The Great Ocean Conveyor:
Thermohaline Circulation
Spring 2012, Lecture 11
1
Seawater Density
• The density of seawater depends primarily on
two factors
o Temperature
• Cold water is denser than warm water
o Salt content (salinity)
• The more salt that is dissolved in seawater, the denser it
is
2
Temperature Variation
• The temperature of the world's ocean is
variable over the surface of the ocean
o Temperature ranges from less than 0°C (32°F) near
the poles to nearly 30°C (84°F) in the tropics
• Seawater is heated from the surface downward
by sunlight
o At depth, most of the ocean is cold
o 75% of the water in the ocean falls within the
temperature range of −1 to +6°C (30 to 43°F)
3
Salinity
• The amount of salts dissolved in water is called salinity
• Salinity is measured in g per 1000 ml and a special symbol is
used: ‰ by weight.
• ‰ is read “parts per thousand”, or more commonly as parts
per mille
• Open ocean water has an average salinity of about 35 0/00
(equivalent to 3.5%)
o 75 percent of the water in the ocean falls within the salinity range of 34
to 35‰
• Scientists use metric measurements and express quantities of
dissolved substances as grams per liter, or 1000 milliliters
• Thus, we use parts per thousand to express salinity
4
Salinity Variation
•
•
•
•
•
Red Sea = 40‰
Mediterranean Sea = 38‰
Average Seawater = 34.7‰
Black Sea = 18‰
Baltic Sea = 8‰
5
Evolution of a Salty Ocean
• The salinity of the ocean has evolved over time
• Early in earth’s history, the oceans were fresh water
• As the result of water circulation within the hydrolgic
cycle, rainwater falling on land dissolves minute
amounts of salt, and carries it slowly to the ocean
• When ocean water evaporates, it leaves salt behind,
gradually increasing the salinity of the ocean
• Until the solubility limit of some substance dissolved
in the ocean is reached, this process continues to
operate
6
Brackish Water
• As water enters the sea, it mixes with salt
water from the ocean
• Much of this mixing occurs in estuaries, such
as Chesapeake and San Francisco Bays
• Water in estuaries is brackish, meaning it has a
salinity between fresh water and open ocean
seawater
7
Baltic Sea
• Baltic
Sea is a brackish inland sea
• Surface discharge is 940 km3/yr
• Sub-surface flow is 475 km3/yr
• Streams contribute 660 km3/yr
• The inflowing salt water stays deep, beneath a halocline
• The halocline is a density induced barrier between the surface
and deep waters
• Little mixing occurs between so the surface and deep waters,
so the salinity of the surface waters is 8‰
8
Black Sea
• The Black Sea has a net positive outflow of
about 300 km3/yr
• There is a return flow of denser, saline water
from the Aegean Sea, a part of the
Mediterranean Sea
• This accounts for the observed 18‰ salinity
9
Mediterranean Sea
• The
Mediterranean is a nearly
land-locked sea, whose
connection to the Atlantic Ocean
is only 14 kilometers wide
• Evaporation greatly exceeds precipitation and river runoff in
the Mediterranean
• In the eastern half, evaporation is especially high
• This creates a pressure gradient, which pushes low-salinity
water from the Atlantic across the basin
• This water warms, and salinity increases, due to evaporation
• The water sinks and returns westward as a sub-surface flow,
where it enters the Atlantic through the Strait of Gibraltar
• The net salinity is higher than the open ocean at 38‰
10
• The Red Sea is an inlet of the Indian
Ocean, and is one of the most saline
bodies of water in the world
• It is bounded by arid lands, except for
an opening to the Arabian Sea through
the Gulf of Aden
• Salinities range from 36‰ in the south
to 41‰ in the north, with an average of
about 40‰
• The high salinity is the result of
extreme evaporation rates, as much as
• The Red Sea was formed by
205 cm/yr, coupled with very little
Arabia pulling away from
freshwater input, from an annual
Africa
rainfall of about 6 cm
Red Sea
• It is part of the Great Rift
Valley system of East Africa
11
Causes of Salinity Variations
• We have seen that salinity is affected by evaporation,
and by mixing of fresh water with seawater
• It is also affected by the formation of ice
• As ice forms in salt water, there is no room in the
crystal for salt
• Salt is squeezed out of the ice structure and the
resulting ice is less salty than when it began to freeze
• In the polar regions, where seawater freezes to form
sea ice, the ice is not as salty as the seawater from
which it formed
12
Layering in Water
• Layers of differing densities can develop in
water
• If the density difference is primarily due to
temperature, the boundary between layers is
known as a thermocline
• If the density difference is primarily due to
salinity, the boundary between layers is known
as a halocline
13
Circulation in the Ocean
• Currents develop in the ocean for three reason:
o Tidal forces – due to gravitational interactions
between earth, the moon, the sun, and other
planetary bodies in the solar system
o Wind – due to pressure differences within the
atmosphere
o Density differences – due to seawater density
differences caused by differences in temperature,
salinity, or a combination of both
14
Thermohaline Circulation
• Density driven currents form many of the main
circulation patterns in the ocean
• The following slides use images and text from
Climatic Research Unit, School of
Environmental Sciences, University of East
Anglia, UK
15
Gulf Stream
• The Gulf Stream (and
its extension, the North
Atlantic Drift) bring
warm, salty water to the
NE Atlantic, warming
western Europe
• One of the water sources for the waters of the Gulf
Stream is water leaving the Mediterranean Sea
through the Strait of Gibraltar at 38‰
• This increases the initial salinity of water in the
Gulf Stream
16
Mixing with Cold Arctic Water
• The water cools, mixes
with cold water coming
from the Arctic Ocean,
and becomes so dense
that it sinks, both to the
south and east of
Greenland
• Note that red indicates surface flow, while blue
indicates sinking or deep water flow
17
Atlantic
Ocean
• If we zoom out,
we see that this
current is part of
a larger system,
connecting the
North Atlantic...
• ...the tropical
Atlantic...
18
Indian and Pacific Oceans
• the South Atlantic...
• ...the Indian and Pacific
Oceans
19
Below the Surface
• If we look below the
surface, water from the
two main sinking
regions spreads out in
the subsurface ocean...
• ...affecting almost all
the world's oceans at
depths from 1000m and
below...
• Note the sinking of waters off Antarctica
20
Global Thermohaline Conveyor Belt
• The
cold, dense water
gradually warms and returns
to the surface, throughout the
world's oceans
• The surface and subsurface
currents, the sinking regions,
and the return of water to the
surface form a closed loop,
the thermohaline circulation
or global thermohaline
conveyor belt
21
Gulf Stream Evaporation
• Waters in the Gulf Stream are very warm, and a great
deal of water evaporates on the way to the North
Atlantic
• Together with the initially high salinity of this water,
this makes the water much more saline, and thus
denser
• Therefore, the critical part of the thermohaline
circulation (THC) is the sinking in the North Atlantic
Ocean, not in the North Pacific
• This makes the THC appear to be self-sustaining
22
Breakdown of THC
• If some event occurs to break this selfsustaining chain of processes, then there is the
potential for the circulation to break down
rapidly (i.e., over several decades) and to
remain in a reduced-circulation state for
several centuries
• What might cause rapid breakdown?
23
Direct Greenhouse Warming Effect
• If the density of the water in the North Atlantic Ocean
were lowered by adding fresh water (rain) and/or by
warming, rapid collapse of the THC can be
envisioned
• Increased rainfall and warming over the North
Atlantic are both expected as a result of increased
greenhouse gas concentrations
• This allows an argument that global warming may
cause a rapid collapse of the thermohaline circulation
to be proposed
24
Weakening of THC in Climate Models
• Intercomparison of changes in the North
Atlantic THC simulated by a number of
different General Circulation Models as a
response to global warming was done by
Gregory et al. 2005
• It showed that the THC weakens gradually in
all of the climate models that were
investigated, but it collapses in none of them
25
Climate Research Unit Conclusion
• “The majority of climate scientist believe that
a critical change in the THC is unlikely to
occur during this century, but the question
cannot be answered with certainty at present.
Due to the potentially serious impact on our
climate of a collapse of the THC, it must be
regarded as a low-risk, high-impact event that
cannot be ignored.”
26
Can THC Be Shut Off?
• It has been suggested that very rapid climate
change might result from drastic alteration of
the THC, perhaps even shutting it down
temporarily
• The present day climate models cannot show
this
• Is there other evidence?
27
Willi Dansgaard
• Danish paleoclimatologist, 1922 – 2011
• Formerly Professor Emeritus of
Geophysics at the University of
Copenhagen
• Dansgaard demonstrated that measurements of the
trace isotopes oxygen-18 and deuterium (heavy
hydrogen) in accumulated glacier ice could be used as
an indicator of climate and atmospheric environment
28
First Deep Ice Core
• The first polar deep ice core drilling expedition
took place in 1966, with the collection of the
American Camp Century Core from Greenland
• In cooperation with other laboratories, Dr.
Dansgaard and his group performed the first
isotopic analysis of the ice and perfected the
methods to date the ice sheets and measure
acidity and dust records, thus demonstrating its
value as an environmental indicator
29
Hans Oeschger
• Swiss paleoclimatologist, 1927 - 1998
• Professor Emeritus, University of Bern
• Dr. Oeschger and his colleagues developed techniques
for measuring radiocarbon on very small samples of
carbon dioxide, oxygen isotopes, and the radiocarbon
dating of ice
30
Carbon Dioxide Bubbles in Ice Cores
• Their measurement of carbon dioxide
concentrations from air bubbles trapped in ice
revealed for the first time the important role that
the world's oceans play in influencing global
climate
• Thus, it is now widely held that it is oceaninfluenced changes in the levels of atmospheric
gases that support the creation of the great
glacial ice caps
31
Dansgaard-Oeschger Events
• Oeschger began his work on isotopes and greenhouse
gases around the same time as Dansgaard initiated his
studies
• Their combined work documented that abrupt climate
swings are associated with changes in atmospheric
greenhouse gases
• The paradigm has come to be known as "DansgaardOeschger events“, or D-O cycles
32
D-O Cycles
• D-O events seem to occur with a period of
1470 years, plus or minus a few percent
• The basic pattern is gradual cooling, followed
by abrupt warming
• D-O cycles are grouped into clusters, known as
Bond cycles
33
Gerard Bond
• American Geologist, 1940-2005
• Late Doherty Senior Scholar at
Lamont Doherty Earth Observatory
of Columbia University
• Bond compared ice-core data with deep sea
sediments
• He counted the number and kinds of benthic
foraminifera in cores
• When forams die, they leave behind microscopic
shells in the sediment
34
Foram Evidence
• Some forams are associated with warm water,
others with cold, and the shells of the different
types are recognizable
• Thus Bond discovered a crude
paleothermometer, which could differentiate
warm periods from cold periods
• Bond discovered that after a large warming
event, the next several D-O cycles would grow
progressively colder
35
Termination of a Bond Cycle
• In the middle of the coldest D-O event in a
Bond Cycle it was observed that rock
fragments of continental origin are found in
abundance in deep-sea cores
• Scientists drilling through marine sediments
can distinguish six distinct events in cores of
mud retrieved from the sea floor, which are
labeled H1-H6 going back in time
36
Hartmut Heinrich
• Marine geologist and climatologist, 1952 present
• Head of the Physics Department, Federal
Maritime and Hydrographic Agency,
Germany
• He found six layers in ocean sediment cores with extremely
high proportions of rocks of continental origin, "lithic
fragments", in the 180 μm to 3 mm size range, which are too
large to be transported by ocean currents
• They are interpreted as having been carried by icebergs or sea
ice which broke off from the large Laurentide ice sheet then
covering North America, and dumped on the sea floor as the
icebergs melted
37
Iceberg Transport
• Armadas of icebergs broke off from glaciers and
traversed the North Atlantic
• The icebergs contained rock mass eroded by the
glaciers, and as they melted, this matter was dropped
onto the sea floor as “ice rafted debris”
• The overall Bond cycle takes 8000-10,000 years,
from one Heinrich event to the next
• What caused the large number of icebergs?
• The Heinrich events represent the period from 70,000
to 15,000 years before present
38
Heinrich Event Graph
• Darker green bars represent the Heinrich events from Hemming 2004, while
the lighter green represents alternative dates for events 2 through 4 from Bond
• Dansgaard-Oeschger events can be seen most clearly in the Greenland delta
39
18O data
Cause of Heinrich Event
• Each Heinrich event clearly represents a
collapse of a large part of a Laurentide Ice
Sheet
• It is accompanied by a significant rise in sealevel, from 1-5 meters
• The mechanism which caused the ice sheet
collapse is still unexplained
40
Ice Sheet Collapse
• Icebergs represent an efficient mechanism for
ice sheet collapse, since they move from high
latitudes with little insolation, to lower
latitudes, with much stronger solar radiation
• The Laurentide Ice Sheet, the source of the
Heinrich glaciers, flowed into the ocean near
60° N latitude
41
Larsen B, Antarctic Peninsula
• An example of the speed at
which an ice shelf can
collapse is shown in the
animated photo file
• The shelf had been stable
for 12,000 years, but ice
equivalent in size to Rhode
Island broke apart in a five
week period
• Larsen B ice shelf breakup in • Breakup was attributed to
meltwater ponds forming on
February, 2002
the surface during the
42
summer months
Larsen B Size
• Size comparison of
Larsen B region with
map of Rhode Island
• Note color difference
between cold freshwater
melt, which is less
dense than salt water,
and the deep blue
oceanwater
43
Abrupt Climate Change
• The speed of the changes seen in the Bond
cycles are much faster than the Milankovitch
mechanisms can account for
• During the 1990’s, there were many papers
published about abrupt climate change, and
many scientists are still working on the
problem, often as their life’s work
44
Greenland Ice Sheet
• The Greenland Ice Sheet is centered near 70° N
• It is a warm ice sheet, near the melting point
• Can the Greenland Ice Sheet behave like the
Laurentide Ice Sheet, and generate a Heinrich
event?
• A key part of the answer deals with the way an
ice sheet flows
45
Ice Flow by Deformation
• Ice sheets which are solidly frozen to their
base can flow only by internal deformation of
ice within the sheet
• Ice at the bottom is fixed, while ice higher in
the sheet breaks and moves
• Such flow is slow
46
The Importance of the Moulin
• If the ice near the base is warmer, it begins to
melt
• Meltwater from the surface may work its way
to the bottom
• Together bottom melting and water from
above can lubricate the glacier, allowing it to
move as a mass, and much faster than
movement by deformation
• This is why moulins are so important!
47
Heat transfer in Glaciers
• The models currently in use for ice sheet heat
transfer involve thermal conduction from the
top to the bottom of the ice sheet
• This would require thousands of years for
heating at the surface to affect the base of the
glacier
• Observations indicate the real lag time is
measured in months
48
Moulin Heat Transport
• Moulins are obviously the source of the rapid
heat transfer
• The problem is that the ice column is below
the freezing point
• How does water from the surface make it to
the base of the glacier without refreezing?
• No mechanism is known, so the modelers
cannot incorporate this mechanism in their
models
49
Cycles and Models
• Although the General Circulation Models
cannot properly imitate Pleistocene climatic
behavior, more specialized models do show a
relationship between D-O cycles, the iceberg
armadas of Heinrich events, and the conveyor
belt currents
• In particular, they suggest the conveyor belt
may turn on and off, a feature the more general
models do not show
50
Which Came First?
• The question of what whether the conveyor
belt on and off has been debated and
researched
• Another very important question is a chicken
and egg problem
• Does an on-off oscillation of the conveyor belt
drive climate, or are climatic changes causing
the conveyor belt to turn on and off?
51
Turning Off the Conveyor
• If a large pulse of fresh water were to flood the
North Atlantic, it would reduce seawater
density
• This might be enough to prevent any sinking
of water in the North Atlantic
• Something similar might happen around the
Antarctic
• It is thought this would bring an abrupt halt to
the conveyor
52
Climate Effects of Conveyor Cutoff
• If the conveyor were turned off, it would affect
climate substantially
• Europe and perhaps northwestern Asia would become
much colder
• Could snow and ice buildup in Europe and
northwestern Asia trigger an overall cooling of the
climate, the first part of the D-O cycle?
• If the conveyor were turned back on, it should
immediately warm Europe, and perhaps cause the
overall warming seen at the end of the D-O cycle
53
Previous THC Cutoff
• Is there any evidence this has happened
before?
• There is a well known period of substantial
cooling in the very late Pleistocene called the
Younger Dryas Stadial
• A stadial is a period of colder temperatures
during an interglacial (warm period) separating
the glacial periods of an ice age
54
Younger Dryas Stadial
• The Younger Dryas stadial was a geologically brief
(1,300) cold climate period between approximately
12,900 and 11,600 years before present
• It was named for an indicator genus, the
alpine/tundra wildflower Dryas octopetala
• It appears the Younger Dryas was most strongly felt
in the northern hemisphere, especially in western
Europe and Greenland
• There is some evidence of re-advancing glaciers in
the Pacific, but the effects were less intense than in
55
areas further east
Lake Agassiz
• It has long been recognized that parts of extreme north-central
U.S. and much large regions of Canada showed evidence of a
very large lake
• William Keating wrote a geologic report describing the Red
River valley of North Dakota in 1823
• He described the extreme flatness of the land, a feature now
associated with lake sediment deposition
• Later Louis Agassiz would tell the world about glacial
advances
• It was later realized that this area had been covered at various
times by a glacial meltwater lake, named in Agassiz’ honor
56
Lake Agassiz Map
• Approximate total extent of
glacial Lake Agassiz
• Flooding started in the
south, and progressed
northward as ice melted
• The entire area shown was
never flooded at once
• Evidence of glacial Lake
Agassiz occurs over an area
of roughly 365,000 square
miles, an area five times the
size of the state of North
Dakota
57
Turning Off the Conveyor
• Claes Rooth proposed in 1982 that a large pulse of fresh water
released into the northern Atlantic as a result of the sudden
switch in the outlet of proglacial Lake Agassiz from the
Mississippi to the St. Lawrence drainage
• This switch was triggered by the retreat of the Laurentian ice
cap, which formed the northern shoreline of the lake
• When an ice dam gave way, the lake surface dropped in a
series of steps by about 100 m
• Water flooded eastward into the northern Atlantic and
presumably reduced the salinity of surface waters there to the
point where deep water could no longer form
58
Younger Dryas Affects
• During the YD, there is much more spatial
heterogeneity as the North became colder and
drier (increasing with latitude) while the South
became warmer and wetter in the opposite
sense
• The global mean cooling during the YD is only
~0.6ºC
• Thus, while the YD was a global scale climate
change event with widespread signatures, it
was not a widespread global cooling event 59
Other Water Depths
• The Younger Dryas event seems to be cause
by a change in surface water temperatures
• What about intermediate to deep water? If the
conveyor shut off, they should be affected as
well
60
Intermediate Water Change
• Jodie Smith et al. (1997), writing in Nature,
reported: “The oxygen isotope ratios in the
coral skeletons change markedly coincident
with the initiation of the Younger Dryas,
suggesting that there were profound changes in
intermediate-water circulation at this time.”
61
Freshwater Flow and Deep Water
• Clark et al. (2001) found that “periods of
increased freshwater flow to the North Atlantic
occurred at the same time as reductions in the
formation of North Atlantic Deep Water, thus
providing a mechanism for observed climate
variability that may be generally characteristic
of times of intermediate global ice volume”
62
Can THC Be Cut Off Now?
• The Younger Dryas event, caused by a
massive infusion of glacial meltwater, is
almost certainly much more severe than what
might happen today
• If the Arctic becomes ice free during the
summer, and heats quickly, the melting of ice
in Greenland is expected to speed up
substantially
63
NADW Cessation
• Such an event would dump fresh water into the
North Atlantic at rates much greater than we
presently see, but probably much less than the
Younger Dryas event
• Thus, we cannot reasonably make a prediction
that the North Atlantic Deep Water (NADW)
would be cut off
• Nevertheless, it would be a high impact event,
and the probability may be more than low risk
64