| Temperature and salinity sampling strategies are studied and compared by
means of the Observing System Simulation Experiment technique in order to
assess their usefulness for data assimilation in the framework of the
Mediterranean Forecasting System. Their impact in a Mediterranean General
Circulation Model is quantified in numerical twin experiments via bivariate
data assimilation of temperature and salinity profiles in summer and winter
conditions, using the optimal interpolation algorithm implemented in the
System for Ocean Forecasting and Analysis. The data impact is quantified by
the error reduction in the assimilation run relative to the free run.
The sampling strategies studied here include various combinations of
temperature and salinity profiles collected along Volunteer Observing Ship
(VOS) tracks, by Mediterranean Multi-sensor Moored Arrays (M3A), a Glider
and ARGO floating profilers. Idealized sampling strategies involving VOS
data allow to recognize the impact of individual tracks. As a result, the
most effective tracks are those crossing regions characterized by high
mesoscale variability and the presence of frontal structures between water
masses.
Sampling strategies adopted in summer–autumn 2004 and winter 2005 are
studied to assess the impact of VOS and ARGO data in real conditions. The
combination of all available data allows to achieve up to 30% error
reductions. ARGO data produce a small impact when alone, but represent the
only continuous coverage of the basin and are useful as a complement to VOS
data sets.
Localized data sets, as those obtained by M3As and the Glider, seem to have
an almost negligible impact in the basin-scale assessment, and are expected
to be more effective at regional scale. |