Download Salinity Patterns in the Ocean

Salinity Patterns in the Ocean
Lynne D Talley
Volume 1, The Earth system: physical and chemical dimensions of global environmental change,
pp 629–640
Edited by
Dr Michael C MacCracken and Dr John S Perry
in
Encyclopedia of Global Environmental Change
(ISBN 0-471-97796-9)
Editor-in-Chief
Ted Munn
 John Wiley & Sons, Ltd, Chichester, 2002
Salinity Patterns in the
Ocean
Lynne D Talley
Scripps Institution of Oceanography, University of
California, La Jolla, CA, USA
Ocean salinity varies geographically and with time. Fresh
water input occurs at the sea surface due to precipitation
and river inflow, reducing salinity. Salinity is increased by
evaporation and also as a by-product of sea ice formation. Because freshening and salinification occur in different
places, salinity at a particular location reflects the upstream
source of the water there. In subtropical latitudes, high surface evaporation creates high salinity near the sea surface.
In subpolar latitudes, high precipitation creates low salinity
near the sea surface. As these waters flow into the ocean
interior, they create layers of high and low salinity.
At mid-depth (i.e., around 1000 to 2000 m deep), outflows
from the highly evaporative Mediterranean and Red Seas
create a vertical salinity maximum in the North Atlantic
and Indian Oceans, respectively. Also at mid-depth in the
subtropical and tropical regions, the relatively fresh, but
dense, surface water from higher latitudes flows in and
creates a vertical salinity minimum, most prevalent in the
Southern Hemisphere and North Pacific. The North Atlantic
is the most saline ocean and the North Pacific the freshest.
Salinity affects sea water density and therefore can be a
controlling factor for the depth of the ocean surface mixed
layer. This depth also depends on wind speed and cooling at
the sea surface. Feedbacks can occur as precipitation and
evaporation are affected by the atmosphere, which in turn is
affected by sea surface temperature. The atmosphere in turn
is affected by sea surface temperature, which is affected by
mixed layer depth.
The distribution of dissolved salts (see Salinity, Volume 1)
in the oceans and adjacent seas varies in space and time.
Salinity is changed near the sea surface by precipitation
and evaporation of fresh water and by salty water produced
sea ice forms and excludes salt. Geographical variations
in inputs create regional differences in salinity at the sea
surface. As seawater circulates down into the ocean away
from the sea surface, it carries these differences along,
creating large-scale salinity patterns throughout the ocean.
Changes in time in the inputs at the sea surface also
affect the salinity distribution. By mapping the salinity
distribution and tracking changes in salinity over time,
much insight is gained into both the basic ocean circulation
and the processes that change the ocean circulation and
temperature distribution.
Salinity is a seawater property related to the amount of
matter, mainly consisting of salts, dissolved in the water
(see Salinity, Volume 1). The original definition of salinity was in terms of grams of dissolved salts per kilogram
of seawater. Salinity is now defined in terms of seawater
conductivity, and is currently calibrated using a standard
solution prepared at a laboratory in the UK. The quantity of
dissolved salts, hence the salinity, affects seawater density,
which also depends on temperature and pressure. Quantifying these dissolved constituents as the single parameter,
salinity, is possible because the source of solids is weathering, which occurs on geological time scales. Ocean circulation and mixing, on the other hand, distribute the solids
throughout the World Ocean on a time scale of no more than
thousands of years, and so the various salt constituents are
essentially well mixed. Therefore, salinity differences are
created only by dilution or concentration as fresh water is
added or removed, or as salty water is rejected from sea ice
as it freezes. Other dissolved non-salt constituents such as
carbon, calcium and silica, which are affected by biological
cycles, have enough spatial variability to affect seawater
density (e.g., Millero et al., 1978); this small variation is
not usually included in salinity and density studies.
In many ocean regions, particularly in the tropics, the
subtropics and the deep ocean, temperature variations are
more important than salinity in changing density. However,
at higher latitudes, the presence of freshened surface waters
resulting from high precipitation and/or ice melt has a
strong influence on whether the water column can be cooled
enough to mix vertically with other layers. Because winter
surface cooling at high-latitudes determines water properties for the global ocean at depths below about 1500 m,
salinity can control the occurrence of convection and its
penetration depth. With a surface layer that is relatively
fresh compared with the underlying water that came from
another ocean region, surface waters can cool to freezing
without overturning. If the high-latitude surface waters are
not much freshened, as is the usual case in the northern
North Atlantic, then overturn to great depth at the end of
winter can occur with even modest surface cooling, leaving
the surface waters well above the freezing point. Because
ocean surface temperatures are part of the forcing for the
atmosphere, changes in high-latitude salinity distribution
can be involved in climate change.
In addition to its effect on seawater density, salinity is
useful in itself as a tracer of ocean circulation and mixing.
Salinity is established in the ocean’s surface layer by local
processes, and therefore salinity variations within the ocean
depend on the surface origin of a parcel of water. Mixing between waters of different salinities occurs within the
ocean, and so the original salinity changes gradually. However, mixing rates are low enough that salinity (and other
chemical tracers) can provide a great deal of information
on the pathways of flow.
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THE EARTH SYSTEM: PHYSICAL AND CHEMICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE
SEA SURFACE FORCING OF SALINITY:
PRECIPITATION, EVAPORATION, RUNOFF
AND ICE PROCESSES
The total amount of salt in the ocean is constant on time
scales of millions of years, and hence is assumed constant
for studies at almost all time scales. However, the total
amount of water in the oceans varies daily due to exchange
at the sea surface. Rainfall, runoff from land, and ice melt
all reduce salinity by increasing the amount of fresh water.
Evaporation increases salinity by decreasing the amount of
fresh water.
Ice formation also locally increases salinity through a
process called brine rejection. As ice forms from seawater,
the dissolved salts are rejected from most of the ice and collect in pockets in the ice. This briny water drips through the
base of the forming ice. More brine is released during slow
ice formation than if the ice forms quickly, so the strength
of brine rejection depends on the local air temperature. The
brine mixes with the seawater beneath the ice, raising the
seawater’s salinity. The net salinity increase depends on the
thickness of the mixing layer below the ice. In shallow locations, such as on shallow continental shelves, salinity might
increase more than over a deep bottom where mixing occurs
to greater depth. Because the water below sea ice is usually
very cold, the addition of salty brine may increase salinity
without increasing temperature. This results in a density
increase in the seawater. This denser water then sinks until
it reaches a depth where surrounding waters have the same
density. Because these deeper waters may have come, at
least in part, from somewhere else (e.g., in the Arctic from
the North Atlantic Ocean), these waters will usually not be
as cold as the new ice-rejected brine. Although warmer,
their density will be equal to the ice-rejected brine because
they are even saltier. That is because the ice-rejected brine
was formed from near-freezing surface waters that initially
had much lower salinity (e.g., because they had a higher
proportion of meltwater and river runoff), and therefore
lower density, than the deeper waters. Although it seems
paradoxical, the ice formation and brine rejection processes
are thus able to create the coldest and freshest waters on a
surface of constant density within the ocean.
Because sea ice has lower salinity than the original
seawater from which it formed, melting of sea ice creates
a fresh surface layer of low-density waters that float over
the more saline waters below.
Ocean salinity patterns at the largest scales are affected
primarily by the annual average of evaporation and precipitation, with additional major effects from ice melt and
freezing at high-latitudes. Evaporation and precipitation,
usually reported in cm year1 , are primarily calculated from
observations collected on a routine basis by most ships. The
difference between evaporation (E) and precipitation (P)
has been calculated from the complete historical data set
compiled from these ship observations (Figure 1). The
range of E–P is about 10 to 10 cm year1 . There is more
evaporation than precipitation in the subtropical regions
between about 15° and 40 ° S and N latitude, under the atmosphere’s large-scale high pressure centers, with descending,
dry air. At higher latitudes, there is more precipitation than
evaporation, under the atmospheric low-pressure centers.
In the tropics, especially at about 10 ° N over the Pacific
and Atlantic, surface air rises in the intertropical convergence zone (ITCZ), forming enormous cumulus clouds that
produce large amounts of rain.
Runoff from land affects the coastal salinity distribution.
On a global scale, runoff is most important at the major
river outlets, indicated in Figure 1. The net fresh water input
is not shown in Figure 1, but the effect of these major rivers
is apparent in the surface salinity distribution considered
next.
SURFACE SALINITY DISTRIBUTION
Salinity at the sea surface (Figure 2) resembles the evaporation–precipitation pattern, with some differences due to
runoff, ocean currents and ice melt. The total salinity range
of the open ocean is about 30–40 parts per thousand or ‰
(see Salinity, Volume 1). The international convention is
that salinity has no units; its value is approximately grams
salt per kilogram of sea water. Many authors continue to use
the practical salinity unit (psu) rather than leaving the quantity unitless. While salinity in coastal estuaries can be much
lower, it does not affect large oceanic-scale patterns. Lowest salinities occur in the Arctic and Antarctic where there is
both net precipitation and seasonal ice melt. Highest salinities occur in the Red Sea and Persian Gulf, both located
in the northwestern Indian Ocean, where net evaporation is
high. High salinity is also found in the Mediterranean Sea.
In the open ocean, high salinity occurs in the subtropical
areas of net evaporation seen in Figure 1. A band of low
salinity underlies the ITCZ at 10 ° N.
The effect of continental runoff is apparent in lowered
surface salinity near the mouths of major rivers such as the
Amazon and the Congo and the numerous large rivers that
empty into the Bay of Bengal, east of India, including the
Ganges and Brahmaputra. Runoff from many rivers around
the Gulf of Alaska in the northeastern Pacific and around
the Arctic Ocean is important in the lowered salinities of
high-latitude ocean regions.
VERTICAL STRUCTURE OF THE SALINITY
DISTRIBUTION
Waters at the sea surface flow down into the interior.
Salinities from the sea surface (Figure 2) carried with
this flow into the ocean interior are helpful in identifying
sources and pathways of the circulation. The two primary
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Figure 1 Evaporation minus precipitation (cm year1 ), annual average. The major rivers of the world are also indicated. (Reproduced using data from
the National Centers for Environmental Prediction Reanalysis Project, Kalnay et al., 1996)
SALINITY PATTERNS IN THE OCEAN
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THE EARTH SYSTEM: PHYSICAL AND CHEMICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE
0°
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SALINITY PATTERNS IN THE OCEAN
mechanisms for this downward flow are: (1) flow following
constant density surfaces that slope gently into the ocean
interior, and (2) vertical convection and brine rejection at
higher latitudes.
Flow down along constant density surfaces is primarily
through a process called subduction, occurring in subtropical regions between about 15° and 40 ° S and N latitude.
In this process, waters in the surface mixed layer circulate equatorward, encountering warmer, less dense waters
to the south, which they are carried beneath. Surface waters
are most saline in the middle of these subtropical regions.
Therefore, all equatorward subtropical regions are characterized by a subsurface vertical maximum in salinity caused
by subduction of more saline waters from the center of
the gyre, as described later (see Ocean Circulation, Volume 1).
Convection occurs in many places, but reaches most
deeply into the ocean at high latitudes. Convection is not
so much a downdraft of water from the surface as it is
a deep mixing, which brings surface water to great depth,
from whence the water spreads laterally along constant density surfaces. Major deep convection areas can be identified
from subsurface properties, such as the salinity distributions
discussed below. In the North Atlantic and Arctic, these
sites are in the Labrador Sea between Labrador and Greenland, in the Greenland Sea east of Greenland and north of
Iceland, and in the Mediterranean Sea. In the North Pacific,
convection in the Okhotsk and Japan Seas is significant
in modifying water properties at mid-depth. In the Southern Hemisphere, convection occurs all around Antarctica at
about 40° to 45 ° S, which is just north of the major current
called the Antarctic circumpolar current. Brine rejection
is important for subsurface water properties primarily in
the Antarctic (Weddell and Ross Seas and other locations
around the continent), the Okhotsk Sea, and the interior of
the Arctic Ocean.
The following definitions are useful for discussing the
salinity distribution in the vertical: (1) halocline – where
the salinity value changes rapidly in the vertical in comparison with changes above and below the halocline;
(2) halostad – where salinity changes very slowly in the
vertical; (3) intermediate water – usually referring to a
minimum or maximum in salinity in the vertical which
is found over a large horizontal area at mid-depth in the
ocean. Equivalent terms for vertical variations in temperature and density are thermocline, thermostat, pycnocline
and pycnostad.
SUBTROPICAL SALINITY STRUCTURE
In the subtropical regions of every ocean, salinity (Figures 3
and 4a) is high close to the sea surface due to strong
evaporation. At the location of the highest surface salinity
(Figure 2), the highest salinity in vertical profiles is at the
5
sea surface. This high salinity water is carried downward
and equatorward through subduction. Therefore, equatorward of the center of the highest surface salinity, there
is a vertical maximum in salinity. This salinity maximum
is at about 50 m depth, which is very shallow compared
with the ocean depth. These subsurface salinity maxima
are sometimes called Subtropical Underwater (Worthington, 1976).
At all locations in the subtropical gyres, and below the
near-surface salinity maximum, salinity decreases smoothly
with depth to a salinity minimum. Temperature also
decreases smoothly and rapidly (Figure 4b). This layer of
changing properties is called the thermocline (region of
high temperature change in the vertical), and is also often
called central water. Central water is the subducted water of
the subtropical gyre; the colder waters come from farther
north within the subtropical circulation, with little input
from the subpolar region. It is clear from Figure 4(b) that
the North Atlantic central waters are the most saline and
the North Pacific the freshest.
Below the central waters, at depths between about
500 and 1500 m, each subtropical gyre (except in the
North Atlantic) has a vertical minimum in salinity. The
salinity minimum in the North Pacific is called North
Pacific intermediate water (Reid, 1965). It arises from
cold, low salinity surface waters in the subpolar region
of the North Pacific, specifically in the Okhotsk Sea
and near the Kuril Islands (Talley, 1991, 1993). These
waters flow southward along the western boundary in the
Oyashio current and enter the subtropical circulation east
of Japan. The existence of the low salinity layer throughout the subtropical circulation reflects the presence of
this subpolar water underlying the waters from subtropical
sources.
The salinity minimum in the Southern Hemisphere is
called Antarctic intermediate water (Reid, 1965). It comes
from low salinity surface waters found just west of Chile
and just east of Argentina (McCartney, 1977). Antarctic
intermediate water in the Pacific Ocean originates primarily from the Chilean source. The Atlantic and Indian Ocean
Antarctic intermediate waters originate from the Argentinean source. The Argentinean waters arise from Chilean
surface waters, which flow eastward through the northern
side of Drake Passage between South America and Antarctica. The Chilean waters are modified somewhat during
their passage into the Atlantic. Therefore, the properties of
the Atlantic and Indian Antarctic intermediate waters differ slightly from those of the Pacific Antarctic intermediate
water.
The salinity structure in the subtropical North Atlantic
is somewhat different. While there is a salinity minimum
at the base of the central water, this results primarily
from the northernmost subducted waters of the subtropical
region, and is not the major fresh intermediate water of
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Figure 3 (a) Vertical cross-section of the Atlantic Ocean, along about 20 ° W. (b) Vertical cross-section of the Pacific Ocean, along about 150 ° W. (c) Vertical
cross-section of the Indian Ocean, along about 90 ° E. (Reproduced using data from the World Ocean Circulation Experiment)
THE EARTH SYSTEM: PHYSICAL AND CHEMICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE
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384.9
34.45
34.65
34.7
34.5 34.35
34.65
34.5534.6
34.55
34.74
1000
34.7 34.76
34.8
34.35
500
334.477 34.71
35.1
34.934.75
33.47
33.46
34.71
34.65 34.7
34.78
34.4
34.76
34.73
34.45
34.6
34.5
34.74
34.71 34.65
1500
34.71
34.7
34.73
34.71
34.73
34.74
34.72
34.74
34.72
2000
34.7
34.73
34.72
2500
34.71
34.7
34.74
3000
34.73
3500
34.72
34.72
4000
4500
5000
1
5500
6000
km 0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
6000
6500
7000
7500
8000
8500
9000
9500
Lat
(c)
Figure 3
−60
(Continued )
−55
−50
−45
−40
−35
−30
−25
−20
−15
−10
−5
0
5
10
15
37.00
36.90
36.80
36.70
36.60
36.50
36.40
36.30
36.20
36.10
36.00
35.90
35.80
35.70
35.60
35.50
35.40
35.30
35.20
35.10
35.00
34.90
34.80
34.78
34.76
34.74
34.73
34.72
34.71
34.70
34.65
34.60
34.55
34.50
34.45
34.40
34.35
34.30
34.20
34.10
34.00
33.90
33.80
33.70
33.60
33.50
33.40
33.30
33.20
33.10
33.00
32.80
32.60
32.40
32.20
32.00
0.10
0.00
THE EARTH SYSTEM: PHYSICAL AND CHEMICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE
77
8
0
SALINITY PATTERNS IN THE OCEAN
0
N.Pacific
500
N.Atlantic
Depth
1000
1500
2000
S.Atlantic
S.Pacific
Indian
2500
3000
33.5
34.0
34.5
35.0
35.5
36.0
36.5
37.0
Salinity
(a)
25
N.Pacific
Potential temperature
20
9
In the Indian Ocean, a dense, high salinity source in the
Red Sea creates a high-salinity intermediate water in the
north, similar to the Mediterranean water of the Atlantic.
Because there are no northern high-latitude sources of water
in the Indian Ocean, there is no fresh intermediate layer
arising from the north. The fresh water at the surface in the
Bay of Bengal arising from major river runoff is much too
warm to sink to mid-depth. Thus, the mid-depth northern
Indian Ocean is dominated by the highly saline outflow
from the Red Sea.
Beneath the low salinity intermediate waters of the North
Pacific and Southern Hemisphere, salinity increases towards
the bottom. The higher salinity of the deep waters of the
Southern Hemisphere and North Pacific results from input
of saline North Atlantic waters. Beneath the saline intermediate water of the North Atlantic and tropical Indian
Oceans, salinity decreases to the bottom. In the South
Atlantic Ocean, low-salinity intermediate water (Antarctic
intermediate water) lies above higher salinity deep waters
from the North Atlantic. Beneath the high-salinity North
Atlantic deep water, salinity decreases to the bottom. The
lower salinity of the bottom waters in the Atlantic and
Indian Oceans results from deepwater sources in the Antarctic that are of lower salinity.
SUBPOLAR SALINITY STRUCTURE
15
N.Atlantic
In high latitude regions dominated by precipitation and ice
melt, salinity is lowest at the sea surface and increases
downward through a halocline (Figure 5, North and South
10
0
5
S.Atlantic
S.Pacific
Indian
500
0
34.0
34.5
35.0
35.5
36.0
36.5
37.0
1000
Salinity
Figure 4 (a) Salinity distribution with depth from each
subtropical gyre. All profiles are chosen from the centers
of the subtropical circulations, at about 25 ° N (Northern
Hemisphere) and 30 ° S (Southern Hemisphere), and from
the center of the basins rather than near the boundaries.
(b) Potential temperature as a function of salinity for each
subtropical ocean basin
Depth
33.5
(b)
1500
2000
N.Atlantic
2500
N.Pacific
S.Pacific
3000
the North Atlantic, which dominates the subpolar region
(described later). The main fresh intermediate water is
displaced in the subtropical gyre by the high salinity outflow
at mid-depth from the Mediterranean Sea. This outflow
creates an intermediate depth salinity maximum in the
vertical.
32.0
32.5
33.0
33.5
34.0
34.5
35.0
35.5
Salinity
Figure 5 Salinity distribution with depth from the subpolar regions. The Northern Hemisphere profiles are from
about 50 ° N. The Southern Hemisphere profile is from the
Pacific Ocean at about 65 ° S
10
THE EARTH SYSTEM: PHYSICAL AND CHEMICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE
Pacific profiles). In the North Pacific and Southern Hemisphere, and also in most of the Arctic Ocean, the halocline
is very sharp, with a large change in salinity over several
tens of meters. Because of the salinity structure, the surface layer can be colder than the water below the halocline.
This pre-conditions the region for sea ice formation. The
cooled surface waters can circulate away from their winter
outcrop locations and appear as a subsurface temperature
minimum between warmer water below from inflow from
other regions and warmer water above from local seasonal
warming.
The North Atlantic’s subpolar gyre is affected by inflow
of a thick layer of saline, warm surface water from the
subtropical gyre. Therefore, the halocline typical of the subpolar Pacific and subpolar Southern Hemisphere is absent
in most of the subpolar North Atlantic (Figure 5, North
Atlantic profile). The saline flow into the central and eastern
subpolar gyre is affected by precipitation, and the surface
waters become gradually fresher as they circulate counterclockwise. When the surface waters enter the Labrador Sea,
they are much fresher than when they first enter the subpolar region. Winter convection to about 1500 m depth in
the Labrador Sea then mixes the relatively fresh water to
intermediate depth. This spreads away from the Labrador
Sea and forms a salinity minimum throughout the subpolar
gyre, beneath the inflowing warm, saline waters. This differs from subpolar regions in the other oceans where the
surface waters are strongly dominated by local fresh water
sources. Therefore, most of the North Atlantic’s subpolar
gyre has a vertical salinity structure similar to a subtropical
structure, with saline waters at the surface and an intermediate depth salinity minimum.
the North Atlantic is the most saline and the North Pacific
the freshest of the major oceans. Because of significant
river inflows, the surface waters of the Arctic are the freshest. The three Southern Hemisphere oceans are connected
around Antarctica and hence their salinities are similar,
midway between the North Atlantic and North Pacific. The
main reason for the differences between the North Pacific
and North Atlantic salinities is the difference in net evaporation per unit area. There is relatively more evaporation in
the North Atlantic’s subtropical regions than in the North
Pacific’s subtropics. The evaporative regions occur where
there are easterly trade winds. The North Atlantic’s trade
winds originate over the deserts of Africa and the MiddleEast. The North Pacific’s trade winds originate over the
much smaller continental areas of Central America and
Mexico. Therefore, the North Atlantic’s trade winds are
drier than the North Pacific’s and cause more evaporation
per unit area. High salinity in the North Atlantic is present
at intermediate depth because of the Mediterranean Sea.
The North Atlantic exchanges water with the Mediterranean
Sea through the Strait of Gibraltar. Evaporation and cooling in the Mediterranean cause the outflowing water to be
much denser than the inflow. This dense outflow sinks to
intermediate depth and creates the mid-depth high salinity that marks North Atlantic waters. This input of high
salinity at mid-depth in the North Atlantic is one reason for the overall higher salinity of the North Atlantic
compared with the North Pacific. Similarly, in the Indian
Ocean, high evaporation in the Arabian region creates very
saline and dense water in the Red Sea, which enters the
Indian Ocean and raises the salinity at mid-depths compared
with the inflowing fresher deep waters from the Antarctic.
TROPICAL SALINITY STRUCTURE
The upper ocean salinity distribution depends on local
precipitation patterns. Thus, a band of low salinity is found
under the high precipitation of the ITCZ at 5–10 ° N in the
Pacific. This band is separated from the saline waters below
it by a halocline. Excess precipitation is also found in the
western Pacific at the equator, creating a halocline there
as well. The eastern equatorial Pacific on the other hand is
dominated by evaporation and so there is no halocline there.
The Bay of Bengal in the Indian Ocean is the site of major
runoff from Asia; this relatively fresh, warm water creates
a shallow halocline throughout the northeast Indian Ocean.
The intermediate and deepwater salinity distributions in
each ocean within 15° of the equator are similar to those
of the adjacent subtropical regions.
GLOBAL SCALE SALINITY PATTERNS
As indicated above, there are significant differences in
salinity between oceans. Considering the total ocean basin,
ROLE OF SALINITY IN CLIMATE CHANGE
Salinity is a major factor in climate change because it
can affect the depth of mixing at the sea surface. If for
instance there is a shallow, fresh layer atop a denser, more
saline layer, even large surface cooling can act only on
the surface layer, creating a large temperature drop without
deep convection. In the absence of a halocline, heat can be
removed from a much thicker layer, which then results in
a smaller temperature change. The processes that set the
salinity of the waters beneath the surface layer, and hence
the strength of the halocline, are complex. This salinity
control of surface-layer processes in the present climate has
been studied, e.g., in the tropical Pacific, as part of the El
Niño and La Niña cycle. In the normal state in the northern
Atlantic, up to and including the region between Greenland
and Norway, surface salinities are not much lower than
those of the underlying waters, permitting overturn to more
than several hundred meters depth. This overturn is part of
the global thermohaline circulation in which deep waters
SALINITY PATTERNS IN THE OCEAN
are produced from surface waters at high-latitudes, mainly
through heat loss to the atmosphere. (The presence of
a permanent and strong halocline in the northern North
Pacific and in the Arctic Ocean prevents deep overturning of
waters there.) During observed decadal climate changes in
the North Atlantic, a halocline sometimes forms as a result
of excessive ice melt and runoff and is associated with
shutdown of deep wintertime convection. This then affects
the sea surface temperature as described previously, and can
then affect the atmosphere. From ocean computer modeling
experiments it is hypothesized that similar controls by
salinity on deep overturn have occurred on much longer
climate timescales.
See also: Broecker, Wallace S, Volume 1; Ocean Circulation, Volume 1.
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