44
DYNAMICAL OCEANOGRAPHY
the adjustment process, the special problem of an initially motionless flow that is
suddenly accelerated by a time-independent wind stress is studied in chapter 9.
The results provide an answer to the first question above. In chapter 10, the linear
stability of a zonal current is studied with focus on two instability mechanisms:
barotropic and baroclinic instability.
2.4.3. Special equatorial phenomena
Fig. 2.4 indicates that there are peculiar equatorial currents both in the Pacific
and Atlantic. Although the wind stress is directed westward, eastward currents
are observed north of the equator. Another special phenomenon is the occurrence,
about once every four years, of El Ni˜ no in the Pacific (and not in the Atlantic)
with a sea-surface temperature anomaly pattern as in Fig. 2.9. This leads to the
following specific questions:
Why are there equatorial countercurrents in the Atlantic and Pacific?
Which physical processes are responsible for the occurrence of El Ni˜ no?
Theory for the steady equatorial ocean circulation, the equatorial free waves
and the equatorial adjustment, is presented in chapter 11. It appears that the answer to the first question is directly related to the meridional gradients of the zonal
wind stress. The material in chapter 11 is necessary to understand the dynamics of
El Ni˜ no as a phenomenon arising through coupled processes between the equatorial Pacific and the global atmosphere. The coupled processes will be presented
in chapter 12 and a basic explanation of the anomaly patterns and time scale of El
Ni˜ no is given.
2.4.4. Phenomena on the planetary scale
In the last chapters, we consider ocean flow phenomena on a planetary
scale. The Antarctic Circumpolar Current (ACC) is special in that there are
no lateral boundaries over a substantial part of the flow domain. This leads
to interesting dynamics controlling the volume transport of the current. The
Arctic Ocean circulation is special because of the strong coupling between
the spatial pattern of the ocean currents and the bottom topography. On this
planetary scale, there is an intricate coupling between the large-scale flow and
the density gradients in the ocean water. The flow transports heat and salt and
thus causes density differences. The density gradients in turn cause pressure
gradients and hence influence the large-scale flow. One of the phenomena
that is caused by this large-scale nonlinear interaction is the thermocline, the
region with a relatively large vertical temperature (and density) gradient in the
ocean (cf. Fig. f:A16). The sharp transitions in the temperature during the
Dansgaard-Oeschger cycles (section 2.3) may have been caused by changes in
DYNAMICAL OCEANOGRAPHY
the adjustment process, the special problem of an initially motionless flow that is
suddenly accelerated by a time-independent wind stress is studied in chapter 9.
The results provide an answer to the first question above. In chapter 10, the linear
stability of a zonal current is studied with focus on two instability mechanisms:
barotropic and baroclinic instability.
2.4.3. Special equatorial phenomena
Fig. 2.4 indicates that there are peculiar equatorial currents both in the Pacific
and Atlantic. Although the wind stress is directed westward, eastward currents
are observed north of the equator. Another special phenomenon is the occurrence,
about once every four years, of El Ni˜ no in the Pacific (and not in the Atlantic)
with a sea-surface temperature anomaly pattern as in Fig. 2.9. This leads to the
following specific questions:
Why are there equatorial countercurrents in the Atlantic and Pacific?
Which physical processes are responsible for the occurrence of El Ni˜ no?
Theory for the steady equatorial ocean circulation, the equatorial free waves
and the equatorial adjustment, is presented in chapter 11. It appears that the answer to the first question is directly related to the meridional gradients of the zonal
wind stress. The material in chapter 11 is necessary to understand the dynamics of
El Ni˜ no as a phenomenon arising through coupled processes between the equatorial Pacific and the global atmosphere. The coupled processes will be presented
in chapter 12 and a basic explanation of the anomaly patterns and time scale of El
Ni˜ no is given.
2.4.4. Phenomena on the planetary scale
In the last chapters, we consider ocean flow phenomena on a planetary
scale. The Antarctic Circumpolar Current (ACC) is special in that there are
no lateral boundaries over a substantial part of the flow domain. This leads
to interesting dynamics controlling the volume transport of the current. The
Arctic Ocean circulation is special because of the strong coupling between
the spatial pattern of the ocean currents and the bottom topography. On this
planetary scale, there is an intricate coupling between the large-scale flow and
the density gradients in the ocean water. The flow transports heat and salt and
thus causes density differences. The density gradients in turn cause pressure
gradients and hence influence the large-scale flow. One of the phenomena
that is caused by this large-scale nonlinear interaction is the thermocline, the
region with a relatively large vertical temperature (and density) gradient in the
ocean (cf. Fig. f:A16). The sharp transitions in the temperature during the
Dansgaard-Oeschger cycles (section 2.3) may have been caused by changes in