14 Internal Solitary Waves System in the Mozambique Channel
267
variability. The shelf slope is also supercritical with respect to the M 2 barotropic
tide (also known as the principal lunar semi-diurnal tide, with a period of 12.42 h),
allowing the existence of critical regions for IT generation at the shelf break. Also
noteworthy, is the fact that the flow in MC is influenced by large, southward propagating eddies, which feed their waters into the Agulhas Current system, and therefore
constitute a major contribution to the overall flow within the channel.
14.2 Oceanic ISWs
14.2.1 Introduction
Internal tidal energy generated at the shelf-break may radiate away either horizontally
in the form of interfacial ITs that propagate along the thermocline, or as internal tidal
beams that propagate into the deep stratified ocean below. Therefore, large interfacial
ITs may form in the thermocline directly above the shelf break and evolve (through
nonlinear processes) to higher-frequency ISWs packets. Another possibility is that
the IT energy, which propagates downward into the deep ocean, may give rise to a
second generation mechanism known as “local generation” (New and Pingree 1992;
Gerkema 2001; New and da Silva 2002; Akylas et al. 2007; Grisouard et al. 2011;
Mercier et al. 2012). Here, a beam (or ray) of IT energy is generated at “critical”
slope regions of the shelf-break where the bottom topographic slope matches the
slope of the ray paths, and propagates at an angle (θ ) to the horizontal into the deep
ocean interior (see Eq. 14.1) where σ is the tidal frequency, f the Coriolis parameter
and N the buoyancy frequency). These rays reflect from the seafloor (Pingree and
New 1989, 1991), and interact with the thermocline from below, causing large IT
oscillations there, and “locally” generating ISWs (far from the shelf-break where the
beam is initially generated). We will show clear evidence (based on remote sensing
SAR images and modelling results) that both generation mechanisms are at work in
MC, which means that there are waves being formed directly at the shelf break, and
further into the open ocean through local generation.
tan(θ) = ±
σ
2
− f
2
N 2 − σ 2
1/2
(14.1)
This hypothesis was originally proposed to explain some ISW observations in the
northern Bay of Biscay, where it was studied for the first time, and where SAR imagery helped to clearly distinguish a bimodal distribution of ISWs. This distribution
results from the direct generation mechanism near the shelf-break and from the local
generation processes taking place some 150 km further into the ocean (New and da
Silva 2002). Nonetheless, the widespread occurrence of the local generation mechanism in other regions of the world’s oceans has been recently put forward with the
contribution of in situ and satellite data. In fact, several other studies have emerged
since, which have also identified locally generated waves. Namely, the southern Bay
267
variability. The shelf slope is also supercritical with respect to the M 2 barotropic
tide (also known as the principal lunar semi-diurnal tide, with a period of 12.42 h),
allowing the existence of critical regions for IT generation at the shelf break. Also
noteworthy, is the fact that the flow in MC is influenced by large, southward propagating eddies, which feed their waters into the Agulhas Current system, and therefore
constitute a major contribution to the overall flow within the channel.
14.2 Oceanic ISWs
14.2.1 Introduction
Internal tidal energy generated at the shelf-break may radiate away either horizontally
in the form of interfacial ITs that propagate along the thermocline, or as internal tidal
beams that propagate into the deep stratified ocean below. Therefore, large interfacial
ITs may form in the thermocline directly above the shelf break and evolve (through
nonlinear processes) to higher-frequency ISWs packets. Another possibility is that
the IT energy, which propagates downward into the deep ocean, may give rise to a
second generation mechanism known as “local generation” (New and Pingree 1992;
Gerkema 2001; New and da Silva 2002; Akylas et al. 2007; Grisouard et al. 2011;
Mercier et al. 2012). Here, a beam (or ray) of IT energy is generated at “critical”
slope regions of the shelf-break where the bottom topographic slope matches the
slope of the ray paths, and propagates at an angle (θ ) to the horizontal into the deep
ocean interior (see Eq. 14.1) where σ is the tidal frequency, f the Coriolis parameter
and N the buoyancy frequency). These rays reflect from the seafloor (Pingree and
New 1989, 1991), and interact with the thermocline from below, causing large IT
oscillations there, and “locally” generating ISWs (far from the shelf-break where the
beam is initially generated). We will show clear evidence (based on remote sensing
SAR images and modelling results) that both generation mechanisms are at work in
MC, which means that there are waves being formed directly at the shelf break, and
further into the open ocean through local generation.
tan(θ) = ±
σ
2
− f
2
N 2 − σ 2
1/2
(14.1)
This hypothesis was originally proposed to explain some ISW observations in the
northern Bay of Biscay, where it was studied for the first time, and where SAR imagery helped to clearly distinguish a bimodal distribution of ISWs. This distribution
results from the direct generation mechanism near the shelf-break and from the local
generation processes taking place some 150 km further into the ocean (New and da
Silva 2002). Nonetheless, the widespread occurrence of the local generation mechanism in other regions of the world’s oceans has been recently put forward with the
contribution of in situ and satellite data. In fact, several other studies have emerged
since, which have also identified locally generated waves. Namely, the southern Bay