 # 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.

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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”

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 freshwater ρ fw (4 °C, 1 atm) = 1000 kg m -3 ρ sw (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 - ρ sw (T,S,1 atm) ~1025 to 1028 kg m -3 oceanographers use “sigma-t” for density σ t (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, σ t profiles σ t 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 (σ t ) T-S data isopycnals – lines of constant density

T-S diagram example water mass 1 T=20 & S=36 water mass 2 T=22 & S=35 σ 1 >> σ 2 water mass 2 is less dense or “buoyant” relative to 1 1 2

water mass 1 T=20 & S=36 water mass 2 T=17 & S=35 σ 1 ~ σ 2 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 NADW AAIW

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 (σ θ ; “sigma theta”) - σ(θ, S,1 atm) - density evaluated with potential temperature θ < T due to adiabatic cooling σ θ > 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 σ θ > σ due to cooler temperature

Readings for next time: Reader pgs 39 – 51 “Density and Pressure in the Oceans” Text Chapter 7

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