Giorgio Budillon

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.

An optimum multiparameter mixing analysis of the shelf waters in the Ross Sea

The analysis of the mixing processes involving water masses on the Ross Sea continental shelf is one of the goals of the CLIMA project (Climatic Long-term Interactions for the Mass balance in Antarctica). This paper uses extended Optimum MultiParameter analysis (OMP), which is applied to four datasets collected during the cruises of 1994/95, 1995/96, 1997/98 and 2000/01 in the Ross Sea (Antarctica). Data include both hydrological, (temperature, salinity, and pressure; T, S, and P, respectively) and chemical parameters (O2, Si(OH)4, PO4, and NO3+NO2). The OMP analysis is based on the assumption that the mixing is a linear process which affects all parameters equally so that each sample shows physical/chemical properties that are the result of the mixing of some properly selected Source Water Types (SWTs). OMP thus evaluates the best set of contributions by all SWTs to each sample, and allows the spatial distribution and structure of the water masses in a basin to be evaluated. Ocean circulation may subsequently be inferred by means of a deeper analysis of the spreading of the water mass. In this study, the “real” Redfield ratios observed in the Ross Sea were used to correct the equations referring to the chemical parameters in accordance with the extended version of OMP. The solutions include some physically realistic constraints. The results allow a detailed description of the water mass distribution, validated through comparison with some “canonical” thermohaline characteristics of the Ross Sea hydrology. In particular our results verify the spreading of the HSSW over the entire continental shelf and emphasize the key role it plays in the ventilation of the deep waters outside the Ross Sea. In addition a description is given of the intrusion of relatively warm waters coming from the open ocean and flowing at some specific locations at the continental shelf break.

Thermohaline structure and variability in the Terra Nova Bay polynya, Ross Sea

Hydrological measurements from three cruises during the summers 1994/95, 1995/96 and 1997/98 in the western sector of the Ross Sea allow summer and year to year changes in heat and salt content in the Terra Nova Bay polynya to be analysed. Changes in the surface layer (upper pycnocline) followed the expected seasonal pattern of warming and freshening from the beginning to the end of the summer. These near-surface changes, expressed as net heating and salting rate, were about 11 W m−2 and -6 mg salt m−2 s−1. The heating changes were substantially lower than the estimated heat supplied by the atmosphere during the summer, which underlines the importance for this season of the advective component carried by the currents in the total heat budget. The year to year differences were about one or two orders of magnitude smaller than the seasonal changes in the surface layer. In the intermediate and deep layers, the summer heat and salt variability were of the same order as or one order higher than from one summer to the next. The differences in sign and magnitude for the heat change in the upper and in the lower pycnocline indicate a weak connection in the summer period between the surface heat fluxes and the deep waters. A local source of very cold water (with temperatures below the surface freezing point) of about 0.3 Sv has been detected close to the Terra Nova Bay coast. It arose out of the interaction of the shallow–intermediate layers of High Salinity Shelf Water with the coastal glaciers. The presence and the variability of this cold water point to the significant role of the thermohaline properties of Terra Nova Bay waters in controlling the floating glacier by governing the basal melting processes.

Fronts and upper ocean thermal variability south of New Zealand

The structure and variability of Southern Ocean fronts south of New Zealand are described based on fifteen summer expendable bathythermograph (XBT) sections obtained between 1994 and 2001. The temperature variability north of 60°S is dominated by meanders and meridional shifts of the Sub-Antarctic Front (SAF), which often bifurcates to form northern and southern branches. The northern branch follows the southern edge of the Campbell Plateau, while the southern branch is found over the abyssal plain of the south-west Pacific Basin. The northern and southern branches of the SAF can be separated by as much as 900 km. Intense eddies or meanders of the SAF displace isotherms by as much as 5 degrees of latitude from their positions when such features are absent. The Polar Front (PF) position is more stable in time, although cold-core features associated with eddies or meanders of the front are occasionally observed between the southern SAF and the PF. The position of the southern ACC front is extremely stable, consistently overlying the 3000 m isobath on the northern flank of the Pacific-Antarctic/south-east Indian Ridge.

Water Masses and Suspended Matter Characteristics of the Western Ross Sea

As part of an interdisciplinary field project performed during the austral summer 1994/95 devoted to examine the physical and bio-geochemical characteristics of the Ross Sea, hydrological stations were operated in the western sector off Victoria Land and water samples were collected at different depths of the water column to determine the particulate matter content. These investigations were carried out in the framework of the CLIMA (Climatic Long-Term Interaction for the Mass Balance in Antarctica) project of the Italian PNRA (National Program of Research in Antarctica).

A study of surface heat fluxes in the Ross Sea (Antarctica)

In the polar regions, dynamical and thermodynamical interactions between atmosphere and ocean are strongly influenced by the presence or absence of the ice cover, which forms an insulating layer over the ocean, hindering sensible heat fluxes and forming an effective barrier to evaporation and thus preventing latent heat loss. In the framework of the CLIMA (Climatic Long-term Interactions for the Mass-balance in Antarctica) project of the Italian PNRA (National Program for Antarctic Research) we focused our attention on the evaluation of the heat fluxes between the ocean and the atmosphere in the Ross Sea, where the ice covers the sea for many months of the year. Wherever the ice cover is absent all year round, such as in leads or polynyas, the air-sea fluxes can be very large, especially in winter when the air-sea temperature differences are strong. In this work heat exchanges between sea and atmosphere, whether ice cover was present or not, were calculated from climatological data obtained from the European Centre for Medium Range Weather Forecasts, while sea ice data were collected from the US National Ice Center and National Climatic Data Center. Each of the terms in the sea surface heat budget were computed for 1994 with a temporal resolution of six hours and a spatial resolution of 0.5° using bulk formulae and obtaining monthly averaged horizontal distributions. The surface heat budget is dominated in November, December, January and February by shortwave radiation, while for the other months the turbulent and conductive heat fluxes dominate the heat exchange between the atmosphere and the sea surface. The annual total heat loss at the surface in 1994 has been estimated at about −90 W m−2 with the highest heat loss occurring close to the coast; the maximum heat loss occurred in May (−217 W m−2) while in January the heat gain by the ocean was 196 W m−2. In addition, weekly averaged values over the whole Ross Sea from 1994 to 1997 were calculated with the same parameterisation in order to study the temporal variability in this basin of each individual component and of the total surface heat budget. For this purpose only the data inside the continental shelf of the Ross Sea were considered in calculating the averaged fluxes. The 1994–97 total heat budget ranges from −87 to −107 W m−2with an average of −96 W m−2; this amount of heat loss was supposed to be compensated for by the heat advected by the Circumpolar Deep Water and its transport was estimated at about 2.9 Sv.

On the dense water spreading off the Ross Sea shelf (Southern Ocean)

In this study, current meter and hydrological data obtained during the X Italian Expedition in the Ross Sea (CLIMA Project) are analyzed. Our data show a nice agreement with previous data referring to the water masses present in this area and their dynamics. Here, they are used to further analyze the mixing and deepening processes of Deep Ice Shelf Water (DISW) over the northern shelf break of the Ross Sea. In more detail, our work is focused on the elementary mechanisms that are the most efficient in removing dense water from the shelf: either classical mixing effects or density currents that interact with some topographic irregularity in order to drop to deeper levels, or also the variability of the Antarctic Circumpolar Current (ACC) which, in its meandering, can push the dense water off the shelf, thus interrupting its geostrophic flow. We also discuss in detail the (partial) evidence of dramatic interactions of the dense water with bottom particulate, of geological or biological origin, thus generating impulsive or quasi-steady density-turbidity currents. This complex interaction allows one to consider bottom particular and dense water as a unique self-interacting system. In synthesis, this is a first tentative analysis of the effect of bottom particulate on the dense water dynamics in the Ross Sea.

Particulate matter and plankton dynamics in the Ross Sea Polynya of Terra Nova Bay during the Austral Summer 1997/98

Abstract The structure and variability of the plankton community and the distribution and composition of suspended particulate matter, were investigated in the polynya of Terra Nova Bay (western Ross Sea) during the austral summer 1997/1998, with the ultimate objective of understanding the trophic control of carbon export from the upper water column. Sampling was conducted along a transect parallel to the shore, near the retreating ice edge at the beginning of December, closer to the coast at the beginning of February, and more offshore in late February. Hydrological casts and water sampling were performed at several depths to measure total particulate matter (TPM), particulate organic carbon (POC), biogenic silica (BSi), chlorophyll a (Chl a ) and phaeopigment (Phaeo) concentrations. Subsamples were taken for counting autotrophic and heterotrophic pico- and nanoplankton and to assess the abundance and composition of microphyto- and microzooplankton. Statistical analysis identified two major groups of samples: the first included the most coastal surface samples of early December, characterized by the prevalence of autotrophic nanoplankton biomass; the second included all the remaining samples and was dominated by microphytoplankton. With regard to the relation of the plankton community composition to the biogenic suspended and sinking material, we identified the succession of three distinct periods. In early December Phaeocystis dominated the plankton assemblage in the well-mixed water column, while at the retreating ice-edge a bloom of small diatoms (ND) was developing in the lens of superficial diluted water. Concentrations of biogenic particulates were generally low and confined to the uppermost layer. The very low downward fluxes, the near absence of faecal pellets and the high Chl a /Phaeo ratios suggested that the herbivorous food web was not established yet or, at least, was not working efficiently. In early February the superficial pycnocline and the increased water column stability favoured the development of the most intense bloom of the season, essentially sustained by micro-sized diatoms (MD). The shift of the autotrophic community toward this size component produced major changes in the composition of particulate matter and determined its export to depth. The particulate organic carbon (POC)/chlorophyll a (Chl a ) and Chl a /Phaeo ratios more than halved, biogenic silica (BSi)/POC and BSi/Chl a strongly increased. Downward fluxes were greatly enhanced (reaching the yearly maximum) and essentially occurred via faecal pellets, underscoring the high efficiency of the herbivorous food web. In late February the deepening of the pycnocline, together with the decrease in light intensity, contributed to halting the diatom bloom. The biomass of small heterotrophs (HNF and MCZ) significantly increased relative to the previous period, favouring the shift toward a mistivorous food web (sensu [Ophelia 41 (1995) 153]) and resulting in the retention of biogenic matter in the superficial layer. Only in early February, with the increase in the size of primary producers (essentially represented by micro-sized diatoms), did the grazing food web become efficient [S. Afr. J. Mar. Sci. 12 (1992) 477], fuelling the long-lived carbon pool and enhancing export to depth (and hence carbon sequestration) via the sinking of large diatoms and high amounts of faecal pellets. The conditions predominating in the Terra Nova Bay polynya in mid-summer probably increased the efficiency of the CO 2 pump, possibly causing the Bay to act as a carbon sink.

REMOCEAN: A Flexible X-Band Radar System for Sea-State Monitoring and Surface Current Estimation

This letter deals with the use of the wave-radar REMOCEAN system for sea-state monitoring starting from images collected in the X-band at two different test sites. In particular, the measurement surveys were carried out at two coastal sites in the Gulf of Naples by means of the installation of the radar on a fixed and on a movable platform, respectively. The effectiveness of the system was also tested by means of a comparison between the REMOCEAN results and the high-frequency coastal radar observations, with emphasis to the sea surface current estimation.

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.