Workshop on Regional Sea Level Change

1. Workshop on Regional Sea Level Change Regional Mean Sea Level Changes in the North Sea Hans von Storch, presenter 20.-22.11.2013 / Hamburg

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): • 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. • 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, 51-67 • 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, 1169-1186. • Albrecht, F., T. Wahl, J. Jensen, R. Weisse, 2011: Determining sea level change in the German Bight. Ocean Dynamics, 61, 2037-2050. • 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, 701-715

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 80-170 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)

8. Data base Data availability • 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 = 0.4286; MTR = 3.23 m- |MTL-MSL| ~ 23 cm • (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 1843-2008 2.01 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 RMSL from different methods black – VM method; green – EOF method • 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) • (Albrecht et al. 2011)

13. Recent changes of regional mean sea level in the German Bight 37-year trends of RMSL from different methods black – VM method; green – EOF method; red – Cuxhaven • 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 1900-1910! • Remains open question • (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 100-120 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 (1993-2009) 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 100-150 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 • Left, Black: RMSL EOF Reconstruction; • Green: RMSL als sum of GMSL and regional SLP contribution • Right: SLP pattern driving RMSL variations rechts; • (Albrecht & Weisse 2012)

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] =g0t+ g1t∙(k-t), k = 0,…H, is an H-year trend fitting the regional mean sea level zt+k = g0t+ g1tH∙(k-t), then we predict a future trend A[t+H,t+2H] = G0t+H+ G1t+H,H∙(k-t-H), by updating the offset of the trend G0t+H = zt and keeping the derivative G1t+H = g1t∙.. • This procedure implies a forecast of the regional sea level Zt+kH =. zt + g1tH∙(k-t-H) • We use H = 5, 10, 15, 20 … years • (Weisse et al. 2013)

19. Possible future changes • A[1975,1989] • a[1960,1974] • Zt+HH for H = 15 • Dots: z1960 – z1989 • a[1976,1990] • A[1991,2005] • Dots: z1976 – z2005 • (Weisse et al. 2013)

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 80-170 years • Changes are in the order of 1.6-2.0 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