Density

- density is a key property of seawater

- one of the most important parameters in ocean dynamics

- the ocean forms layers by density (denser waters deeper)

- density must increase with depth for gravitational stability

- density changes in the vertical inhibit mixing

- density changes in the horizontal drive currents

- mixing is mostly along isopycnals (lines of constant density)

Geography 104 - “Physical Geography of the World’s Oceans”

density is defined as the mass of a substance divided by its volume

seawater density - mass of seawater per volume of seawater

density represented as ρ (Greek letter small “rho”)

density units - expressed as kg m-3 or g cm-3 (1 g cm-3=1000 kg m-3)

two factors regulate the density of freshwater (ρ(T,p))1)temperature – increase in T => decrease in ρ (T > 4 °C)

2)pressure – increase in p => increase in ρ

freshwater density vs. temperature

constant pressure = 1 atm

in a freshwater lake, cooler water (that is very cold) can be less dense than warmer water

three factors regulate the density of seawater (ρ(T,S,p))1)temperature – increase in T => decrease in ρ

2)salinity – increase in S => increase in ρ

3)pressure – increase in p => increase in ρ

equation of state for seawater gives ρ for any combination of T,S,p

seawater is denser than freshwaterρfw(4 °C, 1 atm) = 1000 kg m-3

ρsw(4 °C, 35 psu, 1 atm) = 1028 kg m-3

because- molecular weight of salts greater than water (greater mass)

- presence of salt ions contracts water by a very small amount (reduced volume)

variations in the density of seawater are small- ρsw (T,S,1 atm) ~1025 to 1028 kg m-3

oceanographers use “sigma-t” for densityσt(T,S,P) = ρ(T,S,P) kg m-3 - 1000 kg m-3

example:1027.531 kg m-3 - 1000 kg m-3 = 27.531 kg m-3 (or no units)

T-S distribution of world oceans

T-S processes that can change seawater density

freshwater density vs. temperature

constant pressure = 1 atm

seawater density/freezing depends on T and S

constant pressure = 1 atm

- adding salt decreases the temperature of max density

- temperature of max density increases faster than temp of freezing point - adding salt decreases the freezing point

T-S effects on freezing and density

max density 2 °C

freezes at - 0.4 °C

S = 10

T-S effects on freezing and density

max density 2 °C

freezes at - 0.4 °C

Freezes at - 1.7 °C

S = 10

S = 35

example T, S, σt profiles

σt from T and S values (equation of state)

density must increase with depth

T-S diagram – gives density as a function of T and S

increasing temperature

increasing salinity

increasing density (σt)

T-S data

isopycnals – lines of constant density

T-S diagram example

water mass 1

T=20 & S=36water mass 2T=22 & S=35

σ1 >> σ2

water mass 2 is less

dense or “buoyant”

relative to 1

1

2

water mass 1

T=20 & S=36water mass 2

T=17 & S=35

σ1 ~ σ2

Water masses have the same density so there is no net buoyancy difference

1

2

T-S diagram example

Convection- air-sea cooling & evaporation creates cool and salty surface

waters

- these waters are then denser than those beneath them so

they sink

- occurs on diurnal and annual time scales

- drives very large scale circulation

Convection & the Conveyor Belt

NADW production drives the conveyor

Convection & the Conveyor Belt

AABW

NADWAAIW

the role of pressure on density

increasing pressure (i.e. sinking a water parcel) but not allowing the parcel to exchange heat adiabatic heating density decrease

the role of pressure on density

decreasing pressure (i.e. bringing a water parcel to the surface) not allowing the parcel to exchange heat adiabatic cooling density increase

potential temperature (θ; “theta”) - temperature of a water parcel at sea surface pressure

potential density (σθ; “sigma theta”)- σ(θ,S,1 atm)- density evaluated with potential temperature

θ < T due to adiabatic cooling

σθ > σ due to cooler temperature

seawater is only slightly compressible (~1.5%)

potential temperature and sigma theta data

Important for following water masses in the deep ocean

θ < T due to adiabatic cooling

σθ > σ due to cooler temperature

Readings for next time:

Reader pgs 39 – 51 “Density and Pressure in the Oceans”

Text Chapter 7