From the seafloor to the sea surface: a technical solution for the cabled EMSO-SN1 observatory in the Western Ionian Sea Stefania Sparnocchia*, Mireno Borghini, Sara Alessandro Giordano and Gianni Biasini Durante and Virna Meccia Communication Technology srl Istituto di Scienze Marine, ISMAR Cesena, Italy Consiglio Nazionale delle Ricerche, CNR La Spezia and Trieste, Italy. *[email protected] Abstract—The SN1 observatory is a cabled deep-sea in the vicinity of the Mount Etna, one of the largest and most observatory operating in real-time since 2005 in Southern Italy active volcanoes in Europe [1]. (East Sicily), at 2100 m water depth, 25 km from the harbor of the city of Catania. A structural enhancement of the SN1 observatory is undergoing in the framework of the Italian EMSO MedIT project, including the implementation of a multi- parametric monitoring system for observation in the water column dialoguing with the junction box in SN1. The new system is an inductive mooring collecting data from four water depths, and has been installed in its location on March 18, 2015. Coupling inductive modem, acoustic and fiber optic telemetries, this implementation is innovative and very challenging. The monitoring system of the water column is presented and its expected contribution to the knowledge of the deployment area is highlighted. Keywords—Real-Time Oceanography; Inductive mooring; cabled deep sea observatory; Ionian Sea I. INTRODUCTION A cabled deep-sea observatory located at 25 km from the harbor of Catania (East Sicily) in Southern Italy (Fig. 1) has been operating in real-time since 2005. It is placed at 2100 m water depth on a flat bathymetry zone, about 3 km east from the Malta Escarpment, which is a steep eastward-sloping surface. The prototype consists of two components: the multi- parametric station NEMO-SN1 (TSN branch) equipped with geophysical and environmental sensors, and the NEMO-OȞDE station (TSS branch) equipped with four wideband hydrophones. A 28 km long electro-optical cable links the observatory to a shore laboratory which collects data and supplies power to the underwater instrumentatiion [1]. The observatory is one of the key sites of the large-scale European Research Infrastructure EMSO (the European Multidisciplinary Seafloor and Water Column Observatory) [2] for its intrinsic importance for geo-hazards and for the stepwise development in the Global Monitoring for Environment and Security (GMES) initiative [3]. The site is one of the most seismically active areas of the Mediterranean [4]. In fact, very Fig. 1. Location of EMSO-SN1 observatory. The lower panel corresponds strong earthquakes have taken place in the area in 1169, 1693 to the bold rectangle in the wider map. TSN and TSS are the positions of the two branches and WC-SN1 the position of the multi-parametric monitoring and 1908 causing tsunami waves [5]. Moreoveer, SN1 is located system for observation in the water column. EMSO MedIT is funded by the Italian Ministry of University and Research, Cohesion Action Plan, D.D. 2013/274 (PAC01_00044/1) 978-1-4799-8736-8/15/$31.00 ©2015 IEEE A structural enhancement of the SN1 observatory is variations of the thermohaline properties of waters entering the undergoing in the framework of EMSO MedIT project basin [15, 16]. (http://www.emso-medit.it/en/). It consists in the The change observed at the surface in 1997 also impacted implementation of a multi-parametric monitoring system for the LIW subsurface flow, which was intensified at the western observation in the water column (WC-SN1), based on boundary and in the Strait of Sicily [17].In intermediate layers, inductive modem technology and acoustically dialoguing with the salty LIW enters the Ionian Sea through the Cretan a new junction box. Passage, spreading westward from its formation site, the north- The system will strengthen the capabilities to monitor eastern Levantine Basin [18, 19]. Then, from the Ionian it long-term variability of key physical and biogeochemical enters the Strait of Sicily, usually within the sills south of parameters. Data collected will be shared in near real time Malta. According to [9], a portion of LIW flows northward through the Catania node of the high-speed telecommunication without entering the Strait and possibly re-circulates in the network for University and Scientific Research. Ionian Sea. Possible applications include, for example, studies on At greater depths, the Adriatic Deep Water (ADW) has modifications of the properties of water masses and the marine been recognized for long time as the primary source of EMDW circulation; trends in global sea level; and changes in marine [20, 21, 22] until an additional source of dense water became chemistry, with particular emphasis on the carbon system and active in the Aegean Sea during late eighties and early nineties its role in ocean acidification. [23]. This event is known as the Eastern Mediterranean Transient (EMT). The ADW spreads into the Ionian abyssal II. WATER MASSES AND CIRCULATION IN THE DEPLOYMENT layers following different pathways after overflowing through AREA the Strait of Otranto [24]. The deployment area (Fig. 1) is located in the western Sicilian shelf region, in the Ionian Sea, a crucial basin where III. PRELIMINARY OBSERVATIONS IN THE waters from the Levantine, Aegean, Adriatic and Western DEPLOYMENT AREA Mediterranean seas enter and interact each other. Pre-deployment hydrological observations were conducted during oceanographic cruises on board the R/V Urania of CNR The main water masses circulating in the Ionian Sea are the to characterize the site and surrounding area in October 2013, modified Atlantic Water (AW), identified by a subsurface June 2014 and November 2014. A further survey was done salinity minimum located between 30 m and 200 m depth; the during the EMSO MedIT 03 cruise on board the R/V Minerva Levantine Intermediate Water (LIW), placed between 200 and Uno in March 2015, when WC-SN1 was deployed. 600 m depth and identified by a salinity maximum; and the Eastern Mediterranean Deep Water (EMDW), occupying the The cumulative θ/S diagram and the vertical profiles of abyssal layers below 1600 m. The layer comprised between potential temperature and salinity, measured in 700 m and 1600 m is occupied by a transitional water mass, correspondence of the TSS end-point, are displayed in Fig. 2. with intermediate properties between the LIW and the EMDW. The principal water masses circulating in the basin are present Finally, a further water mass is found in the surface layer, the in the area of study, and their hydrological properties show Ionian Surface Water (ISW) which is warmer and saltier than variations from a profile to the other. the AW and is clearly distinguishable from it in summer [6]. In particular, the LIW is clearly recognized as a salinity Observations in the past two decades have shown that the maximum (S > 38.9), located at about 240 m in June 2014, circulation in this basin is less stable than it was believed 160 m in November 2014 and 340 m in March 2015. The LIW before, presenting a clear variability at seasonal and decadal core is eroded in October 2013, because of an intruding less time scales. salty water mass. Above the LIW, a sequence of layers alternating low and high salinity values is observed. This fact Regarding the surface circulation, the AW enters into the evidences mixing between relatively fresh and warm upper Ionian Sea at the Malta escarpment after crossing the Strait of waters with the saltier and relatively colder LIW below. The Sicily, mainly transported by the Atlantic Ionian Stream (AIS) absolute salinity minimum, found between 35 and 50 m in [7]. Observations during late eighties [6] and early nineties [8] October 2013, June 2014 and November 2014, is a showed the AIS bifurcating after passing the slope north of reminiscence of the AW, diluted by mixing along its path. No Malta, with a strong branch forming an anti-cyclonic meander signature of AW is found in March 2015, suggesting that this northward and a weaker one going south-southeast along the water mass is not crossing the site in this period. Below 1000 slope. Substantial mesoscale activity (eddies and meanders) m to the bottom, where the transitional water and the EMDW was estimated to occur more or less between 15°E and 17°E, are found, salinity progressively reduces from a mean value of interesting the deployment area, where the intermittent 38.76 to 38.74 with a standard deviation equal to 0.02. The cyclonic Messina Rise Vortex (MRV) was documented [9]. In mean value of potential temperature also decreases from 13.66 1997 the surface circulation in the Northern Ionian reversed to C to 13.47 C, with a standard deviation between 0.01 and 0.02. cyclonic [10, 11], suppressing the northern branch of the AIS, and this mode persisted until 2006, when it switched to anti- cyclonic again [12]. The reversal of the surface circulation in the Northern Ionian is the largest documented decadal variability event in this basin, that was associated with changes in the prevalent wind stress curl sign [13, 14], or with Fig. 2. Cumulative θ/S diagram and vertical profiles of Potential Temperature (θ) and Salinity in the position at 37°30.04 ‘ N – 15°23.37’ E. IV. THE WATER COLUMN OBSERVING SYSTEM DESIGN configuration, all the instruments are settled to record data once per hour, except the ADCP at L1 that is acquiring a current The multi-parametric monitoring system in the water profile every 2 hours. column consists of an inductive mooring from 500 m depth to the bottom. It is made of three pieces of cable for inductive TABLE I.