Wind versus air pressure seiche triggering in the Middle Adriatic coastal waters (original) (raw)
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High-frequency atmospherically-induced oscillations in the middle Adriatic coastal area
Annales Geophysicae, 2006
Temporal and spatial characteristics of the resonant coupling between travelling air pressure disturbances and the middle Adriatic coastal waters are examined using a barotropic numerical model for a one year period (July 2000-July 2001). The model is forced by the travelling air pressure disturbances reconstructed from the 2-min resolution air pressure series measured at Split. Six experiments for the studied period are performed, in order to analyse the influence of the speed and disturbance direction on the resonant coupling. The first group of three experiments uses variable disturbance direction, whereas in the second three, a constant direction is employed during the whole experiment. Disturbance direction for the first group of experiments is computed from the 500-mb geopotential data provided by European Center for Medium Range Weather Forecast (ECMWF), as it is found that all of the past extreme events are correlated with them. Each experiment, with variable and constant disturbance direction, is repeated with three different constant values of 10, 20 and 30 m/s for the disturbance speed. The model verification on the Split sea level data reveals that the model reproduces most of the events but also overestimates/underestimates some of them and creates some false events due to the rigid assumption of a constant disturbance speed. The best agreement with data is obtained in the model runs assuming a disturbance speed of 20 m/s. A number of trapped and edge waves have been modelled at the constrictions and along the coast, in particular on a shoal that lies off Split perpendicular to the channel axis. The importance of the disturbance direction to the energy content is highlighted, particularly close to the shore, where the difference may be significant at 2-3 times on average, up to 30 cm in maximum amplitude.
Resonant coupling of a traveling air pressure disturbance with the east Adriatic coastal waters
Journal of Geophysical Research, 2004
1] Exceptional sea level oscillations and strong current reversals, observed in the east Adriatic on 27 June 2003, are analyzed using available meteorological and tide-gauge measurements and numerical model results. It is shown that the variability in the atmosphere-sea system was characterized by high frequencies (0.01-0.1 min À1 ) and sea level and current amplitudes surpassing 1 m and 1 m s À1 , respectively. The event was related to the occurrence of a gravity disturbance in the atmosphere above the Adriatic. The disturbance traveled toward the east-southeast at a speed of 22 m s À1 and was resonantly coupled with a wave in the sea 50 m deep. The resulting forced wave was further amplified when entering funnel-shaped bays opened to it. There are indications that the forcing disturbance and its counterpart in the sea excited normal modes of the bays and harbors in the area. Moreover, nonlinear steepening of the forced wave seemingly occurred, resulting in the formation of high-frequency wave trains. This process, along with the generation of coastal seiches, could explain the observed and modeled differences between the spectra of coastal variability and the spectrum of forcing wave.
Annales Geophysicae, 2005
The paper documents resonant coupling between an air pressure travelling disturbance and the Middle Adriatic coastal waters, examined theoretically by using a barotropic numerical model and then comparing the model to the observed events. The model is forced first with a cosine and box air pressure disturbance travelling with a constant speed and direction but varying its eulerian period/duration, whereas the sensitivity to the disturbance speed is examined by varying it, keeping all the other parameters constant. Larger resonant transfer of energy is documented for shorter disturbances and for the box versus cosine variation, especially in the high-frequency domain. Bands of enlarged sea level amplification are found in front of Stari Grad Bay (6.7 and 11.0 min) close to its natural modes (6.1 and 10.6 min), which may be a reason for the very large amplification and sporadic severe floods observed at the inner bay area. The lack of coincidence in the Stari Grad and Vela Luka observed floods may be partially a result of different disturbance speeds and energy transfer rates due to various depths off the bays, yet some additional factors (basin shape, real disturbance energy, disturbance direction) should be examined in the future.
Resonance in Ploče Harbor (Adriatic Sea)
Frequency was analyzed by applying stationary (spectral) and non-stationary (wavelet and filtering) analyses of the data to extract temporal characteristics of the fundamental seiches with a period of 30 min. Seiches are pertinent throughout the year, but their maximum amplitude doubles (up to 25 cm) during the summer. Modeling studies showed that seiches are primarily a result of incoming waves from the open sea, generated by resonant coupling with air pressure traveling waves. In contrast, direct wind forcing has a minor influence on seiche generation. Seiches endanger ferries and small moored boats as well as large cargo ships in harbors where strong currents (greater than 50 cm s-1) appear during extreme events.
A study of resonant oscillations in the Split harbour (Adriatic Sea)
Estuarine, Coastal and Shelf Science, 2003
The study examines the occurrence of Proudman resonance in front of the Split harbour (Adriatic Sea). The dataset comprises air and sea pressure (sea level) data collected at the harbour entrance during August to October 2000. The interval was characterized by rather strong synoptic disturbances that took place over the harbour. The analyses encompass empirical tools, such as timeseries analysis, high-and band-pass filtering, spectral and wavelet analyses, while the theoretical approach includes the conceptual model of the resonance. Resonance appears in front of the harbour and then propagates inward, covering periods between 7.7 and 28.5 min as a result of complex atmospheric gravity wave structure. Gain between sea level and air pressure equals 0.05À0.40 dbar/hPa (5À40 cm/hPa).
Journal of Geophysical Research, 2000
Low-frequency (0.01-0.1 cpd) variability of air pressure, wind, and sea level is examined through 6-to 8-year records of data collected at three locations along the east coast of the Adriatic and one on the west coast. Seasonal energy spectra show that processes at these timescales are more energetic in winter than in summer. There is substantial wind energy at timescales corresponding to planetary atmospheric waves. In order to explain the stronger-than-isostatic adjustment of sea level at low frequencies to the air pressure forcing, recorded in different parts of the Mediterranean, the present empirical analysis is based on a physically more tractable model, relating sea level slope to the air pressure gradient and wind stress integral. The multiple input regression and the cross-spectral analysis yield a spatially variable response: over the deeper sea region sea level slope is fully explained by isostatic adjustment to the air pressure gradient alone; over the shelf a much stronger-than-isostatic response (-1.7 cm/mbar) is greatly reduced (-1.3 cm/mbar), but not fully accounted for, by the action of wind. Next the multiple linear regression method is carefully reexamined; a simple statistical model is developed to show that in multiple-input linear models with mutually correlated inputs, small errors in one of the inputs produce biased estimates of all the response parameters. The apparent discrepancy between the theoretically predicted and the estimated response is attributed to the bias. 1. Introduction Response of the Mediterranean sea level to the atmospheric forcing has been extensively investigated. Only a few studies, however, are based on time series long enough to permit an analysis of variability at low frequencies. The results obtained for the Adriatic [Pasarid and Orlid, 1992] and some other parts of the Mediterranean, for Naples [Palumbo and Mazzarella, 1982] and the Aegean Sea [Tsimplis and Vlahakis, 1994], showed that oscillations of sea level at frequencies below 0.1 cpd are highly correlated to variations of air pressure; the adjustment of sea level by far surpasses the isostatic one, with sea-level-to-air-pressure gain reaching as much as -1.7 cm/mbar. In the latter two papers the simultaneous action of wind was considered and Copyright 2000 by the A•nerican Geophysical Union. Paper number 2000JC900023. 0148-0227 / 00 / 20003 C 900023509.00 it was concluded that the local wind cannot account for the observed inverse barometer (IB) overshoot. Resuits based on satellite altimetry data [Le Traon and Gauzelin, 1997] showed that not only the local sea level but also the Mediterranean-wide mean sea level significantly surpasses isostatic response to the air pressure variations at low frequencies. In a series of papers [Garrett, 1983; Garrett and Majaess, 1984; Candela et al., 1989; Candela, 1991], results of a two-basin, two-strait model of the Mediterranean Sea brought forward a controlling role of the Gibraltar and Sicily Straits. The constraining effects of the straits lead to a nonisostatic response of sea level to the air pressure forcing at synoptic frequencies, but at lower frequencies there is enough time for sea level to adjust and isostatic response is achieved. An extended, spatially resolvable model of the Mediterranean presented by Candela and Lozano [1994] produced a similar, frequency-dependent response of the mean sea level. In addition, the model reveals adjustment within 11,423 11,424 PASARI(• ET AL' ADRIATIC SEA LEVEL AND METEOROLOGICAL FORCING semienclosed basins to the local air pressure variations which is nearly isostatic at all off-resonant frequencies. The variations of sea level in the Adriatic at timescales between 10 and 100 days are primarily caused by the air pressure oscillations related to planetary atmospheric waves that travel above the Mediterranean [?enzar et al., 1980; Orli5, 1983; Lascaratos and Ga6i5, 1990; Pasari5 and Orli5, 1992]. However, the discrepancy between the isostatic adjustment predicted by the models and the much stronger-than-isostatic response observed in different parts of the Mediterranean suggests that there is another forcing, coherent with the air pressure, which is affecting sea level variability at planetary wave timescales. As wind is the obvious suspect, we shall investigate here the combined action of the wind and air pressure on the Adriatic sea level at low frequencies and reexamine the conclusion reached elsewhere in the Mediterranean that the observed overshoot is not due to the direct action of local wind. In this study the simple and often discussed question, does the sea level in the Mediterranean adjust to slow air pressure disturbances like inverse barometer or not, is addressed again. However, we focus on variability at planetary timescales. Long time series of air pressure, wind, and sea level data collected at four locations in the Adriatic are examined to retain the very slow processes. The problem, usually solved by regressing sea level on local weather variables, is approached here in a somewhat different way. The empirical analysis is based on a physically more tractable model, relating spatial gradients of sea level and air pressure. Finally, a simple statistical model, which may be of a wider interest, is proposed and used to point out the limitations of multiple linear regression, thus providing a novel interpretation of the empirical results. 2. Data The study of long-term variability requires analyzing long data records. In the Mediterranean, the eastern Adriatic coast is one of the areas best covered with tide gauge measurements [Zerbini et al., 1996]. However, 109-113, 1992. Tsimplis, M. N., and G. N. Vlahakis, Meteorological forcing and sea level variability in the Aegean Sea, J. Geophys. Res., 99, 9879-9890, 1994. Tsimplis, M. N., R. Proctor, and R. Wu, J., Wind stress coefficients over sea surface from breeze to hurricane, J. Ceophys. Res., 87, 9704-9706, 1982. Zerbini, S., et al., Sea level in the Mediterranean: A first step towards separating crustal movements and absolute sealevel variations, Global Planet. Change, 1J, 1-48, 1996.
Journal of Geophysical Research Atmospheres, 2007
1] In February 2003, we observed the response of the 40 to 50 m deep northern Adriatic Sea to strong surface forcing by 20 m s À1 winds and 600 W m À2 net upward heat flux resulting from cold bora winds blowing onto a relatively warm sea through gaps in the Croatian mountains. Ocean turbulence throughout the water column was observed with a microstructure profiler and a bottom-mounted, upward-looking, 5-beam, acoustic Doppler current profiler (ADCP). Microstructure-based dissipation rates (e) were close to similarity scaling of the surface wind stress. The surface buoyancy flux, related to the oceanic heat loss, contributed little energy to the turbulence, but led to sustained unstable stratification. The energy-containing range of the turbulence together with the upper end of the inertial subrange, with horizontal scales between a few hundred meters and about 10 m, contained coherent, anisotropic overturning motions aligned with the low-frequency, barotropic ocean currents which carried stress and showed an asymmetry between rare, narrow, faster downdrafts and diffuse, weak updrafts. These motions bear no similarity with Langmuir cells. The turbulence measurements were embedded in surveys of the mesoscale ocean variability. Part of the observations were set in a front a few hundred meters wide with little density contrast. As the bora wind relaxed, the front began to develop a highly stratified ''foot'' undergoing intense mixing. The paper addresses problems of beam spreading and instrumental noise in ADCPs.
Extreme Storms in the Adriatic Sea
Coastal Engineering 1990, 1991
We discuss the application of the third generation WAM wave model to the Adriatic Sea. We focus in particular on one of the extreme storms that produced also heavy flooding in Venice. We discuss the problem of a correct description of the wind fields as a crucial input information to the wave model. After hindcasting the wave conditions during the storm, we use them as input for an estimate of the wave setup towards the shore. We show that its consideration is essential for a proper evaluation of the flood level in the town.
Physics and Chemistry of the Earth, Parts A/B/C, 2009
The paper deals with a possibility to detect the presence and to follow the movement of an atmospheric disturbance that could lead to a meteotsunami formation. The analysis of the four strongest meteotsunami cases in the past 30 years in the Adriatic shows that three of four cases happened in the south-westerly stream on the front side of a large upper-level trough, whereas in one of the cases large-scale streaming was westerly and north-westerly. It was also noted that all cases occurred during summer months and in each case a convective system was present in the area. That led to the assumption that a convective system is responsible for causing the disturbance leading to meteotsunami formation. Since the convective cells can be recognized and their movement can be followed using the nowcasting tools based on satellite data, a method for predicting the occurrence of meteotsunamis in the Adriatic is proposed.
Northern Adriatic meteorological tsunamis: Observations, link to the atmosphere, and predictability
Journal of Geophysical Research: Oceans, 2012
A total of 16 events of tsunami-like sea level oscillations are documented in the northern Adriatic between 1955 and 2010. These oscillations, recorded at the long-term operating Rovinj tide gauge, are characterized by wave heights of up to 60 cm, periods of 20 to 150 min, and duration of 1 to 48 h. The sea level oscillations are found to be coincident with pronounced atmospheric pressure disturbances characterized by a 2-4 hPa air pressure change over 10 min. Convective activity is recognized as the most likely source of atmospheric pressure disturbances. Analysis of propagation speed and direction of the atmospheric pressure disturbances indicates that the sea level oscillations were generated and enhanced via the Proudman resonance over a wide and shallow northern Adriatic shelf. Typical conditions under which pronounced air pressure disturbances occur include an air pressure surface minimum centered over the northern Adriatic, a temperature front at a height of approximately 850 hPa, and a strong southwesterly jet stream with wind speeds reaching 20-30 m/s at a height of approximately 500 hPa over the northern Adriatic. Based on these parameters, a possibility for forecasting tsunami-like sea level oscillations from synoptic conditions is discussed. It appears that under favorable synoptic conditions sea level oscillations are more likely to occur than to not. However, no reliable conclusion on strength of an event can be reached from synoptic conditions only.