A high-resolution parallel ocean model is set up to examine how the sill depth of the Atlantic connection affects circulation and water characteristics in the Mediterranean Basin. An analysis of the model performance, comparing model results with observations of the present-day Mediterranean, demonstrates its ability to reproduce observed water characteristics and circulation (including deep water formation). A series of experiments with different sill depths in the Atlantic–Mediterranean connection is used to assess the sensitivity of Mediterranean circulation and water characteristics to sill depth. Basin-averaged water salinity and, to a lesser degree, temperature rise when the sill depth is shallower and exchange with the Atlantic is lower. Lateral and interbasinal differences in the Mediterranean are, however, largely unchanged. The strength of the upper overturning cell in the western basin is proportional to the magnitude of the exchange with the Atlantic, and hence to sill depth. Overturning in the eastern basin and deep water formation in both basins, on the contrary, are little affected by the sill depth.
The model results are used to interpret the sedimentary record of
the Late Miocene preceding and during the Messinian Salinity
Crisis. In the western basin, a correlation exists between sill depth
and rate of refreshment of deep water. On the other hand, because
sill depth has little effect on the overturning and deep water
formation in the eastern basin, the model results do not support the
notion that restriction of the Atlantic–Mediterranean connection may
cause lower oxygenation of deep water in the eastern basin. However,
this discrepancy may be due to simplifications in the surface
forcing and the use of a bathymetry different from that in the Late
Miocene. We also tentatively conclude that blocked outflow, as found
in experiments with a sill depth
With the model setup and experiments, a basis has been established for future work on the sensitivity of Mediterranean circulation to changes in (palaeo-)bathymetry and external forcings.
Ever since the closure of the connection to the Indian Ocean in the
Middle Miocene, water exchange between the Mediterranean Sea and the
global ocean has been through one or multiple gateways in the
Gibraltar arc
This pattern of surface inflow and deep outflow is known as
anti-estuarine and has been the dominant mode of exchange since at
least the closure of the Indian Ocean connection
In the Neogene and Quaternary sedimentary record, the occurrence of
sapropels has been related to relatively strong precession minima,
clustered in intervals with particularly high eccentricity (100 and
400
The ultimate expression of a strong tectonically driven restriction in
Atlantic–Mediterranean exchange is the MSC, an event which is represented in the sedimentary record by
widespread and voluminous evaporites in the
Mediterranean. Circum-Mediterranean climate changed little during this
event
In the Late Miocene, Atlantic–Mediterranean exchange occurred through
two shallow marine gateways in northern Morocco and southern Spain,
respectively the Rifian and Betic corridors
The purpose of this study is to examine with a high-resolution ocean
circulation model how sill depth of the Atlantic connection, influenced by long-term tectonics, affects
circulation and water characteristics in the Mediterranean Basin. The
physics-based insight thus gained is used to evaluate the Late Miocene
sedimentary record: that of the MSC and, in
particular, that of the period just preceding the actual
crisis. Notwithstanding our focus on the Late Miocene, the basin
bathymetry is – except near the Atlantic connection – that of the
present Mediterranean Sea. The reasons for this are that (1) we
isolate the changes in circulation and water characteristics that are
caused by a different depth of the Atlantic connection, and (2)
a validation of model performance is only possible in a present-day
model setup because quantitative data on water properties and
circulation does not exist for other time periods. Such an assessment
of model performance is necessary because the model has not been
applied to the Mediterranean before. Thus taking the present-day
bathymetry as our reference, the depth of the Strait of Gibraltar is
modified to gain insight into the role of depth of the Atlantic
connection. Even though basin shape was different in the Late Miocene
and the connection to the ocean had a different geometry and location,
our model results form a good starting point for this past time as
well. The reason for this is that, in terms of features that govern
the overall circulation and water properties, the present-day and Late
Miocene basin geometry are largely similar. Both consist of two
subbasins, comprise limited shelf areas and deep-basinal portions,
are subject to net evaporation and possess parts located at relatively
high latitude where cooling of the surface water occurs. Thus, insight
gained about the change in circulation and water properties due to
variation in sill depth is expected to be largely generic. A minimal
model of the Mediterranean with a highly idealized surface forcing has
been shown to be able to capture the important characteristics of
Mediterranean circulation
Water exchange in the present-day Strait of Gibraltar has been studied
extensively because of its influence on Mediterranean water
characteristics and, therewith, circum-Mediterranean climate
The next section will provide a description of the model and boundary
conditions used. In Sect.
Overview of parameter values used to set up the model. SIUS: Smolarkiewicz iterative upstream scheme.
For a good representation of water exchange through a shallow gateway,
a model with a high number of vertical layers in shallow water is
required. A model with vertical sigma coordinates meets this
requirement better than a model with
Model grid and bathymetry. Curvilinear grid coordinates
Vertical mixing coefficients are calculated in the model with the
The bathymetry used in this study (Fig.
Temporal evolution of diagnostic variables for the reference
experiments (SD300, black/grey lines), SD500 (red/orange) and SD50
(blue/light blue).
Initial conditions for temperature and salinity derive from Levitus'
World Ocean Atlas
The surface forcing is idealized and constant in time, and resembles
the forcing used in
The western boundary in the Atlantic part of the model domain is open. For the barotropic mode (2-D) a zero-gradient condition is applied to the free surface elevation, a clamped inflow condition is used for the depth-averaged velocity normal to the open boundary, and a zero velocity is used for the boundary parallel velocity. For the baroclinic mode (3-D), a Sommerfeld radiation condition is applied to the velocity normal to the open boundary and a zero velocity is used, again, for the boundary parallel velocity. The inflow in the external mode compensates for the water volume lost by evaporation in the Mediterranean, keeping water volume constant. To avoid large temperature and/or salinity contrast between the inflow through the open boundary, which has a prescribed salinity and temperature equal to initial conditions, and the Atlantic part of the model, a relaxation back to initial Atlantic conditions is applied in the first 18 columns of the grid (the Strait of Gibraltar starts at the 22nd column). The relaxation timescale in the surface layer varies from 3 months at the westernmost grid cell to 1 month at the easternmost grid cell of the relaxation area, and decreases exponentially in deeper layers.
The results of a reference experiment, with idealized surface forcing
and the sill depth of the Atlantic gateway set to the present-day
depth of 300
The temporal variation of basin-averaged temperature, salinity,
a measure of kinetic energy, and strait transport is illustrated in
Fig.
Vertical profiles of zonal velocity (left), temperature (middle),
and salinity (right) in the middle of the Strait of Gibraltar in the
reference experiment. Grey shading in each frame indicates the range of
variation of the variable in the last 100
At steady state a balance exists between the transport of heat and
salt at the Strait of Gibraltar and the surface forcings in the
Mediterranean, which is reflected by quasi-constant properties of the
exchange. Velocity, temperature and salinity profiles from the Strait
of Gibraltar are shown in Fig.
Figure
Two horizontal and a vertical cross section through the
three-dimensional salinity
A difference in water characteristics does not only exist between the
Atlantic and Mediterranean, a clear difference is also visible between
western and eastern Mediterranean. The Sicily Strait restricts
exchange between the basins. In combination with the fresh water
deficit of the eastern Mediterranean, this drives an eastward surface
flow and westward deep flow, i.e. an anti-estuarine circulation with
respect to the western basin. The western basin has an average
salinity of 38.3
The circulation pattern evidenced by the salinity and temperature
distribution in the Mediterranean is confirmed by the “zonal”
overturning stream function which is calculated along the curvilinear
coordinates (Fig.
On the vertical temperature and salinity profiles, the area east of
25
“Zonal” overturning stream function of the reference experiment.
The contour interval is 0.5
The average
For further analysis of the intermediate and deep water formation,
Fig.
Initial conditions in the model are based on recent observations of
salinity and temperature (Levitus fields). Therefore, the difference
between initial and steady state salinity and temperature is equal to
the difference between modelled and observed values. Due to the simple
constant surface forcings, we cannot expect to capture annual
variability in circulation and deep water formation. Basin-averaged
temperatures are 3.4
In the present-day Mediterranean, deep water formation is strongest
during the winter months
Transport through the Strait of Gibraltar has been the subject of
innumerable studies. Estimates have been based on observations,
numerical modelling and hydraulic control theory. Given the
uncertainty in the fresh water budget of the Mediterranean and the
wide range of approaches, the volume transport at the Strait of
Gibraltar has been estimated at 0.8–1.8
Often, basin-scale circulation models with realistic and idealized
atmospheric forcing have had difficulties reproducing deep water
formation and, in particular, deep overturning in the western basin
In a series of sensitivity experiments, deep water formation has been
compared in models with different latitudinal gradients in the air
temperature. With a reduced latitudinal air temperature gradient, the
mixed layer depth is still largest in the northern part of the western
basin, the Adriatic and the Aegean Sea. However, rates of deep water
formation and the strength of the deep overturning cell are lower
while the formation of intermediate water in the eastern basin and the
strength of the upper overturning cell in both basins are
higher. These results are in agreement with
The strength of the surface overturning cell in the reference
experiment is slightly larger than in most other high-resolution model
studies
The overall good agreement between observed and modelled strait transports, water characteristics and circulation, quantitatively as well as qualitatively, shows that the model setup of the reference experiment captures all important processes of the Mediterranean thermohaline circulation. Having validated the model setup of the reference experiment, we will describe in the next section the changes in Mediterranean water characteristics and circulation due to changes in the sill depth of the Atlantic connection.
The temporal variations of water characteristics and transport in two
experiments with a shallow (50
Latitude-depth-averaged profiles of salinity
The basin averages in Fig.
Strength (left) and depth of overturning extrema (right) of the
upper (red) and deep (blue) zonal overturning cells in the western and
eastern basin. To illustrate temporal changes, light colours indicate the
value after 400
Only where the bathymetry was modified to accommodate a different sill depth,
i.e. between
Latitude-depth-averaged temperatures in Fig.
Modelled inflow (red plus signs) and net flow (blue plus signs) in SD500–SD5. Green crosses show the inflow calculated with hydraulic control theory from the basin-averaged densities of the Atlantic and Mediterranean. Purple crosses indicate the inflow calculated with hydraulic control theory using the average density of inflow and outflow in the gateway instead of basin averages.
Vertical profiles of zonal velocity
The density curves in Fig.
The main features of the basinal circulation can be captured in the
minimum and maximum strengths of the overturning cells and the depths
where these occur in the western and eastern basin. These parameters
are shown as a function of sill depth in Fig.
Compared to the overturning in steady state, deep overturning in the eastern basin is more vigorous before a steady state is reached. In the western basin deep overturning is weaker before a steady state, as are the surface cells in the western and eastern basin. Towards a steady state, deep water formation in the eastern basin slows down when the vertical density gradient in the basin stabilizes. At the same time, intermediate water formation in both basins and deep water formation in the western basin pick up when vertical temperature differences decrease.
The only cell that stabilizes at a different strength depending on
sill depth, is the upper cell in the western basin. The increase in
overturning strength from SD100 to SD500 (
The temperature in the eastern basin is only affected by the magnitude of the
Atlantic-derived inflow in the surface layer near the Strait of Sicily.
Therefore, a reduced Atlantic inflow cannot explain the small temperature
increase from SD500 to SD100 in the eastern basin (Fig.
The magnitude of modelled strait transport as a function of sill depth
is illustrated in Fig.
Even though the Atlantic–Mediterranean exchange appears to be a simple
two-way flow from Fig.
Overview of heat transport through the Strait of Gibraltar. ENSHL: equivalent net surface heat loss.
In the present day, the Mediterranean is a heat sink for the Atlantic, i.e. the
Mediterranean has a net surface heat loss. The present-day net surface heat
loss is estimated to be 5 W m
As noted in Fig.
In SD5 and SD10, the inflow is constant and equal to the net flow,
i.e. there is no outflow. Although outflow is blocked in these two
experiments, neither experiment has reached a steady state
yet. Salinity rises constantly at 11.6
In summary, exchange between the Mediterranean and Atlantic is
proportional to the sill depth until at 10
Even though the usage of a minimal constant forcing is a deliberate choice in
our model setup, it may cause changes in the Mediterranean circulation and
water characteristics compared to a model with a non-constant (seasonal)
forcing. Whereas most deep water formation would be episodic, i.e.
concentrated in the winter months, with a seasonal cycle, a constant forcing
drives a constant deep water formation. Deep water forms year-round in the
northern part of the western and eastern basins due to cooling at the
surface, i.e. a net surface heat loss of 1.3 W m
Strait transport is not significantly affected by the high deep water
temperatures and associated warm Mediterranean outflow. A change in salinity
affects density, and therefore Mediterranean outflow, four times more than
temperature, i.e. a change of 1 g L
In summary, although the constant forcing results in year-round formation of relatively warm deep water, the overall circulation, deep water formation, strait transport, and salinity are sufficiently reproduced to study the impact of sill depth on them.
A comparison of exchange in the model and that predicted by hydraulic
control theory is called for since hydraulic control theory has been
extensively used to describe the Atlantic–Mediterranean
exchange. Hydraulic control theory can be used to calculate the flow
through a narrow strait if the geometry and density difference along
the strait are known. A recent overview of the basic principles
underlying hydraulic control theory and an application to the
Mediterranean can be found in
Besides the modelled strait transport, Fig.
In hydraulic control theory, the velocity and density profiles at the
gateway are envisaged to be a step function with a constant positive
velocity and low density in the upper layer and a constant negative
velocity and high density in the bottom layer. In contrast, profiles
derived from the model (Fig.
When transport is calculated by inserting the average density of
inflow and outflow in Eq. (
Regardless of the densities used, exchange calculated with hydraulic control theory is always larger than modelled transport. The aforementioned vertical mixing between inflow and outflow is one obvious cause of the difference. Another important factor is friction; at the bottom it slows down the outflow and friction between the inflow and outflow slows down both. The Coriolis force does not play a role here due to the narrow width of the gateway.
In summary, transport at shallow sill depths is closer to that predicted by hydraulic exchange theory than at deep sill depths because mixing between inflow and outflow is not as effective in reducing the difference between them. Furthermore, using basin-averaged density differences for calculation of the exchange with hydraulic control gives an overestimation of the exchange because water characteristics at the depths involved in the exchange are not representative of the whole basin.
In experiments with sill depths in the range of 500–5
The influence of sill depth on circulation and water characteristics
found in this study is, mainly due to differences in model setup,
different from that found by
The low resolution used in AMD10 resulted in a 222
As argued in the introduction, we expect that the change in circulation and water properties due to variation in sill depth that we calculated for a basin with the shape of the present Mediterranean forms a starting point for understanding the past as well. In this section we relate our findings to the Late Miocene sedimentary record.
Stable isotopes and faunal changes in the pre-MSC interval of the Late
Miocene suggest that in an interval with increasing salinity, the
average deep water oxygenation decreased steadily
In the western basin, depths up to 1200
Model results thus seem to contradict the notion that restriction of the Atlantic–Mediterranean gateway induces lower oxygenation of deep water in the eastern basin. A lower oxygenation may, however, be inferred for the upper overturning cell in the western basin. Due to the relatively shallow connection between the western and eastern basin, only the western overturning is affected by sill depth in a way that seems consistent with the data. A possible cause of the discrepancy between model results and observations is the present-day bathymetry used in the model. If the connection between both basins was deeper in the Late Miocene, the upper overturning cell could have extended further and deeper into the eastern basin. Without the Sicily Strait, model results are expected to be similar to those in AMD10 who indeed had a single surface overturning cell in the whole Mediterranean.
In the present day, deep water formation is mainly driven by cooling of surface water during the winter. Despite the fact that deep water formation in the model is continuous, the forcing that drives deep water formation is the same. We therefore argue that deep water formation is sufficiently represented in our setup to reproduce possible changes in deep water oxygenation that may occur in a less idealized case.
Another simplification in the surface forcing, on the other hand, may influence the correlation of model results and observations: the inclusion of river discharge in a uniform surface flux equal to E-P-R. Increased river discharge during precession minima is generally accepted to be an important factor in the establishment of low-oxygen conditions during sapropel formation. River discharge is thought to form a fresh-water-lid at the surface, hindering deep water formation by reducing surface layer densities. Due to mixing with more saline water and evaporative concentration, water originating from rivers will lose its fresh water signature when it moves away from the outlet. If the receiving basin is at higher salinity, the density difference between river water and the basin is larger and stratification will be stronger. Although depending on the volume and location of river input, basin circulation, evaporation – precipitation, and the rate of mixing with surrounding water, we can assume that stratification due to river discharge is stronger when the basin-averaged salinity is higher. Hence, deep water formation may be more effectively reduced at lower sill depth. If a river drains into a marginal basin with restricted exchange with the deep basin, stratification is presumably more severe and deep water oxygenation even more strongly reduced.
A spatially heterogeneous distribution of precipitation may have the same effect as river discharge. Precipitation, however, does not give a continuous fresh water input at the same location like river input. Hence, its influence with respect to river discharge is expected to be lower.
Notwithstanding the fact that SD10 and SD5 are not yet in steady
state, it is the first time that a blocked outflow has been observed
in an ocean circulation model in this context. From hydraulic control
theory, one layer flow is predicted to occur only when the sill depth
is a few metres for a gateway with a width of 13
Our model results suggest that if a sill depth of
In this study, a parallel version of POM (sbPOM) has been used to
examine changes in Mediterranean circulation and water characteristics
due to restriction of the Atlantic connection. Model results have
implications for the interpretation of the Late Miocene sedimentary
record in the Mediterranean. Compared to earlier models of the
(Miocene) Mediterranean, the use of a curvilinear grid and parallel
code allows for the use of a higher resolution and more realistic
bathymetry even in long model runs (800
The model setup presented in this study would seem to provide a valuable basis and reference for examination of additional aspects of Mediterranean palaeoconfigurations. This may relate to other aspects of the Miocene evolution, for example, but our setup is also applicable to the Last Glacial Maximum when lower sea level was responsible for a reduction in sill depth.
The main results and implications for the Late Miocene Mediterranean
are the following:
Basin-averaged salinity, temperature and density increase when
the sill is shallower. However, spatial distribution and
inter-basinal differences in water properties in the Mediterranean
are largely unaffected by sill depth. The strength of the upper overturning cell in the western
Mediterranean is proportional to the magnitude of water exchange
with the Atlantic. Overturning in the eastern basin is not
significantly affected by the depth of the sill. Temperature-driven dense water formation operates regardless of
the basin-averaged salinity. At shallower sill depths, the higher
salinity in the Mediterranean results in a stronger salinity-driven
dense water formation in the eastern basin. Modelled strait transport is always smaller than that predicted
by hydraulic control theory. This difference is due to friction,
vertical mixing and a difference between basin-averaged density and
the density of the water involved in the exchange with the Atlantic. Outflow is blocked in (at least the first 800 With the present-day bathymetry, restriction of the
Atlantic–Mediterranean connection does not significantly alter
Mediterranean deep water circulation and refreshening. Hence, model
results do not affirm the hypothesis that deep water ventilation
decreases at shallower sill depths.
The authors would like to thank Rinus Wortel for valuable input on
drafts of this article. The manuscript benefited from reviews by two anonymous reviewers and Mike
Rogerson. R. P. M. Topper was supported by the Netherlands
Research Center for Integrated Solid Earth Science. Computational
resources for this work were also provided by ISES (ISES 3.2.5 High
End Scientific Computation Resources). Figures in this paper were
created using GMT version 4.5.1