14 Internal Solitary Waves System in the Mozambique Channel
281
mainly to northeast (from the south end of the MC) and to southwest (coming from
Madagascar). It was also found that high pressure systems born in the Polar Regions
(known as Moving Polar Highs, MPHs), which are associated with air subsidence in
MC, can play a major influence on the propagation of these waves. This is because
MPHs change the atmosphere’s stability, and influences its vertical structure to form
suitable waveguides, necessary for AGWs to propagate, and can even trigger their
own generation (see da Silva and Magalhaes 2009 for a detailed study).
The sea surface signatures of the AGWs in MC can usually be distinguished
from their oceanic counter parts (discussed in Sect. 14.2 to 14.4) due to their large
dimensions (wavelength and crestlength). However, in many cases, other smallerscale AGWs can also be found to propagate across the MC, including those areas
where oceanic ISWs are frequently present (e.g. see composite map in Fig. 14.5). A
good example can be seen in Fig. 14.9, where a small-scale AGW (labeled S, and
much smaller than the largest packet labeled L) is seen to travel to the southeast, in an
almost perpendicular direction to the larger-scale AGW (labeled L) that is travelling
southwest. In fact, this observation is very similar to the oceanic ISWs presented
in Sect. 14.2, since it has the same characteristics (namely, spatial scales, crests
orientation, propagation direction and geographic location).
Care should be taken when interpreting sea surface manifestations of IWs in SAR
(as mentioned in the Introduction Section). On the one hand, SAR images cannot
detect the presence of cloud structures, which would clearly indicate the atmospheric
nature of AGWs. On the other hand, clouds may not form at all in the presence of
large amplitude AGWs, since the ascending branches of the waves may not meet the
necessary moisture for condensation to occur. In fact, Fig. 14.9 shows that some of the
solitary waves in the trailing edge of the large packet (labeled L) are only partially
recognizable as clouds—the remaining segments are merely seen as sea surface
patterns of banded roughness on a smoother background. In such cases, alternative
methodologies should be taken under consideration to discriminate between oceanic
and atmospheric IWs. A detailed discussion of these issues can be found in the work
presented by Alpers and Huang (2011), where lists of discriminating criteria were
presented based on soliton and SAR theory, and further complemented by additional
information on the ocean and atmosphere.
14.6 Summary
This chapter describes remote sensing data from SARs and image spectrometers to
clearly show that MC is a hotspot for observing ISWs in the ocean and in the atmosphere. Oceanic ISWs were found to be generated directly off the shelf by the
steepening of an interfacial tidal wave, and through a different generation mechanism by which the impact of a tidal beam from below the thermocline generates
ISWs (known as local generation of ISWs). Both mechanisms are associated with
an elevated region of body forcing near the Sofala Bank, which results from strong
281
mainly to northeast (from the south end of the MC) and to southwest (coming from
Madagascar). It was also found that high pressure systems born in the Polar Regions
(known as Moving Polar Highs, MPHs), which are associated with air subsidence in
MC, can play a major influence on the propagation of these waves. This is because
MPHs change the atmosphere’s stability, and influences its vertical structure to form
suitable waveguides, necessary for AGWs to propagate, and can even trigger their
own generation (see da Silva and Magalhaes 2009 for a detailed study).
The sea surface signatures of the AGWs in MC can usually be distinguished
from their oceanic counter parts (discussed in Sect. 14.2 to 14.4) due to their large
dimensions (wavelength and crestlength). However, in many cases, other smallerscale AGWs can also be found to propagate across the MC, including those areas
where oceanic ISWs are frequently present (e.g. see composite map in Fig. 14.5). A
good example can be seen in Fig. 14.9, where a small-scale AGW (labeled S, and
much smaller than the largest packet labeled L) is seen to travel to the southeast, in an
almost perpendicular direction to the larger-scale AGW (labeled L) that is travelling
southwest. In fact, this observation is very similar to the oceanic ISWs presented
in Sect. 14.2, since it has the same characteristics (namely, spatial scales, crests
orientation, propagation direction and geographic location).
Care should be taken when interpreting sea surface manifestations of IWs in SAR
(as mentioned in the Introduction Section). On the one hand, SAR images cannot
detect the presence of cloud structures, which would clearly indicate the atmospheric
nature of AGWs. On the other hand, clouds may not form at all in the presence of
large amplitude AGWs, since the ascending branches of the waves may not meet the
necessary moisture for condensation to occur. In fact, Fig. 14.9 shows that some of the
solitary waves in the trailing edge of the large packet (labeled L) are only partially
recognizable as clouds—the remaining segments are merely seen as sea surface
patterns of banded roughness on a smoother background. In such cases, alternative
methodologies should be taken under consideration to discriminate between oceanic
and atmospheric IWs. A detailed discussion of these issues can be found in the work
presented by Alpers and Huang (2011), where lists of discriminating criteria were
presented based on soliton and SAR theory, and further complemented by additional
information on the ocean and atmosphere.
14.6 Summary
This chapter describes remote sensing data from SARs and image spectrometers to
clearly show that MC is a hotspot for observing ISWs in the ocean and in the atmosphere. Oceanic ISWs were found to be generated directly off the shelf by the
steepening of an interfacial tidal wave, and through a different generation mechanism by which the impact of a tidal beam from below the thermocline generates
ISWs (known as local generation of ISWs). Both mechanisms are associated with
an elevated region of body forcing near the Sofala Bank, which results from strong