Workshop on Regional Sea Level Change Regional Mean Sea Level Changes in the North Sea / Hamburg Hans von Storch, presenter

2 Co-Authors Ralf Weisse, Frauke Albrecht Helmholtz-Zentrum Geesthacht, Center for Materials & Coastal Research, Germany Thomas Wahl, Jürgen Jensen University of Siegen, Research Institute for Water & Environment, Germany Presentation based on (chronological order): 1.Weisse, R., D. Bellafiore, M. Menendez, F. Mendez, R. Nicholls, G. Umgiesser, P. Willems, 2013: Changing extreme sea levels along European coasts. Coastal Eng., accepted. 2.Wahl, T., Haigh, I., Woodworth, P.L., Albrecht, F., Dillingh, D., Jensen, J., Nicholls, R., Weisse, R. and Wöppelmann, G. 2013: Observed mean sea level changes around the North Sea coastline from 1800 to present. Earth-Science Rev., 124, Albrecht, F. and R. Weisse, 2012: Wind and pressure effects on past regional sea-level trends and variability in the German Bight. Ocean Dynamics, 62, Albrecht, F., T. Wahl, J. Jensen, R. Weisse, 2011: Determining sea level change in the German Bight. Ocean Dynamics, 61, Wahl T, Jensen J, Frank T and Haigh I. 2011: Improved estimates of mean sea level changes in the German Bight over the last 166 years. Ocean Dynamics, 61,

3 Motivation  Rates of regional mean sea level changes in the German Bight are not well known.  Currently global sea level projections from IPCC are used for most coastal planning purposes.  Considerable efforts to extend the projection period, but there are also numerous requests for shorter periods (10-50 years).

4 Motivation  Rates of regional mean sea level changes in the German Bight are not well known.  Currently global sea level projections form IPCC are used for most coastal planning purposes.  Considerable efforts to extend the projection period, but there are also numerous requests for shorter periods (10-50 years).  Assessment of regional mean sea level changes in the German Bight over the past about years  Relate regional to global mean sea level changes and other driving factors  Simple approach to assess future regional changes on shorter time scales?

5 Introduction For most coastal planning purposes data from tide gauges are still the most relevant source of information Long but often inhomogeneous (changes in tide gauge datum; measurement techniques; sampling intervals; construction works etc.)

6 Introduction

7 Data base Homogenised set of 13 tide gauges from the German North Sea coast (Wahl et al. 2011) Nationally funded project Analyse von hochaufgelösten Tidewasserständen und Ermittlung des MSL an der deutschen Nordseeküste (AMSeL) (Wahl et al. 2011)

Data base  Two different sources of data (HW/LW./. at least hourly)  Most data from about 1936 onwards  Longest data from Norderney (1901) & Cuxhaven (1843)  Quality check  Correction for local datum shifts  Conversion of MTL to MSL (important where k<<0.5)  Example Emden: - k = ; MTR = 3.23 m - |MTL-MSL| ~ 23 cm Data availability (Wahl et al. 2011) If k=0.5 then MSL = MTL

9 Recent changes of regional mean sea level in the German Bight Objective To construct a sea level index representative for a larger area (German Bight) “Virtual station” (differentiating the time series from the individual stations; then averaging the rates of sea level change between adjacent years & integrate back in time) (Wahl et al. 2011)

10 Recent changes of regional mean sea level in the German Bight  Relative mean sea level change mm/year  Higher rates along the coast of Schleswig- Holstein  Lower rates along the Lower Saxony coast (Wahl et al. 2011)

11 Recent changes of regional mean sea level in the German Bight Questions  Is this type of averaging representative?  How far can we go back (Impact from individual stations)?  Effect of homogenization?  EOF method to exploit for the spatial covariance structure (RMSL as the spatially coherent part of the signal)  Compare with results from virtual station method  Successively extend the period backwards in time and test the sensitivity of the results from the EOF approach for artificial data gaps and datum shifts  Compare with results obtained from original data that formed the basis for the homogenization in the AMSeL project  Albrecht et al. (2011)

12 Recent changes of regional mean sea level in the German Bight  We can reasonably go back until about 1924  For this time interval there is little difference between VM and EOF approach (correlation 0.996; trends 1.64 mm/yr (VM) & 1.74 mm/year (EOF) RMSL from different methods black – VM method; green – EOF method (Albrecht et al. 2011)

13 Recent changes of regional mean sea level in the German Bight  If we go back further in time Cuxhaven becomes dominant (shown here in terms of 37-yr trends; at the end VM & EOF agree reasonably but Cuxhaven deviates, i.e. Cuxhaven is not a good RMSL proxy for this period)  Earlier VM identical to Cuxhaven  Question remains open whether the VM is a good proxy before 1924 or so  Speculation: Cuxhaven influenced by water works in the later period  If this would be the reason for the deviation, then Cuxhaven still might be representative before 1924 as water works mostly later  Exception: First deepening of the navigational channel around !  Remains open question 37-year trends of RMSL from different methods black – VM method; green – EOF method; red – Cuxhaven (Albrecht et al. 2011)

14 Recent changes of regional mean sea level in the North Sea Mean sea level (MSL)  Similar approach for the North Sea  Over the past years absolute MSL in the North Sea increased by about 1.6 mm/year  Comparable to the rates of global MSL rise  For the satellite period ( ) rates in the North Sea are somewhat larger (about 3.7 mm/year) than the global figure (3.20 mm/year)  Present rates of rise are relatively high but still not substantially different from those observed earlier during the last century Figure 1: Standard deviation from de-trended annual MSL time series from 30 tide gauge sites around the North Sea; (b) Sea level index for the Inner North Sea and results from applying SSA smoothing; (c) Sea level index for the English Channel and results from applying SSA smoothing. (Wahl et al.2013)

15 Corresponding changes in extreme sea levels in Cuxhaven Extreme sea level (ESL)  Extreme sea levels increased over the past years in the North Sea  Primarily as a result from a rise in mean sea level  Meteorologically induced components (waves, storm surges) show pronounced variation on time scales of years and decades but no substantial long-term trend  Variations in storm surge and wave climate consistent with those in storm activity over the North Sea Figure 2: Annual mean high water and linear trend at Cuxhaven, Germany (bottom) and corresponding difference between annual 99-percentile and annual mean high water levels (top); In addition an 11-year running mean is shown in the upper panel. (Weisse 2008; Update after von Storch and Reichardt 1997)

16 Relation between regional and global MSL and other drivers Scatterplot between global and regional mean sea level with long term trend included and with long term trend removed

17 Relation between regional and global MSL and other drivers Fluctuations on shorter time scales linked to large scale atmospheric pressure variations  Relation may be used when projecting future regional changes (Albrecht & Weisse 2012) Left, Black: RMSL EOF Reconstruction; Green: RMSL als sum of GMSL and regional SLP contribution Right: SLP pattern driving RMSL variations rechts;

18 Possible future changes  Currently global sea level projections from IPCC are used for most coastal planning purpose  Numerous requests for regional projections on shorter (10-50 year) time scales Extrapolation of observed trends: Persistence of trend, that is If a [t,t+H] = g 0 t + g 1 t∙ (k-t), k = 0,…H, is an H-year trend fitting the regional mean sea level z t+k = g 0 t + g 1 tH∙ (k-t), then we predict a future trend A [t+H,t+2H] = G 0 t+H + G 1 t+H,H∙ (k-t-H), by updating the offset of the trend G 0 t+H = z t and keeping the derivative G 1 t+H = g 1 t∙.. This procedure implies a forecast of the regional sea level Z t+k H =. z t + g 1 tH∙ (k-t-H) We use H = 5, 10, 15, 20 … years (Weisse et al. 2013)

19 Possible future changes (Weisse et al. 2013) a [1960,1974] A [1975,1989] Dots: z 1960 – z 1989 a [1976,1990] A [1991,2005] Dots: z 1976 – z 2005 Z t+H H for H = 15

20 Possible future changes (Cuxhaven) (Weisse et al. 2013)

21 Summary & Conclusions  Relative robust estimates of regional mean sea level changes in the German Bight over the past years  Changes are in the order of mm/year depending on period  The recently accelerated rise of the regional mean sea level of 3-4 mm/year is within the range of previously observed variations  Changes are somewhat higher along the Schleswig-Holstein coast and lower in Lower Saxony  Strong inter-annual to decadal variability mostly related to changes in SLP  First very simple attempt to provide a regional estimate for future regional changes on shorter time scales (10-50 years)  Uncertainties appear to be higher than proposed by IPCC projections