14 Internal Solitary Waves System in the Mozambique Channel
279
There are tangential surface currents associated to these SSH anomalies, whose
rotation directions are indicated by arrows in Fig. 14.8. These currents can be rather
large in the MC (as large as 0.5 ms
−1 according to Swart et al. 2010 and thus of the
order of the ISWs propagation speeds) and their influence can extend in diameter for
some 200 km. In terms of vertical sections the SSH anomalies are associated with
isopycnical deformations, which are in general in the opposite way to the surface
displacements. From Fig. 14.8, it is clear that the eddy-like features associated to
the SSH anomalies interfere with the propagation of the ISWs, apparently causing
an overall refraction pattern (to be discussed in the next paragraphs).
In the southern hemisphere eddies with an anti-clockwise rotation are warm
eddies, whose SSH anomalies are positive at the surface but are characterized by
bowl-like (or bulge) downward displacements of their isopycnicals at depth (isopycnicals can be displaced some 100 m downwards or more, at the core of a warm eddy
near the pycnocline). On the other hand, cold eddies spin in the clockwise direction,
which are characterized by negative SSH anomalies, and have their isopycnicals
displaced upwards near the core.
This means that the eddy-like features identified in Fig. 14.8 are those of a cold
eddy just off the Sofala Bank (in blue), and a warm eddy somewhat farther south
and farther offshore (in red). It can be seen that the ISW crests showed in Fig. 14.8
are clearly interacting with the eddy structures that were measured in the SSH data.
First, the cold eddy closer to the shelf and afterwards the warm eddy farther offshore
(and southwards of the cold eddy) are located directly along the ISWs path. Note
that the sequence of ISW crests shown in Fig. 14.8 also exhibits what appears to be
a clockwise rotation pattern.
In fact, the packets labelled SG1, SG2, LG1, SG3 and LG2 (forming a chronological sequence) progressively propagate more to the south and less to the east, until
they reverse the component of east-west propagation velocity and return back on to
the shore, undergoing a near 135 clockwise rotation. In this interpretation, we considered that the ISWs sequence mentioned above is progressively older in time, with
respect to their generation sites and propagation paths. We believe that the pattern
of ISWs rotating clockwise in their direction of propagation is essentially caused by
interaction with the near-surface currents produced by eddies. However, refraction
may also be caused (at least to some extent) by different stratifications and mixed
layer depths resulting from the relative positions of eddies along the ISWs propagation pathways. The later effect has been explained successfully by Sherwin et al.
(2002) off the northern Portuguese shelf (in the west Iberian Coast), where refraction
of ISWs occurs basically due to an offshore downward tilt of the isopycnicals. In the
Iberian case, similarly to the MC, ISWs were first found to propagate in an off-shelf
direction and then refract back onto the coast some 50 km north of their generation
site (roughly the same distance that ISWs need in MC to refract back to shore).
279
There are tangential surface currents associated to these SSH anomalies, whose
rotation directions are indicated by arrows in Fig. 14.8. These currents can be rather
large in the MC (as large as 0.5 ms
−1 according to Swart et al. 2010 and thus of the
order of the ISWs propagation speeds) and their influence can extend in diameter for
some 200 km. In terms of vertical sections the SSH anomalies are associated with
isopycnical deformations, which are in general in the opposite way to the surface
displacements. From Fig. 14.8, it is clear that the eddy-like features associated to
the SSH anomalies interfere with the propagation of the ISWs, apparently causing
an overall refraction pattern (to be discussed in the next paragraphs).
In the southern hemisphere eddies with an anti-clockwise rotation are warm
eddies, whose SSH anomalies are positive at the surface but are characterized by
bowl-like (or bulge) downward displacements of their isopycnicals at depth (isopycnicals can be displaced some 100 m downwards or more, at the core of a warm eddy
near the pycnocline). On the other hand, cold eddies spin in the clockwise direction,
which are characterized by negative SSH anomalies, and have their isopycnicals
displaced upwards near the core.
This means that the eddy-like features identified in Fig. 14.8 are those of a cold
eddy just off the Sofala Bank (in blue), and a warm eddy somewhat farther south
and farther offshore (in red). It can be seen that the ISW crests showed in Fig. 14.8
are clearly interacting with the eddy structures that were measured in the SSH data.
First, the cold eddy closer to the shelf and afterwards the warm eddy farther offshore
(and southwards of the cold eddy) are located directly along the ISWs path. Note
that the sequence of ISW crests shown in Fig. 14.8 also exhibits what appears to be
a clockwise rotation pattern.
In fact, the packets labelled SG1, SG2, LG1, SG3 and LG2 (forming a chronological sequence) progressively propagate more to the south and less to the east, until
they reverse the component of east-west propagation velocity and return back on to
the shore, undergoing a near 135 clockwise rotation. In this interpretation, we considered that the ISWs sequence mentioned above is progressively older in time, with
respect to their generation sites and propagation paths. We believe that the pattern
of ISWs rotating clockwise in their direction of propagation is essentially caused by
interaction with the near-surface currents produced by eddies. However, refraction
may also be caused (at least to some extent) by different stratifications and mixed
layer depths resulting from the relative positions of eddies along the ISWs propagation pathways. The later effect has been explained successfully by Sherwin et al.
(2002) off the northern Portuguese shelf (in the west Iberian Coast), where refraction
of ISWs occurs basically due to an offshore downward tilt of the isopycnicals. In the
Iberian case, similarly to the MC, ISWs were first found to propagate in an off-shelf
direction and then refract back onto the coast some 50 km north of their generation
site (roughly the same distance that ISWs need in MC to refract back to shore).