Thermohaline Circulation

1
Thermohaline Circulation

• Readings
• Kennett, Chapter 8
• Ocean Circulation - Open Univ. Chapter 6

2
(No Transcript)
3
Subsurface Water Masses

• Convection - Vertical movements of water masses
• Origin - Density differences
• Temperature
• Heat loss
• Salinity
• Evaporation
• Sea Ice formation

4
Water Column Stability/Instability

• Generally the density of ocean water increases
  with depth. In this case, the water column is
  stable.
• A stable water column does not mix vertically
  unless acted on by outside forces (for example,
  winds).

5
Water Column Stability/Instability

• If the density of water decreases with depth, the
  water column is unstable.
• Usually an unstable water column occurs because
  surface water cools and becomes more dense. Less
  commonly, evaporation causes increased salinity
  and thus increased density of surface water.
• An unstable water column does not last long. The
  dense water will sink, and the less dense water
  will rise, resulting in turbulence and vertical
  mixing.

6
The density of seawater is controlled by its
salinity and its temperature.

• Processes that increase the salinity of seawater
  include evaporation and the formation of sea ice.
• When ice crystals form, salt is rejected from
  their structure. The water surrounding the
  crystals becomes saltier. The resulting brine
  seeps out of the forming ice. This brine
  rejection continues for a year or more old sea
  ice is less salty than new sea ice.
• Processes that decrease seawater salinity are
  precipitation at the sea surface, land runoff
  (from rivers, glaciers, etc.), or melting of sea
  ice.

7
The Deep Ocean Properties

• Properties in the subsurface are displayed as
  lines of constant value, for example
• Isopycnals are lines of constant density. They
  show the position of water layers that have
  uniform density.
• Isotherms are lines of constant temperature. They
  show the position of water layers of uniform

8
(No Transcript)
9
Water Masses

• Layers of ocean water with characteristic
  temperature, salinity and density (and, in some
  cases, other properties such as nutrient
  concentrations) are called water masses.

10
Water Masses

• North Atlantic Deep Water
• 1.8 to 4C 34.89 -35.0
• Antarctic Bottom Water
• 0.5 to 2.0C 34.74 to 34.89 
• Antarctic Intermediate Water
• 4 to 5C 34.2

11
Density

• The density, ?, of seawater is function of
  temperature, salinity, and pressure. It increases
  with increasing salinity and pressure, and
  decreases with increasing temperature. The
  density is expressed in units of kg/m3, or
  sometimes g/cm3. Oceanographers use a number of
  different ways to express the density of
  seawater, so you may see the terms density
  anomaly, potential density, (pronounced
  sigma-theta), specific volume, specific volume
  anomaly or others. The most commonly used of
  these are defined below.

12
Sigma ?
Sigma, is a short-hand for seawater density,
where 1000 kg/m3 has been subtracted. So for
1024.32 kg/m3, 24.32 kg/m3. While all the
versions of have units of kg/m3, it is often
reported without units, which is a throwback to
when the definitions included a ratio of the
seawater density to the density of freshwater,
thus rendering the variable dimensionless. Sigma-
t, is density of seawater calculated with in situ
salinity and temperature, but pressure equal to
zero, rather than the in situ pressure and 1000
kg/m3 is subtracted.
Sigma ???????? density of the water at the
surface
13
Sigma ?
Sigma ???????? density of the water at the
surface
14
Note that the Pressure term is ignored in the
Sigma T graph. AABW appears less dense than
NADW. The numbers for each station represent
depth in 100s of meters. E.g., 50 at the
bottom graph 5000m
15
DEEP-WATER FORMATION SITES-places where large
volumes of sea water sink

• NORTH ATLANTIC - NORWEGIAN AND GREENLAND SEAS
• North Atlantic Deep Water (NADW)
• ANTARCTIC - WEDDELL SEA
• Antarctic Bottom Water (AABW)

16
(No Transcript)
17
Bottom Water sources
18
INTERMEDIATE -WATER FORMATION SITES

• AAIW - SOUTHERN OCEAN - NORTH OF THE ANT.
  DIVERGENCE
• NORTH PACIFIC - INTERMEDIATE DEPTH WATER

19
Intermediate Water Sources
20
Salinity driven circulation

• tends to occur in semi-isolated seas in areas of
  net evaporation in the sub-tropics
• Mediterranean -
• Red Sea -

21
Thermohaline Model
22
Thermohaline Model
23
Coriolis Force

• Deflection of moving objects to the right in the
  northern hemisphere and to the left in the
  southern hemisphere.
• This is due the rotation of the Earth.

24
Circulation Tracers

• Conservative
• Those properties that are changed only at the
  margins of the boundaries of the ocean
• Non-Conservative
• Those properties that can be altered by physical,
  chemical or biological processes

25
Non-Conservative TracersO2, PO4 etc.
26
Nonconservative Properties - Dissolved O2
Biological processes
photosynthesis (light)
organic materials O2
CO2 H2O inorganic N (NO3-, NH4) inorganic P
(PO43-)
respiration
Physcial processes
O2dissolved
O2atmosphere
27
Nonconservative Properties - Dissolved O2
O2
O2 ?
atmosphere
sea surface
mixed layer
O2
net photosynthesis
organics
pycnocline
oxygen minimum
sinking
? depth
O2
deep water
rising water
net respiration
organics
burial
sediments
28
Dissolved Oxygen
29
(No Transcript)
30
PO4 distribution in ocean
31
(No Transcript)
32
Atlantic Thermohaline Circulation
33
The North Atlantic
Denmark Strait
Iceland Faeroe
Faeroe Bank Channel
Charlie-Gibbs FZ
34
The North Atlantic
35
The North Atlantic
36
GREAT SALT PUMP

• Fresh water carried from the Atlantic to the
  Pacific - mainly by trades across central
  America. Creates a salt inbalance between the N.
  Atlantic Pacific
• Deep flow - NADW and AABW transport excess salt
  to the Pacific
• Surface return flow - via the Pacific - Indian
  Ocean - S. Atlantic - Gulf stream

37
(No Transcript)
38
(No Transcript)
39
The Southern Hemisphere
40
The Southern Hemisphere
41
(No Transcript)
42
Sea Ice Formation

• SEA ICE - ice that forms from seawater freezing
  at the oceans surface (-2C)
• - typically less than 10 meters thick
• 3 step Formation Process
• 1. starts as tiny needles (spicules) 1
  to 2 cm long forming frazel ice
• 2. Slush - eventually start to
  coalesce
• 3. pancake ice
• Freezing process excludes salt -
  Not pure freshwater, however. Some seawater is
  trapped. Salinity, is usually less than 10 ppt.

43
Sea ice. As sea ice forms, it takes up
freshwater, and excludes most of the salt from
the salty seawater. The water that is left behind
is dense because it is so salty.
44
Implications for Deepwater

• 1. Cooling - initially as sea ice forms - latent
  heat of freezing is lost to the atmosphere,
  cooling the water, warming the atmosphere. Later,
  it prevents further cooling.
• 2. Salt exclusion - salt is excluded from the ice
  - elevating salinity of surface water
• -convection begins as less dense
  waters rise in fingers to replace sinking
  waters
• sea ice expansion is an important process in the
  formation of deep waters at polar latitudes

45
Polynyas

• Coastal Polynyas - Open areas (leads) in the
  sea-ice along the coast
• created by persistent offshore winds which push
  ice away from the coast
• Open Ocean Polynyas -
• Open areas (leads) in the sea-ice created by
  winds or warm currents beneath

46
(No Transcript)
47
Antarctic Intermediate Water
48
Antarctic Intermediate Water
49
MedWater
50
Mediterranean Water
51
Mediterranean Water
52
(No Transcript)