Download Internal waves generated in the Straits of Gibraltar and

Internal Waves Generated in the Straits of
Gibraltar and Messina: Observations from Space
Werner Alpers 1, Peter Brandt 2, and Angelo Rubino 3
1
Institut für Meereskunde, Universität Hamburg, Hamburg, Germany
Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Kiel,
Germany
3
Dipartimento di Science Ambientali, Università Ca’ Foscari, Venezia,
Italy
2
Abstract. The Straits of Gibraltar and Messina are areas where strong internal solitary waves are generated by the interaction of tidal currents with
shallow underwater ridges located within the straits. Remote sensing techniques and numerical simulations have been instrumental in studying the
generation and propagation of internal solitary waves in these sea areas. It
was a Synthetic Aperture Radar (SAR) image acquired by the American
Seasat satellite in 1978 that first revealed that long internal waves are generated in the Strait of Messina. Furthermore, SAR images acquired by the
European Remote Sensing satellite ERS-1 and ERS-2 have revealed that
trains of internal solitary waves generated in the Strait of Gibraltar propagate mostly eastward into the Mediterranean Sea, while westward propagating wave trains can only be supported by a seasonal thermocline.
1.
Introduction
Internal waves are waves of the interior ocean. They can exist when the
water body is stratified, i.e. when it consists of layers of different density.
This difference in water density is mostly due to a difference in water temperature, but it can also be due to a difference in salinity as in the Strait of
Gibraltar. Often the density structure of the ocean can be approximated by
two layers. In this paper we will consider long internal waves propagating
along the main pycnocline.
In the Straits of Gibraltar and Messina the tidal flow pushes the layered
water body over the shallow ridges or sills located within the straits. A lee
depression of the interface is thus generated which crosses the strait when
the tide slackens. As demonstrated theoretically long time ago (Korteweg
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and de Vries 1895) such a travelling disturbance may disintegrate into
trains of rank-ordered solitary waves. To first order, such internal waves
do not produce an elevation of the sea surface as the familiar surface
waves do, but they induce a variable horizontal surface current. This variable surface current gives rise to a change of the sea surface roughness,
which, under favourable viewing conditions, can be detected by visible observations from ships, air- and spacecrafts. From space, internal waves can
be detected very efficiently using a Synthetic Aperture Radar (SAR) which
yields images of the sea surface with a spatial resolution of typically 25
meters. However, also optical, and, to a lesser extent, infrared and ultraviolet images acquired from satellites have been used to observe internal
waves from space.
2.
SAR imaging of internal waves
A linear internal wave propagating in a two-layer ocean causes a variable
surface current which varies in magnitude and direction thus generating
convergent and divergent surface flow regimes. The variable surface current interacts with the surface waves and modulates the sea surface roughness (Alpers 1985). According to Bragg scattering theory, the amplitude of
small-scale sea surface waves, which obey the Bragg resonance condition
(the so-called “Bragg waves”) determines the backscattered radar power.
Due to this hydrodynamic interaction of the Bragg waves with the variable
surface current, their amplitude increases in convergent flow regions and
decreases in divergent flow. As a consequence, the radar signatures of oceanic internal waves consist of alternating bright and dark bands.
However, sea surface manifestations of internal waves are not only visible on radar images. Under favourable viewing conditions they are also
visible on images acquired in the visible, ultraviolet or infrared bands (see
e.g. Artale et al. 1990; Mitnik et al. 2000). The reason is that the backscattered (or reflected) sunlight from the sea surface strongly depends on the
sea surface roughness.
Tidally generated internal waves, which are in general nonlinear and
dispersive, often evolve as trains of solitary waves. They are generated by
the disintegration of long internal waves of tidal period. The resulting
wave packets consist of several solitary waves. Since the first theory on
solitary waves (soliton theory) was developed by Korteweg and de Vries
(1895) hundreds of papers have been published dealing with this subject.
Soliton theories applicable to the description of the generation and propagation of internal solitary waves predict that, if the depth of the upper layer
Internal Waves in the Straits of Gibraltar and Messina
321
is smaller than the depth of the lower layer, the resulting stationary internal
solitary waves are waves of depression, i.e. the pycnocline is pushed down.
In Figure 1 a typical example a nonlinear internal wave packet consisting of three solitary waves of depression is depicted. The color coding denotes the water density and the arrows the velocity. This profile was measured north of the Strait of Messina on 25 October 1995 by a towed
conductivity-temperature-depth (CTD) chain and by a vessel-mounted
acoustic Doppler current profiler (ADCP).
Fig. 1. Density distribution of the water column and distribution of the velocity
north of the Strait of Messina measured by ship-borne sensors during the
passage of a highly nonlinear internal wave packet on 25 October 1995.
The leading edge of a soliton of depression is always associated with a
convergent surface flow region and the trailing edge with a divergent region. At the front of the internal soliton, the amplitude of the Bragg waves
is increased, while at the rear it is decreased. This is the reason why on
SAR images the front section of an internal solitary wave of depression is
bright and the rear section is dark (Alpers 1985).
However, when the wind speed is low and when surface slicks are present, the radar signatures of internal solitary waves may deviate from this
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W. Alpers, P. Brandt and A. Rubino
scheme. When the wind speed is below threshold for Bragg wave generation, strong internal solitary waves often manifest themselves on radar images as bright bands which, most likely, are caused by breaking short-scale
surface waves even in the absence of wind. When surface slicks are present, the internal waves often manifest themselves as dark lines caused by
the accumulation of surfactants in the convergent surface flow regions.
Fig. 2. Astronaut photograph of the Strait of Gibraltar and adjacent sea areas taken
from the Space Shuttle on 11 October 1984, at 12:22 UTC, showing sea
surface manifestations of two internal wave packets propagating out of the
Strait of Gibraltar into the Mediterranean Sea1. The land area in the upper
right-hand section of the image is Spain, the thorn-shaped peninsula at the
eastern entrance of the Strait attached to Spain is the British Crown Colony
Gibraltar and the land area in the lower left is Morocco.
3.
Internal solitary waves in the Strait of Gibraltar
The Strait of Gibraltar connects the Atlantic Ocean with the Mediterranean Sea. The water body in the Strait of Gibraltar and its approaches consists of a deep layer of salty Mediterranean water and an upper layer of
less salty Atlantic water. The mean depth of the interface between these
1
http://www.lpi.usra.edu/publications/slidesets/oceans/oceanviews/slide_13.html
Internal Waves in the Straits of Gibraltar and Messina
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two layers slopes down from about 80 m at the Mediterranean side of the
Strait to about 800 m at the Atlantic side. The relative change of density
across this interface, which is mainly determined by the salinity difference,
is about 0.002 (Lacombe and Richez 1982). The Strait of Gibraltar has a
complex bottom topography including several ridges. The shallowest section in the Strait of Gibraltar is at the Camarinal Sill where the maximum
water depth is 290 m. The interaction of the predominantly semidiurnal
tidal flow with the sills inside the Strait, in particular with the Camarinal
Sill, causes periodic deformations of the halocline in the sill regions (Armi
and Farmer 1985; Farmer and Armi 1988) which then give birth to internal
solitary waves with amplitudes as high as 80 m.
Unfortunately, the first SAR missions (Seasat 1978, Shuttle Imaging
Radar missions SIR A 1981, SIR B 1984, and SIR C/X-SAR 1994) did not
acquire images over the Strait of Gibraltar. However, during the Space
Shuttle flight STS 41-G (the SIR B mission) the US oceanographerastronaut Paul Scully-Power took photos of the Strait of Gibraltar with a
hand-held camera, which showed on 11 October 1984, at 12:22 UTC, impressive sea surface signatures of two internal wave packets (see Figure 2).
Only after the launch of the first European Remote Sensing Satellite (ERS1), in 1991, a large number of SAR images of the Strait became available.
Many of them, in particular those acquired near spring tide, show sea surface manifestations of internal solitary waves (Brandt et al. 1996).
In Figure 3 a typical ERS-1 SAR image of the Strait of Gibraltar is depicted showing sea surface manifestations (or radar signatures) of an internal solitary wave packet consisting of more than 10 long internal waves.
Note that the distance between the solitary waves in the packet decreases
from front to rear and that the strength of the image intensity modulation
of the solitary waves also decreases from front to rear indicating a successive decrease in amplitude of the solitary waves.
4.
Internal solitary waves in the Strait of Messina
The Strait of Messina separates the island of Sicily from the Italian peninsula and connects the Tyrrhenian Sea, in the north, with the Ionian Sea, in
the south. In spite of the small tidal displacements encountered in the
Mediterranean Sea, large gradients of tidal displacements are present along
the Strait of Messina, because the semidiurnal tides in the Tyrrhenian and
Ionian Seas are approximately in phase opposition. These gradients, acting
on the water body constrained by the strait topography, force intense tidal
currents, which can be as large as 3 m/s in the sill region (Vercelli 1925).
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Fig. 3. ERS-1 SAR image acquired on 1 January 1993 at 22:39 UTC (orbit: 7661)
showing sea surface manifestations of a packet of internal solitary waves
generated in the Strait of Gibraltar and propagating eastward into the Mediterranean Sea. The dark line intersecting the packet results from an oil spill,
probably released from a ship. Imaged area: 100 km x 50 km. © ESA
The hydrological peculiarities of the Strait of Messina attracted the attention of many ancient writers and philosophers. Homer (800 B.C.) makes
two monsters, Scylla and Charybdis, responsible for the violent currents in
the strait (Homer, Odyssey, 12th song, line 80-114). Aristotle (384-322
B.C.) argues that hollows in the sea floor and the interaction of two opposing wind-generated currents could produce such intense currents (Aristotle,
Problema Physica, chap. 23) and in the poetry of ancient times, allegories
alluding to the danger of sailing in the Strait of Messina can often be found
('Incidis in Scillam cupiens vitare Charybdim', Ovid, Metamorphosis).
As the Strait of Messina represents a barrier to the free water exchange
between the Tyrrhenian and the Ionian Seas, significant horizontal and vertical density gradients are encountered in this region. According to the
climatological density distribution, at all depths the water south of the
strait is throughout the year denser than north of it. The knowledge of the
presence of horizontal density gradients along the Strait of Messina enabled Defant (1940) to draw a picture of the tidally induced dynamics of
this area: during northward tidal flow, the Ionian water overflowing the sill
spreads under the Tyrrhenian water into the Tyrrhenian Sea. During
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southward tidal flow, the Tyrrhenian water, overflowing the sill, forms a
surface jet that spreads into the Ionian Sea.
Fig. 4. Seasat SAR image acquired on 18 September 1978 at 08:17 UTC showing
sea surface manifestations of northward propagating internal solitary waves
generated in the Strait of Messina (lower section). © NASA
The shallowest section in the sill region, in the centre of the Strait, has a
depth of 90 m. While in the southern part the Strait bottom slopes down
very steeply, to a depth of more than 800 m about 15 km south of the sill,
the northern part has a gentler slope. Here the 400 m isobath is located
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about 15 km north of the sill. Throughout the year, two different layers of
water are encountered in the Strait of Messina: the Tyrrhenian Surface Water and the colder and saltier Levantine Intermediate Water. In the vicinity
of the Strait, these water masses are separated at a depth of approximately
150 m (Vercelli 1925). During most of the year, a seasonal thermocline is
also present in the Strait which overlies this weak stratification.
Fig. 5. ERS-1 SAR image acquired on 11 July 1993 at 9:41 UTC (orbit: 10387)
showing northward as well as southward propagating internal wave packets. Imaged area: 65 km x 65 km. @ ESA
The fact that (1) strong tidally induced currents are encountered in the
Strait, (2) the water body is stratified, and (3) there is a shallow sill in the
Internal Waves in the Straits of Gibraltar and Messina
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center of the Strait which disturbs the tidal flow, suggests that internal
waves should be generated in the Strait of Messina. But it was not before
1978 that internal waves were detected in this strait. The first hint came
from a synthetic aperture radar image which was acquired by the American
Seasat satellite on September 15, 1978 (see Figure 4).
The three rings visible on the Seasat SAR image of the Tyrrhenian Sea
north of the Strait were interpreted as sea surface manifestations of a train
of internal solitary waves propagating northwards (Alpers and Salusti
1983). In the following years internal waves propagating northward as well
as southward have been detected during several oceanographic campaigns
by in-situ measurements (Sapia and Salusti 1987; Nicolò and Salusti
1991). Nonlinear internal waves propagating in a southward direction out
of the Strait of Messina have also been detected on a Landsat 5 thematic
mapper image acquired on 20 July 1984 at 09:30 LT (Artale et al. 1990).
A large number of spaceborne SAR images of the Strait of Messina became available after the launch of the First European Remote Sensing Satellite ERS-1 in 1991 and the Second European Remote Sensing satellite
ERS-2 in 1994 (Alpers et al. 1996). A typical ERS-1 SAR image showing
internal wave packets propagating northward as well as southward out of
the strait is depicted in Figure 5. It was acquired on 11 July 1993 at 09:41
UTC, which was 20 min after the maximum northward tidal flow at Punta
Pezzo (located at the Calabrian coast at the northern exit of the Strait).
The analysis of a large number of ERS-1 and ERS- 2 SAR images acquired between 1991 and 1995 (Brandt et al. 1997) has shown that sea surface manifestations (or radar signatures) of internal waves are observed
more frequently during periods when a strong seasonal thermocline is
known to be present, i.e. during summer. Furthermore, sea surface manifestations of northward propagating internal solitary waves are quite uncommon (Brandt et al. 1997). Since the strong roughness bands are associated with large-amplitude internal solitary waves, the presence of such
strong radar signatures on SAR images could be used as an indicator for an
anomalous density distribution along the Strait of Messina that has its origin in fluctuations of larger-scale circulation patterns. Indeed, on one occasion, as an anomalous horizontal density distribution of the water masses
was present in this region, the presence of strong internal waves north of
the Strait was confirmed by in-situ measurements (Brandt et al. 1999).
This was on 24 and 25 October 1995, when oceanographic measurements
were carried out north and south of the Strait of Messina from the research
vessel Alliance of the SACLANT Undersea Research Centre (Figure 1).
As mentioned before, internal waves generated in the Strait of Messina
have also been detected on optical images. A very impressive example of
such an optical image is depicted in Figure 6. It was acquired by the Ad-
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vanced Spaceborne Thermal Emission and Reflection Radiometer
(ASTER) on NASA’s Terra satellite on 11 August, 2003, when the sun
was located just at right angle to illuminate the internal wave packets.
ASTER has in the visible band a resolution of 15 m. On this image two
wave patterns are visible: a strong one resulting from internal waves generated in the Strait of Messina and a weak one resulting from diffraction of
the primary internal soliton.
Fig. 6. ASTER image acquired on 11 August, 2003, showing a strong southward
propagating internal wave packet and a weak quasi- semicircular internal
wave packet resulting from diffraction of the primary internal soliton
propagating southward along the channel2.
2
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17
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Internal Waves in the Straits of Gibraltar and Messina
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Discussion and conclusion
The advent of remote sensing of the ocean revealed the fascinating presence, in the straits of Gibraltar and Messina, of coherent, large patterns in
the sea surface roughness, which have been recognized as surface manifestations of long internal waves. It was a Seasat SAR image acquired in 1978
that first revealed that internal waves are generated in the Strait of Messina
(Alpers and Salusti, 1983). Since then, a large amount of investigations has
been devoted to the understanding of the nature, the generation, propagation, and dissipation of tidally induced long internal solitary waves. For instance, shore based marine radar observations from the rock of Gibraltar as
well as airborne synthetic aperture radar observations contributed to clarify
the generation and propagation characteristics of the internal solitary
waves in the Strait of Gibraltar (Watson and Robinson, 1990; Richez
1994). Parallel, theoretical investigations addressed the hydrodynamics
and the imaging mechanisms of such oceanic features (Brandt et al., 1996,
1997) thus contributing to deepen our knowledge on the complexity of the
baroclinic dynamics of these sea straits.
Acknowledgements
We thank ESA who provided us with a large number of ERS and Envisat
SAR images acquired over the Straits of Gibraltar and Messina.
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