Andrea Bergamasco

General Circulation of the Eastern Mediterranean

Abstract A novel description of the phenomenology of the Eastern Mediterranean is presented based upon a comprehensive pooled hydrographic data base collected during 1985–1987 and analyzed by cooperating scientists from several institutions and nations (the POEM project). Related dynamical process and modeling studies are also overviewed. The circulation and its variabilities consist of three predominant and interacting scales: basin scale, subbasin scale, and mesoscale. Highly resolved and unbiased maps of the basin wide circulation in the thermocline layer are presented which provide a new depiction of the main thermocline general circulation, composed of subbasin scale gyres interconnected by intense jets and meandering currents. Semipermanent features exist but important subbasin scale variabilities also occur on many time scales. Mesoscale variabilities modulate the subbasin scale and small mesoscale eddies populate the open sea, especially the south-eastern Levantine basin. Clear evidence indicates Levantine Intermediate Water (LIW) to be present over most of the Levantine Basin, implying that formation of LIW is not localized but rather is ubiquitous. The Ionian and Levantine basins are confirmed to form one deep thermohaline cell with deep water of Adriatic origin and to have a turnover time of one and a quarter centuries. Prognostic, inverse, box and data assimilative modeling results are presented based on both climatological and POEM data. The subbasin scale elements of the general circulation are stable and robust to the dynamical adjustment process. These findings bear importantly on a broad range of problems in ocean science and marine technology that depend upon knowledge of the general circulation and water mass structure, including biogeochemical fluxes, regional climate, coastal interactions, pollution and environmental management. Of global ocean scientific significance are the fundamental processes of water mass formations, transformations and dispersion which occur in the basin.

A synthesis of the Ionian Sea hydrography, circulation and water mass pathways during POEM-Phase I

Abstract In this paper we revisit, with a thorough in-depth analysis, the dataset collected in the hydrographic surveys of the international collaborative programme POEM (Physical Oceanography of the Eastern Mediterranean) in the period 1986–1987. The work has two major objectives. The first is to refine the dynamic picture of the Ionian upper thermocline sub-basin scale circulation, rather less definitive than the dynamic picture of the Levantine Sea circulation. The second is to identify the pathways of the major water masses of the basin not only in the near-surface, but also in the intermediate and deep layers. To our knowledge, this is the first work defining the pathways of the Levantine Intermediate Water (LIW) and of the Adriatic Deep Water (ADW)/Eastern Mediterranean Deep Water (EMDW) that characterize the intermediate and deep circulations. The major novel results can be summarized as follows. In the upper thermocline: (1) The Atlantic Ionian Stream (AIS) jet entering the Sicily Straits bifurcates into two main branches at 37°N, ∼ 17°E. It advects the Modified Atlantic Water (MAW) into the Ionian Sea interior. The first branch turning directly southward encloses an overall anticyclonic area comprising multiple centers around which the MAW is advected. (2) The second AIS branch extends further into the northeastern Ionian, where it too turns southward before crossing the entire Ionian Sea meridionally, advecting MAW on its left side and Ionian Surface Water (ISW) on its right. This branch of the jet is confined to the Ionian margin and does not pass around the Pelops gyre. (3) A new water mass, the LSW, is formed in the Levantine basin and enters the Cretan passage, then is first veered cyclonically south of Crete by the Cretan gyre and successively is entrained anticyclonically around the Pelops gyre, and then enters the Aegean Sea. (4) A permanent cyclone located in the northeastern Ionian determines the pathway of mixed Adriatic Surface Water/Ionian Surface Water (ASW/ISW). (5) A permanent cyclone is found in all the surveys near the tip of the Italian boot. This novel analysis of the LIW pathways shows that: (1) The major source of intermediate LIW during the period 1986–1987 was actually in the Levantine Sea. LIW formed there entered the Cretan passage, was veered cyclonically by the Cretan gyre south of Crete and then entered the southern Ionian Sea. The major LIW pathway was westward directly to the Sicilian Straits. (2) Secondary important LIW pathways were determined by the interior structures. The strong Pelops anticyclone was entraining LIW around its periphery and was determining the LIW northward pathway that closely followed the eastern Greek coastline. It was along this pathway that LIW entered the Otranto Strait. A further branch of LIW was entrained and recirculated around the multiple Ionian Anticyclones (IA) of the western Ionian Sea. (3) The Cretan cyclone is a feature confined to the upper thermocline-intermediate layer. It disappears at ∼ 400 dbar while the Pelops anticyclone is strongly barotropic below the upper 100 dbar and penetrates quite intense down to 800 dbar. (4) A further completely novel result concerns the new water mass found in the deep layer that spreads on the 29.15 kg/m3 isopycnal surface. This water mass, characterized by high salinity and high oxygen content, is formed inside the Aegean Sea and is observed to spread out all around the Cretan Arc Straits. The final fully novel result is the demonstration of a second pathway for the ADW exiting from the Otranto Strait that is transformed into EMDW and occupies the abyssal layers of the Ionian Sea interior. The traditional pathway for EMDW is along the isobaths along the western side of Italy but ADW was observed to be exiting from the Otranto Strait in the eastern Hellenic trench at 39.5°N, both during POEM-ON86 and POEM-AS87. This second pathway for EMDW follows isobath contours along the western side of Greece. The two EMDW routes converge and merge between 36°N and 35°N, so producing a deep layer of EMDW that occupies uniformly the abyssal plain of the interior of the Ionian Sea.

The sea surface temperature field in the Eastern Mediterranean from advanced very high resolution radiometer (AVHRR) data: Part II. Interannual variability

A ten-year dataset of Advanced Very High Resolution Radiometer-Sea Surface Temperature (AVHRR-SST) with 18-km space resolution and weekly frequency is used to study the seasonal variability of the Eastern Mediterranean Sea surface field. Three main objectives are addressed in this study. The first is to define the time and space scales of the surface temperature distributions. The second objective is to relate the SST features to the upper thermocline circulation and the third is to compare these features with the observational evidence of the Physical Oceanography of the Eastern Mediterranean (POEM) Programme. The time analysis reveals the presence of a strong seasonal signal characterized by two main seasonal extremes, winter and summer. The transition between the overall zonal distribution of the isotherms (winter) and the mostly meridional pattern of the fronts (summer) occurs very rapidly in May and October. The space analysis shows that the dominant scale is the sub-basin scale and the sub-basin gyres are very well resolved allowing the identification of permanent and semipermanent structures. The results for the two further objectives can be summarized together. The seasonal and monthly SST distributions are strongly correlated with the dynamical structure of the basin upper thermocline circulation. A direct comparison of the September 1987 SST pattern with the corresponding surface temperature map of the POEM-87 survey proves this correlation quantitatively. Furthermore, comparison of the SST monthly climatologies with the POEM circulation scheme shows that all the major currents and the sub-basin gyres are also found consistently in our patterns, with the only exception of the anticyclonic Mersa-Matruh Gyre.

The wind and thermally driven circulation of the eastern Mediterranean Sea. Part II: the Baroclinic case

Compared with other interesting parts of the World Ocean, little is known of the eastern Mediterranean and major issues of the Mediterranean circulation are still unsolved. Among them, the most crucial one is: what is the dominant driving mechanism of the eastern Mediterranean general circulation: (1) the wind stress; (2) the thermohaline surface fluxes; (3) the inflow forcing at the Sicily Straits? What is the relative importance of these three forcing functions? Is it the same in the different sub-basins comprising the eastern Mediterranean? What modelling factors are important for the simulation of the seasonal cycle and is the general circulation overall dominated by the annual mean or seasonal signal? To answer the above questions we have carried out an extensive and thorough series of numerical experiments using a multilevel model of the circulation, suitable for coarse-resolution studies but endowed with active thermodynamics and allowing for realistic geometry (coastlines, islands, bottom relief). The model is used in a three-level version as the minimum one capable of simulating the vertical superposition of different water masses observed in the eastern Mediterranean. The climatological monthly averages of wind-stress, thermal and evaporative fluxes and inflow at Sicily are used to drive the model. In Part I of the present study it was shown that the seasonal cycle present in the wind-stress curl induces a strongly seasonal barotropic circulation comprising the entire eastern Mediterranean. This seasonal gyre reverses from being cyclonic in winter to anticyclonic in summer. The inclusion of baroclinicity, however, profoundly modifies the purely wind-driven, barotropic circulation, eliminating the strong seasonality and the winter-to-summer reversal. The first important result is that the general circulation pattern now consists of a succession of sub-basin-scale gyres, with a seasonal modulation emphasizing the cyclonic centres in winter and the anticyclonic ones in summer, according to the vorticity input by the wind. When surface thermal forcing is included, the winter-to-summer differences become very small and the yearly pattern is dominant. The second important result is that the intercomparisons of the various numerical experiments in which each driving mechanism is studied in isolation from the others allows us to classify unambiguously the three forcing mechanisms in order of relative importance in driving the circulation in the different sub-basins of the eastern Mediterranean. Specifically, for the Ionian Sea and Sea of Crete the dominant forcing is the inflow at the Straits of Sicily while for the Levantine Sea thermohaline fluxes are the main driving function. The wind-stress forcing induces a seasonal fluctuation only in the meandering path of the Atlantic jet entering the Ionian Sea through the Sicily Straits. We finally carry out a ‘central experiment’, the most realistic one in which all forcing functions drive the circulation that we compare quantitatively with other model results and qualitatively with observations. Major features can be recognized and are shown to be persistent all year long. These features are also found in the dynamic heights of the general hydrographic surveys of the Physical Oceanography of the Eastern Mediterranean (POEM) programme. The only POEM feature not reproduced by the model, an intense anticyclonic region in the south eastern Levantine, may be attributed to errors and specifically underestimates, of the available thermal fluxes whose effect is partially overcome by the wind-stress forcing. This anticyclonic cell is in fact obtained when the model is driven by the thermal fluxes alone. Overall, the model results compare well with the observational evidence provided by the POEM surveys and the thermohaline vertical circulation cell reproduced by the model is consistent with the preliminary results of the transient-tracer survey POEM-V-87. Finally, many of the persistent features found in the model circulation patterns can be related to the strong control exerted by the ambient potential vorticity f/H upon the eastern Mediterranean circulation.

Analysis of the seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea from AVHRR data (1984–1992)

Seasonal and interannual variability of the sea surface temperature field in the Adriatic Sea is analyzed from the low-resolution advanced very high resolution radiometer data. The spatial resolution of 18 km allowed analysis of only basin and subbasin scale features. Average monthly and seasonal sea surface temperature fields for the entire studied period (1984–1992) are discussed. The analysis shows the absence of any permanent sea surface thermal features in the Adriatic Sea. The south Adriatic sea surface temperature minimum presumably associated to the cyclonic gyre, previously considered as one of the permanent features, appears to be recurrent, being prominent only in late autumn and early winter, i.e., in the preconditioning and a deepwater formation phases. The major Ionian water inflow is documented in autumn while the thermal signature of the western surface outflow of Adriatic water appears most prominent in winter. The variability of the basin-wide thermal pattern in the Adriatic reveals four distinct seasons, which is different from both the eastern and western Mediterranean, where only two major patterns are recognized. A prominent interannual signal occurs in a northward extension of the warm water plume along the eastern coast, which in some years reaches the northernmost corner of the Adriatic, while in other situations it remains trapped in the south Adriatic cyclonic gyre. The surface thermal signature of the south Adriatic gyre also varies on an interannual timescale, and it was weak or completely absent during the period 1984–1986 while it was rather prominent in the period 1987–1992. A constant trend of sea surface temperature decrease in the center of the south Adriatic gyre and in the northernmost corner of the Adriatic was evidenced over the studied period.

Modeling dense water mass formation and winter circulation in the northern and central Adriatic Sea

In this paper, using an eddy resolving primitive equation model, we attempt to provide quantitative answers to some of the still unresolved or poorly understood dynamical issues related to the general circulation of the northern and central Adriatic Sea. The first question we addressed relates to understanding the effects of the major driving mechanisms on the basin circulation. A series of numerical experiments are carried out to examine the circulation produced by various combinations of different forcing mechanisms. Cold air outbreaks (CAO) associated with the northeasterly bora winds, together with uniform surface cooling and freshwater discharge prescribed near the northwestern corner, give a fairly realistic circulation consistent with the observations. This pattern is comprised of an overall cyclonic gyre in the northern basin, with a strong southward flowing jet along the Italian coastline and a broader northward flow along the Croatian coast. The second question addressed is under what conditions convective mixing extends to the bottom of the Jabuka Pit. While the northern shelf is always uniformly mixed to the bottom, the extent of convection within the Pit depends on the overall stratification prior to onset of the CAO event. Strong subsurface stratification between the intermediate and near-bottom layers prevents deep convection. The third question concerns with the role of the rim current along the Italian coast vs the other circulation components in distributing the North Adriatic Dense Water (NADW) within the basin. In general, the strong density front developed across the shelf break north of the Jabuka Pit restricts the replenishment of the Pit deep layers by the NADW. Rather, the NADW is transported southward along the Italian continental slope in the form of a vein of underflow. The final question addressed is how the thermohaline structure of the water column evolves after the ending of the CAO event, and of the related convection process.

The seasonal steady circulation of the Eastern Mediterranean determined with the adjoint method

Abstract In this paper we use a rather unconventional approach to determine the steady seasonal circulation of the Eastern Mediterranean. Traditional calculations rely either on prognostic models spun-up with different forcing functions or on inverse methods having rather simple dynamics. In the present applications one of the most sophisticated inverse techniques, the adjoint method of control theory, is used to find the model state that is optimally consistent with the model dynamics, with a prescribed climatology and is steady in time. The model used is the GFDL primitive equation model in its fully time-dependent non-linear version forced by seasonal wind-stress fields that are kept steady for each calculation. The prescribed climatology consists of the seasonal hydrographies of the temperature and salinity fields. Steadiness upon the seasonal time scale is required as a term in the cost function of the adjoint that penalizes the tendencies of the prognostic variables. This use of the adjoint method reconstructs the steady seasonal wind-driven circulation in an ocean with a prescribed baroclinic structure. As such, it is equivalent to a prognostic spin-up calculation with steady winds and the robust diagnostic applied, i.e. adding a term that relaxes the temperature and salinity fields to the seasonal climatologies with a time constant of 3 months. To assess the “success’ of these calculations, the success of the inversion must be quantified. The examination of the final data misfits and steady state residuals shows that steady state has indeed been reached. The steady-state residuals are always much smaller than the data misfits and both of them are always small, well below the one standard deviation value for each field. Thus, we can assess that a meaningful solution has indeed been attained. To assess further if these solutions are reasonable, we have carried out for comparison robust diagnostic calculations with a time constant of 3 months. The circulations thus obtained are extremely similar to the adjoint solutions in reproducing the overall patterns as well as the individual sub-basin scale gyres and interconnecting currents and meandering jets. The circulations obtained with the two approaches are also equally strong. However, both the adjoint and the robust diagnostic results produce an overall barotropic transport that is one order of magnitude bigger than that observed. They also both show anomalously strong vortex structures in regions of sharp topographic breaks connecting the deep interior to the shelves, for which no observational evidence is available. These unrealistic features can be explained by taking into account that with the short time scale of 3 months used in both approaches biased solutions may be obtained. These biases are due to inconsistencies between the rough topography used and the smooth climatologies, that lead to a misrepresentation of the important JEBAR effect. This explanation is supported by a further robust diagnostic calculation in which the time constant is increased in the deep layers that gives a circulation intensity much more realistic. Overall, this application of the adjoint method to the GFDL model shows that it can be successfully used to find meaningful optimal solutions. These solutions also prove to be reasonable when compared with analogous robust diagnostics results.

Evidence of dense water overflow on the Ross Sea shelf-break

This paper presents the results of the analysis of hydrological data of a 5-day mesoscale experiment (53 CTD casts) conducted during the XIIIth Italian Expedition to Antarctica (1997–98 cruise) in the framework of the CLIMA (Climatic Longterm Interaction for the Mass balance in Antarctica) Project of the Italian National Programme for Antarctic Research (PNRA). The experiment site was chosen for studying the dense water overflow in relation to the shelf-break in the central Ross Sea, after a large-scale synoptic survey, aimed to detect the general hydrological characteristics of the basin. A classical θ/S analysis was carried out for better understanding of the shelf-slope connection and the interactions between the water masses of this zone: the Circumpolar Deep Water (CDW) coming from the oceanic domain and the Ice Shelf Water (ISW) spreading from the Ross Ice Shelf (RIS) edge. Our results show the evidence of an overflow of dense water, originated on the continental shelf, on the shelf-break. This supercold water signal is found on the continental slope down to 1200 m depth. The shape of this tongue of modified ISW, whose thickness reaches up to 100 m, is very narrow, suggesting that the overflow occurs in very localized areas.

Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basin

ABSTRACT Sclavo, M., Benetazzo, A., Carniel, S., Bergamasco, A., Falcieri, F.M., and Bonaldo, D., 2013. Wave-current interaction effect on sediment dispersal in a shallow semi-enclosed basin The shallow northern Adriatic Sea (namely Gulf of Venice) serves as a model for exploring the interaction of surface gravity waves and oceanic currents and how they influence bottom sediment dispersal and bathymetry evolution. This wave-current interaction effect is investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. COAWST relies on the Regional Ocean Modeling System (ROMS), the Simulating WAves Nearshore (SWAN) model, and the CSTMS (Community Sediment Transport Modeling System) models and routines. The 2-way data transfer between wave and ocean circulation models is synchronous via MCT (Model Coupling Toolkit), with ROMS providing to SWAN the 2-D current field, free surface elevation, and bathymetry. COAWST modeling system is implemented on two computational grids at different horizontal resolution: a parent grid (with resolution of 2.0 km) covering the whole Adriatic Sea and a child grid resolving the Gulf of Venice at a resolution of 0.5 km. Simulated waves and currents are validated against in-situ observations at the CNR-ISMAR Acqua Alta oceanographic tower, located 15 km off the Venice lagoon. The analysis of wave-current interaction effect on sediment dispersal and sea bottom evolution are performed over the 2011 winter season (January-March) with particular focus on the waves generated by dominant and prevailing winds blowing on the Adriatic Sea: Bora and Sirocco. Results show that while the effects on bottom stress may vary depending on wave propagation and current direction, the effects on advective dynamics may become dominant particularly in presence of severe storms with parallel wave propagation and global circulation, which is the case of Bora storms in northern Adriatic Sea.

Downslope flow observations near Cape Adare shelf-break

The analysis of two high resolution hydrological datasets acquired during the 1997 and 2001 summers across the Antarctic continental shelf-break near Cape Adare (Ross Sea) is presented. The main focus of these cruises was the investigation of the overflow of the High Salinity Shelf Water (HSSW). This dense and salty water mass forms along Victoria Land and flows northward, descending the slope near Cape Adare. Water types characterizing the study area are detected through vertical salinity profiles and by the horizontal distributions of the temperature and salinity. Temperature and salinity hydrological sections obtained by means of objective analysis method well describe the water masses interactions at the shelf/slope edge. The 1997 dataset shows evidence of a strong HSSW signature on the slope, but it is difficult to quantify the spatial scales involved in the spreading mechanism, because the overflow takes place at the edge of the investigation area. The 2001 data, collected at the same position with improved spatial and temporal resolution, clearly indicates the absence of a “true” HSSW downslope process. Even though no estimation of the amount of downslope flow can be given at present due to the resolution of the available dataset, it is possible to get a better phenomenological picture of the process by comparing the two years.