density and water masses

Density equation

mass/vol

• density is a function of T,S,P

• p (rho)= 1000kg/m³ for pure water, 1027 kg/m³ for avg ocean water

• sigma= p-1000 => 27 kg/m³ for avg SW

Linear equation of state

average values & expansion coefficients

P₀= 1027 kg/m³, T₀=10C, S=35%

Thermal expansion: a=0.15 kg/(m³C)

salinity contraction: B= 0.78 kg/(m³db)

When can we use a linear EOS?

if t,s,p have small variation

simple process studies

Effects of T and S on density

• T range= 0-25C

• S range= 34-36

• density isopycnals

idealized density profile

Stable- less dense water atop more dense water

stratified- layered water column

• hard to mix a water column if there are strongly stratified layers

• easier to mix along isopycnals

Latitudinal variability in density profiles

• lowest density in tropics but connects to deep water via pycnocline

• poles- little change between surface and deep

potential density

• density usually calculated for a given pressure

• Sigma_T= P_(T,S,0m)

• density calculated with in situ T and S, but pressure at the surface

• not really relevant so not used

• Sigma_{theta= potential T}

  • Theta= density anomaly

  • what it would be if water mass brought to the surface

Adjusting for the impact of compressibility on volume, and pressure on temp

Apparent instability

• potential temp

  • pressure increase cause T increase

  • does not represent change in heat content

  • allows comparison of T of water at one pressure with water at another pressure

  • identify water masses

• potential density :S, 0, P=0

  • pressure increases cause volume decrease

Potential density all 3

T vs S theta

• Blue- 2 parcels that are same density

  • warmer parcel is less dense at theta4

• red- colder parcel is less dense, warmer parcel is more dense

  • b/c colder water more compressible, we find that parcel becomes more dense

Apparent Instability

• referenced to surface

• good for identifying water mass

• select a reference depth within 500m

• when working in top 500m, reference depth is surface

• when working within 500-1500m, reference pressure is 1000 dbar

Conservative water properties

• changed by?

  • physical processes only

• examples

  • T+S

  • major constituents

• importance: can be used to identify water masses

non-conservative water properties

• changed by?

  • changed by biological processes

• examples: DO, nutrients

• importance: can be used to determine age of water mass

Water masses: Atlantic ocean

• get signature T, S at surface

• density controls depth of sinking

• thickness+ horizontal extent of each layer is a function of size of the surface source + rate of formation

NADW

• formed from salty GS water that cools and sinks

• 2-4 C

• S= 34.9-35

• 2000-4000m

AAIW

• forms in roaring 40s in S atlantic

• sinks at convergent front

• 3-4C

• S= 34.2-34.3

• 300-2000m

• high Si signature

AABW

• forms mostly in Atlantic sector of S. Ocean

• -05.-1C

• S=34.6-34.7

• densest water

MIW

• 13C

• S= 38.45

• warm, salty

Pacific ocean water masses

• no deep water forming in N pacific

  

Deep water formation areas