Arctic sea ice is subject to large seasonal variations in extent, thickness and mobility. Most of the Arctic region is covered by sea ice in winter months, with a total area of ∼15 million km 2 ; however, this area declines by a factor of 3 to ∼4.5 million km 2 , by summer (average 2011-2019 National Snow and Ice Data Center, 2020). Moreover, in winter there is about 1.65 million km 2 of landfast sea ice, that reduces to near zero in summer (Li et al., 2020). The presence of landfast ice can significantly affect local hydrodynamics as it prevents interaction between the atmosphere and underlying ocean (Mahoney et al., 2014).While an ice cover shields the ocean from atmospheric forcing, it also exerts additional frictional stress on the surface. Early studies demonstrate that drag between water and ice results in tidal dampening, especially in coastal zones where ice is relatively immobile (Godin, 1986;Kowalik, 1981). Kowalik (1981) showed that the presence of sea ice can lead to tidal amplitude decay and phase delay. More recent studies have looked specifically into the modulation of tides in the Arctic region in response to a seasonally varying ice cover and suggest the effect to be substantial; in some regions changing the amplitude by up to 0.15 m (Kagan & Sofina, 2010;St-Laurent et al., 2008). This implies that it is insufficient to regard tides as constant throughout the year and ignore the influence of sea ice, which is done in most operational tide models. For accurate prediction of Arctic tidal water levels, quantification of seasonal modulation is necessary. In addition, studies have shown that Arctic tides directly affect North Atlantic tides (Arbic et al., 2004(Arbic et al., , 2007,

Tidal marshes play an important role in climate change mitigation through natural coastal protection. The effectiveness of the natural coastal defense by tidal marshes is closely related to their channel network which is in turn greatly influenced by their vegetation cover and shape. Previous research suggests a dual effect of vegetation on marsh topography; stabilizing sediment on the one hand versus promoting erosion and channel incision on the other hand. This study links these effects to different vegetation species, Salicornia procumbens, Spartina anglica, and Puccinellia maritima (further referred to as Salicornia, Spartina, and Puccinellia), by means of a coupled bio‐hydromorphodynamic modeling study. Single species, species‐assemblages, and species shifts were studied, incorporating both species‐specific physical plant properties and spatiotemporal growth strategies. The results indicate the influence of vegetation on the marsh topography to be highly species‐dependent, but also of a very complex nature. Both the presence of Spartina and Puccinellia resulted in significant channel development, whereas Salicornia did not induce topographic change. The combination of several species promoted or reduced channel development depending on the included species. Species‐shifts linked with climatic changes resulted in increased erosion of the existing channel network potentially reducing the protective capacity of the marsh.

Previous studies have demonstrated that tides are subject to considerable changes on secular time scales. However, these studies rely on sea level observations from tide gauges that are predominantly located in coastal and shelf regions and therefore, the large‐scale patterns remain uncertain. Now, for the first time, satellite radar altimetry (TOPEX/Poseidon & Jason series) has been used to study worldwide linear trends in tidal harmonic constants of four major tides (M2, S2, O1, and K1). This study demonstrates both the potential and challenges of using satellite data for the quantification of such long‐term changes. Two alternative methods were implemented. In the first method, tidal harmonic constants were estimated for consecutive 4‐year periods, from which the linear change was then estimated. In the second method, the estimation of linear trends in the tidal constants of the four tides was integrated in the harmonic analysis. First, both methods were assessed by application to tide gauge data that were sub‐sampled to the sampling scheme of the satellites. Thereafter the methods were applied to the real satellite data. Results show both statistically significant decreases and increases in amplitude up to 1 mm/year and significant phase changes up to ∼0.1 deg/year. The level of agreement between altimeter‐derived trends and estimates from tide gauge data differs per region and per tide.

<p>The impact of Arctic sea ice decline on future global tidal and storm surge extreme water levels is unknown. Regional studies show that the impact can be substantial; causing increased erosion and posing higher risks to fragile Arctic ecosystems in low-lying areas. Since Arctic tides and surges influence global water levels, consequences of Arctic sea ice decline will be noticed across the globe. In the ongoing FAST4Nl project, an Arctic Total Water Level model will be used to quantify this impact. The model will be developed as an extension of the operational Global Tide and Surge Model (GTSM) and includes the effect of sea ice on tides.</p><p>Here we present the results of a study on the seasonal variability of the M<sub>2</sub> tide with respect to differences in sea ice cover. The effect of sea ice on the M<sub>2</sub> amplitude was modelled for minimal and maximal sea ice configurations. In addition, tidal harmonic analysis was performed on a global tide gauge data set, supplemented by SAR altimeter derived water levels from the Arctic region. The high along-track resolution of SAR altimeters (300 m) enables to derive water levels from leads in the sea ice. Here, the retrieved sea surface heights within a given region were stacked, in order to obtain a sufficiently large data set for analysis of the predominantly ice-covered areas. This allowed to gain insight in the seasonal modulation of both local and global tides and directly relate these processes to variations in sea ice.</p>