Abstract. Adriatic and Ionian seas are Mediterranean subbasins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basinwide circulation. Altimetric maps showed that the last cyclonic mode started in 2011 but unexpectedly in 2012 reversed to anticyclonic. We related this "premature" inversion to the extremely strong winter in 2012, which caused the formation of very dense Adriatic waters, flooding Ionian flanks in May and inverting the bottom pressure gradient. Using Lagrangian float measurements, the linear regression between the sea surface height and three isopycnal depths suggests that the southward deep-layer flow coincided with the surface northward geostrophic current and the anticyclonic circulation regime. Density variations at depth in the northwestern Ionian revealed the arrival of Adriatic dense waters in May and maximum density in September. Comparison between the sea level height in the northwestern Ionian and in the basin centre showed that in coincidence with the arrival of the newly formed Adriatic dense waters the sea level was lowered in the northwestern flank, inverting the surface pressure gradient. Toward the end of 2012, the density gradient between the basin flanks and its centre went to zero, coinciding with the weakening of the anticyclonic circulation and eventually with its return to the cyclonic pattern. Thus, the premature and transient reversal of Ionian surface circulation originated from the extremely harsh winter in the Adriatic, resulting in the formation and spreading of highly dense bottom waters. The present study highlights the remarkable sensitiveness of the Adriatic-Ionian BiOS to climatic forcing.

Academic, government, and private organizations from around the globe have established High Frequency radar (hereinafter, HFR) networks at regional or national levels. Partnerships have been established to coordinate and collaborate on a single global HFR network (http://global-hfradar.org/). These partnerships were established in 2012 as part of the Group on Earth Observations (GEO) to promote HFR technology and increase data sharing among operators and users. The main product of HFR networks are continuous maps of ocean surface currents within 200 km of the coast at high spatial (1-6 km) and temporal resolution (hourly or higher). Cutting-edge remote sensing technologies are becoming a standard component for ocean observing systems, contributing to the paradigm shift toward ocean monitoring. In 2017 the Global HFR Network was recognized by the Joint Technical WMO-IOC Commission for Oceanography and Marine Meteorology (JCOMM) as an observing network of the Global Ocean Observing System (GOOS). In this paper we will discuss the development of the network as well as establishing goals for the future. The U.S. High Frequency Radar Network (HFRNet) has been in operation for over 13 years, with radar data being ingested from 31 organizations including measurements from Canada and Mexico. HFRNet currently holds a collection from over 150 radar installations totaling millions of records of surface ocean velocity measurements. During the past 10 years in Europe,

Abstract. Previous studies have demonstrated that the salinity in the Levantine basin depends on the intensity of the Atlantic water (AW) inflow. Moreover, its spreading eastward (to the Levantine basin) or northward (to the Ionian Sea) is determined by the Ionian circulation pattern, i.e. by the Adriatic-Ionian Bimodal Oscillating System (BiOS) mechanism. The aim of this paper is to relate salinity variations in the Levantine basin to the salt content variability in the core of the Levantine Intermediate Water (LIW) passing through the Sicily Channel (SC) and its possible impact on the Western Mediterranean Transition -WMT (i.e. the sudden salinity and temperature increase in the deep layer of the AlgeroProvençal subbasin occurring since 2004). From the historical data set MEDAR/MEDATLAS in the Levantine and northern Ionian, we present evidence of decadal occurrences of extreme salinities associated with the varying influx of AW over the last 60 yr. Furthermore, we show that the salinity variations in the two subbasins are out of phase. Highsalinity episodes in the Levantine are a pre-conditioning for the potential occurrence of the events like the Eastern Mediterranean Transient (EMT). Cross-correlation between the salinity time series in the Levantine basin and in the SC suggests that the travel time of the LIW is between 10 and 13 yr. Comparing the timing of the salinity increase associated with the WMT and the salinity in the LIW core in the SC, we estimate that the total time interval needed for the signal propagating from the Levantine to reach the deep mixed layers of the Algero-Provençal subbasin is about 25 yr.We also showed that the extra salt input from the eastern Mediterranean contribute up to about 60 % to the salt content increase in the bottom layer of the western Mediterranean.

International audienceHigh-frequency (HF) coastal radars measure current velocity at the ocean surface with a 30-100 km range and 1-3 km resolution, every 0.25-1 h. HF radars are well suited to many applications, such as search and rescue (SaR), oil-spill mitigation and ecosystem management. Here we present a first organized core of 12 HF radars installed in five sites in four countries (Greece, Italy, France and Spain) within the European MED project, the Tracking Oil Spill and Coastal Awareness (TOSCA) network. Dedicated experiments tested radar capabilities to estimate transport driven by currents, which is the key feature for all the above applications. Experiments involved the deployment of drifters, i.e., floating buoys, acting as proxies for substances passively advected by currents. Using HF radars the search range is reduced by a factor of 1.6 to 5.3 after 24 h. The paper also underlines the importance of sharing common tools for HF radar data processing and the need to mitigate radio frequency interference. The effort can be regarded as an initial step toward the creation of a Mediterranean or European HF radar network, crucial for any European integrated ocean observing system (IOOS)

The performances of a shore-based high-frequency (HF) radar network deployed along the coast of the Venice lagoon (northern Adriatic Sea) are discussed based on a comparison with a single bottom-mounted ADCP deployed in the shallow-water area offshore of the lagoon for a 40-day period in August-September 2005.The analyses, carried out using currents representative of the first meter for the HF radars and 2.5 m for the ADCP, gave rms differences of radial currents in the range of 8.7-14.7 cm s 21 (correlation 0.37-0.82) for the ideal pattern and 8.4-20.5 cm s 21 (correlation 0.14-0.84) for the measured pattern. Good correlation was found between surface current vectors and moored data (scalar correlation up to R 5 0.83, vector correlation r 5 0.78, veering angle 68). Comparison metrics were improved for the low-passed currents. Angular offsets ranged between 168 and 1118. Differences depended primarily on the geophysical variability within the water column. Bearing offsets also contributed because they lead to comparisons with radial velocities at erroneous angular sectors.Radar performances were severely affected by strong northeasterly wind pulses in their early stages. An increased broadband noise, spread over the entire Doppler spectrum across all ranges to the antennas, masked the Bragg peaks and determined the loss in radar coverage, introducing gross underestimations of both radial velocities and total currents.