Thermohaline Circulation

1
The Ocean Thermohaline Circulation Corinne Le
Quéré for Karen J. Heywood (Convenor) Room
01.37a in ENV c.lequere_at_uea.ac.uk For further
reading, see http//www.soc.soton.ac.uk/rapid/
2
Ocean bathymetry (Mondays lecture)
3
Surface ocean circulation (Mondays lecture)
4
Vertical variation in the ocean

• Controlled by density (mass per volume)
• Density (kg m-3) is mass (kg) per unit volume
  (m3).
• Denser water sinks towards the bottom. Lighter
  water rises towards the surface.

5

• Density of pure water at 4oC 1.000 kg/L (1000
  kg/m3)
• Density of sea water is 1.021 to 1.028 kg/L (1021
  to 1028 kg/m3)
• Density of human body is close to 0.99-1.07 (990
  to 1070 kg/m3)
• (fat is 0.90, muscles are 1.06)

Denser water (or objects) sinks towards the
bottom. Lighter water rises towards the surface.
Tourists floating in the dead sea (density 1.30
kg/L or 1300 kg/m3)
6
Vertical variation in the ocean

• Density varies with
• Temperature (Warm water is less dense than cold
  water)
• Salinity (Salty water is denser than fresh water)
• Pressure (High pressure also increases the
  waters density)

7
Vertical variation in the ocean

• Temperature
• How does temperature of sea water vary with depth
  ?
• upper ocean warmed by solar radiation
• near surface layers mixed by wind, surface
  cooling
• ? mixed layer, uniform in temperature.

8
Vertical variation in the ocean

• Temperature
• Below mixed layer, temperature changes rapidly
  with depth ? main thermocline.
• Thermocline separates warmer mixed layer above
  from cooler deep layer below.

9
(No Transcript)
10
Temperature (oC)
32
Depth (m)
-2
Latitude S N
11
(No Transcript)
12

• Salinity
• The other main property of sea water.
• Typically 35 parts of salt to 1000 parts of
  water.
• Salinity has no units!
• Determined by measuring electrical conductivity
  of water.

13
(No Transcript)
14
Temperature (oC)
32
-2
Salinity
36
Depth (m)
30
Latitude S N
15

• Pressure
• Deeper ocean has greater pressure due to the
  weight of the water above.
• Hydrostatic equation next week!

16

• Density
• Warm water is less dense than cold water.
• Salty water is denser than fresh water.
• High pressure also increases the waters
  density.

Temperature (oC)
32
-2
Salinity
36
Depth (m)
30
Density (kg/L)
1.028
1.021
Latitude S N
17
(No Transcript)
18

• An isotherm is a contour of equal temperature.
• An isohaline is a contour of equal salinity.
• An isopycnal is a contour of equal density.
• The thermocline is the depth range with large
  temperature gradient.
• The halocline is the depth range with large
  salinity gradient.
• The pycnocline is the depth range with large
  density gradient.
• can be more than one in the water column.
• usually thermocline, halocline and pycnocline are
  at the same depth range.

19
The Thermohaline Circulation

• Polar regions ocean releases heat to warm the
  overlying air.
• Freezing point of sea water at atmospheric
  pressure is 1.9?C.
• Salt is left behind when the water freezes into
  sea ice. Brine rejection.
• Cold, salty, dense water sinks to the sea bed.
• Warmer water moves to replace it.
• ? thermohaline circulation.

20
(No Transcript)
21
Sea surface temperature in a high resolution
ocean model.
22
The thermohaline circulation, or meridional
overturning circulation. Meridional means
north-south.
23
What does the deep western boundary current
really look like? These are measurements of the
deep current off the east coast of Canada.
24
Two main sources of cold, dense bottom
water Northern North Atlantic North Atlantic
Deep water Around Antarctica Antarctic Bottom
Water
25
The Overturning Circulation in the Southern Ocean
26
(No Transcript)
27
(No Transcript)
28
The Day After Tomorrow depicts what might
happen if the Thermohaline Circulation in the
North Atlantic were to suddenly switch off.
29
The day after tomorrow?Change in surface air
temperature 30 years after a freshwater pulse in
the North Atlantic (Vellinga Wood, 2002)
30
Abrupt climate change happened at the end of the
Younger Dryas cold period about 12k years ago. In
less than a decade, the earth warmed by 15C and
precipitation doubled, according to ice core
evidence. This may have been caused by a change
in the ocean thermohaline circulation.
31
Overturning in the present day ocean. Deep and
strong. Overturning in the ocean during the
last glacial maximum. Shallower and weaker.
32
Almost all the current coupled climate models
predict a reduction in the strength of the
Thermohaline Circulation over the next century.
The strength of the meridional overturning
circulation is measured in Sverdrups (Sv). 1 Sv
a million cubic metres per second of water
passing through.
33
Observations in the Nordic Seas show that there
has been a freshening over the last 50 years.
34
There are probably connections between the
Meridional Overturning Circulation and the North
Atlantic Oscillation. For example, deep water
seems to form in either the Labrador Sea or the
Nordic Seas, depending on the state of the NAO.
35
We are still very uncertain how much heat is
transported meridionally by the atmosphere and
ocean.
Meridional heat transport is measured in
Petawatts, 1015 Watts.
36
There are very few measurements of the meridional
heat transport in the ocean. More on this in the
Practical in week 4!
37
Various estimates of ocean heat transport from
models (lines) and observations (points with
error bars).
PW
38
(No Transcript)
39
The strength of the Atlantic Meridional
Overturning Circulation and heat transport will
be measured during the Rapid project. See Bryden
et al. paper in Nature, set for coursework.
40

• How does the ocean transport heat?
• (1) Wind driven gyres

N
E
W
S
41

• How does the ocean transport heat?
• (2) Thermohaline circulation

42

• How does the ocean transport heat?
• (3) Eddies