Dept. of Oceanology and Environmental Geophysics

Time and Space Evolution of LIW in the Otranto Strait Area A 3D Graphical Visualization

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Paolo SCARAZZATO, Osservatorio Geofisico Sperimentale - Trieste

Summary The time and space evolution of the Levantine Intermediate Water (LIW) in the area of the Strait of Otranto during 1994/1995 is shown by means of a graphical interpolative and displaying technique. The study reveals both a seasonal and an interannual variability which appears in LIW volume, position and salinity maximum. Comparison were also made with data measured in the same area during some POEM cruises performed in the period 1985-1987.

Introduction

The Otranto Strait connects the Adriatic Sea with the Ionian and consequently with the entire Mediterranean Sea and therefore plays an important role in the water exchange processes between the two basins, as revealed by the results of some studies performed in the past decades (Wust, 1961; Ovchinnikov, 1966; Zore-Armanda, 1969, Orlic et al., 1992). However, as they were rather occasional than systematical, a more intense and detailed study (Otranto Project) was recently carried out over the strait area, with six seasonal oceanographic cruises in 15 months and about 20 months of eulerian current measurements by means of moored instruments at 4 to 6 stations across the strait.

The main results are reported by Gacic et al., 1996; however a brief description of the water masses properties which are found in the strait zone will be given here.

In the surface layer the flow through the strait appears to be subject to seasonal fluctuations due to both meteorological factors and thermohaline differences between the Adriatic and Ionian Surface Waters (ASW and ISW). In winter, when the Adriatic waters are denser than the Ionian and south-easterly winds prevail over the region, an inflow (into the Adriatic) of ISW occurs along the Albanian coast and in the central part of the strait, while an outflow of ASW is found along the western side. In summer, when the meteo-oceanographic conditions reverse, outflow predominates along the Italian coast and in the central part of the strait, while the ISW inflow is concentrated in a narrow coastal band on the eastern side.

In the intermediate layer, there is an outflow of Adriatic water along the Italian shelf and slope, while an inflow of highly saline Levantine Intermediate Layer (LIW) occurs in the remaining part of the strait, its main nucleus being centered at about 300 m depth.

Finally, in the bottom layer, the deep water formed in the Adriatic outflows into the northwestern Ionian basin, and subsequently spreads towards and throughout the bottom layer of the Levantine Basin. This water mass, the Adriatic Bottom Water (ABW), was found to be the main component of the Eastern Mediterranean Deep Water (EMDW) (Pollak, 1951).

The aim of the present paper is to follow the temporal and spatial evolution of the LIW, defined as the water with salinity equal or higher than 38.75, during the time interval covered by the Otranto Project. by means of a spatial interpolation technique of the salinity field coupled with a three-dimensional visualization, employing UNIRAS software for gridding, interpolations and displaying.

Experimental

The grid of the hydrological stations occupied during the six oceanographic cruises is shown in Figure 1. The cruises calendar was as follows:

Cruise    Otranto1    Otranto2    Otranto3    Otranto4    Otranto5    Otranto6
From      22/02/94    17/05/94    06/08/94    31/10/94    07/02/95    19/05/95
To        05/03/94    22/05/94    16/08/94    05/11/94    13/02/95    24/05/95

The ships employed were R/V Urania of the Italian Consiglio Nazionale delle ricerche, R/V Aegaeo of the National Centre for Marine Research of Athens, ITS Magnaghi of the Istituto Idrografico della Marina and R/V Alliance of the NATO SACLANT Undersea Research Centre.

The thermohaline measurements were performed by means of a CTD SeaBird 911 (cruises 1 to 5) and a CTD N.Brown Mk3C (cruise Otranto 6), coupled with a 24-bottles Rosette for water sampling. The sensors were calibrated before each cruise and controlled by means of SIS reversing thermometers and frequent discrete salinity determinations by means of an Autosal salinometer.

The CTD data were collected only during the downcast, at a sampling frequency of 24 Hz, while the instrument was lowered through the water mass at a rate of about 1 m/s. The first data processing was carried on board: the data were cleaned to eliminate spikes and misrecordings and subsequently averaged over 1 dbar pressure intervals. Salinity was computed from pressure, temperature and conductivity averaged values, according to the UNESCO (1983) algorithm. The final adjustment of temperature and salinity values was performed on land, after having analized the data sets coming from thermometers readings as well as from salinity determinations.

Figure 1. shows also the area in which the temporal and spatial evolution of LIW was studied; it is delimited by a rectangular grid which was chosen taking into account the following criteria:

- maximum coverage of the investigated area;
- step similar to the stations grid one, both in X and in Y direction.

According to these criteria, the grid was designed with the following features:

- spatial extension:
from 39.5^o N to 40.2^o N in latitude
from 18.5^o E to 19.3^o E in longitude
- grid nodes:
9 in X-direction (longitude)
8 in Y-direction (latitude)
- step:
0.1 degree both in latitude and in longitude

From the salinity data measured at sea the salinity field was subsequently estimated at the locations where the grid lines intersect. This grid fitting was made employing the software (UNIRAS, 1988a) AGL/Interpolations, which is a library of several FORTRAN subroutines that together act as a package for gridding and interpolation of mapping data. Since the LIW layer reaches the maximum depth of about 700 m, this procedure was repeated from the surface layer down to this depth, with a vertical step of 20 m. In this way a 3-dimensional grid of 2592 nodes was obtained, with a total volume of 3703 km³, being 3264 of which occupied by the sea, owing to the bottom topography (see Figure 2).

To visualize the 3-dimensional salinity field so obtained, the AGL/Blocks (UNIRAS, 1988b) software was employed, i.e. a library of FORTRAN subroutines which allow a graphical presentation of spatially varying phenomena using block diagrams. All the maps of the present paper show the resulting "cube" as seen from South-East, as indicated by the arrow of Figure 1.

Results and discussion

The LIW pattern during the investigated period (Figure 3) shows not only a seasonal variability but also an interannual one, with the features of the sixth Otranto cruise (May 1995) quite different from the previous five ones. In fact, the situations met during these cruises do not show remarkable differences: the layer of maximum salinity (from 38.80 to 38.85) is located at a depth ranging from 200 (Otranto 3) to 250 meters (Otranto 5) and occupies the eastern face of the cube or only its south-eastern corner (Otranto 2), while only during the autumn cruise Otranto 4 this layer is restricted to few "spots". On the other hand the maximum thickness of the LIW layer varies from a minimum of 220 m to a maximum of about 300 m (cruises Otranto 2 and 5, both performed during the winter period), while it does not reach the western side of the cube again during the cruise Otranto 4, which appears to be the poorest in LIW.

The LIW features met during the last Otranto cruise show, on the contrary, a sudden change from the previous situations, from three points of view: the salinity maximum value, its position and the volume occupied. In fact the salinity reaches values as high as 38.92, which were never found before, and the depth of this maximum rises to 150 m. At the same time the LIW layer shows a maximum thickness of about 500 m, the volume occupied increases and it appears compressed against the eastern side of the cube.

A better and more immediate understanding of the LIW evolution may be easily reached employing an interesting facility offered by the graphical displaying software. It consists in making visible the inner part of the studied volume simply setting to "undefined" all the grid nodes of the external part: this will make the cells transparent when the grid is countured. Figure 4 shows the inner structure of the six cubes, which were made partially transparent to evidentiate the level of the salinity maximum and the peculiar features found during the sixth cruise.

With this simple contrivance it is also possible to display only the LIW body, setting to "undefined" all the grid nodes where the salinity is less than 38.75. The results are shown in Figure 5, where again the different situation of the May 1995 cruise is evident as well as the minimum LIW volume of the autumn 1994 cruise.

As in the Otranto Strait area several previuos cruises were carried out, mainly in the frame of the POEM program, this kind of technique was employed on these data too, to look at the LIW pattern in previous years. The results are reported in Figure 6, which shows the situations met during the POEM cruises 1 (October 1985), 2 (March 1986) and 5 (August 1987). One can easily see that the LIW amount was always larger than during the Otranto survey, also if the maximum salinity did never reach the high values measured during the May 1995 cruise. On the other hand the LIW layer always reached the westernmost side of the cube, also if during the POEM 2 cruise it was limited to a thin vertical layer facing the southern (Ionian) face of the cube.

Finally, the technique of the "undefined" setting value of the non-LIW grid nodes, allows to compute the LIW percentage simply by evaluating the ratio between the not modified and the total grid nodes number, after having subtracted from the latter one the number of the nodes occupied by the bottom, as shown in Figure 2. The results, summarized in Figure 7, indicate that the LIW amount found during the period covered by the cruises Otranto 1 to 5 (February 1994 - February 1995) ranged with small oscillations from 15 to 22%, while during the second half of the eighties its mean value was higher, ranging from 38 to 49%. On the other hand there are no data enough up to now to state wether the value of 29% found during the last Otranto cruise of May 1995 indicates the beginning of a new period of increase of LIW amount or not.

As a conclusion, it is possible to state that the variability of the Levantine Intermediate Water in the area of the Otranto Strait is both spatial and temporal, the latter being both of seasonal and interannual nature. The variations can be easily followed in terms of LIW volume, position and also maximum salinity value and depth by means of the three-dimensional representation of the salinity field. This technique allows to get an immediate and comprehensive idea both of the field extension and of its structure at any level and/or at any vertical section, making easy the individuation of the water

Acknowledgements

This work was supported by the European Community, under the contract MAS2-CT93-0068 for the project "Hydrodynamics and Geochemical Fluxes in the Strait of Otranto" and by the Italian Consiglio Nazionale delle Ricerche (CNR), contracts 86.00194.02 and 87. 1103.02 for the POEM project.

References

Gacic M., V. Kovacevic, B. Manca, E. Papageorgiou, P.M. Poulain, P. Scarazzato and A. Vetrano (1996): Thermohaline properties and circulation in the Strait of Otranto. Dynamics of Mediterranean Straits and Channels, Bull. Inst. oceanogr., Monaco, n. special 17, CIESM Science Series n. 2, F. Briand Ed., 117-145.

Pollak M.I., 1951: The sources of deep water in the Eastern Mediterranean Sea. J. Mar. Res., 10, 128-15

Orlic M., M. Gacic and P. La Violette, 1992: The currents and circulation of the Ariatic Sea. Oceanol. Acta, 15, 2, 109-124.

Ovchinnikov I.M., 1966: Circulation in the surface and intermediate layers of the Mediterranean. Oceanology, 6, 48-59.

UNESCO, 1983: Algorithms for computation of fundamental properties of seawater. Unesco technical papers in marine science n. 44.

UNIRAS, 1988a: AGL/Interpolations, Version 6, User Guide and Reference Manual, Uniras A/S, Denmark, 80 pp.

UNIRAS, 1988b:AGL/Blocks, Version 6, User Guide and Reference Manual, Uniras A/S, Denmark, 99 pp.

Wust, G., 1961: On the vertical circulation of the Mediterranean Sea. J. Geoph. Res., 66, 3261-3271.

Zore-Armanda M., 1969: Water exchange between the Adriatic and the Eastern Mediterranean. Deep-Sea Res., 16, 171-178.

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