Ivica Janeković

Tidal currents in the northwestern Adriatic: High‐frequency radio observations and numerical model predictions

The results of a two-year long deployment of high frequency radars along the Italian coast of the northern Adriatic are analyzed, to characterize the surface tidal currents. M2 and K1 ellipses are aligned with the basin axis and exhibit large eccentricities in the middle of the basin, decreasing toward the Italian coast. Comparisons are made with a 3D finite-element non-linear numerical model of the tides. Complex correlations between modeled and observed tidal currents show a remarkable agreement in the middle of the basin, with magnitudes reaching 0.985 and average phases of -6.4 deg. However the magnitudes drop to 0.5 within 20-30 km from the Italian coast, where the modeled currents amplitudes are underestimated by 2cm/s and the phases lag the observed phases by 60 deg. This shallow region (less than 30-m deep) is characterized by low-salinity water originating at the Po River and laterally sheared coastal flows. The radars may have captured the influence of stratification or mean sheared flows, both absent from the model. The model parameterization of bottom friction may also inadequately represent the effects of real bottom friction on the vertical current shear.

Extreme cooling and dense water formation estimates in open and coastal regions of the Adriatic Sea during the winter of 2012

Generation of dense waters in the Adriatic Sea during an extreme cooling event in the winter of 2012, including its preconditioning and spreading phases, have been investigated using the one-way coupled ROMS and the ALADIN/HR modeling system. Both climatological and real river fluxes are used in the simulations. Aside from the “convenient” dense water formation areas located at the northernmost Adriatic shelf, we found that a similar amount of dense water, with slightly lower density, was formed in the eastern and deeper Adriatic coastal area, which was subjected to extreme heat losses (up to 2000 W/m2) during peak cooling periods. This part of the Adriatic has been known for extreme cooling during wintertime bora outbreaks ; nevertheless, no ocean model study has previously reproduced dense water formation in this area. The most likely reason for that was an overestimate of river discharges introduced to ocean models. From newly available data, we estimated that the contribution of eastern Adriatic rivers between the Neretva River and Rijeka Bay is more than six times lower than what has been previously documented. Transport of dense water toward the middle Adriatic had a peak value of about 0.6 Sv, while the speed of initial bottom density current surpassed 40–50 cm/s, which is several times faster than past events. Different pathways of the dense water toward the middle and south Adriatic depressions have also been documented. The contribution of the eastern coastal Adriatic area to the overall north Adriatic dense water formation has been quantified and discussed for average and low freshwater load conditions, indicating that this part of the Adriatic is a common place for dense water generation.

Sensitivity of Internal Tide Generation in Hawaii

Energy from the barotropic tide is transferred into the baroclinic tide over topographic gradients, which provides a mechanism for the ocean boundaries to communicate with the deep ocean, to close energy budgets, and as a source of flux affecting nutrient supply and larval transport. Understanding the temporal variability of the conversion from barotropic to baroclinic tides is critical to our understanding of these processes. Using a numerical model and its adjoint, we examine the sensitivity of tidal conversion at Kaena Ridge in Hawaii. We find a sensitivity to changes in the upper ocean due to a phase difference between the pressure anomaly and tidal velocity caused by internal waves generated on the opposite slope of the ridge; however, we also find that conversion is equally as sensitive to local, deep stratification changes.

The Adriatic Sea M2 and K1 tides by 3D model and data assimilation

In the present work we explore the impact of assimilating local tide-gauge and altimetric data on the quality of predicting the major Adriatic tides (M2 and K1). To that end we compute optimal tidal open boundary conditions for a 3D high-resolution finiteelement model by using an incremental assimilation formalism. The essence of the method is the use of two dynamical models where the solution in the complex 3D high-resolution model is sought via assimilation of prediction errors into the simpler 2D model with explicit inverse. In the central numerical experiment, harmonic constants from 12 tide gauges are assimilated and the results are analysed at 31 locations, hence 19 independent ones. The data assimilation contributes to the reduction of maximum amplitude error from 5.6 to 0.5 cm for M2 and from 3.9 to 0.1 cm for K1. The assimilation procedure is repeated by assimilating suitably processed Topex/Poseidon altimeter data, again validating the outcome at 31 tide gauge locations. The result was very similar to the gauge-data assimilation outcome. The model output is also validated with the current data, not used in the assimilation. At two locations and at three depths the model was able to reproduce the major and the minor semi-axes of tidal ellipses, as well as their orientations very well. 2003 Elsevier Ltd. All rights reserved.

Self-Organizing Maps-based ocean currents forecasting system

An ocean surface currents forecasting system, based on a Self-Organizing Maps (SOM) neural network algorithm, high-frequency (HF) ocean radar measurements and numerical weather prediction (NWP) products, has been developed for a coastal area of the northern Adriatic and compared with operational ROMS-derived surface currents. The two systems differ significantly in architecture and algorithms, being based on either unsupervised learning techniques or ocean physics. To compare performance of the two methods, their forecasting skills were tested on independent datasets. The SOM-based forecasting system has a slightly better forecasting skill, especially during strong wind conditions, with potential for further improvement when data sets of higher quality and longer duration are used for training.

Diversity, occurrence, and habitats of the diatom genus Bacteriastrum (Bacillariophyta) in the northern Adriatic Sea, with the description of B. jadranum sp. nov.

Abstract Ongoing phytoplankton monitoring programs over the last 12 years in the northern Adriatic Sea showed that the diatom genus Bacteriastrum is an important component of the pelagic diatom assemblage. The main goals of this study were to identify which Bacteriastrum species occur in the northern Adriatic Sea and describe their distributions. Bacteriastrum comosum and B. hyalinum f. princeps were rare, while B. delicatulum, B. hyalinum, and B. mediterraneum were frequently present, together with the newly proposed species B. jadranum, described herein. This new species is weakly silicified and is mainly characterized by a large aperture between each cell of the filament. Cells are connected to each other by 8–12 setae, which cross at a distance equivalent to the diameters of 2–3 cells. There is no difference in the orientation of the setae on the two terminal cells, hence the species is placed in the section Isomorpha. The new species was recorded throughout the study area, in both coastal and offshore waters, with higher cell abundances in the vicinity of the Po River during the September–October period. The maximum cell abundance of B. jadranum was 244.2×103cells l-1in the surface waters at 44.75666667ºN, 12.75ºE in September 2006, sometimes representing up to 46% of the entire phytoplankton assemblage, as recorded in the surface waters at 45.06833333ºN, 13.515ºE in October 2008.

Chrystal and Proudman resonances simulated with three numerical models

The aim of this work was to study Chrystal and Proudman resonances in a simple closed basin and to explore and compare how well the two resonant mechanisms are reproduced with different, nowadays widely used, numerical ocean models. The test case was based on air pressure disturbances of two commonly used shapes (a sinusoidal and a boxcar), having various wave lengths, and propagating at different speeds. Our test domain was a closed rectangular basin, 300 km long with a uniform depth of 50 m, with the theoretical analytical solution available for benchmark. In total, 2250 simulations were performed for each of the three different numerical models: ADCIRC, SCHISM and ROMS. During each of the simulations, we recorded water level anomalies and computed the integral of the energy density spectrum for a number of points distributed along the basin. We have successfully documented the transition from Proudman to Chrystal resonance that occurs for a sinusoidal air pressure disturbance having a wavelength between one and two basin lengths. An inter-model comparison of the results shows that different models represent the two resonant phenomena in a slightly different way. For Chrystal resonance, all the models showed similar behavior; however, ADCIRC model providing slightly higher values of the mean resonant period than the other two models. In the case of Proudman resonance, the most consistent results, closest to the analytical solution, were obtained using ROMS model, which reproduced the mean resonant speed equal to 22.00 m/s— i.e., close to the theoretical value of 22.15 m/s. ADCIRC and SCHISM models showed small deviations from that value, with the mean speed being slightly lower—21.97 m/s (ADCIRC) and 21.93 m/s (SCHISM). The findings may seem small but could play an important role when resonance is a crucial process producing enhancing effects by two orders of magnitude (i.e., meteotsunamis).

Rogoznica Lake, a Euxinic Marine Lake on the Adriatic Coast (Croatia) that Fluctuates Between Anoxic Holomictic and Meromictic Conditions

Rogoznica Lake is a typical example of euxinic marine lake, situated on the eastern Adriatic Coast, Croatia (43°32′ N 15°58′ E). It is a karstic depression with an area of 10.276 m2, filled with seawater with a maximum depth of 15 m. The lake is stratified during the spring–summer season, both thermally and in its salinity. There are oxic upper water layer and anoxic deeper layer. The mixolimnion varies seasonally in depth and thickness for about 1 m due to influence of meteorological conditions (temperature, wind, rainfall), which also affect the dynamics between the water layers. Rogoznica Lake can be considered as a system with both meromictic and holomictic conditions, which alternate seasonally and affect the lake’s ecology. Anoxic water is usually located below 10 m depth, and it is characterized by high concentrations of reduced sulphur species, nutrients and dissolved organic carbon. In anoxic water and laminated sediment, enrichment of trace metals, especially those that accumulate in sulphidic environment (Fe, Mo), is found. According to sedimentary enrichment of Mo (up to 81 mg kg−1), the lake can be classified as typical anoxic and meromictic environment. Extreme ecological conditions, which prevail in the lake, control the phytoplankton and zooplankton activities. Diatoms are the dominant microphytoplankton group, while oligotrich ciliates and copepods, the heterotrophic zooplankton organisms, play an important role and control the food web of the lake, especially in the post-holomictic–anoxic period. Ciliates by grazing on phototrophic sulphur bacteria at the chemocline transfer organic carbon to higher trophic levels, from anoxic to oxic conditions.

Rogoznica Lake (Croatia), a Unique Euxinic Seawater System on the Adriatic Coast

The characteristics outlines above (a well- studied hydrography-since 1994 ; a stable and constant oxic/anoxic interface ; and a stratified system that has persisted long enough to produce steady-state conditions relative to alkalinity and sulfide concentrations) make Rogoznica lake ideal sites to examine the processes that control the biogeochemistry of anoxic environments. The small size and physically stable nature of the lake allows researchers to examine small scale spatial and temporal variability as well as longer term processes. All our data show how particular sequences of meteorological events, some of which can be regarded as extreme, have affected the internal processes in the Lake. An integration of all results (since 1994 up to now) reveals an interesting possibility that this environment may well potentially serve as valuable sentinels of climate change