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46 A. Ciappa, S. Costabile / Marine Pollution Bulletin 84 (2014) 44–55
analysis is simultaneously performed for multiple receptor points Outputs from 2003 to 2005 were discarded due to the spin-up
equally spaced along the PMA perimeter. It will be shown that period of the model. The surface currents provided by the model
the resulting maps, joined together, provide information on the are daily means spanning the period from 2006 to 2011.
number of receptor points hit by the oil, that is, a measure of the Wind data were extracted every 12 h from re-analysis dataset
extension of the potential damage caused by the oil detected (Kalnay et al., 1996) provided by the National Centers for Environ-
offshore. mental Prediction (NCEP) and Atmospheric Research (NCAR),
The remainder of this paper is organized as follows. Meteo- because of the importance of sub-daily wind forcing in modeling
oceanographic data used in this study and aspects of the imple- the circulation of the Mediterranean Sea (Pierini and Simioli,
mentation of the new method are described in Section 2. Results 1998). In spite of the rather coarse spatial resolution (2.5°), the
of the substitution of the statistical distributions of wind and cur- reliability of NCEP/NCAR winds in the Sicily Channel was verified
rent used in the traditional ‘receptor mode’ with realistic data are by the substantial agreement of their kinetic energy (KE) with
illustrated in Section 3, together with the final results obtained for higher resolution winds (0.25°) shown in Fig. 2, the latter extracted
the Egadi MPA. In Section 4, the position and shape of the sea areas from the European Centre for Medium-Range Weather Forecasts
around the Egadi MPA deserving more frequent oil surveillance are (ECMWF) re-analysis dataset during 2011.
suggested and the main points emerging from data and simula- Winds and currents from 2006 to 2011 were interpolated in
tions are discussed. Finally, Section 5 summarizes the novel results time and space during the calculation of the oil trajectories. The
of this work. wind contribution to the oil transport is commonly estimated to
be 3% of the wind speed (ASCE, 1996). The four cases of maximum
2. Data and methods transport towards the cardinal directions from 2006 to 2011 are
illustrated in Fig. 3, and all occurred in the cold season.
2.1. Meteo-oceanographic data
2.2. The adopted method of oil trajectory analysis
The Egadi archipelago (Fig. 1), with a surface area of 208 square
miles and a perimeter of about 46 miles, is the largest Italian Marine The ‘receptor mode’ trajectory analysis (Galt and Payton, 1983)
Protected Area (MPA) and includes the major islands of Favignana, uses a particular target site as its starting point (receptor site) and
Marettimo and Levanzo (Fig. 1, panel A). The MPA is very close to proceeds to calculate where oil might come from to threaten this
the main tanker routes connecting the Western and the Eastern area. The technique is based on the Lagrangian approach and con-
basins of the Mediterranean Sea and to active oil extraction wells, sists of tracking backwards in time for few days a large number of
with others awaiting a production license in the Sicily Channel particles released at the receptor point by using the bi-dimensional
(source: World Wildlife Fund). The surface circulation in the study and time-reversed Euler scheme. The movement and spreading of
area is characterized by the eastward pathway of the Atlantic Water the oil is controlled by wind drift, current advection, spreading or
(AW), flowing near the surface from the Western to the Eastern diffusion, and ultimately weathering. Wind drift is simulated by
Mediterranean basin. Within the Sicily Channel, AW is advected an additional velocity that is a fraction of the wind speed (typically
by two main seasonal streams (Ciappa, 2009), the Atlantic Ionian 3%), and current advection is given by the current vector because
Stream flowing south-eastward in summer close to the coast of the oil is assumed to move with the water particle it is floating
Sicily (Robinson et al., 1999), and the Atlantic Tunisian Current, pre- on. Spreading and diffusion are incorporated by random walk.
vailing in winter and flowing southward over the Tunisian shelf Weathering processes, i.e. evaporative or decaying processes
(Pierini and Rubino, 2001; Béranger et al., 2004). Current measure- which have the effect of removing part of the oil from the active
ments and hydrographic data indicate a strong seasonal variability components of the spill, have not been incorporated into the
within the Channel (Astraldi et al., 1996, 1998, 1999). Low salinity trajectory analysis. Hence, the spilled oil is considered to be con-
values very close to the surface in the cold period (winter and servative and oil weathering is not included. In the traditional
spring) suggest a significant eastward advection of AW, and the way, ‘receptor mode’ analysis was performed on a seasonal or
higher salt content of the surface waters in the other periods sug- monthly basis, depending on the local wind and current time
gests a weakening of the eastward advection (Béranger et al., 2004). scales. Typically, the statistical description of the wind was sum-
The surface currents in the study area were extracted from a marized in a wind histogram that was assumed uniform over the
numerical experiment in which the circulation in the whole Med- area, and the current field was assumed constant in time and
iterranean Sea was simulated from 2003 to 2011 by using the was set to the mean values in the period (Torgrimson, 1981). The
Princeton Ocean Model (POM). This POM version (Blumberg and reversal path of the oil from the ‘receptor site’ to multiple sources
Mellor, 1987), similar to the model used in a general circulation offshore produces a probability map, i.e. the probability that the oil
study of the Mediterranean Sea (Zavatarelli and Mellor, 1995) found in one of the multiple sources reaches the receptor site, and
except for the use of sub-daily wind forcing, improved spatial res- an arrival time map, i.e. the minimum time for the oil to reach the
olution and few minor modifications, was recently used to investi- site. If oil is detected offshore, these maps provide useful informa-
gate other aspects of the upper circulation of the Mediterranean tion to logistics personnel for the activation of remedial procedures
Sea (Ciappa, 2014). The spatial resolution of the model (regular on the coast and are a natural resource when facing potentially
grid equally spaced 8’ in latitude and longitude) is on the edge of hazardous activities such as a drilling rig or the management of a
the admitted range for solving mesoscale eddies, because the Ross- ship in distress.
by radius of deformation in the Mediterranean Sea varies from a The method proposed in this study differs from the traditional
few kilometers in winter to 12 km in the period of stratification ‘receptor mode’ in two aspects. The first is that it is applied to sev-
(Grilli and Pinardi, 1998). As an additional measure adopted here, eral receptor points at the same time, and probability and arrival
the calculation of the surface current was improved by the assim- time maps are produced for each receptor point. In this case study,
ilation of daily maps of sea surface temperature (SST) in the upper 97 receptor points, at a spacing of 1 km, were selected along the
two layers of the model. SST maps were derived from data acquired perimeter of the Egadi MPA (shown in Fig. 1a). The use of multiple
by the MODerate resolution Imaging Spectroradiometer sensors receptor points provides additional information about the poten-
(MODIS) onboard the Terra and Aqua satellites (downloaded from tial extent of the damage, because in a sea area the number of
http://oceancolor.gsfc.nasa.gov/) and post-processed in order to overlapping plumes originating from the receptor points, which
provide cloud-free daily mosaics covering the Mediterranean Sea. is equal to the number of receptor points potentially reached by