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Predicting sea surface salinity in a tidal estuary with machine learning

Guillou Nicolas, Chapalain Georges, Petton Sébastien

Oceanologia

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2023

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Vol. 65 (2)

318--332

EN

As an indicator of exchanges between watersheds, rivers and coastal seas, salinity may provide valuable information about the exposure, ecological health and robustness of marine ecosystems, including especially estuaries. The temporal variations of salinity are traditionally approached with numerical models based on a physical description of hydrodynamic and hydrological processes. However, as these models require large computational resources, such an approach is, in practice, rarely considered for rapid turnaround predictions as requested by engineering and operational applications dealing with the ecological monitoring of estuaries. As an alternative efficient and rapid solution, we investigated here the potential of machine learning algorithms to mimic the non-linear complex relationships between salinity and a series of input parameters (such as tide-induced free-surface elevation, river discharges and wind velocity). Beyond regression methods, the attention was dedicated to popular machine learning approaches including MultiLayer Perceptron, Support Vector Regression and Random Forest. These algorithms were applied to six-year observations of sea surface salinity at the mouth of the Elorn estuary (bay of Brest, western Brittany, France) and compared to predictions from an advanced ecological numerical model. In spite of simple input data, machine learning algorithms reproduced the seasonal and semi-diurnal variations of sea surface salinity characterised by noticeable tide-induced modulations and low-salinity events during the winter period. Support Vector Regression provided the best estimations of surface salinity, improving especially predictions from the advanced numerical model during low-salinity events. This promotes the exploitation of machine learning algorithms as a complementary tool to process-based physical models.

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Report on the development of the Vistula river plume in the coastal waters of the Gulf of Gdańsk during the May 2010 flood

Zajączkowski M., Darecki M., Szczuciński W.

Oceanologia

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2010

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No. 52 (2)

311-317

EN

The hydrological conditions, suspended matter concentrations and vertical particulate matter flux were measured as the River Vistula flood wave (maximum discharge) was flowing into the southern part of the Gulf of Gdańsk on 26 May 2010. Extending offshore for several tens of kilometres, the river plume was well stratified, with the upper layer flowing away from the shore and the near-bottom water coastwards.

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Transport of the Odra river waters and circulation patterns in the Pomeranian Bay

Beszczyńska-Möller A.

Oceanologia

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1999

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No. 41 (3)

279-308

EN

During several cruises of r/v 'Oceania' in different seasons of 1993-1997 detailed investigations of the Pomeranian Bay were carried out with particular attention to the vicinity of the Odra river mouth. On the basis of CTD soundings as well as quasi-continuous profiling by means of a towed CTD probe, the thermohaline fields were analysed in order to determine the pattern of riverine water transport. The characteristic flow paths under different meteorological conditions were identified, Ekman transport of freshened waters being found to prevail along the coasts of the Pomeranian Bay. Physical phenomena such as the pulsating outflow of the river Odra and the formation of isolated plumes of freshened water were observed. The vertical and horizontal extents as well as the lifetime and speed of movement of the plume-like structures were estimated. A typical plume was a few km in diameter and there were steep horizontal and vertical salinity gradients at the boundaries. As the plume moved away from the mouth, it was transformed and finally vanished. There was strong wind mixing and entrainment into underlying, more saline water at some distance from the channel mouth. Hydrological fronts between riverine and ambient waters frequently formed. Numerous intrusions were found in the temperature and salinity profiles in the frontal zones. The freshwater fraction in the entire volume of the bay waters was estimated for different hydrological situations, the highest values being obtained for the period following the flood event of summer 1997. Under favourable wind conditions, dense, saline waters flowing in from the Arkona and Bornholm Deeps were present in the near-bottom layer at the edges of the bay. Anomalously, waters of higher salinity were found in the Pomeranian Bay in November 1997 as a result of a minor inflow from the Danish Straits.