Workshop on Regional Sea Level Change

Regional Mean Sea Level Changes in the North Sea

20.-22.11.2013 / Hamburg

Hans von Storch, presenter

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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, 51-67

3. 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.

4. Albrecht, F., T. Wahl, J. Jensen, R. Weisse, 2011: Determining sea level change in the German

Bight. Ocean Dynamics, 61, 2037-2050.

5. 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

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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).

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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?

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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.)

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Introduction

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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 = 0.4286; MTR = 3.23 m

- |MTL-MSL| ~ 23 cm

Data availability

(Wahl et al. 2011)

If k=0.5 then MSL = MTL

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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)

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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)

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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)

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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)

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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 1900-1910!

Remains open question

37-year trends of RMSL from different methods

black – VM method; green – EOF method;

red – Cuxhaven

(Albrecht et al. 2011)

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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)

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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 SeaFigure 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)

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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

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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;

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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+ g1

t∙(k-t), k = 0,…H, is an H-year trend fitting the regional mean sea level z t+k = g0t+

g1tH∙(k-t), then we predict a future trend A[t+H,t+2H] = G0

t+H+ G1t+H,H∙(k-t-H), by updating the offset of

the trend G0t+H = zt and keeping the derivative G1

t+H = g1t∙..

This procedure implies a forecast of the regional sea level Zt+kH =. zt + g1

tH∙(k-t-H)

We use H = 5, 10, 15, 20 … years

(Weisse et al. 2013)

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Possible future changes

(Weisse et al. 2013)

a[1960,1974] A[1975,1989]

Dots: z1960 – z1989

a[1976,1990] A[1991,2005]

Dots: z1976 – z2005

Zt+HH for H = 15

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Possible future changes (Cuxhaven)

(Weisse et al. 2013)

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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