Principal scientist: Dr. M.Astraldi (SO-CNR, La Spezia)
Introduction
It is only a few years that the concept of long term variability of the circulation has emerged for the Mediterranean, and now it is common in the literature, where it is reported as a result of experimental data and numerical models. This task is concerned with the study of the seasonal to interannual variability of the Mediterranean basin emerging from existing hydrological and satellite data sets.
A part from the satellite Sea Surface Temperature (SST) images, which offer nowadays a ten-fifteen years time series of surface circulation changes, evidences of significant changes in the hydrographic and current structures of the Mediterranean are becoming more and more frequent for different parts of the basin. The basic question now is: 1) to improve the data sets in critical sea regions where they already exist but there is need to continue to sustain the acquisition effort in order to be able to fully document the climatic evaluations; 2) to analyze the existing hydrographic data sets at the basin level in order to diagnose the changes in the circulation parameters in the whole water column. This work will give the basis to verify the model predictions of changes (Task B.3) and will give the experimental evidence to be able to construct a conceptual model of the causes for seasonal and interannual variability in the Mediterranean Sea.
B.1.1. Study of the seasonal and interannual variability of the Sea Surface Temperature in the Mediterranean.
Responsible: Dr. Lia Santoleri (IFA-CNR, Roma)
Background
A study of the seasonal and interannual variability of the SST for the Mediterranean has been done by Santoleri et al. (1994) for the Western Basin (1982-1990) and by Marullo et al. (1995) for the Eastern one, excluding the Adriatic and the Black Seas (1982 to 1992). Both studies were based on coarse resolution data obtained from the Distributed Active Archive Center (DAAC) of the NASA Ocean Data System (spatial resolution of 18 km and temporal resolution of 1 week). Both studies basically showed that the coarse resolution images can be used to recognize the general pattern of the circulation. In particular, they showed that the three winters of 1983, 1987 and 1992 were characterized by three temperature minima
that were present both in the Western and the Eastern Basin. It is interesting to remark that these three winters were preceded by strong El Nino events. Another feature derived from the analysis of the thermal maps was the increasing trend in the SST during the 1982-1990 period.
With the coarse resolution data, however, single, though important features, such as the Alboran Gyres, are very hard to be detected and followed throughout the year. Actually, the small Mediterranean value of the internal Rossby radius of deformation (5-15 km) requires a higher resolution data set to be discerned by
satellite. In this project we intend to study the seasonal and interannual variability of the SST fields in the whole Mediterranean, including the Adriatic Sea and the Black Sea. This will be done by using a longer time series of the same data set spanning from 1982 to 1995. Moreover, we intend to use a second
data set (the NOAA/NASA Pathfinder), characterized by a higher spatial and temporal resolution (9 km and 1 day). The latter will be obtained with a more refined algorithm (Pathfinder) which takes into account the drift calibration of the AVHHR sensors and therefore is more suitable to study long term trends.
Scientific Objectives
1) To study the seasonal and interannual variability of the sea surface temperature field.
2) To assess the space and scale variability of the main subbasin scale circulation features.
3) To understand the relative role of the heat exchange and ocean dynamics on the long term variability of the basin.
Work Plan
1) Acquisition and extraction of the Mediterranean data from the two data sets.
2) Analysis and intercomparison of the MCSST and Pathfinder data set for the Mediterranean Sea.
3) Study of the seasonal and interannual variability of the Mediterranean Sea and its sub-basins.
Principal Scientist: Mario Astraldi (SO-CNR, La spezia)
Background
Among the known long term changes in the Mediterranean, there are two occurring in the western basin that can bring reliable information on the response of this basins to climate variability. The first one refers to the passage of water through the Corsica Channel, between the Tyrrhenian Sea and the Ligurian-Provencal Basin. In the Corsica Channel there is a current time series initiated in 1985 and lasting, with some interruption, until the present days (1996). The current data show that the transport through the strait is prevailingly northward and it has a marked seasonal variability. The seasonal transport changes significantly from one year to another, and, in the considered period, the current volume across the channel was seen to decrease progressively by more than 70%. A preliminary analysis of the current data indicates that the interannual variability of the transport might be related to a change in the atmospheric-climatic conditions existing over the north-west Mediterranean in winter (Astraldi and Gasparini, 1992). This is confirmed by the response of a circulation model for the Western Mediterranean (Herbaut, 1994), even though there is still an open question on the forcing which could cause this trend. What is known is that the interannual changes of the signal in the Corsica Channel are linked to a progressive improvement of the atmospheric climatic conditions over the Northern Mediterranean in the late eighties, and the continuation of the monitoring will allow us to follow this climatic trend in all its length.
A second signal worth to be followed refers to the Tyrrhenian Sea, that is the deepest, most isolated basin in the Western Mediterranean. This basin is characterized by a circulation particularly energetic at its boundary, and considerably weak in its inner part. The deepest part of the basin is characterized by
a specific structure, a step structure involving the T and S vertical profiles, indicative of the very low energetic conditions existing in the marine environment. Periodic hydrographic measurements initiated in the seventies show that the step structure is sustaining a progressive, long term change, to be related with a positive trend in the atmospheric climatic conditions over the basin (Zodiatis and Gasparini, 1995).
The last crucial region for monitoring the long term variability of the basin is the Sicily Channel which represents the boundary between the two major sub-basins of the Mediterranean. Both the MAW (Modified Atlantic Water) and the LIW (Levantine Intermediate Water) across this Strait carrying the information of the interaction with the atmosphere and its anomalies. It can be then interesting to set up an observational network across the Sicily Channel in order to observe the dynamics and hydrographic properties of the flowing waters and their variability with time. This is particularly important for the very recent events produced by the climate variability in the deep waters of the Eastern Mediterranean (Roether et al., 1996), whose signal should be found in the outflowing bottom waters at the Sicily Channel and it is studied in the experiments described in Task B.4 and B.5.
Workplan
-improve the current time series in the Corsica Channel.
-carry out specific hydrographic measurements in the central Tyrrhenian region.
-start an observational network, involving current measurements and hydrographic stations, in the Sicily Channel.
The work will be partially sustained by the EU-Mast Program and the experimental work will be coordinated within the MATER Project.
B.1.3 Climate variability in the Middle Adriatic Sea
Principal Scientist: Dr. Elio Paschini (IRPEM-CNR, Ancona)
Background
The Meso Adriatic depressions (called Pomo or Jabuka Pits) are a very interesting site for climate studies. This is the deepest area of Northern and Central Adriatic Sea, the maximum depth is about 270 m. and it is divided from the Southern Adriatic Sea by the Pelagosa Sill (about 170 m).
Pomo Pits are filled by the Middle Adriatic Deep Waters (Artegiani et al., 1996) with a characteristic temperature and salinity centered at 11 C and 38.4 psu respectively. From the dissolved oxygen data it was argued that this type of water is renewed in spring by deep water formation processes occurring during the previous winter season in the Northern Adriatic. Three types of conditions can occur every spring.: a) a deep water ,formed in the Northern Adriatic, characterized by cold and low salt content water, fills the pits; b) another kind of deep water formed along the Croatian coast ,characterized by salt and warm waters, fils the pits; c) none of the new deep waters formed during the winter reach the density necessary to get out the old MADW from the Pits.
Pomo Pits are also a nursery area for important commercial fish such as cod, as well as a fishery ground for lobsters. It is clear that environmental parameter variations of the Pomo Pits will affect stock fishes. Since 1977 IRPEM checks that area by quasi-seasonal cruise. The parameters acquired were physical, chemical, biological and fishery. About 40 cruises were carried out in the framework of international ( ASCOP, MEDALPEX, POEM, ELNA), national ( AMEX, PRISMA 1) and IRPEM research programs.
Scientific Objectives
1)- Understand the deep water renewal mechanisms in the Pomo Pits
2)- Determine the correlation between oceanographic conditions and stock fishes
3)-Improve the Pomo Pits decadal oceanographic data-set
Workplan
a) Analysis and collection of meteorological parameters that could affect deep water formation and Po river discharge data since 1977 and analysis of hydrological data set.
b)-Correlation between data-sets ( meteorological versus oceanographic and oceanographic versus stock fish). Attempt, on the basis of previous study, to setup a model to predict the renewal of MADW.
c)- Improve the Pomo Pits data-set. Seasonal cruises will be carried out in the Pomo Pits area. IRPEM own R/V S. Lo Bianco which is normally used for these campaigns. Data collected with a CTD probe are: pressure, conductivity, dissolved oxygen, fluorescence, light transmission and back-scattering as well as derived parameter: salinity, density, etc. Chlorophyll-a, nutrient salts and dissolved oxygen parameters will be measured from bottle samples by laboratory analysis. All data collected will be processed and added to our Pomo Pits data-set.
B.1.4 Diagnostic studies of climate variability in the basin heat storage
Principal Scientist: Dr. Fabio Raicich (ITT, Trieste)
Background
It is now well known that Mediterranean Sea general circulation structures are interannually varying partially due to the atmospheric forcing parameters, described in the Task B.2. Furthermore it is already evident that the Eastern Mediterranean Sea Surface Temperature (SST) is connected to Sahel draughts or wet seasons (Ward, 1992) and that the Mediterranean is an area interested by a train of Rossby waves excited by Monsoon activity which produces adiabatic descent over the Sahara/Mediterranean area (Rodwell and Hoskins, 1996). However, correlation between meteorological forcing and ocean response deduced from historical hydrographic data sets has not been carried out until now in a satisfactory way. The longest atmospheric parameters time series are either decadal (approx. 1980-1995) for operational surface fields or multidecadal for climate indices such as Sahel draught index, Monsoon and El Nino indices, which indicate the presence of certain atmospheric patterns over the Mediterranean area. The historical hydrological data set collected during the Mast-II Project MODB (Mediterranean Oceanic Data Base, MODB, Brasseur, 1995) span a period of almost a century, enabling us to find “climate indices” for the Mediterranean Sea circulation and heat storage. If we look at the major sites of interannual variability in the ocean , e.g. El Nino and Indian Monsoon areas, it is then evident that heat storage is a sensitive indicator of the interannual changes due to atmospheric forcing. Here we would like then to explore the heat storage variability from the MODB historical data set and determine “climatic indices” in selected regions of the basin. This study will serve as a basis to understand the importance of coupled ocean-atmosphere interactions over the Mediterranean basin.
Scientific objectives
The interannual variability of the ocean heat content, subduction rates in key climatic areas and the overall Mediterranean will be examined as it correlates with atmospheric anomalies. The question to be answered is: can the signature of atmospheric anomaly patterns be revealed in the ocean response? If yes, how it correlates with the atmospheric parameters and which ones are the most correlated?
Workplan
-Atmospheric data acquisition and analysis; new COADS analyses and ECMWF reanalyzes fields will be acquired
-MODB data analysis will be carried out in order to compute heat storage anomalies and subsurface temperature time series in selected ocean areas. Furthermore, subduction rates of waters will be also computed
- Anomaly correlation’s of atmospheric surface and lower troposphere anomalies will be computed.
B.1.5 Seasonal and interannual variability of the South Sicilian and South-SouthwestSardinian coasts from historicaldata.
Responsible: Dr. Angelo Perilli (IMC and Ecologia della visione-CNR, Oristano)
Background
Long time series of hydrological data and thermal images have shown interesting oceanographic features along the South Sicilian coasts, in particular in the "Banco Avventura" area, and in the region around the
South Southwest Sardinian coast. In these two areas frequent upwelling, downwelling, filaments, rings and eddies are observed. These phenomena may have a central role to play in the transport of chemical, physical and
biological properties. They generate fronts which present two significant vertical motions: downwelling or transfer of properties from the surface layer (directly heated and modified by the atmosphere) into the deeper water column; upwelling or lifting of nutrient rich waters to the surface. In particular the instabilities originating at the frontal areas may also play an important role in advecting rich nutrient water into the open ocean.
The mechanisms that drive these various oceanographic phenomena, in the Sicilian (Banco Avventura) and Sardinian coasts, and the response to atmospheric forcing, which is an important factor for the Western
Mediterranean basin dynamics, are not yet known. From the MODB historical hydrological data sets, and the MCSST and NOAA/NASA long time series of satellite images we would investigate the space and time scales of the main features in the studied areas. By means of long term atmospheric data sets we will analyze the relationship with meteorological forcing. The information obtained from the previous data sets will be coupled with CZCS historical data that permits the study of the primary production associated with these interesting events.
The complete (physical, biological and atmospheric) data sets will be used with the purpose of investigating the seasonal and interannual variability, and to improve the knowledge of the driving mechanism of these various oceanographic phenomena in these two biologically and physically crucial areas of the Western Mediterranean sea.
Scientific objectives
To study the interannual variability of the upwelling and downwelling regions along the South Sicilian and South Southwest Sardinian coasts from historical hydrological, meteorological and satellite data sets in relation to the atmospheric forcing .
Workplan
- Acquisition and elaboration of MODB and atmospheric data sets.
- Analyses of the MCSST and CZCS data sets.
- Estimation of the space and time scale of the upwelling and downwelling phenomena.
- Assessment of the primary production in the studied areas.
- Evaluation of the seasonal and interannual variability of the upwelling and downwelling.
Principal Scientist: Dr. Salvatore Improta (Dept. Of Physics, Univ. “La Sapienza” of Rome))
Background
We plan to use field and satellite data to investigate problems of some fundamental interest related to the Mediterranean Sea. The first problem concern dense water formation processes. It is well known that violent convective phenomena are observable when cold dry winter wind blows over selected areas of the Mediterranean Sea. Large-scale effects are obviously present but also a local signature of the strong air-sea interaction should be observable, both on SST and on Surface roughness (Schott et al., 1996). By comparing such information with climatological sea surface stratification data, together with heat exchanges rates as obtained in numerical models, recent criterion proposed by Narimousa and Max Worthy allows one to estimate depths and volumes of newly produced dense waters. Preliminary observations related to various zones of the Mediterranean give stimulating results (Buongiono Nardelli et al., 1995; Gremes Cordero, 1998).
About t he second problem of fundamental interest, density currents are caused by the density difference between fluids in many different equations, both in Atmosphere and in Oceans. In the Mediterranean Sea, for example, the density currents are generally driven by salinity and temperatures inhomogeneities. They are deeply influenced by the Coriolis forces, which tend to oppose their spreading. Note how a density current is only a hundred meters thick, but can stand for thousands of kilometers along the continental slope. Increasing its volume by entertainment with the resident water (Astraldi, Gaparini, Salusti 1998). Of particular importance to the ocean circulation are dense bottom currents that flow down over bottom slopes.
Our proposal intend to study a dense bottom current that from the eastern Mediterranean basin goes to sink in the western basin, through the strait of Sicily, its dynamics, and its climatological impacts.
Scientific Objectives
We plan to investigate, using satellite and field data, the open sea convective processes in various geographical zones of the Mediterranean Sea. In particular the relation with the general climatological situation will be examined.
Above the density currents flowing over the sea bottom, we plan to focus our attention on:
- the description of the evolution of the vein in the strait of Sicily;
- the application of analytical models of some peculiar aspects of the vein;
- the simulation of the vein, applying different models.
Workplan
- Acquisition and analysis of realistic heat fluxes, satellite data and climatological information on density stratification in the interesting zones of dense water formation
Consum. Travel Personnel Tot Consum. Travel Personnel Tot
7 7 14 8 7 15
TASK B.2. Heat, water and momentum exchanges at the air-sea interface
Principal Scientist: Dr. Luigi Cavaleri (ISDGM, Venezia).
Introduction
Air-sea interaction processes are of primary importance in forcing the ocean circulation and the associated numerical models. In the past few years, the interannual variability of the heat budgets at the air-sea interface has been established (Garrett et al., 1992). Large fluctuations in heat losses and gains are shown to have periods of several years. This means that the ocean forcing can be variable and water mass formation processes can vary in intensity and quality of the water formed. Direct measurements of air-sea heat budgets (Schiano et al., 1993) have shown large deviations of the bulk formulas from traditional parametrizations derived from other open ocean areas of the world ocean. Furthermore, it is by now evident that the overall heat budget constraint imposed at Gibraltar (plus 7 Watts/m2 entering on the average at the Strait) can be satisfied only on the multiannual time scales and that seasonal heat budget unbalances can occur at the level of the annual cycle. The Mediterranean offer the unique opportunity of “tuning” the air-sea parameterizations given the known long term heat loss at the air-sea interface. The knowledge of the correct air-sea parametrization to be put as air-sea interaction forcing in OGCM’s has been already shown to impact our ability to reproduce correctly the water column stratification (Castellari et al., 1996).
The water budget estimates are indeed another very important question which needs to be stressed. We believe that he ECMWF operational analyses from 1991 have acquired enough horizontal resolution to be able to give reliable estimates of rainfall over the Mediterranean areas. We should then use the precipitation data in order to reevaluate new climatological water budgets or Evaporation minus precipitation values. A first assessment of interannual variability should also be carried out.
Estimates of air-sea kinetic energy exchanges are usually obtained by semi empirical bulk aerodynamic formulae using mean meteorological measurements. Confidence in the actual parametrizations is rather poor and the available data sets show clear deficiencies. In addition, it has been shown that the small scale processes are very important for a better evaluation of the forcing functions and the definition of the marine circulation as well. So, the necessity of having more sophisticated information on the actual basin dynamics is also linked to the possibility of better evaluating the parameters characterizing the air-sea processes.
B.2.1. Yearly variability of the wind drag at the air-sea interface
Principal Scientist: Ing. Luigi Cavaleri (ISDGM-CNR, Venezia)
Background
Large scale circulation models of the oceans, and, more in general, estimates of the large air-sea interactions, often rely on average long term quantities. Only recently circulation models have been driven in some cases by actual time series of the relevant quantities.
This philosophical approach derives from the characteristic time scale of the ocean circulation, which, at least in its large features, is typically orders of magnitude larger than the immediate variability of the forcing. However, the advance of small scale processes, both in the forcing function of the circulation (wind variability) as in the circulation itself (small scale vortices) is becoming more and more evident and there are suggestions that some long term characteristics are connected to shorter term characteristics of the actual basin forcing.
This sub-task deals with the variability (both in time and space) of the wind and of the associated wind stress at the surface. The crucial point is that the stress is a non-linear function of the wind speed. Given the large variability of the wind with respect to its average values, to use average wide speeds to deduce average
stress values can lead to large errors in the stress fields and in the derived circulation. Besides, recent studies have shown the dependence of Tau not only on the wind speed, but also on the state of the sea surface, i.e. on the wave conditions. The physical relationship is well determined and fully computable.
Scientific objectives
-to determine, in spatial and temporal detail, the actual wind forcing acting on the sea surface
-to evaluate the errors due to the usage of climatological or long term averages of the wind field
Workplan
1) Consider the wind field in the Mediterranean Sea from July 1992 to June 1995 and derive the average wind and the associated stress.
2) Derive the stress directly from the single wind fields, and evaluate the corresponding average stress.
3) As 2), but taking into account the effect of the wave field.
Description of team
Institute Personnel Position Man/month
ISDGM-CNR, Venezia Luigi Cavaleri Scientist 3
Luciana Bertotti Scientist 3
Silvestro Curiotto Technician 3
Financial budget *
Total
1997
30
1998
35
Total
65
*All costs are in Millions of Lire
B.2.2. The short and long term variability of the wind field in the Adriatic
Principal Scientist: Dr. L.Bertotti (ISDGM-CNR, Venezia)
Background
The variance of the surface wind field in the Adriatic and over most of Mediterranean regions is large with respect to climatological average values, and this causes problems in the interpretation and the correct usage of the average quantities. Long term time series of the overall fields are required for a proper analysis, but these are not available with the sufficient accuracy, as the meteorological models have achieved the necessary resolution, especially in the Mediterranean Sea, only in recent years. Also the re-analysis of the atmosphere from 1979 till 1994, soon to be completed at the ECMWF will not solve properly this problem because of the usage of T106 model. Cavaleri et al. (1991) have shown that T106 strongly underestimates the wind strength in the Mediterranean Sea, sometimes missing also some key features connected to the cyclogenesis in the western Mediterranean basin.
In connection with another project we have hindcast 30 years of wind fields in the Adriatic Sea. The accuracy of the wind fields has been proved by comparison of both the wind and derived wave fields versus measured data (see Bertotti et al., 1996, Cavaleri et al., 1996).
The wind field are available with high resolution (40 km grid step size) at three hour intervals. They allow therefore a detailed analysis of their variability. While the space variability is often related to the surrounding orography, the time variability is strictly connected to the large scale events in the Mediterranean basin (Cavaleri et al., 1989)
Scientific Objectives
- To determine the yearly and interannual variability of the meteorological situation on the Adriatic Sea
- To determine eventual trends of the average situation in the basin
- To determine events occurred in the time series
The analysis covers the three decades from 1960 till 1988. As the situation on the Adriatic is strictly connected to the general situation in the Mediterranean Sea, the determined variability could be indicative of more general trends over the whole Mediterranean region.
Workplan
Statistical analysis of the wind fields to derive:
a. The long term characteristics, both as a whole and in the single sections of the basin,
b. The trend of the average quantities with a cross-comparison among the results of the single sections,
c. The characteristics seasonal variability,
d. The presence of characteristic patterns associated with specific periods.
Description of team
Institute Personnel Position Man/month
ISDGM-CNR, Venezia Luigi Cavaleri Scientist 4
Luciana Bertotti Scientist 4
Financial budget *
Total
1997
30
1998
20
Total
50
*All costs are in Millions of Lire
B.2.3. Validation of empirical formulae used to estimate the heat budget at the air-seainterface
Principal Scientist: Dr. Maria Elisabetta Schiano (SO-CNR, La Spezia)
