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Lecture 5 (Ch. 5 of text) Properties of Seawater (Part II)
Density and Pressure
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Why is the deep ocean cold?
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Vertical Structure of Temperature
Thermocline
Thermocline is a permanent hydrographic feature of temperate and tropical oceans.
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Seasonal evolution of thermocline at the mid-latitudes
Growing period
Decaying period
Downward heat transport from Sep. to Jan.
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Outstanding question: what sets the depth of the thermocline?
Vertical Structure of Temperature
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Transfer of Heat to the Ocean (heat flux)
Absorption of solar radiation decreases rapidly with depth
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Salinity variations are determined by the addition or removal of H2O from seawater
Processes such as evaporation and sea ice formation will increase the salinityProcesses such as rainfall, runoff, and ice melting will decrease the salinity
What controls the ocean’s salinity?
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Salinity
Temperature
How do the water masses move? c.f. Fig.5.13b
haloclineB
ecom
e u
nch
an
ged
wit
h t
ime
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Pressure in the Ocean (water is not absolutely incompressible)
p g h
0
( )z
p z g dz
( )p zg
z
Hydrostatic Equation
Hydrostatic Balance
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Seawater density is a function of both temperature and salinity (so-called TS diagram)
A
B
ρA < ρB
CρB < ρC
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OCEANWATERMASSES
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Vertical profiles
DENSITY: controls the movement and stability of the ocean water masses
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Vertical circulation driven by density Thermohaline Circulation
(18%)
Tropical oceans: pycnocline ≈ thermocline
Mid-latitudes: pycnocline ≈ halocline
High latitudes: no pycnocline formation
Why? (important)
Den
sit
y
str
ati
ficati
on
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Density: amount of mass per unit volume
T S
Units: kg m-3
Linear Equation for “in situ” Density
Thermal expansion coefficient
Saline contraction coefficient
More on the DENSITY
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But water is slightly compressible
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Density is actually a non-linear function of Temperature, Salinity and Pressure !
-31000 km mt
T S
( , , )pT S
Kg m-3
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Taking into account compressibility effects
Potential Temperature
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Taking into account compressibility effects
Potential Density
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HW#1: Application of Isostasy/Buoyancy Concept (Due date: 17 April)
There is a huge lake with constant depth 100 cm and extension of 500 km. The water surface is still and undisturbed, that is, nothing moves. Now objects A, B, and C (see below for their configurations) are dropped separately and we wait until everything is quiet again. How many cm will the objects be sticking out above or beneath the water surface, if
(a) density of the water is constant at 1.03 g/cm3,
(b) density of the water, for some reasons, increases linearly with depth from 1.03 g/cm3 at the surface to 1.43 g/cm3 at the bottom
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In situ TemperatureTemperature of a particle of water measured at a particular depth and pressure (no correction for compressibility effects)
Surface
Deep ocean
T1
T2
T1=θ1
T2≠θ1
At the ocean surface In Situ and Potential Temperature are the same!
θ1
θ1
Potential TemperatureTemperature that a particle would have if raised adiabatically to the surface of the ocean (corrects for the effects of compression
occurring at great depth make the particle warmer)
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( , )
1000
,
T
pST
In situ Density
( , )
1000
,S p
Potential Density
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Histograms of Temp. and Salinity in the OceansTemperature
Salinity
Natural thermostate mechanism
tropical cirrus clouds resulting from deep convection contribute to long-wave radiative heating of the tropospheric column, and at the same time reduce solar insolation at the sea surface, in this way cooling the ocean. This dual tropospheric, long-wave radiative heating and surface, short-wave radiative cooling role of cirrus is called the thermostat mechanism.
The deep convection occurs only when the SST exceeds 27 C, which is associated with the so-called super-greenhouse effect
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TS Diagram
Tem
pera
ture
Salinity
-31000 km mt Kg m-3
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Distribution of T and S in the Ocean
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Tracking Water Masses on TS diagrams
AABW: Antarctic Bottom Water
AAIW: Antarctic Intermediate
Water
NADW: North Atlantic Deep Water
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Tracking Water Masses on TS diagrams
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Worlds ocean Water Masses
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Properties of Seawater
Mixing (supplements of Ch.5.6)
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Molecular diffusion
Turbulent diffusion
How to mix water masses in the ocean?
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Horizontal Stirring and Mixing
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Horizontal Stirring and Mixing
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Mixing of two water masses with same Density
O1T1 S1 O2T2 S2
2
1
3
Vertical Stirring and Mixing
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y
z +
_
Mixing along surfaces of Constant Density
Surfaces ofconstant density(i.e. isopycnal)
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y
z
+
_
Mixing along surfaces of Constant Density
Along - Isopycnal diffusive mixing
Surfaces ofconstant density
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y
z +
_
Along - Isopycnal diffusive mixing Across - Isopycnal diffusive mixing
Mixing across surfaces of Constant Density
Surfaces ofconstant density
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y
z
b
+
_
the “skew flux”
Diapycnal Mixing
Definitions of Mixing
Surfaces ofconstant density
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y
z
b
+
_
the “skew flux”advection
Diapycnal Mixingturbulent diffusion
Definitions of Mixing
Surfaces ofconstant density
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Diabatic exchanges with the atmosphere at the surface
Adiabatic changes and Mixing in ocean interior
T1 S1 T2 S2
非絕熱
絕熱
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Surface:•Wind stirring and vertical mixing in the surface layer
•Surface fluxes of heat and salt buoyancy fluxes
•Surface Waves
Interior:•Along Isopycnal
eddies and fronts
•Across Isopycnalinternal wave breaking
Bottom:Breaking internal waves over rough topography
Summary of major mixing processes in the Ocean
(Important concepts)
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Ocean Circulation and ClimateMixing energy and dissipation of tides
Mixing rates in the ocean govern the rate at which the ocean absorbs heat and greenhouse gases, mitigating climate. Global climate change forecasts are uncertain in part due to uncertainty in the global average ocean mixing rate. Mixing rates in the ocean vary geographically depending on bottom roughness. Shown are mixing rates observed during an oceanographic survey across the Brazil Basin in the South Atlantic Ocean. Low mixing rates (purple) were found over the smooth topography to the west, and higher mixing rates (colors) over the rough topography to the east (Mauritzen et al. 2002, JGR)
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Properties of Seawater
Dissolved Gases (Ch.5.6)(focus on O2 and CO2)
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Dissolved Gases(ml l-1)Air
Seawater
Total pressure = sum of partial pressures
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Oxygen
Saturation curve
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Main regulator is the activity of organisms (biological oceanography later)
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Dissolved Gases in the OceanOxygen profile
Anoxic environment
compensation depth
Respiration:Animal, plants and microbial decomposition
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Main sources of O2 in the surface layer: photosynthesis and diffusion across the air-sea interfaceWhy does the O2-minimum layer coincide with the pycnocline layer? (important)
Why does the concentration increase with depth toward the deep seas? (important)
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Why is the pH of seawater close to neutral?
10log [ ]pH H
(Seawater pH=7.5-8.5)
pOH ?
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Carbon Dioxide and Carbonate system
Why is this important (important)?
1. Regulates temperature of our planet
2. Important for the ocean biota
3. Regulates the acidity of sea water
The pH of water is directly linked to the CO2 system
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Carbonic Acid
2 2 2 3 3H O CO H CO H HCO
Bicarbonate Ion
Sources for acidity in the ocean
Carbon Dioxide and Carbonate system
23 3HCO H CO
Carbonate (碳酸鹽 )
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At the pH of normal seawater, HCO3- makes up about 80% of the carbon species
More H+ ions need to be releasedless H+ ions need to be released
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(b) Photosynthesis and respiration
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10log [ ]pH H
Carbonate Buffer self-regulating system
Why are the CaCO3 shells dissolved in the cold, deep water, but not in the warm, shallow water (important) ?
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As temperature is low,
The cold water has a higher gas-saturation value
As the water becomes deeper,
The higher pressure also has a higher gas-saturation value
Thus, the dissolved CO2
amount increases and makes the water acidic, and melts the CaCO3 shells that sink to the deep-sea floor.
→NO Calcareous oozes at high latitudes
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Carbon Dioxide and Carbonate system
Why is it important?
1. Regulates temperature of our planet2. Important for ocean biota3. Regulates the pH value of sea water
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CO2
Temperature
70 ppm
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CO2 changes in the last 300 yr
70 ppm
Industrial Revolution
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CO2 changes in the last 50 yr
OceansBiosphere
Rock Weathering
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How much CO2 can be dissolved by the ocean (role of ocean uptake in regulating the global climate)?
Process that control CO2 absorption in the ocean
Chemical
Biological
Physical
Carbon Cycle
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Grand Carbon Cycle
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The Carbonate System
2 2 2 2
23 3
( ) ( )
( ) 2 ( )
H O CO gas H CO aq
H HCO aq H CO aq
2 23 3( ) ( )CaCO s Ca aq CO
from dissolution of Calcium Carbonate
from dissolved CO2 gas
sources of inorganic carbon
Biology and Physics participate in the equilibrium of the carbonate system
NOTE:
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2 2 2 2
23 3
( ) ( )
( ) ( )
CO CO gas H CO aq
HCO aq CO aq
Total dissolved inorganic carbon
this is very small not found in this form
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Total dissolved inorganic carbon
formation and decomposition of organic matter(1)
2 23 3( ) ( )CaCO s Ca aq CO
from dissolution of Calcium Carbonate(2)
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High pH
2 2 2 3 3( )H O CO gas H CO H HCO
Carbon Dioxide and Carbonate system
23 32HCO H CO
Low pH-
+
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Distribution of Carbon species in water
323
[ ]
[ ]
HCO
CO
+ 323
[ ]
[ ]
HCO
CO
-
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Control of pH10log [ ]pH H
23 3( ) ( )HCO aq H CO aq
very rapid reaction in seawater
at equilibrium
23
3
[ ][ ]
[ ]
H COK
HCO
Equilibrium constant
323
[ ][ ]
[ ]
K HCOH
CO
hydrogen ion concentration
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323
[ ]
[ ]
HCO
CO
+ 323
[ ]
[ ]
HCO
CO
-
23
3
[ ][ ]
[ ]
H COK
HCO
hydrogen ion concentration
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Concept of Alkalinity (鹼度 )
3 2 2
23 2 2
( ) ( )
( ) 2 ( )
HCO aq H H O CO gas
CO aq H H O CO gas
23 3[ ] 2[ ]A HCO CO
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Alkalinity
23 3[ ] 2[ ]A HCO CO
22 3 3[ ] [ ] [ ]CO HCO CO
2 2 2 2
23 3
( ) ( )
( ) ( )
CO CO gas H CO aq
HCO aq CO aq
22 3[ ] [ ]A CO CO
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Why is the pH of seawater close to neutral?
10log [ ]pH H
seawaterpH=7.5-8.5