Implementing a High Resolution Numerical Model of the Strait of Gibraltar
by Gianmaria Sannino, Roberto Conversano and Vincenzo Artale
The Mediterranean sea is a basin in which a wide range of oceanic
processes and interactions of global interest occur. Intense evaporation
over the Mediterranean Sea produces dense, salty water that outflows through
the Strait of Gibraltar and can have an impact on the general circulation
of the global ocean and particularly on the formation of deep water in
the North Atlantic. One effect of the outflow from the Mediterranean is
to make the Atlantic Ocean saltier than either the Pacific or Indian Oceans.
The Gibraltar Strait controls the entire fresh water budget of the Mediterranean
sea and for this reason there has been both experimental and theoretical
interest in the physics of the Strait and on monitoring the heat and flux
on the sill. The existence of this strait makes the Mediterranean sea one
of the few regions in the world's oceans where the advective heat and salt
flow is known with sufficient accuracy to permit testing of the available
data sets, the bulk formula and the model results of the surface heat flux.
In recent years, in the context of the III and IV Framework, the European
Community has financed a number of projects aimed at improving knowledge
of the Mediterranean circulation from both numerical and experimental points
of view.
In particular, one task of the MTP-II-MAST Programme (1996-1999) is
dealing with the numerical implementation of a high resolution numerical
model of the Strait of Gibraltar and its parameterization in the Ocean
General Circulation Model.
Motivations
Since the beginning of the century the Strait of Gibraltar has been
a favourite place for oceanographic studies. The dominant oceanographic
feature of the Strait is a two-layer inverse estuarine circulation that
is driven by an excess of evaporation over precipitation and river discharge
into the Mediterranean. In addition, recent field experiments in the Strait
have kindled a renewed interest in two-layer hydraulics and its role in
controlling the mass transport between the Mediterranean and the Atlantic
sea.
As a result of the effects of this hydraulic control, two possible states
have been hypothesised for the Mediterranean Sea/Strait of Gibraltar system:
1) over-mixed with maximal exchange, minimum difference in salinity between
the two layers and a shallow, supercritical inflow at the eastern end of
the Strait, or 2) not over-mixed, with sub-maximal exchange, a bigger salinity
difference and a thicker sub-critical inflow.
The Alboran Sea Circulation is dominated by the Atlantic water entering
the Alboran sub-basin as a shallow buoyant stream that curves southward
most of the time. It then forms a large anticyclonic gyre, which may extend
throughout the whole width of the sub-basin.
From satellite observations the gyre seems to have some variability,
generating a second cyclonic gyre up to the main stream. The anti-cyclonic/cyclonic
structure at the end of the Strait may be related to the geometry of the
basin. Observations have also revealed the appearance of a second anticyclone
in coincidence with the Almeria-Oran front. The variability of this second
anti-cyclonic gyre is more pronounced than that observed in the western
side of the basin. These two-three gyres are considered to be one of the
permanent features of the Western Mediterranean circulation and participate
in the formation of the MAW (Mediterranean Atlantic Water). The precise
criterion for the anti-cyclonic/cyclonic features of the Alboran sea are
still unknown.
The numerical simulation that we show in the figures refers to the evolution
of the Atlantic water into the Alboran basin and the outflow of the Mediterranean
sea into the North Atlantic circulation. Figure 1 shows the Alboran gyre
circulation and the Mediterranean outflow at depth. From a first analysis
of these results we note a good agreement with the observations. However
more experiments and analyses are needed to evaluate whether the resolution
of the model is sufficient to describe this phenomena. In particular, we
hope that our numerical experiments will answer a number of questions,
such as:
- is there a permanent hydraulic control site in the Gibraltar Strait
and, if so, how does it depend on seasonal variations in the Mediterranean
basin?
- What are the principal forcing effects (tide, topography...) that can
generate the variability of the ocean current along the coast of Portugal
and the bifurcation of the Mediterranean outflow at Cape Saint Vincent?
- What are the physical mechanisms that generate gyre variability in
the Alboran circulation?
The numerical simulations were performed using POM (Princeton Oceanographic
Model) a primitive equation numerical model. The model was initialized
with climatological salinity and temperature without wind stress forcing.
The boundary conditions are computed by the Newtonian relaxation terms
at the boundary domains of the model using the monthly temperature and
salinity climatological data. The model runs on the Digital 4100 parallel
platform or on a CRAY J90.
For more information, see the project Web site: http//www.cetiis.fr/mtp/mater
Please contact:
Vincenzo Artale - ENEA
Tel: +39 06 3048 3096
E-mail: artale@casaccia.enea.it
