Coordinator: Dr. Nadia Pinardi

Background

Radiative fluxes are important terms of the energy budget at the sea surface. So far, coarse parametrizations of the processes have been proposed on the basis of the relation between these fluxes and classical meteorological observations. It can be said that the uncertainties of the computed algorithms are often so large to make the energy budget nearly meaningless. The few comparisons performed for the Mediterranean Sea, indicate the existence of a bias between the experimental data and the predicted values. The mean difference between the computed and the measured data seems to be dependent on the specific features of this region. In fact, due to the geographic position and to the regional weather systems, the atmosphere over the Mediterranean is strongly influenced by both anthropogenic and natural aerosols, which play a crucial role in the radiation transmission. In addition, the semi-enclosed nature of the sea and the climatic conditions support high water vapor content in the lower layers of the atmosphere.

The objective of the proposed research is to obtain empirical equations for estimating the air-sea radiative fluxes suitable for the characteristics of the Mediterranean. In particular, attention will be given to the influence of the water vapor on the radiation transmission. In addition, the possibility of obtaining an evaluation of the total heat budget from measurements of skin-bulk sea temperature difference will be analyzed. To perform it, the already available data set of direct observations from the ship will be improved to include some specific measurements suitable to enrich it.

Workplan

1) Improvement of the shipboard measurements methodologies in the seasonal field experiments.

2) Comparison with the predictions and developing of appropriate expressions.

Description of team

Institute Personnel Position Man/month

SO, CNR, Spezia Maria Elisabetta Schiano Scientist 5

Stefania Sparnocchia Scientist 3

Carlo Galli Technician 5

Mireno Borghini Technician 5

Financial budget *	Total
1997	35
1998	35
Total	70

*All costs are in Millions of Lire

B.2.4. Yearly observations of metorological and oceanographic parameters over the sea

Principal Scientist: Dr. Antonio Siccardi (IAN-CNR, Genova).

Background

In the modeling of the marine circulation the input data are usually derived from climatological data sets or from the outputs of prediction models. Many studies demonstrated that, in both cases, the confidence of the meteorological data is rather poor. In fact, the data of the climatological data sets have been prevailingly obtained by merchant ships, so their accuracy is generally scanty. Moreover, the observations have an unequal spatial distributions, so they are not representative of all weather conditions. Conversely, very few tests have been performed to validate the outputs of the predictions models over the Mediterranean Sea. There is the need to provide a continuous monitoring of the principal parameters involved in the heat exchange under many environmental conditions. To realize that, we intend to use a meteo-oceanographic buoy, which sustains the sensors for the measurements of the following parameters: air temperature, humidity, wind intensity and direction, rain amount, air pressure, sea water temperature at surface and other four depths, incident solar radiation, water current at two depths, waves amplitude and direction, using three depth meters and three water level acoustic meters, plus some parameters defining the behavior of the buoy at sea.

The buoy is already operating in the middle of the Ligurian Sea, out of the land influence. This is a crucial region for the evaluation of the heat budget within the Mediterranean, since it is connected with the deep water formation processes. The current data time series collected by the buoy will be compared with the time series existing in the Corsica Channel.

Scientific objective

This subtask will give a contribution to a better understanding of local exchange phenomena as well as to the modeling of air-sea interaction physics in numerical models through the collection of long time series of atmospheric and sea water parameters. Initially the data set will be one year long but it could be extended to longer intervals.

The real time knowledge of sea waves will allow for the introduction of reference data in modeling the wind drag effects on the sea surface. A study will be initiated in order to utilize the deep part of the mooring line for the installation of long term self recording equipment. In parallel, the data will be exchanged between the interested project scientists.

Workplan

1) Improvement of the buoy sensor system. Set up of the procedures of the sensor calibration and the data validation.

2) Collection of a one year long time series of controlled air-sea data.

3) Comparison with the prediction and the historical data.

Description of Team

Institute Personnel Position Man/month

IAN-CNR, Genova Antonio Siccardi Scientist 5

Raffaele Amore Technician 5

Riccardo Mantovani Technician 6

Financial budget *	Total
1997	45
1998	35
Total	80

*All costs are in Millions of Lire

Detailed explanation of costs

1997 1998

Consum. Travel Personnel Tot. Consum. Travel Personnel Tot.

43 2 45 33 2 35

Task B.2.5. Estimates of air-sea interaction heat and water budgets from operational analyses

Principal Scientist: Dr. S. Castellari (IMGA-CNR, Bologna)

Background

Most of the recent work on heat flux fluctuations at seasonal and interannual time scales has been carried out either with COADS data, which have maximum resolution of 1 X 1 degrees squares, and/or low resolution operational surface parameter analyses, which do not adequately resolve the mountain relief of the continents around the basin.

It is then mandatory to reevaluate the “Mediterranean heat flux closure problem” (with means that the long term average heat loss at the air-sea interface of the basin should balance the heat inflow at Gibraltar and evaporation should not exceed climatological values) with higher resolution data sets such as operational analyses from ECMWF at a nominal resolution of 0.5675 X 0.5675 degrees square. Furthermore we would like to further investigate the sensitivity of surface heat budgets estimations to the different bulk formulas parametrizations, as given for example from the observational studies of Task B.2.2. and B.2.3.

Scientific objectives

Analyze and produce a climatological heat and water budget data set for the overall Mediterranean area from the highest resolution surface operational analyses and Reynolds new weekly SST analyses.

Workplan

- Acquire the high resolution (0.5675 X 0.5675 degrees) operational surface fields for a study period from Jan. 1991 until December 1994

- Compute with bulk formulations the long term basin heat loss and do sensitivity experiments to the frequency of the atmospheric forcing fields used (daily versus monthly mean frequencies)

-Use both 1000 mb and surface operational analysis fields and intercompare results

Description of Team

Institute Personnel Position Man/month

IMGA-CNR, Bologna S.Castellari Scientist 5

N.Pinardi Scientist 1

Financial budget *	Total
1997	25
1998	35
Total	60

*All costs are in Millions of Lire

Detailed explanation of costs

1997 1998

Consum. Travel Personnel Tot. Consum. Travel Personnel Tot.

20 5 25 30 5 35

Task B.3 Numerical simulations and process studies of the general circulation

Principal Scientist: Dr. Stefano Pierini (IUN, Napoli)

Introduction

The wind and air-sea process are the main cause of the oceanic surface circulation variability. Moreover, the divergence of the corresponding mass flux leads to a surface topography which, in turn, induces pressure-driven, barotropic current components. In response to this forcing, it appears that the large scale ocean circulation could be highly variable on a daily, seasonal and interannual time scale. We propose to carry out process studies, already initiated in the framework of EU projects, aimed at describing this part of the Mediterranean Sea dynamics and its influence on the dispersion properties of passive tracers and particles.

The theoretical analysis of the Mediterranean Sea circulation by means of general circulation models has only very recently led to detailed descriptions of the global wind-driven, thermohaline and boundary forced circulation schemes (e.g. Roussenov et al.,1995, Zavatarelli and Mellor, 1995, Korres et al., 1996). Parallel to these comprehensive studies, process models have been implemented which focus on a particular forcing, in order to assess in detail its contribution on the global dynamics. As far as the wind-driven component of the circulation is concerned, work was done by Heburn (1987), Pinardi and Navarra (1993) and Pierini and Simioli (1996). In this task we propose to continue the work begun in these studies and diagnose the ocean response to interannual variability in both the wind and buoyancy forcing at the air-sea interface. This work will be done in parallel to the analysis of in situ and satellite data of Task B.1 and comparison with observations will be carried out throughout the Task in order to calibrate and validate the model’s solutions.

B.3.1. General Circulation Modeling studies in the overall Mediterranean Sea

Principal scientist: Dr. N.Pinardi (IMGA-CNR, Bologna)

Background

During the past six years a General Circulation Model (GCM) has been implemented in the Mediterranean Sea with different horizontal and/or vertical resolutions (1/4 X 1/4 or 1/8 X 1/8 degrees in horizontal and 19/31 levels in vertical). The model is derived from the MOM (Modular Ocean Model, Pacanowski et al., 1990) developed by Bryan and Cox in the seventies. The model development consisted mainly of the introduction of sophisticated air-sea fluxes, tuned to the Mediterranean climatology, in order to capture the crucial momentum and heat exchanges the air-water interface. Long time integrations (order of ten up to a hundred years time long integrations) have allowed to study the wind and thermohaline driven general circulation, its seasonal and interannual variability and to construct the first hypothesis on the major driving forces of the circulation and its variability.

It was found that interannual variability can be as large as the seasonal variability and that maximum amplitude is reached in the Eastern Mediterranean (Ionian and Levantine basins). It is also found that winter anomalous conditions have a large impact on the circulation which preserves the memory of anomalous events modifying the regular occurrence of the seasonal cycle. The transports at the basin Straits, and in particular at the Sicily Strait, has been found to be connected local wind forcing conditions. The appearance and disappearance of sub-basin gyres, such a Mersa-Matruh and Shikmona in the Eastern Levantine basin can be traced back to the anomalies of the heat and wind forcing over the region.

Interannual variability simulations for the eighties have been carried out with the low resolution model (1/4 X 1/4 degrees and 31 levels, Roussenov et al., 1995, Pinardi et al., 1997) but it has become evident from the comparison with the relatively high resolution model (1/8 X 1/8 degrees and 31 levels) that they should be repeated at high resolution because of important eddy-mean flow interactions which make the ocean response to atmospheric driving extremely non-linear. Thus it is timely to study in details the interannual variability due to internal nonlinear dynamics with respect to the externally forced interannual variability. Moreover, the simulation should be extended in the nineties where the new data sets collected could be used to validate the model simulations.

Scientific Objectives

- study the interannual variability of the Mediterranean Sea ocean circulation by means of relatively high resolution GCM simulations

- understand the importance of eddy versus externally forced interannual variability

- critically assess the air-sea physics described in the model in comparison with the observations collected in this project

- use the available data sets to validate the model response

-describe with conceptual models the simulations and produce a synthetic picture of the major driving mechanisms at seasonal and interannual time scales

Workplan

- produce long term (1980-1995 approx.) simulations of the ocean circulation with the 1/8 X 1/8 degrees GCM

- intercompare the high and low resolution experiments in coincidence with available observations

- produce a conceptual picture of the general circulation variability in response to atmospheric forcing anomalies and internal dynamics

Description of team

Institution Personnel Position Man/month

IMGA-CNR Nadia Pinardi Scientist 3

S.Castellari Scientist 3

Boursary 12

Financial budget *	Total
1997	30
1998	30
Total	60

*All costs are in Millions of Lire

Detailed explanation of costs

1997 1998

Consum. Travel Personnel Tot. Consum. Travel Personnel Tot.

5 5 20 30 5 5 20 30

B.3.2. Studies on the Wind driven Mediterranean circulation, with particular attention to the Tyrrhenian Sea.

Principal Scientist: Dr. Stefano Pierini (IUN, Napoli)

Background

We choose to focus on the Tyrrhenian Sea area, a particularly interesting basin because it is relatively isolated from the rest of the Mediterranean from the point of view of the wind-induced momentum, and is only indirectly affected by deep and Levantine Intermediate water mass formation (a nice feature if a process model and basic dynamical processes have to be tested). On the other hand this basin is connected to the western and Levantine basins through more or less narrow straits (of Sardinia, Sicily and Corsica), where valuable current meter data are available (e.g. Astraldi and Gasparini, 1992) and new ones are currently being gathered. Therefore the model results can be compared and tuned with the aid of such experimental data. Moreover the bathymetry and geometry of this sea suggest that topographic Rossby waves and modes could be excited by the wind, analogously to what can happen in the Strait of Sicily (Pierini, 1996). Also this aspect will therefore be considered.

We will apply layer models to the Tyrrhenian Sea in a relatively high resolution grid (1/8 of a degree). The wind-forcing will be initially given by the 1980-1988 N.M.C. data so that the new results will be matched with those obtained by Pierini and Simioli (1996). The currents at the open boundaries at the straits will be provided by a coarse resolution model of the whole Mediterranean. Then, in order to test a new data set and to simulate more recent years, the ECMWF wind data will be used, available to this project through the data archiving initiative. At the same time the Eulerian field of currents will be used, along with an appropriate parameterization of the turbulent transport, will be used to compute Lagrangian trajectories. The objective is to analyze the wind-driven dispersion properties of the flow, generalizing the analytical techniques and results developed by Pierini and Zambianchi (1996).

The high frequency variability of the wind will be retained for three reasons. First, the daily variability of the flow, which is usually considered as a noise and so neglected, has an energy content comparable or even larger than the seasonal mean (Pierini and Simioli, 1996). Moreover, it also yields a remarkable seasonal variability itself. We therefore believe that its contribution is an important part of the wind-driven climatology which deserves to be studied in detail, both in its time and spatial characteristics. Second, we want to assess what is the effect that retaining the high frequency content of the flow has in the Lagrangian trajectories and, therefore, on the dispersion at sea. This effect is still to be fully understood, but there are general results indicating that it actually may be important. Our model results are expected to give some answer to this respect. Third, Pierini and Simioli (1996) suggested that the eddy viscosity coefficient in the model could be tuned by comparing the daily transport through a strait, for instance the Strait of Corsica, with experimental current data. We intend to implement this method in collaboration with the colleagues of the SO of La Spezia (Dr. M. Astraldi and G.P. Gasparini), and test its relevance and consistency.

Finally we stress that the work here proposed will be carried out in close cooperation with the experimental group of the ISDGM of La Spezia (Dr. M. Astraldi and G.P. Gasparini) and with the modeling groups of IMGA of Bologna (Dr. N. Pinardi) and ENEA of Rome (Dr. V. Artale). In particular, with the group of ENEA parallel studies in the Tyrrhenian will be carried out in order to obtain a deeper understanding of the role that the free surface has in the wind-driven circulation in comparison with the rigid-lid assumption.

Scientific Objectives

-Detailed description of the seasonal and interannual variability of the dynamics induced by the NMC and ECMWF winds over the Tyrrhenian Sea area. Since the high frequency wind driven fluctuations show a clear also show a clear seasonal statistics with a large energy content, they will be included in the description of the dynamical variability.

-A calibration of the dissipative parameters of the models by means of a comparison between numerical results and transports through the major Straits will be carried out.

-Finally the dispersion properties of such flows will be analyzed through Lagrangian techniques

Workplan

Implementation of layer models in the Mediterranean Sea.

Detailed analysis of the temporal and spatial characteristics of the wind forcings and of the induced flows.

Comparison between numerical results and current meter measurements, especially through the major Straits.

Intercomparison with other numerical model results.

Description of team

Institution Personnel Position Man/month

I U N, Napoli Enrico Zambianchi Scientist 3

Gianni Ficca Scientist 2

Financial budget *	Total
1997	35
1998	35
Total	70

*All costs are in Millions of Lire