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.
The Italian Fixed-Point Observatory Network (IFON) integrates well-established coastal and ocean infrastructures (buoys, platforms, moorings, mast platforms, etc.), most of them providing realtime multidisciplinary monitoring for a number of marine and atmospheric variables. Here, we describe the network characteristics and then discuss an example of its operation during the cold spell of winter 2012. One of the goals of the Italian Flagship Project Ricerca Italiana per il mare (RITMARE) is to create a common, validated IFON database able to fulfil both public and private demands, including validation of remotely sensed data and numerical models, environmental planning and management, and time-series analysis of climate and oceanographic data.
Abstract. 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 European Research Infrastructure Consortium "Integrated Carbon Observation System" (ICOS) aims at delivering high quality greenhouse gas (GHG) observations and derived data products (e.g., regional GHG-flux maps) for constraining the GHG balance on a European level, on a sustained long-term basis. The marine domain (ICOS-Oceans) currently consists of 11 Ship of Opportunity lines (SOOP -Ship of Opportunity Program) and 10 Fixed Ocean Stations (FOSs) spread across European waters, including the North Atlantic and Arctic Oceans and the Barents, North, Baltic, and Mediterranean Seas. The stations operate in a harmonized and standardized way based on communityproven protocols and methods for ocean GHG observations, improving operational conformity as well as quality control and assurance of the data. This enables the network to focus on long term research into the marine carbon cycle and the anthropogenic carbon sink, while preparing the network to include other GHG fluxes. ICOS data Frontiers in Marine Science | www.frontiersin.org September 2019 | Volume 6 | Article 544 Steinhoff et al. ICOS-Oceans Networkare processed on a near real-time basis and will be published on the ICOS Carbon Portal (CP), allowing monthly estimates of CO 2 air-sea exchange to be quantified for European waters. ICOS establishes transparent operational data management routines following the FAIR (Findable, Accessible, Interoperable, and Reusable) guiding principles allowing amongst others reproducibility, interoperability, and traceability. The ICOS-Oceans network is actively integrating with the atmospheric (e.g., improved atmospheric measurements onboard SOOP lines) and ecosystem (e.g., oceanic direct gas flux measurements) domains of ICOS, and utilizes techniques developed by the ICOS Central Facilities and the CP. There is a strong interaction with the international ocean carbon cycle community to enhance interoperability and harmonize data flow. The future vision of ICOS-Oceans includes ship-based ocean survey sections to obtain a threedimensional understanding of marine carbon cycle processes and optimize the existing network design.
The zonal structure of strongly nonlinear inertial western boundary currents (WBCs) is studied experimentally along a straight "meridional" coast in a 5-m-diameter rotating basin by analyzing the "zonal" profile of the meridional velocity field as a function of transport intensity and other dynamical parameters. The return flow that is generated by the surface wind stress curl in the oceanic interior is forced in the rotating basin by the motion of a piston, in the absence of any surface stress. The laboratory setup consists of two parallel rectangular channels separated by an island and linked by two curved connections: in the first channel, a piston is forced at a constant speed u p ranging from 0.5 to 3 cm s Ϫ1 over a distance of 2.5 m, producing a virtually unsheared current at the entrance of the second channel. In the latter, a linear reduction of the water depth provides the topographic beta effect that is necessary for the development of the westward intensification. Nearly steady currents are obtained and measured photogrammetrically over a region of about 1 m 2 . In all of the experiments performed, an appropriate horizontal Reynolds number (Re ϭ /E, where and E are dimensionless numbers measuring the importance of nonlinearity and lateral friction, respectively) is Re k 1. The zonal profile of the meridional velocity is always found to have (away from the viscous boundary layer) a nearly exponential structure typical of inertial WBCs, whose width agrees well with the classical inertial boundary layer length scale ␦ I . A control experiment (with u p ϭ 1 cm s Ϫ1 ) is analyzed in detail: it has the same as the Gulf Stream (GS) but a much smaller E. This implies that the laboratory flow is expected to be geometrically similar to the GS outside the viscous boundary layer, but to differ within it. To assess the effect of such a departure from dynamic similarity, a mathematical model is used that numerically simulates a flow that is fully dynamically similar to the GS. The comparison between the profile thus obtained numerically and the one obtained experimentally shows that they are, indeed, virtually coincident outside the viscous boundary layer, except for a small offset that tends to vanish as Re → ϱ. Moreover, additional sensitivity experiments in which the piston speed, the rotation rate of the basin, the topographic beta effect, and the width of the main channel are varied provide further information on the zonal structure of WBCs.
Abstract. We report on data from an oceanographic cruise in the Mediterranean Sea on the German research vessel Poseidon in April 2014. Data were taken on a west-east section, starting at the Strait of Gibraltar and ending south-east of Crete, as well on sections in the Ionian and Adriatic Sea. The objectives of the cruise were threefold: to contribute to the investigation of the spatial evolution of the Levantine Intermediate Water (LIW) properties and of the deep water masses in the eastern Mediterranean Sea, and to investigate the mesoscale variability of the upper water column. The measurements include salinity, temperature, oxygen and currents and were conducted with a conductivity, temperature and depth(CTD)/rosette system, an underway CTD and an acoustic Doppler current profiler (ADCP). The sections are on tracks which have been sampled during several other cruises, thus supporting the opportunity to investigate the long-term temporal development of the different variables. The use of an underway CTD made it possible to conduct measurements of temperature and salinity with a high horizontal spacing of 6 nm between stations and a vertical spacing of 1 dbar for the upper 800 m of the water column. Data coverage and parameter measuredRepository reference:
Abstract. The North Ionian Gyre (NIG) displays prominent inversions on decadal scales. We investigate the role of internal forcing, induced by changes of the horizontal pressure gradient due to the varying density of the Adriatic Deep Water (AdDW), that 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 named Bimodal Oscillating System. We set up laboratory experiments with a two-layer ambient fluid in a circular rotating tank, where densities of 1000/1015 kg m−3 characterise the upper/lower layer, respectively. From the potential vorticity evolution during the dense water outflow from a marginal sea, we analyse the response of the open-sea circulation to the along-slope dense water flow. In addition, we show some features of the cyclonic/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 the 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. On the other hand, when the density is smaller, 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 the dense water injection, the cyclonic vorticity associated with a current shear develops in the upper layer. The vorticity behaves in a two-layer fashion, thus 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. 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 of the anticyclonic potential vorticity is proportional to the rate of the rise of the interface, namely, to the rate of decrease of 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 the cyclonic to the anticyclonic basin-wide circulation took place following the extremely dense Adriatic water overflow after the harsh winter in 2012. We show how similar are the temporal evolution and the vertical structure 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, as compared to the internal forcing.
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