244
7 Ocean Currents
Bottom Water (AABW). Due to its high density, this water sinks and slowly
spreads northward along the sea bottom. Part of this water, warmed slightly by
mixing with the East and West Drifts water, remains near the surface. When
it encounters less dense water along the Antarctic Convergence, the water sinks
forming the Antarctic Intermediate Water (AAIW).
The continuity of mass requires a compensation for the loss due to downwelling of the Antarctic Bottom Water and the Antarctic Intermediate Water.
This compensation is provided by the Circumpolar Deep Water, upwelling
to the surface, and transporting dissolved nutrients which are the basis for
the food web of this area. A schematic three-dimensional representation in
the Southern Ocean is given in Fig. 7.16. Biological consequences of upwelling
along the Antarctic Convergence are discussed in Chap. 15.
Equatorial Upwelling. As was shown in Sect. 7.3.4, the surface circulation of equatorial water is very complex, consisting of the wind-driven North
and South Equatorial Currents, Equatorial Countercurrent, and Equatorial
Undercurrent. Superimposed on this horizontal transport is the slow vertical movement of water, known as equatorial upwelling. Coriolis deflection,
which changes its sign on both sides of the Equator, induces divergence of the
equatorial water, inducing the upwelling of cold water.
This upwelling is clearly indicated by the presence of a band of water around
the Equator, of 2°-goC colder than the seawater on either side. Upwelling is
stronger in summer than in winter causing high concentration of nitrate during
that season. Within the equatorial band of colder water in the Atlantic and
Pacific Ocean, temperature also varies spatially. The water in the eastern part
of this colder band is cooler than that in the western part. However, due to
tilting of the thermocline to the West (see Fig. 7.11), Ekman transport causes
warm water above the thermocline to ascend to the sea surface in the west.
In contrast, in the east the thermocline is shallow and weak, and even weaker
upwelling transports cold water from beneath the thermocline to the surface.
7.8 Coastal Water Movement
7.8.1 An Overview of Coastal Flows
The land and sea are functionally connected in the coastal zone. At each instant
there exists a dynamic balance between the driving forces and the position,
character and configuration of the shoreline. The most important energy inputs
in the coastal zone are those provided by waves and tides. Resulting beach
morphology is the combined response of beach sediments to wave- and tidegenerated processes, and the interaction between these two processes.
During a tidal cycle the position of the swash zone, surf zone, and shoaling
wave zone shifts with the tide, both vertically and horizontally, causing a change
of intertidal beach profile by each of these processes about every 12 hours. The
rate of migration of the tide across the profile is a function of the tidal range,
7 Ocean Currents
Bottom Water (AABW). Due to its high density, this water sinks and slowly
spreads northward along the sea bottom. Part of this water, warmed slightly by
mixing with the East and West Drifts water, remains near the surface. When
it encounters less dense water along the Antarctic Convergence, the water sinks
forming the Antarctic Intermediate Water (AAIW).
The continuity of mass requires a compensation for the loss due to downwelling of the Antarctic Bottom Water and the Antarctic Intermediate Water.
This compensation is provided by the Circumpolar Deep Water, upwelling
to the surface, and transporting dissolved nutrients which are the basis for
the food web of this area. A schematic three-dimensional representation in
the Southern Ocean is given in Fig. 7.16. Biological consequences of upwelling
along the Antarctic Convergence are discussed in Chap. 15.
Equatorial Upwelling. As was shown in Sect. 7.3.4, the surface circulation of equatorial water is very complex, consisting of the wind-driven North
and South Equatorial Currents, Equatorial Countercurrent, and Equatorial
Undercurrent. Superimposed on this horizontal transport is the slow vertical movement of water, known as equatorial upwelling. Coriolis deflection,
which changes its sign on both sides of the Equator, induces divergence of the
equatorial water, inducing the upwelling of cold water.
This upwelling is clearly indicated by the presence of a band of water around
the Equator, of 2°-goC colder than the seawater on either side. Upwelling is
stronger in summer than in winter causing high concentration of nitrate during
that season. Within the equatorial band of colder water in the Atlantic and
Pacific Ocean, temperature also varies spatially. The water in the eastern part
of this colder band is cooler than that in the western part. However, due to
tilting of the thermocline to the West (see Fig. 7.11), Ekman transport causes
warm water above the thermocline to ascend to the sea surface in the west.
In contrast, in the east the thermocline is shallow and weak, and even weaker
upwelling transports cold water from beneath the thermocline to the surface.
7.8 Coastal Water Movement
7.8.1 An Overview of Coastal Flows
The land and sea are functionally connected in the coastal zone. At each instant
there exists a dynamic balance between the driving forces and the position,
character and configuration of the shoreline. The most important energy inputs
in the coastal zone are those provided by waves and tides. Resulting beach
morphology is the combined response of beach sediments to wave- and tidegenerated processes, and the interaction between these two processes.
During a tidal cycle the position of the swash zone, surf zone, and shoaling
wave zone shifts with the tide, both vertically and horizontally, causing a change
of intertidal beach profile by each of these processes about every 12 hours. The
rate of migration of the tide across the profile is a function of the tidal range,