  This paper proposes a new uncertainty measure, appropriate for quantifying
the performance of transport models in assessing the origin or source of a
given observation. It is found that in a neighbourhood of the observation the
proposed uncertainty measure is related to the invariant dynamical structures
of the model. The paper illustrates the implementation of the proposed
definition to quantify the performance of ocean data sets in the context of a
real oil spill event in the Eastern Mediterranean in 2021.
