Interactive impacts of meteorological and hydrological conditions on the physical and biogeochemical structure of a coastal system

Abstract

<p><strong>Abstract.</strong> The German Bight was exposed to record high riverine discharges in June 2013, as a result of flooding of the Elbe and Weser rivers. Several anomalous observations suggested that the hydrodynamical and the biogeochemical state of the system was impacted by this event. In this study, we developed a biogeochemical model and coupled it with a previously introduced high resolution hydrodynamical model of the southern North Sea, in order to better characterize these impacts, and gain insight into the underlying processes. Performance of the model was assessed using an extensive set of <i>in-situ</i> measurements for the period 2011–2014. We first improved the realism of the hydrodynamic model with regard to the representation of cross-shore gradients, mainly through inclusion of flow-dependent horizontal mixing. Among other characteristic features of the system, the coupled model system can reproduce the low salinities, high nutrient concentrations and low oxygen concentrations in the bottom layers observed within the German Bight following the flood event. Through a scenario analysis, we examined the sensitivity of the patterns observed during July 2013 to the hydrological and meteorological forcing in isolation. Within the region of freshwater influence (ROFI) of the Elbe-Weser rivers, the flood event clearly dominated the changes in salinity and nutrient concentrations, as expected. However, our findings point out to the relevance of the peculiarities in the meteorological conditions in 2013 as well: a combination of low wind speeds, warm air temperatures and cold bottom water temperatures resulted in a strong thermal stratification in the outer regions, and limited vertical nutrient transport to the surface layers. Within the central region, the thermal and haline dynamics interactively resulted in an intense density stratification. This intense stratification, in turn, led to enhanced primary production within the central region enriched by nutrients due to the flood, but reduction within the nutrient-limited outer region, and it caused a wide-spread oxygen depletion in bottom waters. Our results further point to the enhancement of the current velocities at the surface as a result of haline stratification, and intensification of the thermohaline estuarine-like circulation at the Wadden Sea, both driven by the flood event.</p>

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

“…Furthermore, in their recent study, Kerimoglu et al. (2020) confirmed the effect of wind fields on the stratification of the GB during the 2013 extreme event, by simulating the hydrodynamics of the GB under different forcing scenarios. In the present study, a different approach is taken by evaluating the processes that contribute to the increase and decrease of stratification when the position of the river plume changes (section 4.3.1).…”

“…Kerimoglu et al. (2020) have shown that these can be improved by extensive tuning of the horizontal diffusion.…”

“…Furthermore, the model freshwater plume is more persistent compared to observations, resulting in a longer flushing time. Kerimoglu et al (2020) have shown that these can be improved by extensive tuning of the horizontal diffusion.…”

“…Voynova et al (2017) provided important insight on the impact of the event on the physical properties and biogeochemistry of the GB using observational data. Recently, Kerimoglu et al (2020) studied the impact of the extreme event and the particular meteorological conditions on the physical and biogeochemical structure of the GB using a coupled biogeochemical-hydrodynamical model. However, there is still a need for a more detailed understanding of the temporal and spatial variability of the GB hydrodynamics under extreme river discharge events and the specific physical processes contributing to changes in stratification.…”

Long‐term intercomparison of two pCO2 instruments based on ship‐of‐opportunity measurements in a dynamic shelf sea environment

“…The deeper northern part of the North Sea experiences seasonal stratification as summertime warming of the surface establishes a strong pycnocline (Wakelin et al 2012), while the shallower southern part is typically permanently mixed due to strong tidal forcing (Bozec et al 2005). More recent studies have defined biome boundaries on an even finer scale depending on the stratification regime and on the freshwater influence on biogeochemical processes showcasing the high spatial variability in the North Sea (van Leeuwen et al 2015; Kerimoglu et al 2020). While the Lysbris FerryBox‐MBS p CO 2 measurements are available over a large area of the North Sea, in order to check and validate the long‐term MBS p CO 2 record, we focus on the small restricted box in the Skagerrak where many crossovers with the SHS system on Nuka Arctica were identified.…”