322
u
Ro = PL2
Equatorial Dynamics of the Thermocline: The Equatorial Undercurrent
(6.1.3)
However, this parameter is, from (1.4.4), also the measure, c, of the ratio of the
relative vorticity gradient to the planetary vorticity gradient:
u
e = pL2.
( 6.1.4)
Therefore, we can expect that when the distance to the equator, L, becomes
small enough so that the advection of relative momentum becomes important,
i.e., when R 0 is 0(1) and L is O(..JTJ1f3), this also corresponds to the emergence of the advection of relative vorticity becoming as dynamically significant
as the advection of planetary vorticity. We thus expect to lose the geostrophic
approximation to the momentum equation simultaneously with the breakdown
of the Sverdrup balance. This implies that a full reformulation of the dynamics
in the vicinity of the equator is required.
The aspect of the oceanic general circulation of the equatorial region that
is perhaps the most striking is the existence of the Equatorial Undercurrent
(EUC). Although discovered in the nineteenth century, the EUC was essentially forgotten and was rediscovered in modern times only in 1952. The reason
for our initial forgetfulness and the continued ignorance of its existence for so
long is related to its subsurface nature. The current is hidden from direct view
by a surface current normally flowing westward. The undercurrent itself exists
in all three major equatorial oceans as a thin ribbon of water with a width of
about 200 km, at depths of the order of 100--200 m, flowing eastward against
the prevailing trade winds with speeds of the order of 100 cm/s. These speeds
are as large as those of any of the major western boundary currents, and the
current which traverses the entire length of each of the Pacific, Atlantic, and
Indian Oceans is one of the marvels of the general circulation of the ocean.
Figure 6.1.1 shows cross sections of the current's velocity, temperature,
and salinity. The strong eastward velocity has its core at this longitude (155°W,
in the Pacific) about 150 m below the surface and has a vertical thickness of
about 100m. The weaker surface velocity is to the west, i.e., in the direction of
the prevailing wind. The isotherms, as seen in the cross section, become shallow
as they approach the equator in the region outside the EUC, as we would
expect from the theoretical models of the thermocline discussed in Chapter 4,
but in the region of the current they stretch apart, and this bowed shape of the
density surfaces is characteristic of the current wherever it is seen. The salinity
field acts rather as a tracer of the motion, and the tongues in each hemisphere
stretching to the equator suggest that subtropical water flows towards the
equator at depth and enters the current.
The zonal velocity in Fig. 6.1.1 has been calculated using the geostrophic
approximation. This seems a strange thing to do at the equator, but as we see
below, geostrophy for the zonal component of the flow is an adequate approximation. This is underscored by a comparison of direct velocity mea-
u
Ro = PL2
Equatorial Dynamics of the Thermocline: The Equatorial Undercurrent
(6.1.3)
However, this parameter is, from (1.4.4), also the measure, c, of the ratio of the
relative vorticity gradient to the planetary vorticity gradient:
u
e = pL2.
( 6.1.4)
Therefore, we can expect that when the distance to the equator, L, becomes
small enough so that the advection of relative momentum becomes important,
i.e., when R 0 is 0(1) and L is O(..JTJ1f3), this also corresponds to the emergence of the advection of relative vorticity becoming as dynamically significant
as the advection of planetary vorticity. We thus expect to lose the geostrophic
approximation to the momentum equation simultaneously with the breakdown
of the Sverdrup balance. This implies that a full reformulation of the dynamics
in the vicinity of the equator is required.
The aspect of the oceanic general circulation of the equatorial region that
is perhaps the most striking is the existence of the Equatorial Undercurrent
(EUC). Although discovered in the nineteenth century, the EUC was essentially forgotten and was rediscovered in modern times only in 1952. The reason
for our initial forgetfulness and the continued ignorance of its existence for so
long is related to its subsurface nature. The current is hidden from direct view
by a surface current normally flowing westward. The undercurrent itself exists
in all three major equatorial oceans as a thin ribbon of water with a width of
about 200 km, at depths of the order of 100--200 m, flowing eastward against
the prevailing trade winds with speeds of the order of 100 cm/s. These speeds
are as large as those of any of the major western boundary currents, and the
current which traverses the entire length of each of the Pacific, Atlantic, and
Indian Oceans is one of the marvels of the general circulation of the ocean.
Figure 6.1.1 shows cross sections of the current's velocity, temperature,
and salinity. The strong eastward velocity has its core at this longitude (155°W,
in the Pacific) about 150 m below the surface and has a vertical thickness of
about 100m. The weaker surface velocity is to the west, i.e., in the direction of
the prevailing wind. The isotherms, as seen in the cross section, become shallow
as they approach the equator in the region outside the EUC, as we would
expect from the theoretical models of the thermocline discussed in Chapter 4,
but in the region of the current they stretch apart, and this bowed shape of the
density surfaces is characteristic of the current wherever it is seen. The salinity
field acts rather as a tracer of the motion, and the tongues in each hemisphere
stretching to the equator suggest that subtropical water flows towards the
equator at depth and enters the current.
The zonal velocity in Fig. 6.1.1 has been calculated using the geostrophic
approximation. This seems a strange thing to do at the equator, but as we see
below, geostrophy for the zonal component of the flow is an adequate approximation. This is underscored by a comparison of direct velocity mea-