OC210 TOPIC 1: SEAWATER PROPERTIES
Online Reference Material: Scripps Ocean210 Class at http://gyre.ucsd.edu/sio210/ “Properties
of Seawater” and Textbbok “Introduction to Physical Oceanography” by Robert Stewart at
http://www-ocean.tamu.edu/education/common/notes/contents.html. “Chapter 6”.
I. Importance of Density to Water Movement
1. The density of seawater largely controls its tendency to move vertically
-if density of water at surface is higher than below, the water parcel will sink to a level of
its own density
-in this situation the water column is ‘unstable’
-if density of water at surface is lower than below, the water parcel will not sink
-in this situation the water column is ‘stable’
-in this situation it takes energy input (usually from the wind) to "push" water
downward
-e.g., like submerging a rubber duck in bathtub (you supply energy)
2. Sinking of surface water generally occurs where there is cold air to cool water at surface
-this situation found at high latitudes, near the poles
-at these polar sites, surface waters cool and become dense enough to sink thousands of
meters (as we’ll see colder water is denser than warm water)
-sinking of surface waters is a very important mechanism to replenish waters in the ‘deep
sea’
3. In contrast, for most of the ocean (within ~50° of the equator) the surface waters are much
warmer and less dense than the cold waters found at depth
-under these conditions surface waters do not sink and thus there is no direct contact with
waters in the deep sea
4. The “deep sea” refers to the portion of the ocean below a depth of about 1500m (1.5 km)
-how deep is the ocean?....average of 4000m (~3 miles)
-in contrast, the layer of warm (10-30°C) surface water typically is only 100m deep
-since the water sinking to the deep sea is found near the poles and is cold at the surface,
the deep sea is also cold, typically <2°C
II. Seawater Properties that Control Density
1. Density is defined as the mass of water per unit volume
-use rho (ρ)as the symbol for density
-density units are grams per cubic centimeter (gms / cm3), kilograms per liter (kg/ L) or
kilograms per cubic meter (kg/m3)
2. Useful Conversions:
1 cubic meter (m3) = 1000 liters (L)
1 L = 1000 cubic centimeters (cm3)
thus 1 m3 = 1,000,000 cm3 = 106 cm3
3. Water density
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-density of freshwater at 4°C is 1.0000 gms/cm3 or 1.000 kg/liter or 1000 kg/m
1
-one gm of mass is defined as the weight of one cm3 of freshwater at 4°C
-why do you think “gram” is defined at a specific temperature?
4. Density range in ocean is from about 1.020 to 1.070 gms/cm3
-the changes in density are caused mainly by variations in pressure, salinity and
temperature
-colder water more dense
-saltier water more dense
-higher pressure causes density increase
-pressure increases with depth due to the mass of water above
5. Density (ρ) variations in the ocean are in the parts per thousand range
-that is, between 1.020 and 1.070 gm/cm3 the density changes by 50 parts per thousand
-(multiply 1.020 and 1.070 by 1000 and subtract one from the other)
-we also use σ (Sigma) to denote density, where σ = (ρ - 1)*1000
-sigma is useful to express the small changes in density (parts per thousand) that
we observe in the ocean
-thus if density of water is 1.025 gms/cm3, then ρ = 1.025 gms/cm3 and σ = 25
-remember that to convert a σ value to density (ρ) then divide by 1000 and add 1,
3
-e.g. σ = 28 means ρ = 1.028 gms/cm or 1028 kg/m3
A. Salinity Effects
1. Salt in seawater makes it denser than freshwater
-how much salt is in seawater?
- Typically seawater contains between 33 to 37 gms of salt per kilogram of seawater
-although the extremes of observed salinity range is 28 to 40 gms/kg
2. Experiment:
a. Start with freshwater at 4.00°C and a density of 1.000 gm/cm3 (σ = 0 )
-add 35 gms of salt to one liter (1 kg) of this freshwater (35 gms per 1000 gms)
-this resulting salinity is not 35 but closer to 33.8, Why?
-because 35 gms salt/ 1.035 kg seawater = 33.8
-seawater is denser than freshwater because of added mass of dissolved ions
3. Typically, the salinity decrease from the surface ocean to deep waters is very small, from
about 36 in the surface to ~34.7 in the deep water
-thus there is a correspondingly very small density decrease with depth of about 0.0012
gms/cm3 or 1.2 gm/liter due to only the salinity change
B. Temperature Effects
1. Temperature changes affects seawater density
-seawater density increases as water cools
-as water cools, H2O molecules pack more closely together (because the molecules are
vibrating less at lower temperatures) and take up less volume
-same number of water molecules in smaller volume yields higher density
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2. How much does the seawater density increase upon cooling from 20° to 0°C?
-seawater density increases from 1.0240 gm/cm3 (1024.0 kg/m3) at 20°C to 1.0273
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gm/cm (1027.3 kg/m3) at 0°C (at a constant salinity = 34 ‰)
-thus σ increases from 24.0 to 27.3
3. Globally averaged, there is a about a 20°C temperature decrease from the surface to bottom of
ocean which causes about a 3.1 gms/liter increase in density
-Notice that the density increase with depth caused by the temperature decrease more
than offsets the smaller density decrease with depth (about 1.2 gms/liter) caused by the average
salinity decrease with depth.
4. Thus the density effects of the temperature decrease with depth dominates over the salinity
decrease and makes the deeper water more dense than surface water
-this means that in most regions the ocean is a stable fluid, that is, it will take energy to
mix the ocean vertically
-this is not necessarily the situation in the polar regions
C. Density as a function of T-S
1. The Equation of State relates density to temperature (T), salinity (S), and pressure (P) (Fig 1)
-equation of state is experimentally determined in the lab
-and is therefore referred to as an “empirical” relationship
-the equation has lots of terms and thus is very cumbersome to use for “manual”
calculations
2. Putting the equation of state into a spreadsheet, however, makes it relatively easy to calculate
density from temp and salinity and pressure
-this will be a Homework Problem
3. Density is a calculated, not measured, characteristic of seawater.
4. The equation of state indicates that there is not a linear relationship between changes in
density and changes in temperature and salinity
-i.e., there are exponents in the T and S terms in the equation
5. The salinity of seawater also affects it’s freezing point temperature and it’s temperature of
maximum density (Fig 2)
-freshwater has maximum density at 4°C and freezing point at 0°C
-seawater at S=35 has maximum density at ~ –3.5°C and freezing point at ~ -2°C
-relationship between temperature of max density and temperature of freezing versus
salinity explains why it is easier to form ice on a lake (freshwater) than on a bay (seawater)
D. Pressure Effects
I. Pressure Effect on Temperature
1. Pressure (P) increases water temperature because the water molecules are packed tighter and
collide with each other more frequently and these collisions produce heat
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-if you take a parcel of seawater that has a temp of 0.00°C at the surface and submerge it
to 4000m, without losing any heat to the surroundings (called adiabatic warming), then the insitu water temperature will increase
-the term in-situ means measured at the actual location, in this case, by recording a
thermometer reading at 4000m.
2. The rate of in-situ temperature increase with depth for seawater ranges from about 0.05°C per
1000m at the surface to ~0.15°C per 1000m at 4000m.
-use 0.10°C per 1000m as an average rate of adiabatic heating in the ocean
3. The Potential Temperature of water is the temperature the seawater would have if it was
moved from its in-situ depth to the surface without any loss or gain of heat, i.e. adiabatically
-so the potential temperature of a water parcel that has an in-situ temperature of 0.40°C
at 4000m is about 0.00°C
0.40°C - 4000m*(0.10 /1000m) = 0.00°C
-this means that the act of moving the water parcel from the surface to 4000m accounted
for about 0.4°C of its measured in-situ temperature
-Potential Temperature is denoted by symbol theta (θ)
-thus θ = In-situ Temperature – Pressure * (∆temp/∆ Pressure)
4. Why do oceanographers use potential temperature, rather than in-situ temperature, to compare
the temperature properties of different water parcels in the deep ocean?
-Comparing potential temperature, rather than in-situ temperature, allows one to estimate
whether the two water parcels at different depths in the ocean could have had the same
temperature when those water parcels were at the surface.
5. For two water parcels that have the same potential temperature but different in situ
temperatures, no source of heat was need to cause the difference in in-situ temperature
-that is, the depth (pressure) difference alone was enough to cause the in situ temperature
difference
6. Note: To calculate theta (θ)we have to measure both in-situ temperature and pressure.
-oceanographers do this using a device called a CTD which measures the in-situ
conductivity (C) (which is related to salinity), temperature (T) and pressure which can be
converted to depth (D)
7. The pressure exerted by the entire column of air at the surface of the earth is equal to 1
atmosphere of pressure
-10m of seawater exerts approximately the same pressure as the entire column of air
-for every 10m increase in depth the pressure in the ocean increase by ~1 atmosphere
-for every 1m increase in depth in the ocean the pressure increases by 1 dbar (Fig. 3)
8. Units of Pressure
1 bar = 0.987 atmospheres
10 m seawater = 1 bar
1m seawater = 1 decibar (dbar or 0.1 bar)
1 atmosphere = 760 mm Hg
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II. Pressure Effect on Density
1. As pressure (P) increases, so does water density (Fig. 3)
-the water molecules pack together tighter as pressure increases
-the pressure increase with depth, due to the weight of the water above, and causes the
greatest density changes in seawater with depth (greater than the density changes due to
temperature and salinity changes)
2. If we take a surface water mass with density of 1.028 gm/cm3 and sink it to 5000m depth, the
in-situ density increases to ~1.050 gm/cm3 (Fig. 3)
-this 0.022 gm/cm3 (22 kg/m3) density increase is due to pressure increase only, not
temperature or salinity changes
-in comparison, the density changes due to the typical temperature and salinity
differences between surface and deep water were +0.0031 and –0.0012 gm/cm3, respectively.
3. The symbol sigma-t (σt) describes density of a seawater parcel assuming it was at the sea
surface to remove pressure effects on density
-changes in σt are due only to changes in T and S but not P
-thus σt = (ρsurface - 1) *1000
4. Tables of density (σt) as a function of T and S are available (Fig 4)
5. Connecting the Temp and Salinity characteristics of water masses that have the same density
yields a curved line (Fig 4)
-lines of constant σt are curved on a plot of T vs S
-thus water can have different T and S characteristics yet still have the same density
-one water mass can be warmer and saltier than another but have same density
-least dense water is warmest and freshest
-densest water is coldest and saltiest
6. The density term used most by oceanographers is potential density (σθ)
-σθ is the density of a water parcel that it would have at the sea surface (like σt) but also
corrected for the pressure effect that increases in-situ temperature
-that is, a pressure (depth) change affects both in situ density and temperature
7. The σθ of a water parcel is the density it would have at the surface correcting for both the
density and temperature increase that is caused by the pressure increase with depth
-thus σθ uses potential temperature, rather than in-situ temperature, to calculate density
8. Oceanographers most often use potential density (σθ) to describe the density of subsurface
water parcels
9. Using σθ allows oceanographers to:
a. compare the density of water masses in the deep sea with the density of surface waters
which could have been the source of the deep water
b. determine the stability of a column of water, which is independent of pressure effects
on temperature and density
c. determine the depth distribution of equal potential density “surfaces” or contours
along which water can mix without having to change density and, thus, overcome stability
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10. The σ , σt and σθ of water at the surface of the ocean are equal.----right?
11. Do we actually measure the density of seawater?
-No! We measure the salinity, temperature and pressure of seawater in-situ and then
calculate density from the equation of state which accounts for the change in density due to T, S
and compressibility of water.
III. TEMPERATURE, SALINITY AND DENSITY DISTRIBUTIONS
A. Temperature Distribution
1. Entire temperature range in ocean is ~ -1.9°C (freezing point at 35 ‰) to +40°C
-ocean has much temp range than air which varies from
-60°C to +60°C
2. Surface water temperatures vary much more than deep water temperatures (Fig. 5)
-most of the ocean is warm in the surface and cold at depth with an overall temperature
range of –1.5° to 29° C
-most deep water (>1500m) between –1.5° and 4°C
3. The potential temperature decrease with depth is greatest in top 1000m (Fig. 6)
-the depth region where temp decrease is greatest with depth is called the thermocline
-the rate of change of temperature with depth is called the temperature gradient
-a gradient expresses the rate of change of one variable relative to another variable,
-i.e., in this case the depth gradient in Potential Temperatrue = ∆θ/∆Z
-the steepness of the depth gradient in temperature depends on location
-it is greatest in the warm tropical ocean and least in the cold polar ocean
4. The ocean is divided into three layers based on temperature (Fig. 6)
-surface mixed layer which is warm and uniform in temp (typically 0 to ~100m)
-thermocline where temp decrease is largest (100m to ~1500m)
-deep sea where temp is cold and fairly uniform (1500m - 5000m)
5. Isotherms are lines connecting locations of constant temperature (Fig. 5)
-deep, colder temps found at surface at higher latitudes
-this is a hint that deep water comes from the surface in polar regions only
-If ocean had constant salinity, how would the contours of constant potential density
compare to isotherms?
6. Surface ocean warmest at equator and coldest at poles (FIG 7)
-mean sea surface temperature (SST) is about 18°C..range from -1 to 30°C
-the poleward decrease in SST is about equal to depth decrease in temperature
-is this a coincidence?
7. Isotherms of SST tend to follow latitude lines
-east-west variability in SST much less than north-south variability
-generally isotherms follow the latitudinal trends in solar radiation received at earth's
surface (Fig 8)
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-at what latitude is the highest temperature versus latitude gradient (∆T/∆latitude) found
in the Pacific ocean?
-what is the likely trend in surface density?
Class Exercise: Graph the trend in annual average SST versus latitude between 60°S and 60°N
(at 10° intervals) along 180°W (use information in Fig. 7).
-How would a similar graph look for the potential temperature at 1000m? (see Fig. 5)
B. Salinity Distribution
1. Salinity (S) range in the surface ocean is about 28 to 37 (gms salt per kg seawater) (Fig. 9)
-highest values in the Subtropical Gyres (20°- 35°)
-very large pools of salty surface water in all ocean basins
-lowest surface S at high latitudes, especially Arctic Ocean
-typically higher S in the Atlantic than Pacific ocean
-contours of constant S are called isohalines
-most of the world ocean has a salinity range of 32 to 36
2. What controls surface salinity distribution?
-mainly the relative rates of evaporation (E) versus precipitation (P) (Fig 10)
-when E>P, then surface ocean salinity increases
-when P>E, then surface ocean salinity decreases
-highest E-P at mid-latitudes
-causes high S in Subtropical Gyres
-river input at high northern latitudes is important
-causes low S in Arctic ocean
-the units for E and P are in m/yr or cm/yr and represent the amount (depth) of water lost
to evaporation (E) or added by precipitation (P) per unit time
3. Salinity does not vary monotonically with depth, like temperature does (Fig 11)
-salinities, generally, are greater in the surface than at depth—Why?
-overall there is little S variation in the deep sea (about 34.4 to 35 ‰)
-there is a major subsurface salinity minimum at 1000m in the southern oceans
-where is the salinity of the deep water similar to that of surface water?
-contours of equal salinity called isohalines
4. How does a subsurface salinity minimum (or maximum) occur?
-it can’t be the result of only vertical mixing between surface and deep water
because there is higher S above and below
-Is low S at depth caused by in-situ processes?
-NO, salinity is neither produced nor consumed once the water parcel moves
away from the surface (where evaporation and precipitation affect salinity)
-A subsurface minimum (or maximum) in S is caused by currents moving surface water
into this depth region
-For example, the S minimum at 1000m between 20°S and 50°S is caused by low salinity
surface water at about 50-60°S sinking to about 1000m and moving equatorward (this water mass
is called Antarctic Intermediate Water) (Fig. 11)
C. Density Distribution
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1a. In the surface ocean, temperature changes dominate density changes so the average surface
density increases poleward as surface water gets colder (Fig 12)
-at surface σ = σt = σθ because there is no pressure effect at surface
-remember σ = (ρ - 1)*1000
b. A poleward surface temperature decrease from 29° to 0° C (see Fig. 7) corresponds to a
density increase from 1022 to 1028 kg/m3, (σ = 22 to 28 ) which approximately equals the
density range observed (Fig 12)
-similarity between surface distribution of temperature and density suggests that
temperature, rather than salinity, largely determines the surface density distribution
c. Contrastingly a poleward decrease in surface salinity range from 36.5 to 33 ‰ decreases
density only from 1026 to 1023 kg/m3 (σ = 26 to 23)
-opposite to the poleward temperature effect on surface density
d. Generally surface temperature changes, rather than salinity changes, dominate surface density
changes
-i.e. surface temp changes yield twice the density change derived from surface salinity
changes
-thus in surface ocean, distribution of equal density contours (called isopycnals) looks
like distribution of isotherms
2. In the deep sea, the typical potential density range is much smaller (27.7 to 28) than in the
surface ocean (Fig 13)
-this density range is 20x less than the σθ range observed in surface waters
3. In the deep sea, both potential temperature and salinity distributions have significant effects on
potential density
4. When oceanographers are particularly interested in the potential density distribution in the
deep sea, they often calculate potential density relative to a deep depth (say 4000m) rather than
relative to the surface (which is done for σθ )
-they do this to reduce the errors in the potential density calculation
-if potential density is calculated relative to the in situ density at 4000m, then it is
expressed as σ4 and has larger values (~45) (Fig. 13)
-What differences are there between the distributions if σθ and σ4 in Fig. 13?
IV. STABILITY
1. Generally, the potential density of seawater increases with depth (Fig 13)
-on average σθ ranges from 24.5 to 27.8 between surface and bottom of ocean
-Is this surprising?
-No, colder deep water is denser than warm surface water
2. That σθ (not σ) increases with depth indicates that density increase it is not a pressure effect
-since σθ eliminates pressure effects on both density and temperature
3. Because σθ increases with depth, then the ocean generally is a stable fluid
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-that is, there is resistance to vertical motion of water in the ocean because vertical
motion would require mixing denser water upward or less dense water downward
-however, in polar regions with very low temperature changes over depth, this is
resistance to vertical mixing is significantly reduced
-in these regions, the isopycnal are more vertical (Fig 13)
4. Does this mean there is no vertical motion in the ocean?
-No. Energy derived from winds can overcome stability and vertically mix water
-seasonal cooling of surface waters can also eliminate stability and cause vertical mixing
(convection), especially during fall and winter
-mixing caused by winds and convection can extend to several hundred or even
thousands of meters in the polar oceans
-generally, however, the increase in potential density with depth observed in most of the
ocean means that horizontal mixing of water is easier to accomplish (and thus many times faster)
than vertical mixing
-parcels of water tend to mix along isopycnal surfaces where there is constant potential
density
-since isopycnals dip downwards in certain regions of the ocean (Fig. 13), mixing along
isopycnals can result in “vertical” mixing
5. Stability is a measure of the resistance to vertical water movement and is defined as the rate of
change of density with depth, i.e. the depth gradient of potential density
-the rate of change of density relative to change in depth is mathematically expressed
as ∆σθ/∆Z
6. Why do we use σθ to calculate stability and not in-situ density (σ)?
-pressure effects on density do not affect stability
-the in-situ density increase that sinking water parcels experience due to increased
pressure is lost if they return to their original position
7. The higher the stability of the water column, generally the lower the vertical mixing rates
because of the greater resistance to vertical movement of parcels of water (other things being
equal)
8. Example: You fill a 5000m deep beaker with seawater (homogeneous T and S).
-You measure the in-situ density versus depth and it increases from 1028 at the top to
about 1050 kg/m3 at the bottom of the beaker.
-Is the water in the beaker stable, i.e., is there any resistance to vertical motion?
9. The importance of using σθ (potential density), rather than σt, to examine the stability of the
water column is demonstrated by the very deep waters in ocean trenches (Fig. 14)
-in this situation, σt indicates an decrease in density with depth whereas σθ indicates a
very slight increase in potential density with depth
-σt changes imply an unstable water column whereas σθ changes imply a stable (slightly)
water column
-why would σ4 be better for this example?
10. Generally, the stability vs depth curve has a maximum in the thermocline of ocean at about
100-250m (Fig 15)
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-the stability maximum is represented by the maximum in the potential density gradient
with depth (∆σθ/∆Z)
-stability is low in surface mixed layer (due to wind mixing of the water) and lowest in
deep sea (where there are small temperature and salinity gradients)
-that is, ∆σθ/∆Z approaches 0 in the mixed layer and deep sea
11. Note: oceanographers often assume that depth is a negative number when expressed in an
equation, i.e., altitude above the earth’s surface is expressed as a positive distance above earth’s
surface and depth below the earth’s surface is expressed as a negative distance
-However, I usually do not follow this convention in my notes.
12. Surface heating and precipitation promote water column stability by lowing the density of
surface seawater whereas cooling and evaporation diminish stability by increasing surface
density
14. Vertical profiles of stability generally are greater in the tropics than in the polar regions (FIG
16)
-surface waters in polar regions are colder and thus denser, thus density difference
between deep water and surface water is less in polar regions
-warm surface water in tropics means density difference between deep water and surface
water is greater than in polar regions
15. Only in polar regions (poleward of 60°), where density of surface water approaches that of
deeper water is there the opportunity for surface waters to sink downward into the deep sea (see
Fig. 13)
-the polar regions are a deep water "window" to atmosphere where water destined to
wind up in deep sea resides temporarily at the surface before sinking to depth
-the increased density found in polar surface waters is primarily due to colder temps
16. The atmospheric conditions in these regions, mainly cold air temperatures, cool surface
waters and make them denser than water below
-cooled surface seawater will then sink to a depth where its potential density matches
that of surrounding water
17. In the far north Atlantic, near Greenland, and around Antarctica, primarily in the Weddell
Sea, the surface waters get cold enough in winter to sink to great depths (2000m to the bottom)
V. KEY POINTS
1.Range in temperature and salinity of surface ocean is much greater than in the deep sea.
2. Seawater density depends on pressure, temperature and salinity and increases with increasing
P, S and decreases with increasing T.
3. Sea surface temperatures are greatest near the equator and decreases poleward, generally
following latitude lines.
4. Potential temperature corrects the measured in-situ temperature for the heating due to the
pressure increases resulting from increasing pressure (depth).
5. Surface salinity highest in subtropics and is caused by an excess of E over P.
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6. Temperature changes in upper ocean (<1000m) are the primary cause of density changes,
whereas in deep sea both salinity and temperature contribute significantly to density changes.
7. Density of surface waters increases poleward generally following isotherms.
8. Potential Density corrects in situ density for density and temperature changes due to pressure.
9. Potential Density generally increases with depth and causes ocean to be a stable fluid.
10. Stability is the rate of change of density with depth and is greatest in tropics and least in polar
regions. The depth region where stability is greatest is in the thermocline.
11. At high latitudes cold surface waters can sink to great depths because of low stability.
QUESTIONS
1. If the observed density range for surface waters is about 1022 to 1028 kg/m3, then which of
the three factors (temperature, salinity or pressure) primarily controls the density of surface
seawater?
2. Calculate the volume change that occurs when a seawater parcel is cooled from 20° to 0°C and
the density has increased from 1024 to 1027 kg/m3 . Assume the initial mass of seawater was
1000.0 kg and the salinity was kept constant. What percent change in volume does this
represent?
3. What would be the approximate temperature, at the sea surface, of a parcel of seawater that
was raised from 2000m and had an in-situ temperature of 4.00°C? What would be it’s
approximate potential temperature at 2000m?
4. Estimate the surface temperature in Puget Sound based on information in presented in Figure
7? What processes would cause the observed temperature to deviate from this expectation?
5. Where does the range of surface and deep (>2000m) water plot in T-S coordinates? What is
the corresponding range in potential density?
6. Calculate the stability of the water column at the equator between 200 and 800m (in the main
thermocline) based on information in Fig 16. Check the units.
7. Why is the depth gradient of in-situ density much higher than the depth gradient of potential
density? (see Fig. 15)
8. Why should σθ be greater than σt everywhere in the ocean except at the surface?
9. Does the temperature of the ocean change over time? Explain why the answer depends on the
relevant time scale.
10. Explain the cause of the in situ density distribution at 500m shown in Fig. 12. Explain why
the potential density distribution would or would not look similar.
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