THESIS ON CIVIL ENGINEERING F29 Spatio-Temporal Variability of the Baltic Sea Wave Fields ANDRUS RÄÄMET PRESS TALLINN UNIVERSITY OF TECHNOLOGY Faculty of Civil Engineering Department of Mechanics Institute of Cybernetics The dissertation was accepted for the defence of the degree of Doctor of Philosophy in Engineering on 6 May, 2010 Supervisor: Prof. Tarmo Soomere Institute of Cybernetics and Department of Mechanics Tallinn University of Technology, Estonia Opponents: Dr. Arno Behrens Coastal Research Institute GKSS Geesthacht, Germany Prof. Sirje Keevallik Marine Systems Institute at Tallinn University of Technology Tallinn, Estonia Defence of the thesis: 22 June 2010 Declaration: Hereby I declare that this doctoral thesis, my original investigation and achievement, submitted for the doctoral degree at Tallinn University of Technology, has not been submitted for any academic degree. /Andrus Räämet/ Copyright: Andrus Räämet, 2010 ISSN 1406-4766 ISBN 978-9949-23-008-2 3 EHITUS F29 Läänemere lainetuse tingimuste ajalis-ruumiline muutlikkus ANDRUS RÄÄMET Contents List of tables............................................................................................................. 6 List of figures ........................................................................................................... 6 Glossary ................................................................................................................. 10 Introduction ........................................................................................................... 11 Surface waves in the changing world ................................................................. 11 The Baltic Sea – a challenge for surface wave research ..................................... 13 Outline of the thesis ............................................................................................ 17 Approbation of the results .................................................................................. 18 Acknowledgements ............................................................................................ 19 1. Wave climate studies in the North Atlantic region and the Baltic Sea .... 20 1.1. North Atlantic ........................................................................................ 20 1.2. North Sea ............................................................................................... 26 1.3. Instrumental wave measurements in the northern Baltic Sea ................ 27 1.4. Visual observations from the eastern coast of the Baltic Sea ................ 30 1.5. Wave climate studies for the northern Baltic Sea .................................. 33 2. Wave model and wind data ......................................................................... 38 2.1. Introduction ........................................................................................... 38 2.2. State-of-the-art wave models ................................................................. 39 2.3. WAM model setup ................................................................................ 41 2.4. Wind forcing .......................................................................................... 42 2.5. Model performance................................................................................ 44 3. Wave statistics and seasonal variations ...................................................... 51 3.1. Introduction ........................................................................................... 51 3.2. Distribution of wave heights .................................................................. 52 3.3. Wave fields in extreme storms .............................................................. 56 3.4. Seasonal variability ............................................................................... 60 3.5. Stormy and calm seasons ....................................................................... 63 4. Wave climate changes .................................................................................. 66 4.1. Introduction ........................................................................................... 66 4.2. Long-term trends in significant wave height ......................................... 67 4.3. Wave heights over stormy and ice seasons ............................................ 73 4.4. Modelled extreme wave heights ............................................................ 76 4.5. Wave periods and directions .................................................................. 77 Conclusions ............................................................................................................ 84 Summary of the results ....................................................................................... 84 Main conclusions proposed to defend ................................................................ 86 Recommendations for further work .................................................................... 87 Bibliography .......................................................................................................... 90 Abstract ................................................................................................................ 100 Resümee ............................................................................................................... 101 Appendix A: Curriculum Vitae ......................................................................... 102 Appendix B: Elulookirjeldus ............................................................................. 106 5 List of tables Table 1. Average observed or measured and hindcast wave properties at the measurement sites in the northern Baltic Proper and the Gulf of Finland (Broman et al., 2006; Zaitseva-Pärnaste, 2009). For visual observation sites the average of daily mean values is presented (Soomere and Zaitseva, 2007) ............................... 46 Table 2. Correlation coefficients (r), biases (in cm) and standard deviations (STD, in cm) between significant wave height time series based on different winds at Almagrundet ........................................................................................................ 48 Table 3. Correlation coefficients (r), biases (in cm) and standard deviations (STD, in cm) between measurements and significant wave height time series based on different winds in the northern Baltic Proper (NBP) ......................................... 49 Table 4. Correlation coefficients between the annual mean wave heights at Vilsandi, Pakri and Narva-Jõesuu. The upper right cells show correlations for 1957−2008 (also separately for 1957−1986 and 1987−2008 for Vilsandi and Narva-Jõesuu); the lower left cells show the relevant p-values (in italic) .............. 68 Table 5. Correlation coefficients (r), biases (in cm) and standard deviations (STD, in cm) between numerically simulated and observed time series of the annual mean wave heights at Vilsandi, Pakri and Narva-Jõesuu ............................ 69 Table 6. Correlation coefficients (r), biases (in cm) and standard deviations (STD, in cm) between numerically simulated and observed time series of the mean wave heights at Vilsandi, Pakri and Narva-Jõesuu, calculated for the time periods from 1 July to 30 June of the subsequent year. ..................................................... 75 List of figures Figure 1. Location scheme of the Baltic Sea .................................................... 14 Figure 2. Location of coastal observation sites (filled circles) and instrumental measurement sites (crossed circles) from where the data is used in this study ....... 28 Figure 3. Weather map from the EMHI showing air-pressure isolines (a) and the corresponding wind field restored from the geostrophic wind database (b) at 03:00 GMT, 7 November 1996 ............................................................................... 43 Figure 4. Wave heights (thin line) at Almagrundet (A), October 2000. Grey and bold lines show the hindcast of the WAM model forced with MESAN (M) and geostrophic (G) winds. The biases and standard deviations (cm) are: biasM-G = 23.4; biasA-M = 24.1; biasA-G = 47.4; STDM-G = 37.7; STDA-M = 48.3; STDA-G = 72.1 ..... 45 Figure 5. Measured (thin line) and hindcast with the use of geostrophic winds (bold line, no MESAN winds available for this time) significant wave heights at Almagrundet in December 1986. The bias and standard deviation (STD) between 6 the modelled (average 1.34 m) and measured (1.52 m) data were 18.7 and 63.9 cm, respectively. The correlation coefficient was 0.78. Note that the bias and STD between the observed and modelled data at Almagrundet in 1999 were 19 and 45 cm, respectively, for the HYPAS model and MESAN winds (Jönsson et al., 2002)………. .......................................................................................................... 45 Figure 6. Scatter plot of measured and numerically simulated wave heights at Almagrundet in 1991. The brightness scale shows the number of wave conditions in pixels with dimensions of 0.05×0.05 m. The overall bias is 21.1 cm (observed waves are generally higher) and the STD is 54.1 cm. ............................................. 46 Figure 7. Wave heights (www.fimr.fi/en/tietoa/veden_liikkeet/en_GB/aaltoennatyksia/, thin line) in the northern Baltic Proper at the location of the FMI wave buoy (wb) in December 1999. Notations are the same as for Figure 4. The biases and standard deviations (cm) are: biasG-M = 11.7; biaswb-M = 77.0; biaswb-G = 68.0;