Abstract
Wind direction measurements from anchored floating objects (buoys) are disturbed by the movement of an agitated sea surface. This article describes corrected wind direction measurements that ...account for a rotated and tilted anemometer, in which the rotations around the Euler angles “roll,” “pitch,” and “yaw” are simultaneously measured on coastal buoy “Vida” in Slovenian waters of the Gulf of Trieste. The wind data analysis is composed of a short-term analysis during a 4-h summer storm and a long-term analysis of wind direction for an 8-yr period. In the short-term analysis, the time evolution of the spectral composition of the anemometer’s translation and wind speeds revealed the dominance of 3–4-s oscillations that are mostly caused by waves. Anemometer velocity may account for up to 5% of apparent wind speed. In the long-term analysis, wind direction measurements were validated against those from two other stations in the Gulf of Trieste, as well as from the Acqua Alta tower in front of the Venice lagoon; all three are located in Italian waters. Short-term analysis of a summer storm and long-term measurements from four stations show that correcting the wind direction with the buoy’s orientation accounts for about 10°–12° of the error in wind direction.
Seawater Icicles of the Adriatic Sea Malacic, Vlado; Zagar, Nedjeljka
Bulletin of the American Meteorological Society,
06/2019, Letnik:
100, Številka:
6
Journal Article
Recenzirano
Odprti dostop
The marine icicles that form on coastal constructions (e.g., piers and railings) are very common in polar and subpolar areas. The occasional formation of icicles in the coastal zones of the ...Mediterranean Sea occurs in relation to cold-air outbreaks from the polar region, such as the one presented in this paper in February 2018. The air temperatures over the northern Adriatic Sea, the northernmost part of the Mediterranean, dropped below −2.1°C, a necessary condition for seawater to freeze, with salinity between 38.0 and 38.5 PSU. The formation of icicles on the coastal structures was further enabled by the bora wind and related high seas along the coast. Measurements presented in this paper confirm that the icicles in the Bay of Piran in the Gulf of Trieste (45.55°N) were formed from the seawater. The measured salinity level of the melted icicles, around 9 PSU, is a typical value reported for marine icicles in polar regions.
Under the emerging features of interannual-to-decadal ocean variability, the periodical reversals of the North Ionian Gyre (NIG), driven mostly by the mechanism named Adriatic-Ionian Bimodal ...Oscillating System (BiOS), are known as impacting on marine physics and biogeochemistry and potentially influencing short-term regional climate predictability in the Eastern Mediterranean. Whilst it has been suggested that local wind forcing cannot explain such variability, aspects of the alternative hypothesis indicating that NIG reversals mainly arises from an internal ocean feedback mechanism alone remain largely debated. Here we demonstrate, using the results of physical experiments, performed in the world's largest rotating tank and numerical simulations, that the main observed feature of BiOS, i.e., the switch of polarity of the near-surface circulation in the NIG, can be induced by a mere injection of dense water on a sloping bottom. Hence, BiOS is a truly oceanic mode of variability and abrupt polarity changes in circulation can arise solely from extreme dense water formation events.
We present an adaptation and application of frequent subgraph mining (FSM) in a time series of spatial multi-level directed graphs depicting probabilistic transitions of water masses between ...neighboring sea areas within a given time interval. The directed graphs are created from the results of the numerical model, the Mediterranean Ocean Forecasting System. We assign unique labels (geographical locations) to vertices of the multi-level directed graphs. Then, we add the edge labels as discretized values of the probabilities of transitions between vertices. This modification allows the use of the established algorithm gSpan to search for frequently directed subgraphs in the sequence of such directed graphs. Thus, we obtain both general and specific subgraphs, such as convergences, divergences, and paths of the ocean currents in the numerical model. The resulting substructures, revealed by directed subgraphs, match oceanographic structures (gyres, convergences/divergences, and paths) deduced from field observations, and can also serve as a tool for the validation of the numerical model of circulation in the sea.
Although small in size, the Gulf of Trieste (GoT), a marginal coastal basin in the northern Adriatic Sea, is characterized by very complex dynamics and strong variability of its oceanographic ...conditions. In April–May 2012, a persistent, large-scale anticyclonic eddy was observed in the GoT. This event was captured by both High Frequency Radar (HFR) and Lagrangian drifter observations collected within the European MED TOSCA (Tracking Oil Spill and Coastal Awareness) project. The complexity of the system and the variety of forcing factors constitute major challenges from a numerical modeling perspective when it comes to simulating the observed features. In this study, we implemented a high-resolution hydrodynamic model in an attempt to reproduce and analyze the observed basin-wide eddy structure and determine its drivers. We adopted the Massachusetts Institute of Technology General Circulation Model (MITgcm), tailored for the GoT, nested into a large-scale simulation of the Adriatic Sea and driven by a tidal model, measured river freshwater discharge data and surface atmospheric forcing. Numerical results were qualitatively and quantitatively evaluated against HFR surface current maps, Lagrangian drifter trajectories and thermohaline data, showing good skills in reproducing the general circulation, but failing in accurately tracking the drifters. Model sensitivity to different forcing factors (wind, river and tides) was also assessed.
The attenuation coefficients of photosynthetically active radiation (PAR) were extracted from the vertical profiles of PAR in coastal waters (the Gulf of Trieste, Northern Adriatic). The vertical ...profiles were collected roughly twice per month from July 2011 to December 2015, and the PAR values just above the sea surface were compared with the PAR data measured on a buoy.This research supports the nonlinear fit with the biexponential expression for the dependence of PAR with depth, yielding a much better match with the data than the fit with a mono-exponential expression. However, another reasoning for biexponential attenuation is because it functions as a solution for a homogeneous differential equation of a second order. The method for estimating the water type is offered with an analysis of the attenuation coefficients of PAR. It was found that for a particular location (the Gulf of Trieste), the attenuation coefficient in a mono-exponential decrease of PAR is 0.19-0.21 m-1, while for a biexponential decrease of PAR, the coefficient of the long-range attenuation is 0.12-0.14 m-1 and that of short-range attenuation is 0.8-0.9 m-1. This leads to the conclusion that most water columns match coastal water type 1, while the surface layer is represented by coastal water types 7 or 9. From the estimate of the water types, the coefficients of downward irradiation were inferred as 0.19 m-1∓0.01 m-1 (long-range attenua-tion) and 3.0 m-1∓0.7 m-1 (short-range attenuation). These coefficients can determine the heat source inside the water column.
Koeficijenti slabljenja fotosintetski aktivnog zračenja (PAR) izvučeni su iz vertikalnih profila PAR-a u obalnim vodama (Tršćanski zaljev, Sjeverni Jadran). Okomiti profili sakupljani su otprilike dva puta mjesečno od srpnja 2011. do prosinca 2015. godine, a vrijednosti PAR-a neposredno iznad morske površine uspoređivane su s podacima PAR-a izmjerenim na uvali.Ovo istraživanje podržava nelinearno uklapanje s bieksponencijalnim izrazom za ovisnost PAR-a o dubini, što daje puno bolju podudarnost s podacima nego uklapanje s mono-eksponencijalnim izrazom. Međutim, drugo je obrazloženje bieksponencijalnog slabljenja zato što ono funkcionira kao rješenje za homogenu diferencijalnu jednadžbu drugog reda. Ponuđena je metoda za procjenu vrste vode s analizom koeficijenata prigušenja PAR. Utvrđeno je da za određeno mjesto (Tršćanski zaljev) koeficijent slabljenja u mono-eksponencijalnom smanjenju PAR iznosi 0,19–0,21m–1, dok je za bieksponencijalno smanjenje PAR-a koeficijent dugog prigušenja je 0,12–0,14m–1, a slabljenje kratkog dometa 0,8–0,9m–1. To dovodi do zaključka da se većina vodenih stupova podudara s obalnim vodama tipa 1, dok površinski sloj predstavljaju obalni tipovi voda 7 ili 9. Iz procjene tipova voda zaključeni su koeficijenti zračenja prema dolje kao 0,19m–1±0,01m–1(dugo prigušenje) i 3,0m–1±0,7m–1(prigušenje kratkog dometa). Ovi koeficijenti mogu odrediti izvor topline unutar vodenog stupca.
Understanding animal movement patterns is not only important for providing insight into their biology, but is also relevant to conservation planning. However, in aquatic and wide-ranging species such ...as cetaceans, this is often difficult. The common bottlenose dolphin (Tursiops truncatus) is the most common cetacean in the northern and central Adriatic Sea and has been the focus of long-term studies in some areas. All of the studied local populations show a relatively high degree of site fidelity, but their movements, ranging patterns or connectivity are not well understood. On 24 and 26 April 2014 a single adult bottlenose dolphin was observed and photographed alive off the Slovenian coast. The same individual was found dead on the shores of Goro, Italy, on 5 May 2014, about 130 km from the two sighting locations. The well-marked dorsal fin made the identification straightforward. The dolphin was found freshly dead, suggesting it had died very recently prior to being found. This indicates that the reported movement was a real one, rather than an artefact of currents. Although single cases cannot provide the basis for making population-level inferences, our observation shows that northern Adriatic bottlenose dolphins can make substantial movements in short periods of time and suggests that such movements could be more common than currently documented. Comparisons among photo-ID catalogues and stranding events can be highly informative, as they can provide useful information with implications for the cross-border conservation of mobile marine predators.
In this study, we investigate the dynamics of the bottom layer of the southern Adriatic Sea (eastern Mediterranean basin) by merging experimental measurements and numerical simulations. We ...hypothesize that the recently observed continuous density decrease over time, which was basically related to a temperature increase, and the following sudden density rise, which was caused by the intrusion of very dense water masses (cold but relatively fresh), constitute one cycle of a general “saw‐tooth” pattern: the alternation of long‐lasting and almost linear density decreases (mixing phases) and sudden density increases (dense water intrusion phases). The model results, which provide a basin‐scale view of the process, corroborate this theory because they satisfactorily reproduced the observed oceanographic features. We describe the almost linear density decrease in terms of local mixing fostered by the advection of flow instabilities that originate from the large‐scale quasi‐permanent cyclonic circulation. Conversely, diffusive processes play a minor role in determining the bottom layer thermohaline variability. The interpretation of the experimental findings, supported by the numerical simulations, suggests that similar dynamics might be observed in other basins characterized by similar bathymetric and hydrodynamic features.
Key Points
Local mixing fostered by flow instabilities induces long‐term, positive thermohaline trends in the bottom of the southern Adriatic Sea
Only exceptional production and sinking of large amounts of very dense shelf water can cause drastic bottom water renewal
The hypothesized saw‐tooth modulation of the deep‐water thermohaline properties could be observed in similar basins in the world ocean
High resolution high frequency (HF) radar observations of surface currents in the Gulf of Trieste (GoT) are presented and compared to moored subsurface current measurements and to high‐resolution ...simulations from the Northern Adriatic Princeton Ocean Model (NAPOM) oceanographic model. Dominant circulation modes of the GoT were resolved and numerical model capabilities in reconstructing them were assessed. The time frame covers March 2011 through October 2012. NAPOM reconstructs the dominant surface circulation features observed by the radar, such as the general basin‐wide cyclonic circulation scheme and the coastal jet outflowing the GoT, but is in general less energetic than radar currents. Comparisons between radar, ADCP, and model currents suggest that the model underestimates originate within the low‐frequency, diurnal, semidiurnal, and inertial bands, and that both radar and model currents do not reproduce the diurnal tidal ellipse structure observed by the moored current meter. However, radar‐model results for the semidiurnal tides are spatially consistent. Using empirical orthogonal function (EOF) decomposition, the coherent spatial scales and corresponding temporal scales were extracted. Findings suggest that HF radar current observations complement model simulations in regions of enhanced topographic variability where variance of model currents at the surface is distorted by the effects of the sigma layer grid. On the contrary, model results complement radar observations in areas with poor radar coverage, and furthermore provide spatial and temporal continuity of ocean state forecasts.
Key Points
Dominant surface current patterns in the Gulf of Trieste were studied
Capabilities of high‐resolution regional circulation model were assessed
HF radars and model are complementary tools for identifying circulation features
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