ALBERT

Description: Statistics of transit and residence times in the Adriatic Sea surface, a semi-enclosed basin of the Mediterranean, are estimated from drifter data and Lagrangian numerical simulations. The results obtained from the drifters are generally underestimated given their short operating lifetimes (half life of ∼40 days) compared to the transit and residence times. This bias can be removed by considering a large amount of numerical particles whose trajectories are integrated over a long time (750 days) with a statistical advection–dispersion model of the Adriatic surface circulation. Numerical particles indicate that the maximum transit time to exit the basin is about 216–260 days for particles released near the northern tip of the Adriatic, and that a particle entering on the eastern Otranto Channel will typically exit on the other side of the channel after 170–185 days. A duration of 150–168 days is estimated as the residence time in the Adriatic Basin.

Description: Data from 38 Argo profiling floats are used to describe the subsurface Mediterranean currents for the period 2003–2009. These floats were programmed to execute 5-day cycles, to drift at a neutral parking depth of 350 m and measure temperature and salinity profiles from either 700 or 2000 m up to the surface. At the end of each cycle the floats remained at the sea surface for about 6 h, enough time to be localised and transmit the data to the Argos satellite system. The Argos positions were used to determine the float surface and subsurface displacements. At the surface, the float motion was approximated by a linear displacement and inertial motion. Subsurface velocities estimates were used to investigate the Mediterranean circulation at 350 m, to compute the pseudo-Eulerian statistics and to study the influence of bathymetry on the subsurface currents. Maximum speeds, as large as 33 cm/s, were found northeast of the Balearic Islands (Western basin) and in the Ierapetra eddy (Eastern basin). Typical speeds in the main along-slope currents (Liguro-Provençal-Catalan, Algerian and Libyo-Egyptian Currents) were ~20 cm/s. In the best sampled regions, the pseudo-Eulerian statistics show typical subsurface circulation pathways which can be related to the motion of Levantine Intermediate Water. In general our results agree with the qualitative subsurface circulation schemes proposed in the literature, except in the southern Ionian where we found westward-flowing subsurface currents. Fluctuating currents appeared to be usually larger than the mean flow. Subsurface currents were found to be essentially parallel to the isobaths over most of the areas characterized by strong bathymetry gradients, in particular, in the vicinity of the continental slopes.

Description: Adriatic and Ionian seas are Mediterranean sub-basins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide 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.

Description: This paper is the outcome of a workshop held in Rome in November 2011 on the occasion of the 25th anniversary of the POEM (Physical Oceanography of the Eastern Mediterranean) program. In the workshop discussions, a number of unresolved issues were identified for the physical and biogeochemical properties of the Mediterranean Sea as a whole, i.e., comprising the Western and Eastern sub-basins. Over the successive two years, the related ideas were discussed among the group of scientists who participated in the workshop and who have contributed to the writing of this paper. Three major topics were identified, each of them being the object of a section divided into a number of different sub-sections, each addressing a specific physical, chemical or biological issue: 1. Assessment of basin-wide physical/biochemical properties, of their variability and interactions. 2. Relative importance of external forcing functions (wind stress, heat/moisture fluxes, forcing through straits) vs. internal variability. 3. Shelf/deep sea interactions and exchanges of physical/biogeochemical properties and how they affect the sub-basin circulation and property distribution. Furthermore, a number of unresolved scientific/methodological issues were also identified and are reported in each sub-section after a short discussion of the present knowledge. They represent the collegial consensus of the scientists contributing to the paper. Naturally, the unresolved issues presented here constitute the choice of the authors and therefore they may not be exhaustive and/or complete. The overall goal is to stimulate a broader interdisciplinary discussion among the scientists of the Mediterranean oceanographic community, leading to enhanced collaborative efforts and exciting future discoveries.

Description: In the framework of the EU-funded MFSTEP project, autonomous drifting profilers were deployed throughout the Mediterranean Sea to collect temperature and salinity profile data and to measure subsurface currents. The realization of this profiler program in the Mediterranean, referred to as MedArgo, is described and assessed using data collected between June 2004 and December 2006 (including more than 2000 profiles). Recommendations are provided for the permanent future implementation of MedArgo in support of operational oceanography in the Mediterranean Sea. More than twenty drifting profilers were deployed from research vessels and ships-of-opportunity in most areas of the Mediterranean. They were all programmed to execute 5-day cycles with a drift at a parking depth of 350 m and CTD profiles from either 700 or 2000 m up to the surface. They stayed at the sea surface for about 6 h to be localised by, and transmit the data to, the Argos satellite system. The temperature and salinity data obtained with pumped Sea-Bird CTD instruments were processed and made available to the scientific community and to operational users in near-real time using standard Argo protocols, and were assimilated into Mediterranean numerical forecasting models. In general, the cycling and sampling characteristics chosen for the MedArgo profilers were found to be adequate for the Mediterranean. However, it is strongly advised to use GPS and global cellular phone telemetry or the future Argos bi-directional satellite system in order to avoid data compression and losses, for the continuation of the Mediterranean drifting profiler program.

Description: Within the framework of the EGITTO/EGYPT program, the spatial structure and the temporal variability of the surface circulation in the eastern basin of the Mediterranean Sea was studied by means of satellite-tracked drifters. A total of 97 drifters drogued to 15-m nominal depth were released between September 2005 and March 2007, either regularly along ship-of-opportunity routes (Sicily Channel) or within specific structures during dedicated campaigns (Levantine sub-basin). After editing and low-pass filtering, the drifter trajectories were used to estimate pseudo-Eulerian statistics: mean current, variance ellipses, mean and eddy kinetic energies. A statistical analysis was also performed dividing the dataset in two extended seasons (winter and summer). This study completes previous ones in the Sicily Channel and in the Ionian. Several veins are evidenced, together with a seasonal variability inducing a reversal of the circulation in the southern part of the Ionian. However, in this latter area, data are too scarce and the dynamics too complex to achieve a circulation pattern yet. Eastward, the general circulation is described as a counterclockwise flow along the Libyo-Egyptian and Middle East slopes. Part of this flow is deflected toward the open sea by the anticyclones generated alongslope by the Libyo-Egyptian current (Libyan and Egyptian eddies), by the wind (Ierapetra) or by the topography (over the Eratosthenes Seamount and off Latakia). The entrainment of this flow around successive eddies (paddle-wheels effect) results in an open sea eastward transport of Atlantic water. When the Libyan eddies (anticyclonic) are close to the slope, the westward current is stronger than the mean eastward current and the circulation is temporally and locally reversed. The strong variability induced by the eddies and the meteorological conditions hinder evenly covering all the study area. Additional deployments are required to improve further our understanding of the circulation in this basin.

Description: Data from 38 Argo profiling floats are used to describe the intermediate Mediterranean currents for the period October 2003–January 2010. These floats were programmed to execute 5-day cycles, to drift at a neutral parking depth of 350 m and measure temperature and salinity profiles from either 700 or 2000 m up to the surface. At the end of each cycle the floats remained at the sea surface for about 6 h, enough time to be localised and transmit the data to the Argos satellite system. The Argos positions were used to determine the float surface and intermediate displacements. At the surface, the float motion was approximated by a linear displacement and inertial motion. Intermediate velocities estimates were used to investigate the Mediterranean circulation at 350 m, to compute the pseudo-Eulerian statistics and to study the influence of bathymetry on the intermediate currents. Maximum speeds, as large as 33 cm/s, were found northeast of the Balearic Islands (western basin) and in the Ierapetra eddy (eastern basin). Typical speeds in the main along-slope currents (Liguro-Provençal-Catalan, Algerian and Libyo-Egyptian Currents) were ~20 cm/s. In the central and western part of Mediterranean basin, the pseudo-Eulerian statistics show typical intermediate circulation pathways which can be related to the motion of Levantine Intermediate Water. In general our results agree with the qualitative intermediate circulation schemes proposed in the literature, except in the southern Ionian where we found westward-flowing intermediate currents. Fluctuating currents appeared to be usually larger than the mean flow. Intermediate currents were found to be essentially parallel to the isobaths over most of the areas characterized by strong bathymetry gradients, in particular, in the vicinity of the continental slopes.

Description: A novel method for three-dimensional variational assimilation of Lagrangian data with a primitive-equation ocean model is proposed. The assimilation scheme was implemented in the Mediterranean ocean Forecasting System and evaluated for a 4-month period. Four experiments were designed to assess the impact of trajectory assimilation on the model output, i.e. the sea-surface height, velocity, temperature and salinity fields. It was found from the drifter and Argo trajectory assimilation experiment that the forecast skill of surface-drifter trajectories improved by 15 %, that of intermediate-depth float trajectories by 20 %, and moreover, that the forecasted sea-surface height fields improved locally by 5 % compared to satellite data, while the quality of the temperature and salinity fields remained at previous levels. In conclusion, the addition of Lagrangian trajectory assimilation proved to reduce the uncertainties in the model fields, thus yielding a higher accuracy of the ocean forecasts.

Description: In the framework of the EU-funded MFSTEP project, autonomous drifting profilers were deployed throughout the Mediterranean Sea to collect temperature and salinity profile data and to measure subsurface currents. The realization of this profiler program in the Mediterranean, referred to as MEDARGO, is described and assessed using data collected between June 2004 and March 2006 (including more than 1500 profiles). Recommendations are provided for the permanent future implementation of MEDARGO in support of operational oceanography in the Mediterranean Sea. More than twenty drifting profilers were deployed from research vessels and ships-of-opportunity in most areas of the Mediterranean. They were all programmed to execute 5-day cycles with drift at a neutral parking depth of 350 m and CTD profiles from either 700 or 2000 m up to the surface. They stayed at the sea surface for about 6 h to be localised by, and transmit the data to, the Argos satellite system. The temperature and salinity data obtained with pumped Sea-Bird CTD instruments were processed and made available to the scientific community and to operational users in near-real time using standard ARGO protocols, and were assimilated into Mediterranean numerical forecasting models. In general, the cycling and sampling characteristics chosen for the MEDARGO profilers were found to be adequate for the Mediterranean. However, it is strongly advised to use GPS and global cellular phone telemetry or the future Argos bi-directional satellite system in order to avoid data compression and losses, for the continuation of the Mediterranean drifting profiler program.

Description: Within the framework of the EGITTO/EGYPT programs, the spatial structure and the temporal variability of the surface circulation in the eastern basin of the Mediterranean Sea was studied with satellite-tracked drifters. A total of 97 drifters drogued to 15-m nominal depth were released between September 2005 and March 2007, regularly along ship-of-opportunity routes in the Sicily Channel and within specific structures during dedicated campaigns in the Levantine sub-basin. This study complements previous ones in the former and in the Ionian, but it is the first one in the latter. After editing and low-pass filtering, the drifter trajectories were used to estimate pseudo-Eulerian statistics: mean current, variance ellipses, mean and eddy kinetic energies. A statistical analysis was also performed dividing the dataset in two extended seasons (winter and summer). A branching behaviour of the surface water after passing through the Sicily Channel is evidenced, together with a seasonal variability inducing a reversal of the circulation in the southern part of the Ionian. In the Levantine, the surface circulation describes an eastward flow along the Libyan and Egyptian slopes (the Libyo-Egyptian Current: LEC) that continues in a cyclonic circuit along the Middle East and Turkish slopes. This general alongslope circuit can be perturbed locally and temporally by the numerous anticyclonic eddies that co-exist in the Levantine, mainly created by the instability of the LEC in the south (Libyo-Egyptian Eddies: LEEs), but also by the wind (Ierapetra and Pelops), and by the topography (over the Eratosthenes Seamount and off Latakia). The most frequent perturbation is the entrainment of part of the flow seaward: the LEEs close to the slope can interact with the LEC, which then spreads more or less around them, so that a series of contiguous LEEs (paddle-wheel effect) can possibly result in an eastward offshore transport (the so-called Mid Mediterranean Jet). Additionally, when LEEs are close enough to the slope, most of the surface flow is spread seaward. Along the slope the current is then induced by the LEEs southern side which results in a westward current. Locally and temporally the circulation along the slope can thus be reversed.