workshop on regional sea level change regional mean sea level changes in the north sea...
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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