Oceanic Island Wake Flows in the Laboratory - Modeling Atmospheric and Oceanic Flows - page 269

2008]. Exhaustive experimental investigations on rotat-
ing and stratified wakes which take into account asym-
metrical island shapes or multiple islands interaction are
missing. This is probably a new goal to achieve in the
near future.
A common hypothesis for laboratory models of oceanic
wake is to assume a steady and uniform upstream current.
According to the standard Strouhal number, typical shed-
ding periods of island wakes in the deep ocean should
vary from one to several weeks. For such time intervals
the variability of surface currents is not negligible and
the upstream flow may change in direction and intensity.
Hence, the transient response of the flow could become
significant and affect the wake pattern. Indeed, when the
upstream current is accelerated from rest, the first pattern
to emerge in the lee of a cylindrical island is a sym-
metric dipole, also called the starting dipole [ Afanasyev
and Korabel , 2004], while regular and periodic shedding
of opposite sign vortices will appear later on. The tem-
poral variability of the incoming flow will disturb the
quasi-steady vortex street and induce a wider variety of
wake patterns. An unsteady wake forcing will then be
a technical challenge for more realistic experiments on
rotating wakes.
The role of local wind stress as a driver of oceanic
vortices in the lee of mountainous island was recently
highlighted in numerical studies of the Hawaiian [ Calil
et al. , 2008; Yoshida et al. , 2010; Kersalé et al. , 2011] or
the Madeira [ Couvelard et al. , 2012; Caldeira et al. , 2014]
wake. For the Hawaiian case, the interaction between the
North Equatorial Current and the archipelago is enough
to generate eddies, but high-resolution winds stress curls
are needed to get the correct intensities of the vortices as
observed from the altimetry. For the Madeira case, the
wind wake appears to be the main contributor to the gen-
eration of the oceanic eddies in the island wake [ Caldeira
et al. , 2014]. As far as experimental investigations are con-
cerned, combining both an upstream low and a local wind
forcing on a rotating turntable seems hard to achieve and
here we probably reach the limits of laboratory models.

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Acknowledgments. The author gratefully acknowl-
edges all the collaborators, G. Perret, S. Teinturier,
A. Lazar, T. Dubos, J. M. Chomaz, R. Caldeira, and
C.Dong,whotookpartintheexperimentalandnumerical
investigations on rotating shallow-water wakes. Warm
thanks to the Coriolis team S. Vuiboud and H. Didelle
for their irreplaceable expertise on rotating experiments.
The author is also grateful to the French ANR, the
6th European Commission (EC) framework program
Hydralab, and the sustainable development chair of the
Ecole Polytechnique, who funded a large part of our
studies these past years.