Despite an extensive bibliography for the circulation of the Mediterranean Sea and its sub-basins, the debate on mesoscale dynamics and their impacts on bio-chemical processes is still open because of their intrinsic time scales and of the difficulties in their sampling. In order to clarify some of these processes, the "Algerian BAsin Circulation Unmanned Survey-ABACUS" project was proposed and realized through access to the JERICO Trans National Access (TNA) infrastructure between September and December 2014. In this framework, a deep glider cruise was carried out in the area between the Balearic Islands and the Algerian coast to establish a repeat line for monitoring of the basin circulation. During the mission a mesoscale eddy, identified on satellite altimetry maps, was sampled at high-spatial horizontal resolution (4 km) along its main axes and from the surface to 1000 m depth. Data were collected by a Slocum glider equipped with a pumped CTD and biochemical sensors that collected about 100 complete casts inside the eddy. In order to describe the structure of the eddy, in situ data were merged with next generation remotely sensed data: daily synoptic sea surface temperature (SST) and chlorophyll concentration (Chl-a) images from the MODIS satellites, as well as sea surface height and geostrophic velocities from AVISO. From its origin along the Algerian coast in the eastern part of the basin, the eddy propagated northwest at a mean speed of about 4 km/day, with a mean diameter of 112-130 km, mean amplitude of 15.7 cm; the eddy was clearly distinguished from the surrounding waters thanks to its higher SST and Chl-a values. Temperature and salinity values over the water column confirm the origin of the eddy from the Algerian Current (AC) showing the presence of recent Atlantic water in the surface layer and Levantine Intermediate Water (LIW) in the deeper layer. The eddy footprint is clearly evident in the multiparametric vertical sections conducted along its main axis. Deepening of temperature, salinity and density isolines at the center of the eddy is associated with variations in Chl-a, oxygen concentration and turbidity patterns. In particular, at 50 m depth along the eddy borders, Chl-a values are higher (1.1-5.2 μg/l) in comparison with the eddy center (0.5-0.7 μg/l) with maximum values found in the southeastern sector of the eddy. Calculation of geostrophic velocities along transects and vertical quasi-geostrophic velocities (QG-w) over a regular 5 km grid from the glider data helped to describe the mechanisms and functioning of the eddy. QG-w presents an asymmetric pattern, with relatively strong downwelling in the western part of the eddy and upwelling in the southeastern part. This asymmetry in the vertical velocity pattern, which brings LIW into the euphotic layer as well as advection from the northeastern sector of the eddy, may explain the observed increases in Chl-a values.
A rare long time series of hydrographic profiles and moored current meter data, collected from 1995 to\ud 2008 in Terra Nova Bay polynya, are used in combination with meteorological data, acquired by an\ud Automatic Weather Station, and remote sensing data from a Special Sensor Microwave/Imager. The\ud behaviour of Terra Nova Bay coastal polynya in terms of air–ice–sea interactions and the consequent High\ud Salinity Shelf Water production are detailed. The katabatic regime that characterizes Terra Nova Bay\ud polynya is investigated and different types of events are distinguished on the bases of their duration and\ud intensity. The more frequent katabatic events take place during the winter season from April to October,\ud blowing on average 1–7 h, with speed between 25 and 56 m s−1 and they abruptly end in just a few\ud hours. The link between the persistence of the wind and the opening of the polynya is showed. In\ud particular, an increase of the open water percentage in correspondence with each katabatic event of long\ud duration is detected. Terra Nova Bay polynya appears characterized by two different periods of activity\ud during the winter season. A period characterized by a considerable sea-ice free area and by an increase in\ud salinity along the water column (from July to November), which is preceded (from March to June) and\ud followed (from December to February) by a period in which the polynya is still open but the salinity of\ud the water column decreases. While the period between July and November appears related to a\ud maximum efficiency of Terra Nova Bay polynya in the sea-ice production, the period from March to June\ud marks a “partial” functioning of the polynya. During March–June, the polynya is partially free of ice and\ud consequently the brine is released but, at this time of year, it is merely increasing the salinity of the upper\ud layer of the ocean, reducing the stratification, but not causing High Salinity Shelf Water to be formed
Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation and ventilates the abyssal ocean. In recent decades, AABW has warmed, freshened and reduced in volume. Ross Sea Bottom Water (RSBW), the second largest source of AABW, has experienced the largest freshening. Here we use 23 years of summer measurements to document temporal variability in the salinity of the Ross Sea High Salinity Shelf Water (HSSW), a precursor to RSBW. HSSW salinity decreased between 1995 and 2014, consistent with freshening observed between 1958 and 2008. However, HSSW salinity rebounded sharply after 2014, with values in 2018 similar to those observed in the mid-late 1990s. Near-synchronous interannual fluctuations in salinity observed at five locations on the continental shelf suggest that upstream preconditioning and large-scale forcing influence HSSW salinity. The rate, magnitude and duration of the recent salinity increase are unusual in the context of the (sparse) observational record.
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