Wind and Wave Correlation in the Greek Seas. N.Aspiotis, S.Sofianos, N.Skliris and A.Lascaratos University of Athens, Department of Applied Physics

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1 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 219 Wind and Wave Correlation in the Greek Seas N.Aspiotis, S.Sofianos, N.Skliris and A.Lascaratos University of Athens, Department of Applied Physics Abstract In the present study, the wind-wave field in the Greek seas and its relation to the wind regime in the region for the period 01/ /2005 is investigated. Both data sets are derived from operational tools, namely the TRITON wave forecast system and SKIRON weather forecast system of the University of Athens. Analysis of the wave height time-series in the different sub-basins showed that the number of intense wave events is larger in the Ionian Sea compared to the Aegean Sea. Furthermore, the mean value of the maximum wave height in the Ionian Sea is higher, although the maximum wave height was recorded in the Aegean Sea (Ionian 9.02m, North Aegean 11.20m and South Aegean 11.80m). The wind and wave fields are well correlated (correlation coefficient over 0.86 in all sub-basins) and the wave fields lag the corresponding wind field by eighteen hours in the Ionian and fifteen hours in the Aegean Seas. The small difference of the wind/wave correlation in the various sub-basins can be attributed to the presence of swell in the region. It has also been found that intense wave events are associated with certain wind directions, different from basin to basin. Keywords: wind, wave, swell, Greece ΠερΙληψη Στη συγκεκριμένη εργασία μελετώνται τα ανεμογενή κύματα στις Ελληνικές θάλασσες και η σχέση τους με το καθεστώς του ανέμου για την περίοδο 01/ /2005. Τα κυματικά και ανεμολογικά δεδομένα έχουν προκύψει από επιχειρησιακά συστήματα και συγκεκριμένα, το σύστημα πρόγνωσης κυμάτων ΤΡΙΤΩΝ και το σύστημα πρόγνωσης καιρού ΣΚΙΡΩΝ του Πανεπιστημίου Αθηνών. Η ανάλυση των χρονοσειρών των υψών κύματος στις διάφορες υπολεκάνες έδειξε ότι στο Ιόνιο λαμβάνει χώρα μεγαλύτερος αριθμός έντονων κυματικών φαινομένων από ότι στο Αιγαίο. Επιπλέον, η μέση τιμή των μεγίστων υψών των κυμάτων στο Ιόνιο είναι μεγαλύτερη παρόλο που το μέγιστο παρατηρούμενο ύψος κύματος κατεγράφη στο Αιγαίο (Ιόνιο 9.02m, Βόρειο Αιγαίο 11.20m και Νότιο Αιγαίο 11.80m). Η συσχέτιση των πεδίων ανέμου και κυμάτων είναι ιδιαίτερα υψηλή (συντελεστής συσχέτισης πάνω από 0.86 σε όλες τις υπολεκάνες) αλλά τα πεδία των κυμάτων υστερούν χρονικά σε σχέση με αυτά του ανέμου 18 ώρες στο Ιόνιο και 15 ώρες στο Αιγαίο. Η μικρή αυτή διαφορά συσχέτισης κύματος- ανέμου στις διάφορες υπολεκάνες μπορεί να αποδοθεί στην εμφάνιση αποθαλασσίας(swell) στη συγκεκριμένη περιοχή. Επίσης βρέθηκε ότι τα έντονα κυματικά επεισόδια σχετίζονται με συγκεκριμένες διευθύνσεις ανέμου, διαφορετικές από λεκάνη σε λεκάνη.

2 220 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Introduction Due to the very complicated geography and topography of the Greek seas and the strong variability of the wind in the region, in terms of speed and direction, the wind-wave field presents important spatial and temporal variability. This is investigated in the different sub-basins (North Aegean, South Aegean and the Ionian Seas) using results from wave and wind operational tools, namely the TRITON wave forecast sys- Table 1. Results from the statistical analysis of waves for the period 01/ /2005 Maximum wave height observed Standard deviation during intense events Mean of maximum height Number of intense wave events during intense events Standard deviation Mean significant wave height N.Aegean Sea 1.62m 1.18m 4.32m m 11.20m S.Aegean Sea 1.97m 1.07m 4.37m m 11.80m Ionian Sea 2.09m 1.12m 4.42m m 9.02m Figure 1. Distribution of the intense wave events in the investigated time period for the North Aegean, South Aegean and the Ionian Seas.

3 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 221 tem and SKIRON weather forecast system, developed and maintained in the University of Athens. In the present work, we focus mainly on the intense wave events in each area and their correlation to the dominant speed and direction of the local wind. Method The core of the TRITON Aegean and Ionian Seas wave forecast system is based on the WAM model code (WAMDI group, 1988; Komen et al., 1994). WAM (CYCLE 4) is a third generation wave model, which computes spectra of random short-crested wind-generated waves. The WAM code used in the Aegean and Ionian Seas forecast system was modified in order to get reliable wave forecasts especially in coastal areas, dissipation by depth induced wave breaking, etc.(g. Korres, 2002). The Aegean and Ionian Seas wave forecast system runs daily (1 cycle) using atmospheric data (wind speed and direction etc.) from the SKIRON weather forecast system (Kallos, 1997) with a spatial resolution of 0.24 x 0.24 deg and provides 72-hours forecast ahead. The wind data that have been used in this study are also derived from the SKI- RON system. The wave forecast system issues 66-hour forecasts of significant wave height and direction. The Aegean and Ionian Seas wave model is nested to the Mediterranean and Black Sea wave model. It covers the geographical area 19E 30E and 33N-43N with a spatial resolution of 1/10 x 1/10 deg. In the present study we investigate sixteen months of wind and wave data (01/ /2005) considering only the first 24 hours of forecasting. The resulting time series were statistically analyzed focusing mainly on the relation between the wind field and the generated waves in each area under investigation. The time series were also analyzed in the form of intense wave events, defined here as those Figure 2. Wind/wave correlation as a function of time-lag (in hours) for the North Aegean (blue), South Aegean (green) and the Ionian Seas (red).

4 222 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Figure 3. Percentage of intense wave events corresponding to different directions of the wind field for the N. Aegean and S. Aegean and the Ionian Seas. periods that the wave height exceeds the mean value plus one standard deviation. Results The analysis of the time series revealed several interesting aspects of the spatio-temporal variability of the wave field in the region. During the time period that we examined, sixty-seven intense events occurred in the Ionian Sea, sixty in the South Aegean and fifty-nine in the North Aegean. The Ionian Sea presents higher mean value of maximum significant wave height in these events than the Aegean Sea although the maximum observed wave height occurred in the Aegean Sea (table 1). From the temporal distribution of the intense wave events, during the time period that we examined, it is clear that the wave field follows the seasonal variability of the wind (figure 1). Most of the events occur during winter with only a small number of events occurring during summer in all sub-basins. The correlation of the wind and wave time series was found large (Ionian Sea 0.865, South Aegean Sea 0.880, North Aegean Sea 0.925), with a significant time lag between the wind and wave fields. In the Ionian Sea the wave lags the wind by eighteen hours while in the Aegean Sea subbasins the lag is fifteen hours (Figure 2). Another finding is that certain wind directions in each area under investigation are related to the generation of intense wave events (Figure 3). In the Aegean more preferable wind direction to gen-

5 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 223 erate intense wave events is the northern one. Thirty six percent of the generated main wave events in the North Aegean and thirty-eight in the South Aegean were generated by northerly winds, while sixty five percent of the events were generated by winds of the northern quadrant (north, northeast and northwest) direction. On the other hand, in the Ionian the distribution is more even with most preferable direction the south-eastern (twenty two percent). Conclusions Investigation of sixteen months wind and wave fields in the Greek Seas, using operational modelling results, showed an important spatial and temporal variability. The basic findings include a high correlation of the two fields with a time lag of fifteen to eighteen hours. In the Ionian Sea the significant wave height was found higher and a larger number of intense wave events take place, in comparison to the Aegean Sea. The distribution of intense wave events in the different sub-basins is related to different directions of the wind field. Further investigation, in space and time, of the wind-wave variability in the region using numerical model results as well as available observation will help establish the dominant patterns and better understand the mechanisms involved. References Kallos, G., 1997: The Regional weather forecasting system SKIRON. Proceedings of the Symposium on Regional Weather Prediction on Parallel Computer Environments, October 1997, Athens, Greece. Pp 9. Komen, G.J, L.Cavaleri, M.Donelan, K.Hasselmann, S.Hasselmann, P.A.E.M.Janssen, 1994: Dynamics and Modelling of Ocean Waves. Cambridge University Press, 532 pp. Korres G., 2002: see model details in WAMDI group, 1988: The WAM model a third generation ocean wave prediction model. J. Phys. Oceanogr., 18,

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