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Ann. Geophys., 21, 167-187, 2003
www.ann-geophys.net/21/167/2003/
© European Geosciences Union 2003
Multivariate Empirical Orthogonal Function analysis of the upper thermocline structure of the Mediterranean Sea from observations and model simulations
S. Sparnocchia1, N. Pinardi2, and E. Demirov3
1Istituto Talassografico di Trieste-CNR, Viale R. Gessi 2, 34123 Trieste, Italy
2Corso di Scienze Ambientali, University of Bologna, Ravenna, Italyieste, Italy
3Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
Abstract. Multivariate vertical
Empirical Orthogonal Functions (EOF) are calculated for the entire
Mediterranean Sea both from observations and model simulations, in order to
find the optimal number of vertical modes to represent the upper thermocline
vertical structure. For the first time, we show that the large-scale
Mediterranean thermohaline vertical structure can be represented by a limited
number of vertical multivariate EOFs, and that the "optimal set" can
be selected on the basis of general principles. In particular, the EOFs are
calculated for the combined temperature and salinity statistics, dividing the
Mediterranean Sea into 9 regions and grouping the data seasonally. The
criterion used to establish whether a reduced set of EOFs is optimal is based
on the analysis of the root mean square residual error between the original
data and the profiles reconstructed by the reduced set of EOFs. It was found
that the number of EOFs needed to capture the variability contained in the
original data changes with geographical region and seasons. In particular,
winter data require a smaller number of modes (4–8, depending on the region)
than the other seasons (8–9 in summer). Moreover, western Mediterranean
regions require more modes than the eastern Mediterranean ones, but this result
may depend on the data scarcity in the latter regions. The EOFs computed from
the in situ data set are compared to those calculated using data obtained from
a model simulation. The main results of this exercise are that the two groups
of modes are not strictly comparable but their ability to reproduce
observations is the same. Thus, they may be thought of as equivalent sets of
basis functions, upon which to project the thermohaline variability of the
basin.
Key words. Oceanography: general
(water masses) – Oceanography: physical (hydrography; instruments and
techniques)
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