14 Internal Solitary Waves System in the Mozambique Channel
265
waves are the solitary waves of finite amplitude and permanent form, which owe
their existence to a delicate balance between linear wave dispersion and nonlinear
wave steepening. These are usually found in rank-order packets (a consequence of
their speed being an increasing function of amplitude) and they have been observed
to travel for considerable amounts of distance without significant changes in form.
In turn, this is a clear indication that these waves can in fact be responsible for
transferring considerable amounts of energy throughout large distances, which is
one of the main reasons why they are so important in the dynamics of the coastal
oceans and in the lower troposphere.
On the one hand, ISWs in the ocean have been proved to significantly contribute to
ocean mixing, either during breaking processes in their final stages of propagation,
or due to their highly turbulent character as they propagate along the pycnocline
(Pinkel 2000; Moum et al. 2003; van Haren et al. 2005; Lamb and Farmer 2011). In
fact, recent studies have shown that IWs contribute considerably to shelf dynamics
in terms of vertical heat fluxes and ocean-shelf mass transports (Shroyer et al. 2010a,
2010b). Mixing due to IWs has also been shown to be important from a biological
point of view. Several studies have shown that the dissipation of ISWs may constitute
a primary mixing mechanism, and that it may be sufficient to induce strong upward
fluxes of nutrients across the thermocline and produce bursts of primary production
(see e.g. Sandstrom and Elliott 1984; Pingree et al. 1986). Da Silva et al. (2002)
also suggested that enhanced primary production, could simply occur due to the
increase and decrease in available light experienced by phytoplankton, during up
and downward advection induced by IWs.
On the other hand, atmospheric IWs of solitary form—sometimes also termed Atmospheric Gravity Waves (AGWs)—have been shown to constitute a serious hazard
to aircrafts operating at low altitudes and small speeds (see e.g. Christie and Muirhead
1983; Bedard et al. 1986). Some large-scale AGWs may also have a significant role
in establishing low-level circulation and determining the vertical structure of wind,
temperature and moisture. They can even modify and trigger convective motions
and induce significant rain (Reeder et al. 1995). Their ability to transfer energy and
momentum (together with the fact that they propagate for several hundreds of kilometres) makes them an important topic for research, especially since their influence
can also extend into weather forecast and climate models.
Furthermore, ISWs can sometimes occur in regions where both mediums (ocean
and atmosphere) can support simultaneous propagation. In these cases, care should
be taken to avoid miss-interpretation. It is also important to recognize that atmospheric internal waves may not always be associated with cloud structures, since
the ascending branches of the waves may not find the necessary conditions (high
moisture in the atmosphere) for condensation to occur (see e.g. Crook 1986). In
this case, there are no guaranteed methods to discriminate between oceanic and atmospheric ISW sea surface signatures in SAR images. However, recent research
based on solitary wave and radar imaging theory has provided useful criteria (sometimes with complementary information on the ocean and atmosphere) to help in the
distinguishing process (see Alpers and Huang 2011).
265
waves are the solitary waves of finite amplitude and permanent form, which owe
their existence to a delicate balance between linear wave dispersion and nonlinear
wave steepening. These are usually found in rank-order packets (a consequence of
their speed being an increasing function of amplitude) and they have been observed
to travel for considerable amounts of distance without significant changes in form.
In turn, this is a clear indication that these waves can in fact be responsible for
transferring considerable amounts of energy throughout large distances, which is
one of the main reasons why they are so important in the dynamics of the coastal
oceans and in the lower troposphere.
On the one hand, ISWs in the ocean have been proved to significantly contribute to
ocean mixing, either during breaking processes in their final stages of propagation,
or due to their highly turbulent character as they propagate along the pycnocline
(Pinkel 2000; Moum et al. 2003; van Haren et al. 2005; Lamb and Farmer 2011). In
fact, recent studies have shown that IWs contribute considerably to shelf dynamics
in terms of vertical heat fluxes and ocean-shelf mass transports (Shroyer et al. 2010a,
2010b). Mixing due to IWs has also been shown to be important from a biological
point of view. Several studies have shown that the dissipation of ISWs may constitute
a primary mixing mechanism, and that it may be sufficient to induce strong upward
fluxes of nutrients across the thermocline and produce bursts of primary production
(see e.g. Sandstrom and Elliott 1984; Pingree et al. 1986). Da Silva et al. (2002)
also suggested that enhanced primary production, could simply occur due to the
increase and decrease in available light experienced by phytoplankton, during up
and downward advection induced by IWs.
On the other hand, atmospheric IWs of solitary form—sometimes also termed Atmospheric Gravity Waves (AGWs)—have been shown to constitute a serious hazard
to aircrafts operating at low altitudes and small speeds (see e.g. Christie and Muirhead
1983; Bedard et al. 1986). Some large-scale AGWs may also have a significant role
in establishing low-level circulation and determining the vertical structure of wind,
temperature and moisture. They can even modify and trigger convective motions
and induce significant rain (Reeder et al. 1995). Their ability to transfer energy and
momentum (together with the fact that they propagate for several hundreds of kilometres) makes them an important topic for research, especially since their influence
can also extend into weather forecast and climate models.
Furthermore, ISWs can sometimes occur in regions where both mediums (ocean
and atmosphere) can support simultaneous propagation. In these cases, care should
be taken to avoid miss-interpretation. It is also important to recognize that atmospheric internal waves may not always be associated with cloud structures, since
the ascending branches of the waves may not find the necessary conditions (high
moisture in the atmosphere) for condensation to occur (see e.g. Crook 1986). In
this case, there are no guaranteed methods to discriminate between oceanic and atmospheric ISW sea surface signatures in SAR images. However, recent research
based on solitary wave and radar imaging theory has provided useful criteria (sometimes with complementary information on the ocean and atmosphere) to help in the
distinguishing process (see Alpers and Huang 2011).