Answer the following question in a short, coherent, well-written paragraph. Attach your essay as a Word document.
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1. estimate the density of a mixture of two samples of seawater that
have the same density but different temperatures and salinities,
2. describe and sketch changes in the seasonal thermocline at
mid-latitudes through the year,
3. plot temperature and salinity as a function of depth and identify
the thermocline and halocline,
4. list five different water masses and describe how they form,
5. relate surface convergence and divergence to downwelling
6. describe the properties of water masses in each ocean basin,
7. describe and sketch the motion of water in the Ekman layer,
8. diagram the formation of surface current gyres,
9. locate the major surface currents on a map of the ocean basins,
10. explain the process of western intensification,
11. relate patterns of surface convergence and divergence to
downwelling and upwelling, and
12. sketch the Great Ocean Conveyor Belt.
CHAP T E R OUT L INE
7.1 Ocean Structure 182
7.2 Thermohaline Circulation and
Water Masses 186
7.3 The Layered Oceans 189
7.4 What Drives the Surface Currents? 191
7.5 Ocean Surface Currents 193
7.6 Current Characteristics 197
7.7 Eddies 199
7.8 Convergence and Divergence 200
7.9 The Great Ocean Conveyor Belt 204
7.10 Changing Circulation Patterns 204
Diving In: Ocean Drifters 207
7.11 Measuring the Currents 206
Key Terms 211
Study Problems 211
Sea surface temperature (SST) simulation created by scientists at the
National Oceanic and Atmospheric Administrationfs Geophysical Fluid
Dynamics Laboratory using a coupled atmosphere-ocean model.
Currents and eddies off the southern tip of Africa are evident.
E arth is surrounded by two great oceans: an ocean
of air and an ocean of water. Both are in constant
motion, driven by the energy of the Sun and the
gravity of Earth. Hidden below the oceanfs surface is its
structure. If we could remove a slice of ocean water in the
same way we might cut a slice of cake, we would find that,
like a cake, the ocean is a layered system. The layers are
invisible to us, but they can be detected by measuring the
changing temperature and salt content, and by calculating
the density of the water from the surface to the ocean floor.
This layered structure is a dynamic response to processes that
occur at the surface: the gain and loss of heat, the evaporation
and addition of water, the freezing and thawing of ice, and
the movement of water in response to wind. These surface
processes produce a series of horizontally moving layers of
water, as well as local areas of vertical motion. Surface currents
carry heat from one location to another, altering Earthfs
surface temperature patterns and modifying the air above. The
interaction between the atmosphere and the ocean is dynamic;
as one system drives the other, the driven system acts to alter
the properties of the driving system.
In this chapter, we will study both the surface processes
and their below-the-surface results in order to understand why
the ocean is structured and how its structure is maintained. We
will also explore the formation of the oceanfs surface currents.
We follow these currents as they flow, merge, and move away
from each other. We examine both horizontal and vertical
circulation, and consider ways in which they are linked to the
overall interaction between the atmosphere and the ocean.
7.1 Ocean Structure
Variation of Temperature
With very few exceptions, the temperature
of seawater decreases with depth. Effectively
all of the energy available to heat the
ocean comes from incoming solar radiation.
Consequently, only a thin surface layer
of the ocean is heated directly because of
how rapidly solar radiation is absorbed with
depth (fig. 7.1). Nearly half of the total solar
energy at the sea surface is absorbed within
10 cm of the surface, and all of the infrared
energy is absorbed within about a meter of
the surface. A typical seawater temperatureversus-
depth profile consists of three gtemperature
layersh (fig. 7.2):
.3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4
Energy (arbitrary units)
Figure 7.1 A simplified plot of total solar energy in seawater as a function of depth. Area
below each curve is representative of the percent of available solar energy at the surface
that reaches that depth. A little over half of the total solar energy at the surface is absorbed
in the upper 1 m of water (only 45% penetrates to a depth of 1 m). Infrared energy is
absorbed particularly rapidly, with nearly all of it absorbed in the upper 1 m of water.
. a surface layer tens to a few hundreds of meters thick, called
the mixed layer;
. a region called the thermocline, extending from the bottom
of the mixed layer to a depth of about 1000 m (3280 ft); and
. the region from the base of the thermocline to the sea floor.
The mixed layer is an isothermal layer.a layer of constant
temperature. The thickness of the mixed layer is variable. It
depends on the depth to which the surface water is mixed by
turbulence caused by waves and wind. The mixed layer can be
as thick as 200.300 m (~650.1000 ft) at mid-latitudes in the
open ocean, whereas in protected coastal waters in the summer,
it can be as little as 10 m (~33 ft) thick. Between about 200.300 m
and 1000 m depth is the thermocline, where temperature
decreases rapidly throughout much of the ocean. This layer is
also frequently known as the gpermanent thermoclineh because
seasonal changes in climate at the surface do not influence water
temperature at these depths. Below 1000 m depth is the third,
and largest in volume, layer, which extends to the sea floor.This
deep water is nearly isothermal, temperature decreases very
slowly with depth, and it is uniformly cold everywhere. Roughly
75% of the water in the ocean has a temperature between 0 and
4‹C (table 7.1 and fig. 7.3). The discovery that the deep water
of the ocean, even in tropical regions, is very cold was made in
the eighteenth century. The obvious conclusion that followed
was that deep seawater must originate in polar regions, where
cold, dense surface water sinks and flows toward the Equator
along the ocean floor.
The details of actual temperature-versus-depth profiles
vary considerably depending on latitude and season of the year
(fig. 7.4). At mid-latitudes, the temperature and depth of the
mixed layer undergo seasonal changes with the formation of a
shallow seasonal thermocline in the summer and its disappearance
in the winter (fig. 7.4a). Throughout the winter, when strong
7.1 Ocean Structure 183
winds produce deep mixing and surface-water temperatures
are cold, the mixed layer may extend all the way to the top of
the permanent thermocline, producing an essentially vertical
temperature profile in the upper 200.300 m (fig. 7.5, March).
Throughout the summer, as surface temperatures rise and winds
decrease, the mixed layer will become more shallow and a
strong (steep temperature gradient) seasonal thermocline can
develop above the permanent thermocline (fig. 7.5, August).
At low latitudes, surface temperatures are warm and constant
throughout the year. Consequently, there is no development of
seasonal thermoclines and the three distinct temperature layers
are quite stable (fig. 7.4b). At high latitudes above about 60‹,
there is no permanent thermocline (fig. 7.4c). However, weak
seasonal thermoclines can develop in the summer. At high latitudes
there is often a layer of cold water 50.100 m below the
surface. Overall, the presence of three temperature layers in
the ocean is illustrated in the plot of seawater temperature in the
Pacific Ocean versus depth and latitude shown in figure 7.6.
Variation of Salinity with Depth
As discussed in chapter 5, the salinity of surface seawater
varies as a function of latitude in a relatively stable pattern
(review figs. 5.2 and 5.3). However, it is more difficult to draw
general conclusions about vertical profiles of salinity versus
depth. Depending on latitude, salinity may be relatively constant,
decrease, or increase with depth down to about 1000 m.
Below about 1000 m depth, the influence of surface processes is
minor and salinity is fairly constant. The total salinity range of
75% of the ocean is between 34.5 and 35.0 ppt (table 7.2 and
0 4 8 12 16 20 24
Figure 7.2 Simplified profile of temperature with depth in the
ocean. A shallow mixed layer with relatively constant temperature
overlies the thermocline, where temperature decreases rapidly with
depth. Beneath the thermocline, below a depth of about 1000 m,
temperature is fairly constant and cold.
Table 7.1 Distribution of Seawater Temperature
.2.0 6409 4.7
0.2 57,990 42.4
2.4 40,065 29.3
4.6 11,849 8.7
6.8 6059 4.4
8.10 4222 3.1
10.12 2632 1.9
12.14 2358 1.7
14.16 1340 1.0
16.18 985 0.7
18.20 685 0.5
20.22 638 0.5
22.24 475 0.3
24.26 426 0.3
26.28 489 0.4
28.30 269 0.2
30.32 2 0.001
.2 0 2 4 6 8 10 12 14
16 18 20 22 24 26 28 30 32
60,000 Figure 7.3 Distribution of seawater
temperature. Histograms indicate the
number of cubic kilometers of seawater
in each 2‹C range.
184 Chapter 7 Ocean Structure and Circulation
found at high latitudes. In either case, this marked
change in salinity with depth is called the halocline
(fig. 7.8). The general distribution of seawater salinity
in the Pacific Ocean is illustrated in the plot of
salinity versus depth and latitude shown in figure 7.9.
Variation of Density with Depth
Variations in temperature, salinity, and pressure
(depth) combine to control the density of seawater.
Seawater density is inversely proportional to temperature
and directly proportional to salinity and
pressure. This can be expressed as (ª = increases,
and « = decreases):
Density ª as temperature «
Density ª as salinity ª
Density ª as pressure ª
In general, variations in temperature and
salinity are more influential in determining seawater
density than are variations in pressure (or
depth). Consequently, we can consider density
to be a function of temperature and salinity. The
stratification of the ocean by temperature and
salinity that we have just discussed results in a
stratification of the ocean into density layers also.
A typical seawater density-versus-depth profile
consists of three gdensity layersh (fig. 7.10):
.. the mixed layer, a surface layer tens to a few hundreds of
Midlatitudes Low latitudes
0 5 10 15 20
0 5 10 15 20 25
.5 0 5
(a) (b) (c)
March May July August
2 4 6 8 10 12 14 16
Figure 7.4 Simple temperature-versus-depth profiles for three latitude zones: (a) mid-latitudes with significant seasonal variation, (b) low latitudes
where climate tends to be uniformly warm through the year, and (c) high latitudes where climate tends to be uniformly cold through the year.
Figure 7.5 Detailed variation in the depth of the seasonal thermocline and the
temperature of the surface mixed layer at mid-latitudes. Solid lines indicate the
growth of the seasonal thermocline as it increases in strength and shoals during
the summer, and dashed lines indicate its decay as it becomes deeper and weaker
in winter. Note the different scales compared to figure 7.4.
fig. 7.7). In the upper 1000 m, areas where salinity decreases
with depth are typically found at low and middle latitudes
whereas areas where salinity increases with depth are typically
7.1 Ocean Structure 185
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8500 9500
8000 9000 10,000 10,500 11,000 11,500 12,000 12,500 13,000 13,500
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5
Figure 7.6 Ocean temperature versus depth and latitude. North-south profile though the Pacific Ocean along the 150‹W meridian. Temperature
decreases with depth. The depth of the surface mixed layer and the permanent thermocline are less at low latitudes than at mid-latitudes because
winds are generally weaker and seasonal temperature contrasts are less at low latitudes. The black represents seafloor bathymetry.
Figure 7.7 Distribution of seawater salinity. Histograms indicate the number of cubic kilometers of
seawater in each 0.1 ppt range.
. a region called the pycnocline (which closely follows the
thermocline), extending from the bottom of the mixed layer
to a depth of about 1000 m (3280 ft); and
. the region from the base of the pycnocline to the sea floor.
1. What processes control the depth of the mixed layer?
2. Explain the difference between a seasonal thermocline
and the permanent thermocline.
3. How does the seasonal thermocline change during
4. Describe the basic characteristics of a temperatureversus-
depth profile at high latitudes, mid-latitudes,
and low latitudes.
5. Describe the total salinity range of seawater.
6. What is the halocline?
7. How does seawater density depend on temperature
186 Chapter 7 Ocean Structure and Circulation
7.2 Thermohaline Circulation
and Water Masses
If the density of the water increases with depth, the water column
from the surface to that depth is stable. If there is higherdensity
water on top of lower-density water, the water column is
unstable. An unstable water column cannot persist; the denser
surface water sinks and the less-dense water at depth rises to
replace the water above it. When dense water from the surface
sinks and reaches a level at which it is denser than the water
above but less dense than the water below, it spreads horizontally
as more water descends. At the surface, water moves horizontally
into the region where sinking is occurring. The dense water
that has descended displaces deeper water upward, completing
the cycle. Because water is a fixed quantity in the oceans, it cannot
be accumulated at one location or removed at another location
without movement of water between those locations. This
concept is called continuity of flow. This motion, caused by
variations in density due to differences in temperature and salinity,
is called thermohaline circulation. Areas of thermohaline
circulation where water sinks are called downwelling zones;
areas of rising waters are upwelling zones. Downwelling is a
mechanism that transports oxygen-rich surface water to depth,
where it is needed for deep-living animals. Upwelling returns
low oxygen-content water with dissolved, decay-produced
nutrients to the surface, where the nutrients act as fertilizers to
Table 7.2 Distribution of Seawater Salinity
<33.0 226 0.17
126.96.36.199 44 0.03
188.8.131.52 28 0.02
184.108.40.206 28 0.02
220.127.116.11 44 0.03
18.104.22.168 80 0.06
22.214.171.124 40 0.03
126.96.36.199 86 0.06
188.8.131.52 97 0.07
184.108.40.206 266 0.19
220.127.116.11 483 0.35
18.104.22.168 1058 0.77
22.214.171.124 1152 0.84
126.96.36.199 1896 1.38
188.8.131.52 4112 3.00
184.108.40.206 6515 4.76
220.127.116.11 12,301 8.98
18.104.22.168 44,235 32.31
22.214.171.124 32,761 23.93
126.96.36.199 7989 5.83
188.8.131.52 14,211 10.38
184.108.40.206 2396 1.75
220.127.116.11 1094 0.80
18.104.22.168 870 0.64
22.214.171.124 668 0.49
126.96.36.199 765 0.56
188.8.131.52 539 0.39
184.108.40.206 434 0.32
220.127.116.11 325 0.24
18.104.22.168 362 0.26
22.214.171.124 144 0.11
126.96.36.199 178 0.13
188.8.131.52 199 0.15
184.108.40.206 173 0.13
220.127.116.11 148 0.11
18.104.22.168 192 0.14
>36.5 786 0.57
33 34 35 36 37
Increasing salinity (ñ)
Figure 7.8 Simplified profiles of salinity with depth in the ocean
for high and low latitudes. In each case, there is a surface mixed
layer of relatively constant salinity. Beneath the mixed layer at high
latitudes, salinity decreases rapidly to a depth of about 1000 m.
Beneath the mixed layer at low latitudes, salinity increases rapidly
to a depth of about 1000 m. The region of rapid change in salinity is
called the halocline.
Figure 7.10 Simplified profile of density with depth in the ocean.
A shallow mixed layer with relatively constant density overlies the
pycnocline, where density increases rapidly with depth (this layer
corresponds closely to the thermocline shown in figure 7.2). Beneath
the pycnocline, density increases slowly to the sea floor.
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8500 9500
8000 9000 10,000 10,500 11,000 11,500 12,000 12,500 13,000 13,500
1.023 1.024 1.025 1.026 1.027 1.028
Figure 7.9 Seawater salinity versus depth and latitude. North-south profile through the Pacific Ocean along the 150‹W meridian. Most of
the variation in salinity is found in surface water with high values at low and middle latitudes and low values at high latitudes. Beneath about
1000.1500 m, salinity shows only minor variations. The black represents seafloor bathymetry.
promote photosynthesis and the production of more oxygen in
the sunlit surface waters. Upwelling and downwelling can also
be caused by wind-driven surface currents. When the surface
waters are driven together by the wind or against a coast, a
surface convergence is formed. Water at a surface convergence
sinks, or downwells. When the wind blows surface waters away
from an area or a coast, a surface divergence occurs and water
upwells from below. Surface convergences and divergences are
discussed further later in this chapter.
The speed of upwelling and downwelling water is about
0.1.1.5 m (0.3.5 ft) per day. Compare these flows to ocean
surface currents, which reach speeds of 1.5 m (5 ft) per second.
Horizontal movement at mid-ocean depth due to thermohaline
flow is about 0.01 cm (0.004 in) per second. Water caught in this
slow but relentlessly moving cycle can spend 1000 years at the
greater ocean depths before it again reaches the surface.
If the water column has the same density over depth, it has
neutral stability and is termed isopycnal. A neutrally stable
water column can easily be mixed vertically by wind, wave
action, and currents. If the water temperature is unchanging over
depth, the water column is isothermal; if the salinity is constant
over depth, it is isohaline.
There are many different combinations of temperature and
salinity that produce the same seawater density. This can be
illustrated in a temperature-salinity diagram, or T-S diagram,
where lines of equal density, or isopycnals, are drawn for many
combinations of temperature and salinity (fig 7.11). As the salinity
188 Chapter 7 Ocean Structure and Circulation
Figure 7.12 ARGO float measurements north of the Canary Islands
in the North Atlantic (about 31.37‹N, 25.27‹W) along vertical
profiles of (a) temperature and (b) salinity. (c) T-S plot of the data
collected can be used to identify specific water masses. Notice the
greater range of temperature and salinity found in surface waters
(red) compared to deep waters (violet).
0 5 10 15 20 25
1 5 10 15 20 25
Cycle number/relative temperature
Float ID# 1900036
34.5 35.0 35.5 36.0 36.5 37.0 37.5
1 5 10 15 20 25
2000 1500 1000 500 0
34.5 35.0 35.5 36.0
Upper North Atlantic deep water
36.5 37.0 37.5
Cycle number/relative salinity
(lines of equal density in gcm.3)
33.5 34.0 34.5 35.0 35.5 36.0 36.5
75% A . 25%B
50%A . 50%B
75% A . 25%B
50%A . 50%B
Figure 7.11 The density of seawater, measured in grams per cubic
centimeter, is abbreviated as ƒÏ (rho) and varies with temperature
and salinity. Many combinations of salinity and temperature produce
the same density. Low densities are at the upper left and high densities
are at the lower right. The straight line is the mixing line for waters A
and B, both with the same density. A mixture of A and B lies on the
mixing line and is more dense than either A or B.
increases, the density increases; as the temperature increases, the
density decreases. Salinity can be increased by evaporation or
by the formation of sea ice; it can be decreased by precipitation,
the inflow of river water, the melting of ice, or a combination
of these factors.
FOR MORE INFORMATION ON THIS PAPER VISIT……
Any seawater sample can be plotted on a T-S diagram as a
single point and the density of the sample can be determined from
the isopycnals. Seawater samples from a common source that all
plot very near each other on a T-S diagram, with a narrow range
of temperature and salinity, define a water type. When two water
types (e.g, A and B in fig. 7.11) having the same density, but different
values of salinity and temperature, are mixed, they form a
new water type that lies on a mixing line (a straight line drawn
from point A to point B). The location of the new water type on
the mixing line will depend on the relative amounts of the two
original water types that were combined to form the mix. Notice
that the isopycnals are curved. Consequently, the mixture of two
water types with the same density will produce a new water type
with a density greater than either of the two original water types,
and the newly created mixed water will sink. This mixing and
sinking process is known as caballing and it occurs wherever
surface waters converge and mix.
A water mass is a large body of water that has similar
values of temperature and salinity throughout. Water masses
can be identified by measuring water temperature and salinity
along vertical profiles in the ocean and plotting these data on a
T-S diagram (fig. 7.12a and b). A water mass will be represented
on a T-S diagram by closely clustered data points (fig. 7.12c).
Surface-water masses typically have greater variability in temperature
and salinity than do deeper water masses.
7.3 The Layered Oceans 189
zone extends to 200 m (650 ft), and the intermediate zone lies
between 200 and 2000 m (1000 and 6500 ft). Deep water is
found between 2000 and 4000 m (6500 and 13,000 ft), and
bottom water is below 4000 m (13,000 ft).
The Atlantic Ocean
The properties of the layers of water making up the Atlantic
Ocean are shown in figure 7.13. At the surface in the North
Atlantic, water from high northern latitudes moves southward,
while water from low latitudes moves northward along the
coast of North America and then east across the North Atlantic.
These waters converge in areas of cool temperatures and high
precipitation at approximately 50‹. 60‹N in the Norwegian Sea
and at the boundaries of the Gulf Stream and the Greenland and
Labrador Currents. The resulting mixed water has a salinity of
about 34.9ñ and a temperature of 2‹. 4‹C. This water, known
as North Atlantic deep water (NADW), sinks and moves southward.
North Atlantic deep water from the Norwegian Sea moves
south along the east side of the Atlantic, while water formed
at the boundary of the Labrador Current and the Gulf Stream
flows along the western side. Above this water at 30‹N, a lowdensity
lens of very salty (36.5ñ) but very warm (25‹C) surface
water remains trapped by the circular movement of the major
oceanic surface currents. Between this surface water and the
North Atlantic deep water lies water of intermediate temperature
(10‹C) and salinity (35.5ñ). This water is a mixture of surface
water and the upwelled colder, saltier water from the subtropical
regions. It moves northward to reappear at the surface south of
the convergence in the North Atlantic.
1. Describe the concept of continuity of flow.
2. How do stable and unstable water columns differ?
3. What is the relationship between upwelling and
downwelling? Between surface divergence and convergence?
4. What is the difference between a water type and a
5. Explain how water types with different temperature and
salinity can have the same density.
7.3 The Layered Oceans
Oceanographers have taken salinity and temperature measurements
with depth at many locations and for many years. Gradually,
they accumulated sufficient data to identify the layers of
water that make up each ocean and the surface source of the
water forming each layer. The structure of an ocean is determined
by the properties of these layers. Each layer received its
characteristic salinity, temperature, and density at the surface.
The waterfs density controls the depth to which the water sinks;
the thickness and horizontal extent of each layer are related
to the rate of its formation and the size of the surface source
region. Water that sinks from the surface to spread out at depth
and slowly mixes with adjacent layers eventually rises at another
location. In all cases, water that sinks displaces an equivalent
volume of water upward toward the surface at some other location
so that the oceansf vertical circulation is continuous.
The layers of water just described are associated with depth
zones: surface, intermediate, deep, and bottom. The surface
10 ‹C 34.2ñ
Figure 7.13 North-south cross section of the
Atlantic Ocean illustrating major water masses
and their general movement. Areas of surface
convergence and divergence associated with
downwelling and upwelling are also shown.
190 Chapter 7 Ocean Structure and Circulation
the layers difficult to distinguish. Antarctic bottom water forms
in small amounts along the Pacific rim of Antarctica, but it
is quickly lost in the great volume of the Pacific Ocean. The
deeper water of the South Pacific Ocean is the water of the Antarctic
circumpolar flow. Because the North Pacific is isolated
from the Arctic Ocean, only a small amount of water comparable
to North Atlantic deep water can be formed. In the extreme
western North Pacific, convergence of the southward-flowing
cold water from the Bering Sea and the Sea of Okhotsk and
the northward-moving water from the lower latitudes produces
only a small volume of water that sinks to mid-depths. There is
no large source of deep water similar to that found in the North
Atlantic. Warm, salty surface water occurs at subtropical latitudes
(30‹N and 30‹S) in each hemisphere, and Antarctic intermediate
water is produced in small quantities, but its influence is small.
Deep-water flows in the Pacific are sluggish, and conditions are
very uniform below 2000 m (6600 ft). The slow circulation of
the Pacific means that it has the oldest water at depths where
age is measured as time from the waterfs last contact with the
surface. Residence time for deep water in the Pacific is about
twice that of deep water in the Atlantic.
The Indian Ocean
The Indian Ocean is principally an ocean of the Southern Hemisphere
and has no counterpart of the North Atlantic deep water.
Small amounts of Antarctic bottom water are soon mixed with
the deeper waters to form a fairly uniform mixture of Antarctic
circumpolar water brought into the Indian Ocean by the Antarctic
circumpolar current. There is a small amount of Antarctic
intermediate water, and in the subtropics, a lens of warm, salty
water occurs at the surface.
The Arctic Ocean
The Arctic Ocean basin is unique. About one-third of its area
is covered by extensive continental shelves, the widest of any
ocean basin. Two basins, the Eurasian to the east and the Canadian
to the west, occupy the central portion of the ocean; they
are separated by the Lomonosov Ridge extending due north
The density of Arctic Ocean water is controlled more by
salinity than by temperature. Its surface layer is formed from
low-salinity water entering from the Bering Sea, fresh water
from Siberian and Canadian rivers, and seasonal melting of sea
ice. The surface layer from these combined sources is about
80 m (250 ft) deep and has a low salinity (32.5ñ) and a low
temperature (.1.5‹C). Below the surface layer, salinity increases
with depth in the halocline layer, 200 m (650 ft) thick, to reach
34.5ñ at its base. The cold, salty water of the halocline layer
is produced by annual freezing and formation of sea ice over
the continental shelves bordering the ocean. This water sinks
and moves across the shelves to spread out in the central ocean
basins. West of Spitsbergen, Norway, North Atlantic water (2‹C
and 35ñ) enters the Arctic Ocean and is cooled as it flows
under the halocline and fills the Arctic Ocean basins. This water
Near the equator, the upper boundary of the North Atlantic
deep water is formed by water produced at the convergence
about 40‹S. This is Antarctic intermediate water
(AAIW). Because it is warmer (5‹C) and less salty (34.4ñ) than
the North Atlantic deep water, it is less dense and remains above
the denser and saltier water below. Along the edge of Antarctica,
very cold (.0.5‹C), salty (34.8ñ), and dense water is produced
at the surface by sea ice formation during the Southern Hemispherefs
winter. This is Antarctic bottom water (AABW), the
densest water in the oceans. This water sinks to the ocean floor
and flows slowly northward, creeping beneath North Atlantic deep
water, as it continues on through the deep South Atlantic ocean
basins west of the Mid-Atlantic Ridge. Antarctic bottom water
does not accumulate enough thickness to be able to flow over
the mid-ocean ridge system into the basins on the African side
of the ridge; it is confined to the deep basins on the west side of
the South Atlantic and has been found as far north as the equator.
At the same time, the North Atlantic deep water between
the Antarctic bottom and intermediate waters rises to the oceanfs
surface in the area of the 60‹S divergence. As it reaches the surface,
it splits; part moves northward as South Atlantic surface
water and Antarctic intermediate water; part moves southward
toward Antarctica, to be cooled and modified to form Antarctic
bottom water. A mixture of North Atlantic deep water and
Antarctic bottom water becomes the circumpolar water for the
Southern Ocean that flows around Antarctica. The Antarctic
circumpolar water becomes the source of the deep water found
in the Indian and Pacific Oceans. In this way, the Atlantic Ocean
and its circulation play a defining role in the structure and circulation
of all the oceans.
Warm (25‹C), salty (36.5ñ) surface water in the South
is also caught by the circular current pattern at the
surface and is centered about 30‹S. South of the southern tips
of South America and Africa, the water flows eastward, driven
by the prevailing westerly winds, which move the water around
and around Antarctica.
Water from the Mediterranean Sea has a temperature of about
13‹C and a salinity of 37.3ñ as it leaves the Strait of Gibraltar.
This water, mixing with Atlantic Ocean water, forms an intermediate
density water, Mediterranean Intermediate Water (MIW),
also known as Mediterranean Outflow Water (MOW), that sinks in
the North Atlantic to a depth of approximately 1000 m (3300 ft).
The influence of Mediterranean water can be traced 2500 km
(1500 mi) from the Strait of Gibraltar before it is lost through
modification and mixing.
FOR MORE INFORMATION ON THIS PAPER VISIT……
Because the Atlantic Ocean is a narrow, confined ocean of
relatively small volume but great north-south extent, the water
types are readily identifiable and the movement of the layers can
be followed quite easily. In addition, the bordering nations of the
Atlantic have had a long-standing interest in oceanography, so
the vertical circulation and layering of the Atlantic are the most
studied and the best understood of all the oceans.
The Pacific Ocean
In the vast Pacific Ocean, waters that sink from relatively small
areas of surface convergences lose their identity rapidly, making
7.4 What Drives the Surface Currents? 191
7.4 What Drives the
When the winds blow over the oceans, they set the surface water
in motion, driving the large-scale surface currents in nearly
constant patterns. The density of water is about 1000 times
greater than the density of air, and once in motion, the mass of
the moving water is so great that its inertia keeps it flowing.
The currents flow more in response to the average atmospheric
circulation than to the daily weather and its short-term changes;
however, the major currents do shift slightly in response to seasonal
changes in the winds. The currents are further modified by
interactions between the currents and along zones of converging
and diverging water. The major surface currents have been called
the rivers of the sea; they have no banks to contain them, but
they maintain their average course.
Because the frictional coupling between the ocean water
and Earthfs surface is small, the moving water is deflected by
the Coriolis effect in the same way that moving air is deflected
(see chapter 6). But because water moves more slowly than air,
it takes longer to move water the same distance as air. During
this longer time period, Earth rotates farther out from under the
water than from under the wind. Therefore, the slower-moving
water appears to be deflected to a greater degree than the overlying
air. The surface-current acted upon by the Coriolis effect is
deflected to the right of the driving wind direction in the Northern
Hemisphere and to the left in the Southern Hemisphere. In
the open sea, the surface flow is deflected at a 45‹ angle from
the wind direction, as shown in figure 7.14.
upwells along the edge of the continental shelves, mixing with
the water formed during freezing, and exits the Arctic as water
of 0.5‹C temperature and 34.9ñ salinity along the edge of the
shelf adjacent to Greenland. This exiting water moves south
along the coast of Greenland and enters the North Atlantic south
of Greenland and Iceland, where it combines with Gulf Stream
water to form North Atlantic deep water.
Mixing between waters in the ocean is most active when turbulence
and energy of motion are available to stir the waters and
blend their properties. At the sea surface, wind-driven waves and
currents supply energy for mixing, and the tides create currents
at all depths. The large eddies that may form at the boundaries of
currents also stir together dissimilar waters, acting to homogenize
them. When surface currents coverage, mixing at current boundaries
may produce caballing of the mixed water. When currents
and their associated turbulence are weak, mixing is reduced.
Mixing by diffusion occurs continually at the molecular level,
but diffusion is much weaker than mixing by turbulent processes.
If a parcel of water is displaced vertically by turbulence,
buoyancy forces tend to return the parcel to its original density
level. Therefore, vertical mixing between the water types that
form the oceansf internal layers is weak. Horizontal mixing is
more efficient because it requires less energy than vertical mixing.
A parcel of water displaced horizontally along a surface of constant
density remains at its new position and shares its properties
with the surrounding water.
In areas under warm, high-salinity surface water with an
appreciable salinity and temperature decrease with depth,
internal vertical mixing processes occur despite the stability
of the water column. Vertical columnar flows, approximately
3 cm (5 in) in diameter, are called salt fingers; they develop
and mix the water vertically, causing a stair-step salinity and
temperature change with depth. This phenomenon is caused by
the ability of seawater to gain or lose heat faster by conduction
than it gains or loses salt by diffusion. This causes the density
of the vertically moving water to change relative to that of
the surrounding water, and the moving water is propelled either
up or down. Salt fingers mix water over limited depths, creating
homogeneous layers 30 m (100 ft) thick. These layers exist
from about 150.700 m (500.2300 ft) deep and are estimated
to occur over large areas of the oceans when the required
conditions are present.
1. Identify the water masses of the Atlantic Ocean. What is
the origin of each? In which direction does each flow?
2. Why is the layering of the Pacific Ocean less dramatic than
the layering of the Atlantic Ocean?
3. In what ways is the water of the Indian Ocean similar to
the water of the South Atlantic Ocean?
4. What is the origin of Arctic Ocean deep water?
Figure 7.14 The Ekman spiral and Ekman transport. Water motion
in the surface gEkman Layerh is due to wind stress.
192 Chapter 7 Ocean Structure and Circulation
The Ekman Spiral and Ekman Transport
Wind-driven surface water sets the water immediately below it
in motion. But because of low-friction coupling in the water, this
next deeper layer moves more slowly than the surface layer and
is deflected to the right (Northern Hemisphere) or left (Southern
Hemisphere) of the surface-layer direction. The same is true for
the next layer down and the next. The result is a spiral in which
each deeper layer moves more slowly and with a greater angle
of deflection to the surface flow (fig. 7.14). This current spiral is
called the Ekman spiral, after the physicist V. Walfrid Ekman,
who developed its mathematical relationship. The spiral extends
to a depth of approximately 100.150 m (330.500 ft), where the
much-reduced current will be moving in the opposite direction
to the surface current. The surface layer of water corresponding
to the Ekman spiral is known as the Ekman layer. Over the
depth of the spiral, the average flow of the water set in motion
by the wind, or the net flow, moves 90‹ to the right (Northern
Hemisphere) or left (Southern Hemisphere) of the surface wind.
This is known as Ekman transport. This relationship is in
contrast to the surface water, which moves at an angle of 45‹ to
the wind direction.
The major surface currents in the ocean are driven primarily
by the trade winds, blowing in a westerly direction toward the
equator, and the westerlies, blowing in an easterly direction
away from the equator (review figure 6.20). Surface water is
driven at a 45‹ angle to the direction of these winds (fig. 7.15).
Thus, the trade winds drive surface currents that flow from
east to west on either side of the equator. When these currents
reach the western boundaries of ocean basins, they are deflected
away from the equator and move to higher latitudes
where they enter the region of the westerlies. Once
under the influence of the westerlies, the surface
currents are then driven back across the ocean
basins from west to east. When these eastwardflowing
currents reach the eastern boundaries of
the ocean basins, they are largely deflected back
toward the equator where they again come under
the influence of the trade winds. This completes a
full cycle of surface currents that rotate clockwise
in the Northern Hemisphere and counterclockwise
in the Southern Hemisphere. These large, circularmotion,
wind-driven current systems are known
as gyres (fig. 7.16). In high southern latitudes,
no land separates the Atlantic, Pacific, and Indian
Oceans; here, the surface currents, driven by the
westerlies, continue around Earth in a circumpolar
flow around Antarctica.
Because of Ekman transport, a portion of the
wind-driven surface water
center of each of the large, circular current
Figure 7.15 Wind-driven transport and resulting surface currents
in an ocean bounded by land to the east and to the west. The currents
form large oceanic gyres that rotate clockwise in the Northern
Hemisphere and counterclockwise in the Southern Hemisphere.
Figure 7.16 There are five major ocean gyres: (a) Indian Ocean, (b) North Pacific,
(c) South Pacific, (d) North Atlantic, and (e) South Atlantic gyres. Each has a strong and
narrow gwestern boundary current,h and a weak and broad geastern boundary current.h
just described (fig. 7.16). A convergent lens of surface
elevated as much as 2 m (6.5 ft) above the equilibrium
and this lens depresses the underlying denser water.
7.5 Ocean Surface Currents 193
7.5 Ocean Surface Currents
The currents that make up the large oceanic gyre systems and
other major currents have been given names and descriptions
based on their average positions. These are presented here ocean
by ocean and can be followed on figure 7.18. As you follow
these current paths, review their associations with the large gyre
systems and their overlying wind belts.
Pacific Ocean Currents
In the North Pacific Ocean, the northeast trade winds push
the water toward the west and northwest; this is the North
Equatorial Current. The westerlies create the North Pacific
Current, or North Pacific Drift, moving from west to east.
Note that the trade winds move the water away from Central
and South America and pile it up against Asia, whereas the
westerlies move the water away from Asia and push it against
the west coast of North America. The water that accumulates in
one area must flow toward areas from which the water has been
removed. This movement forms two currents: the California
Current, moving from north to south along the western coast of
North America, and the Kuroshio Current, moving from south
to north along the east coast of Japan. The Kuroshio and California
Currents are not wind-driven currents;
they provide continuity of flow and complete
a circular motion centered around
30‹N latitude. This circular, clockwise
flow of water is called the North Pacific
gyre. Other major North Pacific currents
include the Oyashio Current, driven by
the polar easterlies, and the Alaska Current,
fed by water from the North Pacific
Current and moving in a counterclockwise
gyre in the Gulf of Alaska. Little
exchange of water occurs through the
Bering Strait between the North Pacific
The thickness of the surface lens is about 1000 times greater
than the elevation of the lens above sea level. This is because the
difference in density between the surface water and the deeper
water is only about 1/1000 of the density difference between
air and water at the sea surface. The surface slope of the mound
increases as deflected water moves inward until the outward
pressure driving the water away from the gyre center equals the
Coriolis effect, acting to deflect the moving water into the raised
central mound. At this balance point, geostrophic flow is said
to exist, and no further deflection of the moving water occurs.
Instead, the currents flow smoothly around the gyre parallel to its
elevation contours. See figure 7.17 for a diagram of this process.
1. If a north wind blows across the sea surface, what
direction does the surface current flow in the Northern
Hemisphere? In the Southern Hemisphere?
2. What is the direction of Ekman transport in question 1?
3. Why is the sea surface elevated in the interior of the major
4. Why do gyres rotate in opposite directions in the two
V Fc Fg
Fc Fc Fc
F Fg F g g
Lens of accumulated
Figure 7.17 Geostrophic flow (V) exists around a
gyre when Fc, the inward deflection force due to the
Coriolis effect, is balanced by Fg, the outward-acting
pressure force created by the elevated water and
gravity. This example is of a clockwise gyre in the
Oyashio BBeerriinngg Alaska
Current Equatorial Counter
South Equatorial Current
East Australian Peru
Antarctic Circumpolar Antarctic Circumpolar
Figure 7.18 The long-term average flow of
the major wind-driven surface currents.
194 Chapter 7 Ocean Structure and Circulation
ward movement of South Atlantic surface water across the
in a net flow of surface water from the Southern
to the Northern Hemisphere. This flow is balanced
by a flow of water at depth from the Northern Hemisphere to the
Southern Hemisphere. This deep-water return flow is the North
Atlantic deep water. Again, the equatorial
currents are displaced
northward, although not as markedly as in the Pacific Ocean.
The Sargasso Sea marks the middle of an ocean gyre. It is
located in the central North Atlantic Ocean, and its boundaries
are the Gulf Stream on the west, the North Atlantic Current
to the north, the Canary Current on the east, and the North
Current to the south. The circular motion of the
a lens of clear, warm, downwelling water
1000 m (3000 ft) deep. The region is famous for the floating
mats of Sargassum,
a brown seaweed, stretching across its
surface. The extent of the floating seaweed frightened early
sailors, who told stories of ships imprisoned by the weed and
sea monsters lurking below the surface. Except for the floating
Sargassum, with its rich and specialized
the clear water is nearly a biological
Indian Ocean Currents
The Indian Ocean is mainly a Southern Hemisphere ocean. The
southeast trade winds push the water to the west, creating the
South Equatorial Current. The Southern Hemisphere westerlies
still move the water eastward in the West Wind Drift.
The gyre is completed by the West Australia Current moving
northward and the Agulhas Current moving southward
along the east coast of Africa. Because this is a Southern
Hemisphere ocean, the currents are deflected left of the wind
direction, and the gyre rotates counterclockwise. The northeast
trade winds in winter drive the North Equatorial Current to
the west, and the Equatorial Countercurrent returns
eastward toward Australia. Again, these equatorial currents
are displaced approximately
5‹N. With the coming of the wet
and its west winds, these currents are reduced.
The strong seasonal
monsoon effect controls the surface flow
of the Northern
Hemisphere portion of the Indian Ocean. In
the winds blow the surface water eastward, and
in the winter, they blow it westward (fig. 7.19). This strong
seasonal shift is unlike
anything found in the Atlantic or the
Arctic Ocean Currents
The relentless drift of water and ice in the Arctic Ocean moves
in a large clockwise gyre driven by the polar easterly winds.
This gyre is centered not on the North Pole, as early explorers
but is offset over the Canadian basin at 150‹W and
80‹N (fig. 7.20). Although the currents and the winds move the
ice slowly at 0.1 knot (2 mi/day), Arctic explorers trying to reach
the North Pole found that they traveled south with the drifting ice
and water at speeds almost equal to their difficult progress north.
The Arctic Ocean is supplied from the North Atlantic
by the Norwegian Current; some of this flow enters west of
and the Arctic Ocean; no current exists that is comparable to the
Oceanfs Norwegian Current, which moves warm water
to the Arctic Ocean.
In the South Pacific Ocean, the southeast trade winds move
the water to the left of the wind and westward, forming the South
Equatorial Current. The westerly winds push the water to the
east. The current formed, the Antarctic Circumpolar Current, moves
continuously around Earth. The tips of South America
deflect a portion of this flow northward on the east sides of the
South Pacific and South Atlantic Oceans. As in the North Pacific,
continuity currents form between the South Equatorial Current
and the Antarctic Circumpolar Current. The Peru Current, or
Humbolt Current, flows from south to north along the coast of
South America, while the East Australia Current can be seen
moving weakly from north to south on the west side of the ocean.
These four currents form the counterclockwise South Pacific gyre.
The North Pacific and South Pacific gyres form on either side
of 5‹N because the meteorological equator, or doldrums belt, is
displaced northward from the geographic equator (0‹), owingto
the unequal heating of the Northern and Southern Hemispheres.
Also between the North and South Equatorial Currents, in the
zone of the doldrums is a current moving in the opposite direction,
from west to east. This is a continuity current known as the
Equatorial Countercurrent, which helps to return accumulated
surface water eastward across the Pacific. Under the South Equatorial
Current is a subsurface current flowing from west to east
called the Cromwell Current. This cold-water continuity
also returns water accumulated in the western Pacific.
Atlantic Ocean Currents
The North Atlantic westerly winds move the water eastward
as the North Atlantic Current, or North Atlantic Drift. The
northeast trade winds push the water to the west, forming the
North Equatorial Current. The north-south continuity currents
are the Gulf Stream, flowing northward along the coast of
North America, and the Canary Current, moving to the south
on the eastern side of the North Atlantic. The Gulf Stream is
fed by the Florida Current and the North Equatorial Current.
The North Atlantic gyre rotates clockwise. The polar easterlies
provide the driving force for the Labrador and East Greenland
Currents, which balance water flowing into the Arctic Ocean
from the Norwegian Current.
In the South Atlantic, the westerlies continue the West Wind
Drift. The southeast trade winds move the water to the west, but
the bulge of Brazil deflects part of the South Equatorial Current
northward into the Caribbean Sea and eventually into the Gulf of
Mexico, where it exits as the Florida Current and joins the Gulf
Stream. A portion of the South Equatorial
Current moves south
of the Brazilian bulge along the western side of the South Atlantic
to form the Brazil Current. The Benguela Current moves
along the African coast. The South Atlantic gyre is
complete, and it rotates counterclockwise.
Because much of the South Equatorial Current is deflected
across the equator, the Equatorial Countercurrent appears only
weakly in the eastern portion of the mid-Atlantic. The north195
Spitsbergen, but most flows along the coast of Norway and
moves eastward along the Siberian coast into the Chukchi Sea.
A small inflow of water entering the Arctic through the Bering
Strait brings water from the Bering Sea to join the eastward
flow along Siberia and the large Arctic gyre. The western side
of the gyre crosses the center of the Arctic Ocean to split north
of Greenland. Here, the larger flow forms the East Greenland
Current flowing south and taking Arctic Ocean water into the
North Atlantic. The lesser flow moves along the west side of
to join the Labrador Current and move south along
the Canadian coast.
Outflow from Siberian rivers is caught in the eastward
flow of water and ice along Siberia. Eventually, this discharge
joins the gyre, distributing sediments and pollutants throughout
Northeast Monsoon (January)
Southwest Monsoon (July)
North Equatorial Current
South Equatorial Current
Southwest Monsoon Current
South Equatorial Current
t Monsoon (July)
Figure 7.19 Indian Ocean monsoonal circulation. In the winter, high pressure over the continent creates dry monsoon winds roughly out of
the northeast that drive water in the Northern Hemisphere portion of the ocean to the west. In the summer, low pressure over the continent
creates wet monsoon winds roughly out of the southwest that reverse the surface currents in the North Hemisphere portion of the ocean.
Figure 7.20 The circulation in the Arctic Ocean is driven by the
polar easterlies, which produce a large, clockwise gyre. Water enters
the Arctic Ocean from the North Atlantic by way of the Norwegian
Current and exits to the Atlantic by the East Greenland Current and
the Labrador Current.
East Greenland C.
West Greenland C.