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54 A. Ciappa, S. Costabile / Marine Pollution Bulletin 84 (2014) 44–55
trajectory tracking as well, because statistical distributions of wind by the intersection of the three main tanker routes around the
and current are not adequate to establish a sound statistical basis MPA with the areas of greatest threat of oil pollution. The route fol-
for quantifying the likely oil trajectories. The results of the compar- lowing the northern coast of Sicily is the least frequented one but
ative test effected in this study show that, in the short term requires monitoring all year long. The route following the southern
(5 days), part of the oil trajectory variability is filtered out if statis- coast of Sicily is the most critical because it is highly frequented,
tical distributions of the wind and mean current fields are used. As very close to the MPA and a source of threat all year long. The third
consequence, considering that multi-annual sequences of wind route, close to the Tunisian shore, is the most frequented but the
and current data are produced by numerical models, the second threat is limited to the months of November and December.
point is that results of backward trajectory analysis can be The oil trajectory method illustrated in this study, based on the
improved if based on data available at higher spatial resolution. ‘receptor mode’ trajectory analysis, consists of tracking backwards
As an example, results of this study based on sub-daily wind fields in time Lagrangian elements from the receptor points to multiple
at 2.5° resolution and daily current data at 8’ resolution can be sources offshore. This technique is a valid alternative to the for-
greatly improved by data produced by higher resolution models. ward-in-time integration in computational terms. Indeed, the
The third point is a source of uncertainty arising from the fact that seeded points lie on the coast or around the MPA rather than off-
oil trajectory statistics are extracted from a limited number of shore, and the number of Lagrangian elements tracked is largely
years, and adequate statistics were reached after 5 years in the Gulf reduced.
of Finland (Soomere et al., 2014). A general improvement coming
from the evolution of the modeling techniques and the increasing Acknowledgements
quality of the data collected year after year will be the possibility of
managing longer datasets. As the final point, the choice of the Grateful acknowledgement is made to the institutions that in
backward-in-time tracking done in this study is a valid and compu- different countries provide free access to the models and data used
tationally efficient alternative to the use of the forward-in-time
in this study: the Ocean Model group at Princeton University
technique. Indeed, release points uniformly covering the 3 3° (POM), National Center for Environmental Prediction (NCEP) and
domain of Fig. 6 (spaced at 1 min) used in forward tracking would
National Center for Atmospheric Research (NCAR), the Oceancolor
be about 300 times the number of receptor points used in this team of National Aeronautics and Space Administration (NASA). A
study.
special thank to Alex Cooper for the revision work. The authors are
grateful to the Editor and the reviewers for their constructive sup-
5. Conclusions port that improved the manuscript.
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