SIO 210 Physical properties of seawater (Lectures 2 and 3) First lecture: 1. Depth and Pressure 2....

SIO 210 Physical properties of seawater (Lectures 2 and 3)

First lecture:

1. Depth and Pressure

2. Temperature

3. Heat

4. Potential temperature

Second and third lectures:

1. Salinity

2. Density

3. Freezing point, sea ice

4. Potential and neutral density, stability, Brunt-Vaisala freq.

5. Sound speed

6. Tracers: Oxygen, nutrients, transient tracers

Talley SIO 210 (2014)

1. Salinity• “Salinity” in the oldest sense is the mass of matter (expressed in

grams) dissolved in a kilogram of seawater = Absolute salinity

• Units are parts per thousand (o/oo) or “psu” (practical salinity units), or unitless (preferred UNESCO standard, since salinity is mass/mass, but this has now changed again, in 2010)

• The concept of salinity is useful because all of the constituents of sea salt are present in almost equal proportion everywhere in the ocean.

• This is an empirical “Law of equal proportions”

• (There are really small variations that are of great interest to marine chemists, and which can have a small effect on seawater density, but we mostly ignore them; note that the new definition of salinity in TEOS-10* takes these small variations into account.)

*TEOS-10 is “Thermodynamic Equation of State 2010”


Salinity

• Typical ocean salinity is 34 to 36 (i.e. 34 to 36 gm seasalt/kg seawater)

• Measurements:

Oldest: evaporate the seawater and weigh the salts

Old: titration method to determine the amount of chlorine, bromie and iodine (prior to 1957)

Modern: Use seawater conductivity, which depends mainly on temperature and, much less, on salinity, along with accurate temperature measurement, to compute salinity.

Modern conductivity measurements:

(1) in the lab relative to a reference standard

(2) profiling instrument (which MUST be calibrated to (1))


Sea Salt: what is in it?

Millero et al. (Deep-Sea Res. I, 2008)Talley SIO 210 (2014)

Conductivity and salinity profiles

DPO Figure 3.2

X

Data from northern (subpolar) N. Pacific


Salinity

Bottles for collecting water samples

Autosalinometer for running salinity analyses relative to standard seawater

CTD (conductivity, temperature, pressure) for measuring conductivity in a profile (on the fly)

DPO Chapter S16Talley SIO 210 (2014)

Salinity accuracy and precision

Accuracy Precision

Old titration salinities (pre-1957)

0.025 psu 0.025 psu

Modern lab samples releative to reference standard

0.002 psu 0.001 psu

Profiling instruments without lab samples

? ?


“Absolute salinity” (TEOS-10)Absolute salinity = reference salinity + correction for other stuff

SA = SR + δSA

Reference salinity SR = 35.16504/35 * Practical salinity = 1.0047*psu

Reference salinity has been corrected for new knowledge (since 1978) about sea water stoichiometry as well as new published atomic weights.

The correction δSA is for dissolved matter that doesn’t contribute to conductivity variations: silicate, nitrate, alkalinity

Millero et al. (2008), IOC, SCOR and IAPSO (2010), McDougall et al. (2010)Talley SIO 210 (2014)

Surface salinity

DPO Figure 4.15

Note range of values and general distribution


Surface salinity (psu) in winter (January, February, and March north of the equator; July, August, and September south of the equator) based on averaged (climatological) data from Levitus et al. (1994b).

Surface salinityAquarius satellite salinity tour


http://podaac.jpl.nasa.gov/AnimationsImages/Animations

Atlantic salinity section

DPO Figure 4.11bTalley SIO 210 (2014)

What sets salinity? Precipitation + runoff minus evaporation (cm/yr)

NCEP climatology DPO 5.4

Salinity is set by freshwater inputs and exports since the total amount of salt in the ocean is constant, except on the longest geological timescales


Return to Atlantic potential temperature:what about this inversion – why is it vertically stable?


S. Atlantic (25°S)

X

We now see how the water column can be stable with a temperature minimum, since there is also a large salinity minimum.Talley SIO 210 (2014)

2. Seawater density Seawater density depends on S, T, and p

= (S, T, p) units are mass/volume (kg/m3)

Specific volume

(alpha) α= 1/ units are volume/mass (m3/kg)

Pure water has a maximum density (at 4°C, atmospheric pressure) of

(0,4°C,1bar) = 1000 kg/m3 = 1 g/cm3

Seawater density ranges from about 1022 kg/m3 at the sea surface to 1050 kg/m3 at bottom of ocean, mainly due to compression


Equation of state (EOS) for seawater

• Common way to express density is as an anomaly (“sigma”)

(S, T, p) = (S, T, p) - 1000 kg/m3

• The EOS is nonlinear• This means it contains products of T, S,

and p with themselves and with each other (i.e. terms like T2, T3, T4, S2, TS, etc.)


Seawater densitySeawater density is determined empirically with lab measurements.

New references: TEOS-10 and Millero et al. (linked to notes)

UNESCO tables (computer code in fortran, matlab, c) (see link to online lecture notes)

From Gill textbook AppendixTalley SIO 210 (2014)

• (S, T, p)• Changes in as a function of T,S,p:

• Thermal expansion coefficient

• Haline contraction coefficient

• Adiabatic compressibility

Equation of state for seawater

€

dρ =∂ρ

∂TdT +

∂ρ

∂SdS +

∂ρ

∂pdp

= −αρdT + βρdS + κρdp

€

α =−1

ρ

∂ρ

∂T

€

β =1

ρ

∂ρ

∂S

€

κ =1

ρ

∂ρ

∂p

See course website link for correction to DPO Section 3.5.5


Generally positive for seawater

Positive

Positive

Seawater density dependence on pressure

DPO Figure 3.4For water parcel at 0°C, S = 35 (psu)


Dependence of density on T, P

T p


Seawater density, freezing point

DPO Figure 3.1Talley SIO 210 (2014)

Where does most of the volume of the ocean fit in temperature/salinity space?

DPO Figure 3.1

75% of ocean is 0-6°C, 34-35 psu

50% is 1.3-3.8°C, 34.6-34.7 psu (=27.6 to 27.7 kg/m3)

Mean temperature and salinity are 3.5°C and 34.6 psu


3. Digression to freezing point and sea ice• Freezing point temperature decreases with

increasing salinity• Temperature of maximum density decreases

with increasing salinity• They cross at ~ 25 psu (brackish water).• Most seawater has maximum density at the

freezing point

• Why then does sea ice float?


Sea ice and brine rejection• Why then does sea ice float? (because it is

actually less dense than the seawater, for several reasons…)

• Brine rejection: as sea ice forms, it excludes salt from the ice crystal lattice.

• The salt drips out the bottom, and the sea ice is much fresher (usually ~3-4 psu) than the seawater (around 30-32 psu)

• The rejected brine mixes into the seawater below. If there is enough of it mixing into a thin enough layer, it can measurably increase the salinity of the seawater, and hence its density

• This is the principle mechanism for forming the densest waters of the world ocean.


http://www.youtube.com/watch?v=CSlHYlbVh1c

http://www.youtube.com/watch?v=CSlHYlbVh1c

Surface density (winter)


Surface density sq (kg m–3) in winter (January, February, and March north of the equator; July,August, and September south of the equator) based on averaged (climatological) data from Levitusand Boyer (1994) and Levitus et al. (1994b).

4. Potential density: compensating for compressibility

Adiabatic compression has 2 effects on density:

(1) Changes temperature (increases it)

(2) Mechanically compresses so that molecules are closer together

As with temperature, we are not interested in this purely compressional effect on density. We wish to trace water as it moves into the ocean. Assuming its movement is adiabatic (no sources of density, no mixing), then it follows surfaces that we should be able to define. This is actually very subtle because density depends on both temperature and salinity.


Potential density: compensating for compressibility

Sigma-t: This outdated (DO NOT USE THIS) density parameter is based on temperature and a pressure of 0 dbar

t = (S, T, 0)

Potential density: reference the density (S, T, p) to a specific pressure, such as at the sea surface, or at 1000 dbar, or 4000 dbar, etc.

= 0 = (S, , 0)1 = (S, 1, 1000)…..4 = (S, 4, 4000)


Potential density: compensating for compressibility

Potential density: reference the density (S, T, p) to a specific pressure, such as at the sea surface, or at 1000 dbar, or 4000 dbar, etc.

First compute the potential temperature AT THE CHOSEN REFERENCE PRESSURE

Second compute density using that potential temperature and the observed salinity at that reference pressure.

= 0 = (S, , 0)1 = (S, 1, 1000)…..4 = (S, 4, 4000)


Potential density profiles ( & 4): note different

absolute range of values because of different ref. p


An important nonlinearity for the EOS

• Cold water is more compressible than warm water

• Seawater density depends on both temperature and salinity. (Compressibility also depends, much more weakly, on salinity.)

• Constant density surfaces flatten in temperature/salinity space when the pressure is increased (next slide)


Potential density: density computed relative to 0 dbar and 4000 dbar

4


Potential density: reference pressures

Therefore 2 water parcels that are the same density or unstably stratified close to the sea surface will have a different relationship at high pressure


Atlantic section of potential density referenced to 0 dbar ( )

Note deep potential density inversion - need to use deeper reference pressures to show vertical stability


Atlantic section of potential density referenced to 0 dbar (sea surface):

Need to use deeper reference pressures to check local vertical stability (e.g. 4) Talley SIO 210 (2014)

Atlantic section of potential density referenced to 4000 dbar: 4

Potential density inversion vanishes with use of deeper

reference (4 ): in fact, extremely stable!!Talley SIO 210 (2014)

Isopycnal analysis: track water parcels through the ocean

• Parcels move mostly adiabatically (isentropically). Mixing with parcels of the same density is much easier than with parcels of different density, because of ocean stratification

• Use isopycnal surfaces as an approximation to isentropic surfaces


Isopycnal analysis: an isopycnal surface from the Pacific Ocean

Depth Salinity

WHP Pacific Atlas (Talley, 2007)Talley SIO 210 (2014)

Isopycnal analysis: an isopycnal surface from the Pacific Ocean

Potential temp. Salinity


Neutral density n

• To follow a water parcel as it travels down and up through the ocean:

• Must change reference pressure as it changes its depth, in practical terms every 1000 dbar

• Neutral density provides a continuous representation of this changing reference pressure. (Jackett and McDougall, 1997)

• Ideal neutral density: follow actual water parcel as it moves, and also mixed (change T and S). Determine at every step along its path where it should fall vertically relative to the rest of the water. This is the true path of the parcel.

• Practically speaking we can’t track water parcels.


Atlantic section of “neutral density”: n


Brunt-Vaisala frequency• Frequency of internal waves (period is time between

successive crests, frequency is 1/period or 2p/period)• Internal waves are (mostly) gravity waves• Restoring force depends on g (gravitational

acceleration) AND• Restoring force depends on the vertical stratification• So frequency depends on g and stratification

17 19


Brunt-Vaisala frequency

• The ocean stratification is quantified by the measured value of /z

• The stratification creates a restoring force on the water;if water is dispaced vertically, it oscillates in an internal wave with frequency

N = sqrt(-g/ x /z) If the water is more stratified, this

frequency is higher. If less stratified, frequency is lower.



Practical calculation of /z to get exact frequency, and also an exact measure of how stable the water column is:

Use a reference pressure for the density in the middle of the depth interval that you are calculating over (for instance, you might have observations every 10 meters, so you would reference your densities at the mid-point of each interval, I.e. change the reference pressure every 10m.

Calculation (right panel) is noisy since it’s a derivative



Values of Brunt-Vaisala frequency:0.2 to 6 cycles per hour

These are the frequencies of “internal waves”

Compare with frequency of surface waves, which is around 50-500 cycles per hour (1 per minute to 1 per second)

Internal waves are much slower than surface waves since the internal water interface is much less stratified than the sea-air interface, which provide the restoring force for the waves.


Seawater is compressible/elastic supports compressional waves or pressure waves

Sound speed:

Adiabatic compressibility of seawater (if compressibility is large then c is small; if compressibility is small then c is large:

5. Ocean acoustics: sound speed


Sound speed cs and Brunt-Vaisala frequency (N) profiles

cs

N


Ocean acoustics

• Sound is a compressional wave• Sound speed cs is calculated from the

change in density for a given change in pressure1/cs

2 = /p at constant T, S

This quantity is small if a given change in pressure creates only a small change in density (I.e. medium is only weakly compressible)

• Sound speed is much higher in water than in air because water is much less compressible than air


Sound speed profile: contributions of temperature and pressure to variation of cs

• Warm water is less compressible than cold water, so sound speed is higher in warm water

• Water at high pressure is less compressible than water at low pressure, so sound speed is higher at high pressure

• These competing effects create a max. sound speed at the sea surface (warm) and a max. sound speed at great pressure, with a mininum sound speed in between

• The sound speed minimum is an acoustic waveguide, called the “SOFAR” channel


Sound speed profile: contributions of temperature and pressure to variation of c

At Ocean Weather Station PAPA in the Gulf of Alaska at 39°N

DPO Figure 3.7

T S c

Pressure contribution

Temperature contribution SOFAR channel


c = 1448.96 + 4.59T – 0.053T2 + 1.34 (S – 35) + 0.016p (gives c in m/s if T in °C, S in psu, p in dbar)

• c increases ~5m/s per °C• c increases ~1m/s per psu S• linear increase with pressure/depth

Sound speed c has a complicated equation of state (dependence on T, S, p), but approximately:

Typical sound speed profiles in open ocean

Sound speed equation


Sound channel, or SOFAR channel (a wave guide)

Mid-latitude

High-latitude


6. Tracers

• Use tracers to help determine pathways of circulation, age of waters

• Conservative vs. non-conservative• Natural vs. anthropogenic

• We will return to this topic in “Typical distributions” lectures


Tracers on isopycnal surfaces

Oxygen Chloroflourocarbons


Tracers on isopycnals

3He 14C


SIO 210 Physical properties of seawater (Lectures 2 and 3) First lecture: 1. Depth and Pressure 2....

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Transcript of SIO 210 Physical properties of seawater (Lectures 2 and 3) First lecture: 1. Depth and Pressure 2....