Processes of Stratification and Destratification During An Extreme River Discharge Event in the German Bight ROFI

Onur Kerimoglu,Hans Burchard,Markus Kreus,Peter Holtermann,Ulf Gräwe,Christian Winter,Fatemeh Chegini,Knut Klingbeil,Marius Becker

doi:10.1029/2019jc015987

Abstract

AbstractProcesses of stratification and destratification in the German Bight region of fresh water influence (ROFI) are investigated following an extreme river discharge event in June 2013. For this purpose, a high‐resolution baroclinic ocean model is set up and validated against field data. The model results are used to study the temporal and spatial variability of stratification and the duration of persistent stratification in 2013. The relevant processes affecting stratification are investigated by analyzing the potential energy anomaly budget, with a focus on mixing and tidal straining. It is shown that the stratification in the German Bight is highly affected by the spring‐neap tidal cycle, with generally less stratification at spring tides due to dominant tidal mixing. It is also shown that the location of the river plume can modify this pattern. During spring tides, if the river plume is confined to the eastern region, stratification decreases significantly, as expected, due to the dominance of mixing over tidal straining. On the other hand, if the river plume moves toward deeper regions at spring tides, strong tidal straining becomes present. In this condition, mixing is weak, and the dominant tidal straining results in persistent stratification.

Highlights

Stratification plays a major role in the functioning of coastal ecosystems
The relevant processes affecting stratification are investigated by analyzing the potential energy anomaly budget, with a focus on mixing and tidal straining
If the river plume is confined to the eastern region, stratification decreases significantly, as expected, due to the dominance of mixing over tidal straining

Summary

Introduction

Stratification plays a major role in the functioning of coastal ecosystems. Stratification affects vertical mixing and impacts momentum, heat, and salt fluxes as well as other constituents such as suspended particulate matter (Pietrzak et al, 2011). In a ROFI, the interaction between tides, wind, and freshwater buoyancy input determines the state of stratification on multiple time scales. The variability of the wind fields affect the state of stratification (Marques et al, 2010). The variability in river discharge influences the stratification on monthly to annual time scales (Marques et al, 2010). The amount of riverine discharge can vary considerably, especially at extreme precipitation events During these events, the stratification of the region increases considerably, resulting in long‐term, large‐scale consequences on the coastal systems (Voynova et al, 2017). Understanding the processes that impact stratification during these events is essential for understanding the time scales of the extreme river discharge effects on coastal seas

Methods

Results

Conclusion