Lecture 5 (Ch. 5 of text) Properties of Seawater (Part II)

Density and Pressure

Why is the deep ocean cold?

Vertical Structure of Temperature

Thermocline

Thermocline is a permanent hydrographic feature of temperate and tropical oceans.

Seasonal evolution of thermocline at the mid-latitudes

Growing period

Decaying period

Downward heat transport from Sep. to Jan.

Outstanding question: what sets the depth of the thermocline?

Vertical Structure of Temperature

Transfer of Heat to the Ocean (heat flux)

Absorption of solar radiation decreases rapidly with depth

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?

Salinity

Temperature

How do the water masses move? c.f. Fig.5.13b

haloclineB

ecom

e u

nch

an

ged

wit

h t

ime

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

Seawater density is a function of both temperature and salinity (so-called TS diagram)

A

B

ρA < ρB

CρB < ρC

OCEANWATERMASSES

Vertical profiles

DENSITY: controls the movement and stability of the ocean water masses

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

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

But water is slightly compressible

Density is actually a non-linear function of Temperature, Salinity and Pressure !

-31000 km mt

T S

( , , )pT S

Kg m-3

Taking into account compressibility effects

Potential Temperature

Taking into account compressibility effects

Potential Density

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

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)

( , )

1000

,

T

pST

In situ Density

( , )

1000

,S p

Potential Density

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

TS Diagram

Tem

pera

ture

Salinity

-31000 km mt Kg m-3

Distribution of T and S in the Ocean

Tracking Water Masses on TS diagrams

AABW: Antarctic Bottom Water

AAIW: Antarctic Intermediate

Water

NADW: North Atlantic Deep Water

Tracking Water Masses on TS diagrams

Worlds ocean Water Masses

Properties of Seawater

Mixing (supplements of Ch.5.6)

Molecular diffusion

Turbulent diffusion

How to mix water masses in the ocean?

Horizontal Stirring and Mixing

Horizontal Stirring and Mixing

Mixing of two water masses with same Density

O1T1 S1 O2T2 S2

2

1

3

Vertical Stirring and Mixing

y

z +

_

Mixing along surfaces of Constant Density

Surfaces ofconstant density(i.e. isopycnal)

y

z

+

_

Mixing along surfaces of Constant Density

Along - Isopycnal diffusive mixing

Surfaces ofconstant density

y

z +

_

Along - Isopycnal diffusive mixing Across - Isopycnal diffusive mixing

Mixing across surfaces of Constant Density

Surfaces ofconstant density

y

z

b

+

_

the “skew flux”

Diapycnal Mixing

Definitions of Mixing

Surfaces ofconstant density

y

z

b

+

_

the “skew flux”advection

Diapycnal Mixingturbulent diffusion

Definitions of Mixing

Surfaces ofconstant density

Diabatic exchanges with the atmosphere at the surface

Adiabatic changes and Mixing in ocean interior

T1 S1 T2 S2

非絕熱

絕熱

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)

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)

Properties of Seawater

Dissolved Gases (Ch.5.6)(focus on O2 and CO2)

Dissolved Gases(ml l-1)Air

Seawater

Total pressure = sum of partial pressures

Oxygen

Saturation curve

Main regulator is the activity of organisms (biological oceanography later)

Dissolved Gases in the OceanOxygen profile

Anoxic environment

compensation depth

Respiration:Animal, plants and microbial decomposition

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)

Why is the pH of seawater close to neutral?

10log [ ]pH H

(Seawater pH=7.5-8.5)

pOH ?

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

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 )

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

(b) Photosynthesis and respiration

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) ?

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

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

CO2

Temperature

70 ppm

CO2 changes in the last 300 yr

70 ppm

Industrial Revolution

CO2 changes in the last 50 yr

OceansBiosphere

Rock Weathering

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

Grand Carbon Cycle

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:

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

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)

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-

+

Distribution of Carbon species in water

323

[ ]

[ ]

HCO

CO

+ 323

[ ]

[ ]

HCO

CO

-

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

323

[ ]

[ ]

HCO

CO

+ 323

[ ]

[ ]

HCO

CO

-

23

3

[ ][ ]

[ ]

H COK

HCO

hydrogen ion concentration

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

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

Why is the pH of seawater close to neutral?

10log [ ]pH H

seawaterpH=7.5-8.5