Download Ocean and Climate II

Ocean and Climate II
 Salinity
 Thermohaline circulation
 The great conveyor belt
http://www.gerhardriessbeck.de/
Temperature, Salinity, Pressure
Three layer model:
1. mixed layer,
T,S,P≈constant
2. Cline layer,
decrease of T,
increase of S and P,
3. abyss layer,
moderate change
of T, S, P
Hydrographic profile along Atlantic Ocean
25oW longitude
Temperature profile every
2.5oC. High line density
indicates the thermocline,
abyssal range shows only
slow temperature decline.
Salinity profile in psu, every
0.5 psu (>35psu) to 1 psu
(<35psu). The strongest
gradient in salinity appears
to exist in the thermocline
layer as indicated before.
Temperature gradient
T  z   Tp  Tm  Tp  e

z
z*
empirical values : mean surface temperature : Tm  19.60C
polar temperature at abyss depth : Tm  1.20C
25.00
T  z   1.20C  18.80 C  e

z
z*
Temperature oC
z*  500m
20.00
15.00
10.00
5.00
0.00
10.00
100.00
Depth m
1000.00
Latitudinal change of temperature and salinity
The latitudinal temperature and salinity pattern of the oceans indicates a sharp
temperature drop towards higher latitude with slight decrease of salinity (melting ice).
Empirical approximations for    T , S 
 T , S    ref  
1 d
T  
 ref dT
T  1  104 K 1
Thermal expansion coefficient
(Temperature dependence of density)
S 
1
 ref

d
dS
 S  7.6  104 psu1
Salinity dependence of density
   0   ref   T T  T0    S S  S0 
o≈26.7kg/m3 for T0 =15oC and S0 =36psu
are reference numbers, extracted from
the T-S contour plot. Conditions change
and differ for different depth layers.

K
c
2
sound
Example
Calculate the density of ocean water for equatorial water temperatures
of T=28oC and a salinity of S=35psu and for polar temperatures of T=4oC
at the same salinity of 35psu with a reference density ref = 1000 kg/m3!
   0   ref   T T  T0    S S  S0 
T0  150 C  288K T  1  104 K 1
S0  36 psu  S  7.6  104 psu1
  26.5
kg
kg
4
1
4
1



S  36 psu

1000

10
K

288
K

T

7
.
6

10
psu
3
3
m
m
T  280 C  301K
  26.5
kg
kg
kg

2
.
06

24
.
44
m3
m3
m3
T 40C  277 K
  26.5
S  35 psu
 warm  1024
kg
m3
The density increases with
decreasing temperature at
constant salinity!
S  35 psu
kg
kg
kg

0
.
34

26
.
84
m3
m3
m3
 cold  1027
kg
m3
Ocean water density
Buoyancy and Density
Archimedes principle: A body wholly or partially submerged in a fluid
is buoyed up by a force equal to the weight of the displaced fluid!
For floating object :  Fi  0
i
Fbuoyancy  Fpressure  Fgravity  WH 2O
WH 2O  mH 2O  g   H 2O  g  V
Wobject   object  g  V
 object   H O
2
 object   H O
2
Object will sink
Object will float with a fraction
x of its volume submerged:
x
Vsubmerged
Vobject
object

H O
2
Example
Ice has a density of ice=916.7kg/m3 compared to
water which has a higher density H2O=1000kg/m3,
calculate the buoyancy force FB and the fraction x
of the submerged volume of a typical iceberg with
a total volume of V=900,000 m3.
 object 916.7
x

 0.917
H O
1000
2
FB   H 2O  g  Vsubmerged   H 2O  g  x  V
kg
m
FB  1000 3  9.81 2  0.917  900,000m 3
m
s
m
FB  8.1  109 kg 2  8.1  109 N  8.1MN
s
With sea water density of sw≈1025kg/m3 the fraction x would be smaller, since the
submerged volume would be reduced, but the buoyancy force would remain the same.
Thermohaline circulation
Warm water ocean currents such as the Gulf current originate in tropic ocean
regions flowing north. The warm surface water cools to the atmosphere and has
a high evaporation rate. Cooling increases the density as shown before,
evaporation increases the salinity and therefore further increases the density of
the ocean water. The high density salt water sinks to lower layers driving the
returning deep water flow, forming the so-called ocean conveyor belt.
The layer structure
of ocean changes in
polar regions due to
the down-welling of
high density water
with high salinity.
The decrease of temperature and
salinity cause down welling of
salty surface water from ocean
currents; the mixed layer reaches
towards much larger depths and
couples directly with the cold
abyss layer, which is at the same
low temperature level. The
sinking water triggers a deep cold
water flow directed towards the
equatorial zones of low latitude.
Pronounced deep mixing zones appear at latitudes
of 70oN but only 50oS. The high latitude reach of
70oN is due to the warm water flow of Gulf stream.
Consider a water volume element at the ocean surface of 50m depth and 2500m2
surface moving north as part of the gulf current. Emerging from the Gulf of Mexico
the water has a density of 1=1024kg/m3, reaching the Arctic Ocean, evaporation
has increased the density to 2=1027kg/m3. Calculate the buoyancy of this body of
salty water compared to an averaged ocean water density if H2O=1026kg/m3 and
determine the location from the density map at which it is completely submerged.
Water flows at surface
x1 
object
 H 2O

1024
 0.998
1026
FB   H 2O  g  Vsubmerged   H 2O  g  x  V
kg
m
FB1  1026 3  9.81 2  0.998 125,000m 3  1,255,552,724 N
m
s
object 1027
Density is too high, water sinks down
x2 

 1.001
 H 2O 1026
FB2  1,259,390,633 N
Density Conditions near ocean surface
1024 kg/m3
1026 kg/m3
1027 kg/m3
Latitude
kg
x  1;   1026 3
m
Corresponds to 45oN latitude. Real conditions
are determined by the interplay between
temperature and salinity conditions!
Observations near the arctic coast
Salinity for AO1 58o N
Salinity for A24
A24
AO1
Thermohaline circulation
near Greenland coast,
visualized by the high salinity
turn-over current at 3000 m.
Thermohaline
circulation near
Greenland coast,
visualized by the high
salinity turn-over
current at 2000 m.
Temperature around Antarctica
Salinity conditions around Antarctica
Decrease in salinity at surface (50 m depth) due to mixing with melting
water; increase in salinity at 800 m depth due to thermohaline circulation.
Pacific Ocean salinity conditions
Double salinity layer due to thermohaline
currents shown in distribution at different
depths and in depth profile.
Thermohaline circulation is one of the
drivers of the ocean conveyor belt!
The great Ocean Conveyor Belt
http://pmm.nasa.gov/education/videos/thermohaline-circulation-great-ocean-conveyor-belt
Threat to Europe? The THOR project