The Mediterranean Sea is considered a hot spot of global warming because it has been changing faster than the global ocean, creating a strong impact on the marine environment. Recent studies agree on ...the increase in the sea level, in the sea surface temperature, and in the sea surface salinity in the Mediterranean Sea over the last two decades. In this research, the possible interconnection between these and other parameters that contribute to the regulatory effect of the sea on the climate are identified and discussed. Spatio-temporal variability of four oceanographic and air–sea interaction parameters (sea-level, sea surface temperature, sea surface salinity, and freshwater flux) are estimated over the last 27 years by performing the empirical orthogonal function analysis. Climatic trends, and interannual and decadal variability of the different datasets are delineated and described in the whole Mediterranean and in its sub-basins. On the climatic scale, the Mediterranean and its sub-basins behave in a coherent way, showing the seal level, temperature, salinity, and freshwater flux rise. On the interannual scale, the temporal evolution of the sea level and sea surface temperature are highly correlated, whereas freshwater flux affects the variability of sea level, temperature, and the salinity field mainly in the Western and Central Mediterranean. The decadal signal associated with the Northern Ionian Gyre circulation reversals is clearly identified in three of the four parameters considered, with different intensities and geographical extents. This signal also affects the intermediate layer of the Eastern Mediterranean, from where it is advected to the other sub-basins. Decadal signal not associated with the Northern Ionian Gyre reversals is strongly related to the variability of main sub-basin scale local structures.
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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.
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The Mediterranean sea Borzelli, Gianluca Eusebi; Borzelli, Gianluca Eusebi; Gacic, Miroslav ...
2014., 2014, 2014-02-26, 2014-03-10, Volume:
202
eBook
Surface, intermediate, and deep-water processes and their interaction in time and space drive the major ocean circulation of the Mediterranean Sea. All major forcing mechanisms, such as surface wind ...forcing, buoyancy fluxes, lateral mass exchange, and deep convection determining the global oceanic circulation are present in this body of water. Deep and intermediate water masses are formed in different areas of the ocean layers and they drive the Mediterranean thermohaline cell, which further shows important analogies with the global ocean conveyor belt. The Mediterranean Sea: Temporal Variability and Spatial Patterns is a comprehensive volume that investigates the temporal and spatial variability patterns in the ocean basin. Volume highlights include: * Discussions of state-of-the-art physical and biogeochemical properties of the Mediterranean Sea * Multiple physical ocean circulation processes, both in time and spatial scales (basin, sub-basin, and mesoscale) * How different regional phenomena in the sea influence the biogeochemistry of the basin and the ocean dynamics * Spatio-temporal variability of the surface circulation in the western Mediterranean * Deep-water variability and inter-basin interactions in the eastern Mediterranean Sea * Understanding the link between global ocean circulation patterns and the global climate The Mediterranean Sea will be a valuable resource for geoscientists, oceanographers, and meteorologists.
A network of high‐frequency (HF) radars was installed in the northern Adriatic in the second half of 2007, aimed to measure surface currents in the framework of the North Adriatic Surface Current ...Mapping (NASCUM) project. This study includes a detailed analysis of current measurements from February to August 2008, a period in which three radars were simultaneously operational. Current patterns and temporal evolutions of different physical processes were extracted by using self‐organizing map (SOM) analysis. The analysis focused on subtidal frequency band and extracted 12 different circulation patterns on a 4 × 3 rectangular SOM grid. The SOM was also applied on a joint data set that included contemporaneous surface wind data obtained from the operational hydrostatic mesoscale meteorological model ALADIN/HR. The strongest currents were recorded during energetic bora episodes, being recognized by several current patterns and having the characteristic downwind flow with magnitudes exceeding 35 cm/s at some grid points. Another characteristic wind, the sirocco, was represented by three current patterns, while the remaining current structures were attributed to weak winds and the residual thermohaline circulation. A strong resemblance has been found between SOM patterns extracted from HF radar data only and from combined HF radar and wind data sets, revealing the predominant wind influence to the surface circulation structures and their temporal changes in the northern Adriatic. These results show the SOM analysis being a valuable tool for extracting characteristic surface current patterns and forcing functions.
Key Points
Surface current patterns in the northern Adriatic were extracted
Pronounced wind forcing is dominant in subtidal surface dynamics
SOM proved to be a valuable tool for extracting characteristic current fields
The North Ionian Gyre (NIG) displays prominent inversions on
decadal scales. We investigate the role of internal forcing induced by
changes in the horizontal pressure gradient due to the varying ...density of
Adriatic Deep Water (AdDW), which spreads into the deep layers of the
northern Ionian Sea. In turn, the AdDW density fluctuates according to the
circulation of the NIG through a feedback mechanism known as the bimodal
oscillating system. We set up laboratory experiments with a two-layer
ambient fluid in a circular rotating tank, where densities of 1000 and 1015 kg m−3 characterize the upper and lower layers, respectively. From the
potential vorticity evolution during the dense-water outflow from a marginal
sea, we analyze the response of the open-sea circulation to the along-slope
dense-water flow. In addition, we show some features of the
cyclonic and anticyclonic eddies that form in the upper layer over the slope
area. We illustrate the outcome of the experiments of varying density and
varying discharge rates associated with dense-water injection. When the
density is high (1020 kg m−3) and the discharge is large, the kinetic
energy of the mean flow is stronger than the eddy kinetic energy. Conversely, when the density is lower (1010 kg m−3) and the discharge
is reduced, vortices are more energetic than the mean flow – that is, the
eddy kinetic energy is larger than the kinetic energy of the mean flow. In
general, over the slope, following the onset of dense-water injection, the
cyclonic vorticity associated with current shear develops in the upper
layer. The vorticity behaves in a two-layer fashion, thereby becoming
anticyclonic in the lower layer of the slope area. Concurrently, over the
deep flat-bottom portion of the basin, a large-scale anticyclonic gyre forms
in the upper layer extending partly toward a sloping rim. The density record
shows the rise of the pycnocline due to the dense-water sinking toward the
flat-bottom portion of the tank. We show that the rate of increase in the
anticyclonic potential vorticity is proportional to the rate of the rise of
the interface, namely to the rate of decrease in the upper-layer thickness
(i.e., the upper-layer squeezing). The comparison of laboratory experiments
with the Ionian Sea is made for a situation when the sudden switch from
cyclonic to anticyclonic basin-wide circulation took place following
extremely dense Adriatic water overflow after the harsh winter in 2012. We
show how similar the temporal evolution and the vertical structure are in
both laboratory and oceanic conditions. The demonstrated similarity further
supports the assertion that the wind-stress curl over the Ionian Sea is not
of paramount importance in generating basin-wide circulation inversions
compared with the internal forcing.
The Malta-Sicily Channel is part of the Sicily Channel system where water and thermohaline properties between the Eastern and Western Mediterranean basins take place. Several mesoscales features are ...detached from the main circulation due to wind and bathymetric forcing. In this paper, surface circulation structures are studied using different remotely sensed datasets: satellite data (absolute dynamic topography, Cross-Calibrated Multi-Platform wind vector analysis, satellite chlorophyll and sea surface temperature) and high frequency radar data. We identified high frequency motions (at short time scales—hours to days), as well as mesoscale structures fundamental for the understanding of the Malta-Sicily Channel circulation dynamics. One of those is the Malta-Sicily Gyre; an anticyclonic structure trapped between the Sicilian and Maltese coasts, which is poorly studied in the literature and often confused with the Malta Channel Crest and the Ionian Shelf Break Vortex. In order to characterize this gyre, we calculated its kinetic properties taking advantage of the fine-scale temporal and spatial resolution of the high frequency radar data, and thus confirming its presence with an updated version of the surface circulation patterns in the area.
Maps of the remotely sensed Absolute Dynamic Topography (ADT) and Sea Surface Temperature (SST) over the Eastern Mediterranean in the period June 1993 to June 2001 show a dramatic and sudden change ...in the sea surface structure. This change, which is presumably associated with the relaxation of the Eastern Mediterranean Transient (EMT), had taken place by the middle of 1997 and consisted of a complete reversal of the Ionian upper‐layer circulation from anticyclonic to cyclonic. In the EMT phase, SST and ADT maps suggest enhanced communication between the Ionian and the Aegean basins as well as Aegean waters spreading northward along the eastern Ionian flank. Negative wind vorticity in the Ionian suggests that the switch of the upper‐layer circulation from anticyclonic to cyclonic cannot be explained in terms of the wind forcing. Therefore, we suggest the predominance of the baroclinic vorticity production term in determining the upper‐layer circulation inversion.
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•The mechanism and implications of the BiOS are reported.•BiOS links the deep thermohaline cell to the circulation of the North Ionian Gyre.•The inversions of the North Ionian Gyre affects the ...properties of the Mediterranean Sea.
Over the past four decades, the understanding of Mediterranean oceanography has evolved considerably. From a purely stationary view, there has been a shift to a concept that considers the Mediterranean as a highly dynamic sea in which ocean-typical processes occur on smaller spatial and temporal scales. The recent discovery of the mechanism called BiOS (Adriatic-Ionian Bimodal Oscillating System) has further highlighted the highly variable nature of Mediterranean oceanography and the interconnectedness of its sub-basins.
The BiOS is a mechanism by which the deep thermohaline cell of the eastern Mediterranean, originating in the southern Adriatic, is connected to the upper circulation of the Northern Ionian Gyre via positive feedback, causing a decadal circulation change that leads to a redistribution of salt throughout the Mediterranean.
The effects of this variability are manifold: decadal modulations of the intensity of winter convection in the southern Adriatic, with variations in the volume and thermohaline properties of the dense water produced; variations in preconditioning in the eastern Mediterranean, affecting the salinity of the intermediate Levantine waters; influence on biodiversity in the Adriatic due to the import of Lessepsian organisms or those of western Mediterranean and Atlantic origin; variations of the trophic regime in the Ionian Sea due to the different vertical dynamics of the nutricline during the cyclonic or anticyclonic circulation of the Northern Ionian Gyre; significant contribution to the preconditioning of the northwestern Mediterranean Sea by the salinity variability of the Levantine Intermediate Water.
In this study, the BiOS mechanism, the underlying theory, and the implications for the oceanographic features of the Mediterranean Sea are presented, based on the extensive literature published in the last three decades and some new analyses.
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9.
Ocean Exchange and Circulation Gačić, Miroslav; Bensi, Manuel
Water (Basel),
03/2020, Volume:
12, Issue:
3
Journal Article
Peer reviewed
Open access
The great spatial and temporal variability, which characterizes the marine environment, requires a huge effort to be observed and studied properly since changes in circulation and mixing processes ...directly influence the variability of the physical and biogeochemical properties. A multi-platform approach and a collaborative effort, in addition to optimizing both data collection and quality, is needed to bring the scientific community to more efficient monitoring and predicting of the world ocean processes. This Special Issue consists of nine original scientific articles that address oceanic circulation and water mass exchange. Most of them deal with mean circulation, basin and sub-basin-scale flows, mesoscale eddies, and internal processes (e.g., mixing and internal waves) that contribute to the redistribution of oceanic properties and energy within the ocean. One paper deals with numerical modelling application finalized to evaluate the capacity of coastal vegetated areas to mitigate the impact of a tsunami. The study areas in which these topics are developed include both oceanic areas and semi-enclosed seas such as the Mediterranean Sea, the Norwegian Sea and the Fram Strait, the South China Sea, and the Northwest Pacific. Scientific findings presented in this Special Issue highlight how a combination of various modern observation techniques can improve our understanding of the complex physical and biogeochemical processes in the ocean.
•Good agreement between meso-zooplankton DVM and ADCP backscattering strength (Sv).•Zooplankton migration is highly conditioned by environmental conditions.•MLD, NPP and light (solar and lunar) ...influence vertical position of zooplankton.•Sv and ADCP vertical velocity highlight different types/ranges of DVM.•DVM is not mandatory behaviour but depends on food availability in deeper layers.
From the eleven-year data record at the Southern Adriatic Observatory (E2M3A), we study for the first time in the Adriatic Sea the strength of the backscatter signal from an Acoustic Doppler Current Profiler (ADCP) in relation to zooplankton vertical movement. The signal represents well the general behaviour of the zooplankton to sink at dawn and to emerge at the sunset. The modulation of the signal along the year is linked in particular to the different environmental conditions and the presence of different zooplankton groups. The correlation between the backscatter signal and distinct zooplankton groups (copepods, euphausiids, ostracods, appendicularians, salps) shows that in different seasons and environmental conditions, different groups are responsible for the strength of the signal. Furthermore, the backscatter signal, interpreted in terms of the qualitative composition of the zooplankton community, provides important information on the behaviour and habits of zooplankton in the southern Adriatic. Poorly known aspects of zooplankton behaviour are described in more detail, such as: recovery of zooplankton vertical distribution after mixing events, vertical migration in conditions of increased food availability at depth due to vertical mixing, modulation of vertical migration in dependence of the lunar phase and vertical migration in relation to deep chlorophyll maximum position. In addition, the present study emphasizes the possible influence of gelatinous zooplankton at the open sea on high backscatter values during the warmer part of the year.
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