Lagrangian transport in a microtidal coastal area: the Bay of Palma, island of Mallorca, Spain

Hernández-Carrasco, Ismael; López, Cristóbal; Orfila, Alejandro; Hernández-García, Emilio
Nonlinear Processes in Geophysics 20, 921-933 (2013)

Coastal transport in the Bay of Palma, a small region in the
island of Mallorca, Spain, is characterized in terms of
Lagrangian descriptors. The data sets used for this study are
the output for two months (one in autumn and one in
summer) of a high resolution numerical model, ROMS, forced
atmospherically and with a spatial resolution of 300 m.
The two months were selected because its different wind
regime, which is the main driver of the
sea dynamics in this area. Finite-size Lyapunov Exponents (FSLEs)
were used to locate semi-persistent
Lagrangian coherent structures (LCS) and to understand the
different flow regimes in the Bay. The different wind
directions and regularity in the two months have a clear impact
on the surface Bay dynamics, whereas only topographic features
appear clearly in the bottom structures. The fluid interchange
between the Bay and the open ocean was studied by computing
particle trajectories and Residence Times (RT) maps. The escape rate of
particles out of the Bay is qualitatively different,
with a 32$%$ more of escape rate of particles to the
ocean in October than in July, owing to the different
geometric characteristics of the flow. We show that LCSs
separate regions with different transport properties by
displaying spatial distributions of residence times on synoptic
Lagrangian maps together with the location of the LCSs.
Correlations between the time-dependent behavior of FSLE and
RT are also investigated, showing a negative
dependence when the stirring characterized by FSLE values moves
particles in the direction of escape.

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