. We conclude that the subpolar gyre strength is a key predictor of inter-annual variability of North Sea water-mass pathways, origin, and properties.

How to cite: Eisbrenner, E., Chafik, L., and Döös, K.: Connecting the North Atlantic subpolar gyre and the northern European shelf seas: a satellite-based Lagrangian perspective, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1761, https://doi.org/10.5194/egusphere-egu22-1761, 2022.

11:30–11:37
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EGU22-2452
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ECS
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Virtual presentation
Ange Ishimwe et al.

Numerical models of marine hydrodynamics have to deal with processes exhibiting a wide range of timescales. These processes include fast external gravity waves and slower internal fully three-dimensional motions. In order to be both time-efficient and numerically stable, the temporal scheme has to be chosen carefully to cope with the characteristic time scale of each phenomenon. An usual approach is to split the fast and slow dynamics into separate modes. The fast waves are modeled with a two-dimensional system through depth averaging while the other motions, where characteristic times are much longer, are solved in a three-dimensional. However, if the splitting is inexact, for instance in projecting the fields in a new 3D mesh, this procedure can lead to improper results in regards to the physical properties such as mass conservation and tracer consistency. In this work, a new split-explicit Runge-Kutta scheme is adapted and developed for the Discontinuous-Galerkin Finite Element method in order to obtain a new second-order time stepping, yielding more accurate results. This method combines a three-stage low-storage Runge-Kutta for the slow processes and a low-storage one of two-stage for the fast ones. The 3D iterations are not affecting the surface elevation, hence an Arbitrary Lagrangian Eulerian implementation is straightforward. Water volume and tracers are conserved. A set of test cases for baroclinic flows as well as a laboratory application demonstrate the performance of the scheme. They suggest that the new scheme has little numerical diffusion.

How to cite: Ishimwe, A., Lambrechts, J., Legat, V., and Deleersnijder, E.: A Split-Explicit Runge-Kutta methods for 3D hydrodynamic equations for coastal applications, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2452, https://doi.org/10.5194/egusphere-egu22-2452, 2022.

11:37–11:50
break

Fri, 27 May, 13:20–14:50

Chairpersons: Alexander Osadchiev, Jian su

13:20–13:21
Estuarine processes

13:21–13:28
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EGU22-2890
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ECS
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On-site presentation
Erika Henell et al.

In this talk we present new insights into the overturning circulation of the Baltic Sea. Based on a state-of-the-art 3D numerical model for the entire Baltic Sea, we analyse the estuarine circulation and water mass transformation in salinity space. State and diagnostic variables are binned to salinity classes and conservatively averaged already during model runtime such that exact budgets for local isohaline volumes can be evaluated. Derived maps in salinity space for entrainment velocities, diahaline diffusive fluxes as well as physical and spurious numerical mixing contributions will be shown. The latter is based on a unique separation and quantification of mixing due to turbulence parameterisations and discretisation errors from the applied numerical advection schemes. Finally it is demonstrated that integration of the different new local diagnostics confirms existing bulk theories (e.g. Knudsen theorem, universal law of estuarine mixing).

How to cite: Henell, E., Burchard, H., Gräwe, U., Gröger, M., and Klingbeil, K.: Analyzing Diahaline Exchange and Mixing in the Baltic Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2890, https://doi.org/10.5194/egusphere-egu22-2890, 2022.

13:28–13:35
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EGU22-6491
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ECS
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On-site presentation
Marlein Geraeds et al.

Strongly forced salt wedge estuaries are known to demonstrate significant variability in stratification, currents, and mixing within a tidal cycle. Such estuaries also show strongly spatially varying patterns of stratification and mixing, and the processes causing this spatial and temporal variability are known to interact. As a consequence, the dynamics of these estuaries are distinctly different from well-mixed and partially mixed estuaries, and intratidal processes may play a more central role in the evolution of stratification.

The Rhine-Meuse estuary is an example of a strongly forced salt wedge estuary. It is very dynamic, mesotidal, and stably stratified. In this study, we investigate how individual intratidal estuarine processes contribute to the evolution of stratification in the Rhine-Meuse estuary. Data from recent shipboard measurements are used to assess their relative influence and highlight the potential importance of interactions between these processes.

From measurements in the Rhine-Meuse estuary we find that the availability of salt is determined by tidal advection of the salt wedge. Additionally, exchange flows transfer salt from high-density to low-density regions such as harbour basins and side branches while the salt wedge is advected through the estuary. The combination of the barotropic tidal asymmetry imposed at the river mouth and turbulence damping at the pycnocline results in strong shear and subsequent formation of mid-depth jets at the onset of flood. These mid-depth jets contribute to the transfer of salt by transporting salt from regions of higher momentum to regions of lower momentum. Furthermore, the measurements suggest that several bathymetric transitions locally generate internal wave activity, although the resulting turbulent mixing is not strong enough to erode the persistent salt wedge structure.

These findings underline the importance of interactions between intratidal processes on different spatial scales and their effect on the evolution of stratification in the Rhine-Meuse estuary. As an extension to our findings, measurements in the Rhine region of influence (ROFI) are used to further examine the role of the seaside forcing on the individual physical processes and the resulting intratidal variability of stratification in the estuary.

How to cite: Geraeds, M., Pietrzak, J., Verlaan, M., Katsman, C., and Hulsen, L.: The importance of interactions between intratidal processes for the evolution of stratification in a dynamic salt wedge estuary, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6491, https://doi.org/10.5194/egusphere-egu22-6491, 2022.

13:35–13:42
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EGU22-4680
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On-site presentation
Manuel Diez-Minguito and Hans Burchard

The Total Exchange Flow (TEF) analysis framework computes the exchange flow in estuaries using isohaline coordinates. The TEF represents a paradigm for exchange flow estimates that is consistent with (steady-state) Knudsen-bulk values, and naturally allows quantifying mixing, which in turn controls the inflow and outflow fluxes of water and salinity.

This study provides preliminary estimates of TEF along the Guadalquivir River Estuary (Spain) at seven notable cross-sections during low river flows. The analysis combines observations recorded between 2008 and 2011 by a real time monitoring network and analytical model output for a well-mixed M2+M4 tidal flow with oscillating salinity. Exchange profiles and volume and salinity transports sorted by salinity classes are computed.

The results indicate that bulk along-channel TEF estimates decrease upstream. Incoming and outgoing water volume transports are about 10% larger than previous estimates based on gravitational circulation only. The largest net incoming water volume transport, viz. approx. 300 m3s-1, is attained at the lower part of the estuary, near where the largest salinity gradient is observed. This value is about 12-fold the normal river flow from the head dam at Alcalá del Río. Its corresponding representative TEF bulk salinity value is 20 psu, whereas the representative value for outflows at the same location is about 16 psu. In the middle part of the estuary, incoming TEF bulk volume values below 150 m3s-1 are obtained. As expected, negligible values are obtained in the upper part of the estuary near the head dam. Mixing completeness is larger than 75% at all cross-sections, thereby evidencing the poorly-stratified character of the Guadalquivir estuary.

Regarding the effects of tidal asymmetry, the inclusion of the M4 tidal constituent in the analysis does not improve significantly the TEF estimates (less than 1% at all cross-sections), although it might be the case in other estuaries or coastal seas. This ensues that the covariance between salinity and current seems to play a more important role in exchange flow in the Guadalquivir estuary. 

How to cite: Diez-Minguito, M. and Burchard, H.: Total Exchange Flow in the Guadalquivir River Estuary, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4680, https://doi.org/10.5194/egusphere-egu22-4680, 2022.

13:42–13:49
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EGU22-3440
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ECS
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On-site presentation
Xiangyu Li et al.

The relationship between the diahaline mixing, the diffusive salt transport, and the diahaline exchange flow is examined using salinity coordinates. The diahaline inflow and outflow volume transports are defined in this study as the integral of positive and negative values of the diahaline velocity. A numerical model of the Pearl River Estuary (PRE) shows that this diahaline exchange flow is analogous to the classical concept of estuarine exchange flow with inflow in the bottom layers and outflow at the surface. The inflow and outflow magnitudes increase with salinity, while the net transport equals the freshwater discharge Qr after sufficiently long temporal averaging. In summer, intensified diahaline mixing mainly occurs in the surface layers and around the islands. The patchy distribution of intensified diahaline velocity suggests that the water exchange through an isohaline surface can be highly variable in space. In winter, the zones of intensification of diahaline mixing occur mainly in deep channels. Apart from the impact of freshwater transport from rivers, the transient isohaline mixing is also controlled by an unsteadiness term due to estuarine storage of salt and water volume. In the PRE, the diahaline mixing and exchange flow show substantial spring-neap variation, while the universal law of estuarine mixing m=2SQr (with m being the sum of physical and numerical mixing per salinity class S) holds over longer averaging period (spring-neap cycle). The correlation between the patterns of surface mixing, the vorticity, and the salinity gradients indicates a substantial influence of islands on estuarine mixing in the PRE. 

How to cite: Li, X., Lorenz, M., Klingbeil, K., Chrysagi, E., Gräwe, U., Wu, J., and Burchard, H.: Diahaline Mixing and Exchange Flow in A Large Multi-outlet Estuary with Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3440, https://doi.org/10.5194/egusphere-egu22-3440, 2022.

13:49–13:56
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EGU22-12000
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ECS
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Virtual presentation
Cinthia Arellano et al.

Understanding the circulation in the narrow (5-10 km) and shallow (15 m depth) Paracas bay in the Peruvian Upwelling System and the mechanisms that set up stratification are key steps for the forecast of extreme events affecting the highly exploited coastal ecosystem of the bay. Using a CROCO offline dynamical downscaling from 10 km to 500 m resolution in the bay, sensitivity experiments were carried out to investigate the impact of local forcing (diurnal wind variability, solar heat flux, river discharge, tides) and of parameterizations of the solar radiation absorption. According to observations, the bay stratification is strongest in summer, with a temperature vertical difference of ∼ 4-5 °C. Results show that stratification increases slightly (<0.5 °C) when diurnal wind variability was introduced. Parameterization of shortwave absorption using chlorophyll-dependent water type affects mainly the bottom temperature in the bay. Furthermore, the freshwater flow associated with the Pisco River discharge only affects the salinity (and stratification) in the bay during periods of weak winds, when low salinity water is transported southward and then mixed in the bay. It is also shown that the shape of the wind forcing, which is difficult to obtain in small bay surrounded by orographical obstacles,  has a major impact on the stratification.

How to cite: Arellano, C., Echevin, V., Colas, F., Chamorro, A., and Merma, L.: Modelling the stratification and circulation in a shallow embayment off Peru: the case of Paracas bay, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12000, https://doi.org/10.5194/egusphere-egu22-12000, 2022.

13:56–13:57
River plumes

13:57–14:04
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EGU22-719
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ECS
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Highlight
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Virtual presentation
Alexander Osadchiev et al.

Wind forcing is the main driver of river plume dynamics. Direction and magnitude of wind determine position, shape, and size of a river plume. The response of river plumes on wind forcing was simulated in many numerical modeling studies; however, in situ measurements of this process are still very scarce. In this study, we report the first direct measurements of frontal movement of a small river plume under variable wind forcing conditions. Using quadcopters, we performed nearly continuous daytime aerial observations of the Bzyb river plume located in the non-tidal Black Sea. The aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing. We assessed spreading patterns of the plume and evaluated movement velocity of its outer border with unprecedentedly high spatial (~ 10 m) and temporal (~ 1 minute) resolution, which was not available in previous studies based on in situ measurements and satellite observations. Based on the collected data, we evaluated the time of response of plume spreading dynamics on changes in wind forcing conditions. The advection velocity of the outer plume border shows linear relation to wind speed with very small response time (10-20 minutes). The reversal between upstream/downstream plume spreading occurs during several hours under moderate wind forcing conditions. These reversals involve only near-field part of the plume, which cause detachment of the far-field part of the plume. The obtained results are crucial for understanding and simulating spreading dynamics of small river plumes worldwide.

How to cite: Osadchiev, A., Sedakov, R., and Barymova, A.: Response of a small river plume on wind forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-719, https://doi.org/10.5194/egusphere-egu22-719, 2022.

14:04–14:11
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EGU22-7299
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ECS
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Virtual presentation
Vladimir Rogozhin and Alexander Osadchiev

The Barents Sea is a shelf marginal sea of the Arctic Ocean. The river runoff in the Barents Sea is small (260 km3 per year) and does not have a significant effect on hydrophysical processes with the exception of the southeastern part of the sea, where the large Pechora River flows with an annual runoff of 130–160 km3 (Gordeev et al., 1996) . Most of the annual Pechora runoff flows into this shallow water area, also called the Pechora Sea, during the summer flood in June-July. Because of this, the hydrological regime of the Pechora Sea in the warm season seems to largely depend on the processes of propagation and mixing of the Pechora plume.

In this work, the study of the structure and variability of the Pechora River plume in 2017-2021 was carried out based on the data of detailed field measurements, wind reanalysis, as well as satellite images for the first time. Seasonal increases in the Pechora plume area (up to 35,500 km²) were recorded in the Pechora Sea in 2018 and 2020. At the same time, the salinity in the plume increased from 9-10 psu near the Pechora Bay to 18-20 psu at the outer boundary of the plume, the plume thickness was 12-15 m. The Pechora plume flowed into the Kara Sea through the Kara Gates Strait it was recorded for the first time. Analysis of the literature data on salinity in the studied region showed that in similar periods in 2017 and 2019, the Pechora plume was noticeably less pronounced, i.e., it had a much smaller area. Apparently, such variability is caused by the influence of external influences (wind, river runoff, tides), which requires further study.

How to cite: Rogozhin, V. and Osadchiev, A.: Spreading of the Pechora river plume in the southeastern part of the Barents Sea in 2017-2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7299, https://doi.org/10.5194/egusphere-egu22-7299, 2022.

14:11–14:18
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EGU22-5768
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ECS
Lennart Keyzer et al.

The position and properties of a river plume strongly depend on the wind conditions. Upwelling-favourable winds force the plume offshore, while the freshwater layer thins. Downwelling-favourable winds confine the plume against the coast, resulting in a thicker freshwater layer. The Rhine Region Of Freshwater Influence (ROFI) along the Dutch coast is not only modulated by winds, but also influenced by tides and bottom friction. The near-field region is dominated by tidal plume fronts, which are formed by the release of freshwater. The mid- to far-field region of the plume switches between stratified conditions on neap tide and well-mixed conditions on spring tide. Furthermore, tidal straining induces a semidiurnal cycle in the stratification. In this study, we investigate how winds influence this shallow frictional river plume. We use an unstructured high-resolution 3D model to perform idealized simulations of a spring-neap cycle, forced with constant upwelling- or downwelling-favourable winds. It is shown that the state of the ROFI depends on the combined effect of tides and winds. In the absence of winds, a multi-frontal system forms on neap tide, which is strongly stratified. On spring tide, however, stronger tidal currents in combination with stronger salinity gradients result in separate, buoyant fronts that propagate northwards. The background plume is well-mixed vertically, due to strong tidal mixing. Under downwelling winds, flood currents are aligned with the wind direction, resulting in faster propagating fronts. As a result, separate freshwater lenses arise on neap tide. In contrast, upwelling winds hinder the alongshore propagation of tidal plume fronts, which results in a multi-frontal system on spring tide. Furthermore, we show that winds change the semidiurnal cycle in the stratification, induced by tidal straining. Under downwelling winds, this cycle is enhanced due to stronger cross-shore salinity gradients. Under upwelling winds, there is less variability in the stratification within the tidal cycle. Understanding the dominant physical processes of this complex system is important for transport of, amongst others, sediments, nutrients and pollutants in coastal systems.

How to cite: Keyzer, L., Pietrzak, J., Snellen, M., Katsman, C., Zijl, F., Afrasteh, Y., Guarneri, H., Verlaan, M., Klees, R., and Slobbe, C.: Variations in the wind-driven response of the Rhine ROFI during a spring-neap cycle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5768, https://doi.org/10.5194/egusphere-egu22-5768, 2022.

14:18–14:25
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EGU22-6649
Recirculation and trapping of tidal plume fronts and internal wave generation in the Rhine ROFI
(withdrawn)
Julie D. Pietrzak et al.
14:25–14:32
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EGU22-5275
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ECS
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On-site presentation
Source-to-front mixing in a tidally pulsed river plume: mechanisms and evolution
(withdrawn)
Preston Spicer et al.
14:32–14:39
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EGU22-4878
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Virtual presentation
Olivier Marchal and Alan Condron

The fate of buoyant water discharged into the coastal ocean from river or meltwater runoff is a problem of preeminent interest in coastal management, physical oceanography, and climate research. In the northern hemisphere, buoyant water introduced into the ocean is expected to turn to the right of the discharge location and form a buoyant gravity current flowing along the coast. Whilst the fate of coastal discharges has been intensively studied from field observations, laboratory experiments, analytical theories, and numerical simulations, the mechanisms responsible for the observed instability of buoyant currents produced from coastal discharges are not completely understood.

Here the instability of a coastal buoyant current produced from the discharge of glacial meltwater is studied using idealized numerical experiments with a primitive-equation circulation model. It is found that glacial water released from the coast produces a surface plume in front of the release location, turns to the right, and leads to a buoyant current flowing at a speed of O(1 m/s) along the coast. Over the course of the numerical integration, the coastal current becomes unstable and develops dipolar vortices along its offshore boundary. The vortices are the largest near the glacial water inflow and migrate away from the discharge location at a speed of O(1 cm/s). They are asymmetric, with the region of cyclonic flow occupying a larger area than the region of anticyclonic flow, given them the appearance of breaking “backwards” relative to the direction of the current. The vortices occur for both free-slip and no-slip conditions imposed along the coast, suggesting that their development is not associated with vorticity continuously generated by the frictional retardation of the flow at the boundary. Numerical results are compared to laboratory results published in the literature and interpreted in the light of the theory of the baroclinic instability in a two-layer model (a surface buoyant layer flowing relatively to a deeper heavier layer). The implications of our results for the simulation of buoyant coastal discharges with coarse-resolution ocean models, such as used in climate studies, are clarified.

How to cite: Marchal, O. and Condron, A.: On the Instability of Buoyant Coastal Currents, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4878, https://doi.org/10.5194/egusphere-egu22-4878, 2022.

14:39–14:50
break

Fri, 27 May, 15:10–16:40

Chairpersons: Alexander Osadchiev, Nils Christiansen, Ute Daewel

15:10–15:11
Biogeochemistry

15:11–15:18
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EGU22-3634
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ECS
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On-site presentation
Feifei Liu et al.

Identifying the mechanisms driving the variability of the air-sea exchange of carbon dioxide (CO2) in the North Sea is necessary to evaluate the consequences of human interventions such as coastal alkalinity enhancement (OAE) on this societally important ecosystem. For this purpose, the three-dimensional coupled physical-biological model SCHISM-ECOSMO, encompassing a carbonate chemistry module, is employed to present the local physical-biogeochemical processes as well as the exchange processes across scales and compartments. Here we present model results for a 5-year simulation (2002-2004), which are shown to agree well with the observations, indicating a net CO2 uptake in the northern North Sea (NNS) over the year while a net source of CO2 to the air in summer in the southern North Sea (SNS). In the NNS, the ‘Continental Shelf Pump’ mechanism, attributing to the seasonal stratification and efficient carbon export, determines the CO2 exchange, making the ocean a net sink despite the high temperature in summer that contributes to an enhancement of the CO2 release. In contrast, the temperature-driven release of CO2 outcompetes the biological CO2 drawdown in the shallower SNS. In this region, the tidal mixing prevents seasonal stratification. As a result, the CO2 generated via remineralization gets quickly in contact with the atmosphere. In addition, the interannual variability of the CO2 flux is assessed based on the 5-year simulation, which is mainly associated with the variations of the hydrodynamic conditions and productions induced by changes of meteorological conditions.

How to cite: Liu, F., Schrum, C., and Daewel, U.: Drivers of the air-sea CO2 flux variability in the North Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3634, https://doi.org/10.5194/egusphere-egu22-3634, 2022.

15:18–15:25
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EGU22-4096
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ECS
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Virtual presentation
Xiaoyan Wei et al.

Strong interannual variability in pre-spring nitrate concentrations have been observed in shelf seas. These variations contribute to interannual variability in net primary production during the following spring and summer and thus also regulate the biological uptake of atmospheric carbon dioxide. Most of the nitrate required by shelf seas to maintain their high productivity is supplied from the open ocean.  In this study, we investigate for the first time the importance of variability in the depth of winter mixing along the shelf edge and subsequent wind-driven across-shelf Ekman transport in setting the pre-spring nitrate concentrations across the NW European shelf.  

Monthly EN4 temperature data is used to identify the maximum depth of winter mixing along the NW European shelf edge between 2000 and 2016 and monthly climatologies of nitrate from the World Ocean Atlas are used to estimate the resulting concentration of surface water nitrate. The wind-driven across-shelf nitrate transport for each sector of the shelf during late winter is then derived using wind velocities from ERA5. The deepest winter mixing close to the shelf edge occurs in the Rockall Trough. However, the largest interannual variations in winter mixed layer depth occur in the Bay of Biscay and Porcupine Sea Bight. The smallest variations are seen in the Faroe-Shetland channel. The estimated nitrate concentration near the shelf edge after winter mixing (i.e., recharge of surface water nitrate concentrations) shows strong interannual variability in the Bay of Biscay and Porcupine Sea Bight (interannual differences of 1- 2.5 mmol/m3), comparable to variability that has been observed on the shelf in this region. Further north, despite deeper winter mixing, there are much smaller interannual differences in surface water nitrate concentrations at the end of winter. This is because the vertical gradients in nitrate concentration within the range of maximum winter mixed layer depths are much weaker. Over the northern sectors of the NW European Shelf large interannual differences in wind-driven Ekman transport drive between 1 and 6 mmol N m-1 s-1 on-shelf during the winter. Further south wind-driven nitrate transport during the winter is typically off-shelf, weaker (typically 1 mmol N m-1 s-1), and much less variable between years. 

How to cite: Wei, X., Hopkins, J., Oltmanns, M., Johnson, C., and Inall, M.: Oceanic and atmospheric drivers of interannual variability in shelf sea nitrate supply, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4096, https://doi.org/10.5194/egusphere-egu22-4096, 2022.

15:25–15:32
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EGU22-8463
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Virtual presentation
Lorenzo Pasculli et al.

Ocean straits connect basins characterized by different fluid properties. They are important exchange areas showing peculiar phenomena that often strongly influence physical as well as biogeochemical exchange processes. These are mostly unique for each strait, depending on its specific bathymetry, local air-sea interactions, and remote forcing. Consequently, different methods for observing straits dynamics and fluxes have been developed. Starting from analogue current meters, technological development has led to the use of increasingly complex instruments, such as acoustic as well as microwave devices. Hence, in situ measurements are complemented by remote sensing methods to accurately determine current velocities across the straits. The advent of very high-resolution numerical models, which are able to reproduce small-scale features of the near surface as well as of the interior water masses, yielded a strong improvement in the understanding of straits dynamics. Starting from the 1960s, much work has been devoted at developing an integrated approach to the study of sea straits, including in situ and remote sensing observations, and modelling analysis. In this work, we examine different methods used to observe, monitor and simulate the dynamics of sea straits and their biogeochemical impact, focusing particularly on integrated approaches.

How to cite: Pasculli, L., Falcieri, F. M., Chiggiato, J., Schroeder, K., García Lafuente, J., Sammartino, S., Sánchez-Garrido, J. C., and Rubino, A.: Across the Straits: a review of methods to compute mass and nutrients transports through straits and channels, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8463, https://doi.org/10.5194/egusphere-egu22-8463, 2022.

15:32–15:33
Anthropogenic influence

15:33–15:40
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EGU22-11852
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ECS
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On-site presentation
Jonni Lehtiranta and Tuomas Kärnä

The fate of plastic litter in seasonally ice-covered waters is an area of active research. The ice will transport any litter affixed to it, and the drift of sea ice differs substantially from the flow of surface currents, especially in marginal seas. This work studies typical drift patterns of marine litter in water, on ice, and in realistic circumstances where seasonal ice melts leaving marine litter suspended in flowing water.

The drift of litter in the Baltic Sea is simulated using the OpenDrift software package using oceanic drift from NEMO 4.0. A simple module was written to advect passive tracers that might be transported on sea ice or by sea water. The particles move with ice or surface water depending on the prevailing ice conditions. The results are analysed and compared to drift buoy results.

How to cite: Lehtiranta, J. and Kärnä, T.: Modelling the transport of marine litter on Baltic sea ice and surface water, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11852, https://doi.org/10.5194/egusphere-egu22-11852, 2022.

15:40–15:47
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EGU22-425
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ECS
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Virtual presentation
Igor Zhdanov et al.

The present study addresses spatial and temporal distribution of floating microplastics in the Kara Sea. The studies were carried out in two cruises: on board the R/V «Akademik Mstislav Keldysh» in October 2020 and 2021, and the cruise on the R/V «Akademik Ioffe» as part of the MIPT Floating University in July-August 2021.  A neuston net was used for sampling surface microplastics, each catch lasted an average of 30 minutes. Only particles up to 0.5 mm in size (detection limit with the naked eye) are considered in this work. In total, 85 samples were processed, including the samples in Kara Gates, estuaries of the Ob, Yenisei, Pyasina rivers, Baydaratskaya Bay, the central and northern parts of the Kara Sea (the northernmost point is 82.5 °N). All particles were analyzed on a Fourier transform infrared-spectrometer «Perkin Elmer spectrum two».

A number of trends have been found. First, microplastics concentrations in plumes of Siberian Rivers were significantly lower than in the sea water (0-0,032 items/m3 in summer and 0-0,02 items/m3 in autumn). Second, higher concentrations were observed in the sea water (it can be noted that microplastic particles were found at the northernmost point of the expedition route). There was a local maximum in the Kara Gates Strait (1,53 items/m3), which can be explained by the local hydrophysics features, as well as by the general intensification of the processes in straits. Also a noticeable decrease of the microplastics concentration in the autumn in the same areas. Chemical composition of microplastics was diversified , all polymers with positive buoyancy were found: polyethylene, polypropylene, polystyrene and polyurethane foam. The most common were polyethylene and polypropylene, which correspond the level of their use. Particles of all morphological types were found. There was no significant seasonal variability in the polymer composition.

Brief conclusions: Although the Great Siberian Rivers are one of the main sources of microplastics in the Kara Sea, they seem to dilute the total number of particles. The most considerable contribution is made by the Barents Sea waters coming from the Kara Gates Strait. A similar conclusion can be made due to the observed decrease in the concentrations of microplastics when moving eastward from the Kara Gates. There is a slowdown in the rate of microplastics inflow in the autumn period. There is no seasonal changes in the polymer composition of microplastic particles and their morphological composition.

How to cite: Zhdanov, I., Pakhomova, S., Mekhova, O., Pogojeva, M., Berezina, A., Novikov, M., Gettikh, N., Evenkova, T., Osadchiev, A., Stepanova, N., Semiletov, I., and Yakushev, E.: "Microplastic pollution of the Kara Sea surface in different seasons.", EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-425, https://doi.org/10.5194/egusphere-egu22-425, 2022.

15:47–15:54
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EGU22-1212
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ECS
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On-site presentation
Nils Christiansen et al.

The aerodynamic drag of wind turbine rotors creates downstream wake structures in the atmosphere, which represent decreasing wind speed and increasing turbulence behind wind turbines. In the marine environment, these atmospheric wakes entail attenuated wind forcing at the sea surface boundary and thus imply consequences for wind-driven processes in the ocean dynamics. Based on the unstructured-grid model SCHISM, this study presents a new cross-scale hydrodynamic model setup for the southern North Sea, which enables to simulate wake effects in the marine environment at high resolution. We introduce an observational-based empirical approach to parameterize the atmospheric wakes in the hydrodynamic model and simulate the seasonal cycle of the summer stratification in consideration of the current state of European offshore wind farm development. The simulations show the emergence of large-scale structural changes in local hydro- and thermodynamics due to the wind speed reductions caused by offshore wind farms. The wake effects lead to spatial variability of the mean horizontal currents and, in particular, affect the stratification strength during the summer season. Our results aim to advance understanding of how coastal systems adapt to anthropogenic stressors such as offshore wind farms and raise awareness of potential changes to the future ocean. In particular, large-scale changes in stratification suggest potential consequences for biogeochemical processes and marine ecosystem dynamics.

How to cite: Christiansen, N., Daewel, U., Djath, B., and Schrum, C.: Emergence of large-scale hydrodynamic structures due to atmospheric offshore wind farm wakes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1212, https://doi.org/10.5194/egusphere-egu22-1212, 2022.

15:54–16:01
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EGU22-3636
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Highlight
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On-site presentation
Ute Daewel et al.

The North Sea has become a focus of renewable energy production with an increasingly large number and size of offshore wind farms (OWFs) planned in the German and British sectors in far deeper waters than before. As the North Sea is also a complex ecosystem that is strongly driven by hydrodynamical features such as tidal fronts and seasonal stratification, these large OWFs can be expected to impact the ecosystem dynamics in the area. Here, we use the coupled ecosystem model ECOSMO, previously used and validated for the area, to explore the consequences of large scale OWFs for marine ecosystem productivity. The model is forced with results from two model simulations of a high-resolution regional climate model, one with and one without implemented wind-farm parameterization using a near future wind farm scenario that includes existing and planned OWFs. Our major research focus lies on the large-scale, integrated effects imprinted on the ocean physics and ecosystem by changes in the atmospheric conditions rather than small scale processes. The simulations were integrated over the time period of one year and the average system response was analysed. The model shows a clear and direct response to the modifications in the atmosphere with respect to surface current speed, sea surface elevation and vertical transport depending on the wind direction. However, these immediate impacts are not visible in the ecosystem variables. Instead, the ecosystem shows an integrated (over the year) response related to the general modifications in stratification, transport pattern and bottom shear stress. It becomes evident that we cannot conclude a general increase/decrease pattern of change in ecosystem productivity, instead we can see changes in both spatial distribution and phenology of the lower trophic level ecosystem components, which we expect to be relevant for fish connectivity pattern and early larval survival for economically relevant fish species. 

How to cite: Daewel, U., Akhtar, N., Christiansen, N. G., and Schrum, C.: Future offshore windfarm effects on ecosystem productivity: upscaling to the southern North Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3636, https://doi.org/10.5194/egusphere-egu22-3636, 2022.

16:01–16:08
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EGU22-11267
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ECS
Internal waves on a steep shelf and their influence on coherent underwater acoustic communications
(withdrawn)
Elizaveta Khimchenko and Alexander Shatravin