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water level reduction below a certain threshold is considered an NSS (e.g., [6-8]). Most 
of these studies either consider an official threshold given by respective authorities or 
apply a threshold mirroring the commonly used threshold for positive storm surges at the 
respective gauge. Another approach is to analyse the distribution of the excess, i.e., the 
detrendet non-tidal residuals of the water levels ([9,10]). Other studies created an index 
(SSI) describing the mean of the three lowest independent NSS minima per-year, along the 
considered periods, e.g., [11]. 
The most important parameters that affect an ELW are local winds, remote winds 
(large-scale atmospheric circulation), astronomic tides, bathymetry of the area, currents, 
and waves [12]. The individual impact of these variables varies for different locations. 
Gurumurthy et al. [4] for example showed that the local winds affect the NSS potential 
further up an estuary whereas at more open locations, such as the NY Harbour [4] or the 
Port of Buenos Aires [7], remote winds play a more important role. In more enclosed 
basins, such as the Mediterranean [11] or Baltic Sea [9], an NSS is often triggered by a 
preceding positive storm surge (due to the effects of seiches) and generally lasts shorter 
than its positive counterpart. This is because the (sub-)basins fill up during the positive 
storm tide and induce a rapid drop in water level once the onshore wind forcing vanishes. 
Local topographic conditions can also have an influence on the occurrence of NSSs. 
It might determine whether the surge is triggered by fast-moving strong storms, as it is 
the case for the port of Buenos Aires [7], the Baltic [9] and the North-East US coast [13], or 
rather by long-lasting and stationary atmospheric pressure systems which would be the 
case in Alaska [8] or the southern North Sea coast of the Netherlands [9]. 
To our knowledge, possible future changes in NSSs have been very rarely analysed in 
scientific literature. Conte et al. [11] found that the effect of climate change in this respect is 
small for the Mediterranean Sea. 
In the presence of ongoing global warming, the related global sea-level rise (SLR) 
is of particular relevance for phenomena such as NSSs and resulting ELWs. The most 
comprehensive overview on projections of future global mean SLR is given in the IPCC 6th 
Assessment Report (AR6) [14]. For the majority of coastal regions, the median of future 
regional mean SLR lies within £20 cm of the median of the projected global mean SLR until 
2100 [13]. Projections for individual tide gauge locations can be accessed via an interactive 
tool hosted by NASA [14-16]. At the gauge Cuxhaven (located in the mouth of the Elbe 
estuary), the projected SLR for year 2050 relative to the reference period 1995-2014 is in 
the range between 0.25 and 0.29 m for the median of all SSP-scenarios [14-16]. However, 
for the year 2100 the projected SLR is strongly differing across socio-economic pathways 
(SSPs) and associated emission scenarios, with a median and 66% confidence interval of 
(i) 0.51(0.32- 0.74) m for SSP1-2.6, (ii) 0.64 (0.46-0.88) m for SSP2-4.5, (iii) 0.73 (0.52-1.00) m 
for SSP3-7.0, and (iv) 0.85 (0.61-1.16) m for SSP5-8.5 [14-16]. The median of the projections 
of regional mean SLR at Cuxhaven is up to 8 cm higher than the median of global mean 
SLR [14-16]. 
As future SLR will influence water level and therefore ELW in the Elbe estuary, the 
range and development of projected SLR needs to be considered in our investigation. 
[o assess SLR impacts in estuaries, advanced hydrodynamic numerical models are used 
as a common tool which can consider various driving forces and their interaction. In a 
review of [17] several hydrodynamic numerical studies on the effect of SLR on estuarine 
parameters and processes are summarised. For the Elbe estuary, the impact of SLR on storm 
tides [18] and mean tidal parameters [19] was investigated. However, to our knowledge, 
the impact of SLR on ELWS in the Elbe estuary has not yet been studied. 
Future changes in atmospheric circulation have already been analysed in a number 
of studies: In the recent past, a shift to more zonal rather than meridional flow over most 
parts of Northern and Central Europe has been observed [20]. This has been found to 
be continuing throughout different future scenarios [21]. Following this shift, there is an 
increase in westerly flows which can be seen in the global climate model (GCM) [22,23] as 
well as in higher resolution regional climate model (RCM) simulations [23,24]. This increase