The correct representation of the 18.61 year nodal tide is essential for an interpretation of the evolution of mean sea level, as errors cause misleading bias. The nodal tide is currently estimated by applying correction factors in harmonic analysis, which are derived from the equilibrium tide. From the equilibrium tide, correction values f for amplitude and u for phase are determined, which alter lunar tidal constituents, depending on the nodal cycle. This approach has proven to be valid for many tide gauges, even though the impact of the nodal tide in shelf seas has been shown to differ from their theoretical correction value. Hence, tidal constituents from tide records in the North Atlantic shelf region were analyzed for their nodal amplitude and phase lag with a new multiple, non-linear regression approach, which is able to approximate the nodal modulation quantitatively and its agreement to the theoretical equilibrium tide. Results show an overestimation of the lunar M and N constituents by the equilibrium of more than 2.7% in the Wadden Sea, while O and K are underestimated by 1 to 4.6%, which would produce an error of 2 to 5 cm e.g. in the German Wadden Sea. Additionally, a process-based model of the North Sea was applied at the diurnal minimum and maximum of the nodal cycle to calculate the spatial distribution of f and u. Model results reproduce a regionally varying pattern of f and u, indicating how the amplitude modulation of nodal constituents in shallow areas is distributed.

In many places around the world, tide gauges have been measuring substantial non-astronomical changes. Here we document an exceptional large spatial scale case of changes in tidal range in the North Sea, featuring pronounced trends between -2.3 mm/yr in the UK and up to 7 mm/yr in the German Bight between 1958 and 2014. These changes are spatially heterogeneous, suggesting a superposition of local and large-scale processes at work within the basin. We use principal component analysis to separate large-scale signals appearing coherently over multiple stations from rather localized changes. We identify two leading principal components (PCs) that explain about 69% of tidal range changes in the entire North Sea including the divergent trend pattern along UK and German coastlines, which suggest movement of the region’s semidiurnal amphidromic areas. By applying numerical and statistical analyses, we can assign a baroclinic (PC1) and a barotropic large-scale signal (PC2), explaining a large part of the overall variance. A comparison between PC2 and tide gauge records along the European Atlantic coast, Iceland and Canada shows significant correlations on time scales of less than 2 years, which suggests an external and basin-wide forcing mechanism. By contrast, PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters. In particular, from an analysis of observed density profiles, we suggest that an increased strength and duration of the summer pycnocline has stabilized the water column against turbulent dissipation and allowed for higher tidal elevations at the coast.

In March 2019 cyclone Idai led to a compound flooding event in Mozambique combining high river runoff and storm induced water level extremes and causing damages up to 2 billion USD and more than 1200 fatalities. The co-occurrence of storm surges, wind waves, and flooding through heavy precipitation and runoff increases the risk of flooding and exacerbates the impacts along the vulnerable Southern African coasts. To mitigate the associated high-impacts, it is essential to know the probability of theses compound events (multivariate extreme event analysis) and understand the processes driving them (Wahl et al. 2015). In the project CASISAC*, we propose a regionalized multivariate assessment of extreme events to model extremes as combination of storm surge, waves, river discharge and high precipitation at southern African coasts (Namibia, South Africa, Mozambique). We develop a multivariate statistical model based on copulas to represent and analyze the physical mechanism underlying compound events and their return periods under present day climate conditions. The African regions are particularly poor sampled by tide gauges and available records include large gaps. To overcome the data scarcity, ancillary data from high-resolution ocean model hindcasts (Schwarzkopf et al. 2019) based on the NEMO model will complement the analysis. An integrated analysis of observational records from tide gauges in combination with ocean model hindcasts allows us to regionalize compound extreme events corresponding to certain return periods along the entire coastline. In this presentation, we will show preliminary results of the multivariate extreme value model analysis chain. *CASISAC (Changes in the Agulhas System and its Impact on Southern African Coasts: Sea level and coastal extremes) is funded by the German Federal Ministry of Education and Research (BMBF) under the grant number 03F0796C. Schwarzkopf et al. (2019): The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations. In: Geoscientific Model Development, Vol. 12, 7, 3329-3355, doi: 10.5194/gmd-12-3329-2019. Wahl et al. (2015): Increasing risk of compound flooding from storm surge and rainfall for major US cities. In: Nature Climate Change, Vol. 5, 12, 1093-1097, doi: 10.1038/nclimate2736.