Density - density is a key property of seawater - one of the most important parameters in ocean...
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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”
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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)
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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
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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
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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
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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)
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T-S distribution of world oceans
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T-S processes that can change seawater density
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freshwater density vs. temperature
constant pressure = 1 atm
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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
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T-S effects on freezing and density
max density 2 °C
freezes at - 0.4 °C
S = 10
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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
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example T, S, σt profiles
σt from T and S values (equation of state)
density must increase with depth
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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
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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
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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
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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
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Convection & the Conveyor Belt
NADW production drives the conveyor
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Convection & the Conveyor Belt
AABW
NADWAAIW
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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
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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
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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%)
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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
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Readings for next time:
Reader pgs 39 – 51 “Density and Pressure in the Oceans”
Text Chapter 7