OC 210 TOPIC 2: CIRCULATION IN THE DEEP SEA (Fall 2009)
Readings: Ocean Circulation Chap 6: sections 6.3-6.7.
Intro to Physical Oceanography (on-line): Chap 13.
PART 1
Objectives for this Topic:
1. Identify the major water masses that input water to the Deep Sea.
2. Identify and quantify the presence of these deep water masses based on their seawater
properties (T, S, nutrients, O2).
3. Describe the general circulation pathway in the Deep Sea.
4. Use natural and anthropogenic chemical compounds (tracers) to estimate the rate of deep
water circulation.
5. Understand the impact of deep water circulation on climate and anthropogenic CO2 uptake.
A. How does the formation of deep waters affect the earth’s climate?
1. The cooling of northward flowing warm surface water (initially via the Gulf Stream) in
the far N. Atlantic has two effects (Fig. 1)
a. It increases the density of the water to the point where it can sink to great depth
(~2000-3000m) and enter the circulation of the Deep Sea
b. It releases heat from the ocean to the atmosphere
i. This transfer of heat from the surface ocean to the atmosphere warms the
air masses over the N. Atlantic
ii. Air temperatures in lands surrounding the N America are warmer than
land areas outside this region at similar latitudes because of this oceanair heat transfer
c. Climatologists theorize that a slow down in this deep water formation process
played a key role in the occurrence of Ice Ages during the last 1 million years.
2. A reduction in the formation rate deep water in the N. Atlantic would have a cooling
effect on air temperature over the N. Atlantic Ocean and surrounding land areas (Europe,
Canada) (Fig. 2)
a. Less deep water formed means less heat would be released to the atmosphere and
air temperatures would cool
3. One concern among climatologists and oceanographers is about future changes in deep
water formation rates and the subsequent impact on the earth’s heat budget.
a. One idea is that global warming will increase ice melt on the continents
surrounding the N. Atlantic (e.g., Greenland, Canada, Scandinavia, etc.)
b. This increased ice melt will lower the salinity of the surface waters, decreasing
their density and make it more difficult for surface waters to sink as they cool.
c. This reduction in deep water formation rate will, in turn, slow the rate at which
surface currents (e.g., Gulf Stream) transport heat from the tropics to the polar
latitudes.
d. This reduced poleward heat flow will likely make the tropics warmer and the
polar latitudes colder. (Fig. 2)
i. This is the movie Day After Tomorrow scenario…..but a much less
dramatic version.
e. Most ocean circulation models predict a future slowing of deep water formation
in the N Atlantic as a result of a predicted surface salinity decrease due to
increased ice melting in polar regions caused by the warming effects of the
anticipated increase in atmospheric greenhouse gas concentrations. (Fig. 3)
f. Currently, it is difficult to determine the likelihood that this will happen and the
magnitude and timing of the consequential temperature change (i.e., the accuracy
of the model predictions is difficult to determine)
4. Generally, oceanographers agree that the rate of deep water formation in the N Atlantic is
a very important factor in maintaining the present distribution of temperature on earth
and human induced perturbation of this formation rate could have significant climatic
consequences.
B. Characteristics of the Seawater in the Deep Sea
1. Generally, the Deep Sea is the portion of the ocean at depths greater than ~1500m where the
water is cold and homogeneous in its properties (compared to surface waters) (Fig. 4).
2. Typical Water Properties in the Deep Sea
 Cold temperatures, typically -1° to 3°C
 Low salinity (than most surface waters), typically 34.5-35
 High potential densities, σ = 27.7 to 28.0 (primarily due to cold temperatures)
 High nutrient concentrations and low dissolved oxygen concentrations as a result of
respiration
- Remember: no photosynthesis occurs in the Deep Sea because it is much too
deep for sunlight to penetrate
- The main source of oxygen in the ocean is the dissolution of oxygen gas in
air, which occurs at the surface of the ocean.
4. Current speeds are very slow, typically ~0.1 cm/sec
 usually too slow to measure directly with current meters so circulation pathways are not
as well known as for surface ocean
 circulation pathways have largely been determined by theory and by using properties of
seawater (like , S, oxygen, nutrients) to trace circulation pathways
 however, in a few places (e.g., along the western edge of the deep N. Atlantic (where
North Atlantic Deep Water is flowing southward) current speeds can be fast enough (>1
cm/s) to measure directly
5. The average water residence time in the Deep Sea is about 800 years
- thus, it takes ~800 years for the average water parcel in the Deep Sea (depths > 1500m)
to return back to the polar regions of ocean’s surface where they originally sank.
- if we divide the path length of deep water flow by the average time it takes to
cover this path (~800 years), we find that the mean current velocity is ~0.1 cm/s (or 30km
per yr or 1/3º per yr)
C. Geographic Regions where Surface Waters sink into the Deep Sea
1. Deep water is formed when surface water gets very dense (primarily cold and, to a lesser
degree, salty) and sinks to depths >2000m. This process occurs in regions where the air
temperatures are very cold and water column stability ( /Z) is low (at polar latitudes).

2. Two primary sites of deep water formation are the far north Atlantic Ocean (off the coasts of
Greenland, Labrador and Norway) and in the far south Atlantic Ocean off the coast of
Antarctica primarily in the Weddell Sea. (Fig. 5).
-Very little deep water forms off the coast of Antarctica (Ross Sea) in the South Pacific
Ocean. No deep water forms in the North Pacific.
- No deep water forms in the Indian Ocean.
3. In the far North Atlantic, deep water is produced at three locations (Fig. 5)
- Greenland (or Irminger) Sea, off the eastern coast of Greenland (~80% of deep water
formation in the N Atlantic)
- Norwegian Sea, off the coast of northern Norway (some of this deep water may
originate in the Arctic Ocean)
- Labrador Sea, between east coast of Labrador and western coast of Greenland
4. The surface water that ultimately cools to become deep water in the far N. Atlantic originated
as warm and salty tropical currents that flowed northward via the Florida Current, Gulf Stream
and N. Atlantic Current (Fig. 1)
- During winter, these surface waters eventually cool sufficiently by the time they reach
the far N. Atlantic (near Greenland, Labrador and Norway) to reach densities that allow
surface water to sink into the Deep Sea
5. The cumulative deep water formed by the sinking of surface waters in all three regions in the
N. Atlantic is called North Atlantic Deep Water (or NADW).
- The average rate of NADW formation is about 15 Sv (15x106 m3/s)
6. At the other end of the Atlantic Ocean, in the Weddell Sea off the coast Antarctica (across from
the tip of S. America) is the other primary site where deep waters are formed (Fig. 5).
7. In the Weddell Sea, deep waters are formed as a result of three processes (Fig. 6)
a. upwelling of deep cold water caused by winds
b. cooling at the surface during winter because of extremely cold air temperatures
c. increase in salinity due to sea ice formation in shallow ice shelves off the Antarctic coast.
- during ice formation the ice rejects most of the salt and thus makes the remaining
sea water saltier and, thus, denser.
8. The deep water formed off of Antarctica is called Antarctic Bottom Water (or AABW)
 as the surface water sinks, it entrains (mixes with) upwelling deep water in about a 50/50
mix of sinking surface water and upwelling deep water. (Fig. 6)
 the estimated rate of AABW formation is ~20 Sv (~10Sv of sinking surface water and
~10Sv of entrained deep water)
 AABW is dense enough to sink to the bottom of the Atlantic Ocean (denser and deeper
than NADW)
9. The entrainment of deep water during the formation of AABW increases its nutrient
concentrations and lowers its oxygen concentration (Why?)
-thus AABW has lower dissolved O2 and higher nutrients (CO2, PO4, and NO3) than
NADW (which is a deep water with a surface water source, where typically nutrient
concentrations are low and oxygen concentrations are high).
10. Deep water formation rates of both NADW and AABW are sporadic both in time and space
 more deep water is formed during years with cold winter air temperatures and less is
formed during years with warm winters
 because deep water is formed in relatively small geographic areas (that are fairly
inhospitable), it can be difficult to locate the exact formation region in any given year
 it is difficult to directly measure deep water formation rates in any one year
-there are better estimates of longer term formation rates (over several years) that are
obtained from moored current meters or from the concentration of chemical “tracers”
in the deep water
 our best estimates of deep water formation rates are ~15 SV for NADW and
~ 20 Sv for AABW yielding a total deep water input rate to the Deep Sea of about 35 Sv
(35x106 m3/s)
11. There is no deep water formed in the Indian Ocean and very little in the Pacific Ocean
 in the Indian Ocean, surface waters do not get cold enough to sink (the N. Indian ocean
doesn’t extend past ~20ºN)
 in the Pacific, surface waters have lower salinity than in the Atlantic which makes it them
less dense and, thus, more difficult to sink
 the N. Pacific does not extend geographically as far north as the N. Atlantic and thus
surface waters cannot be cooled as effectively
 in the S. Pacific off coast of Antarctica (Ross Sea), some deep water is formed but at a
significantly slower rate than the rate of AABW formation in the Weddell Sea
D. Formation of Intermediate Waters
1. Water masses that become dense enough at the surface of the ocean to sink, but are not dense
enough to sink to great depths are called Intermediate Waters.
-Intermediate Waters typically sink to depths of about 600 to 1500m, which is either in
the thermocline or at the upper boundary of the Deep Sea.
2. The most important Intermediate Water is called Antarctica Intermediate Water (AAIW).
 AAIW is formed at around 50ºS - 60°S in all the ocean basins (Atlantic, Indian and
Pacific) at the location of the Antarctic Polar Front at the northern edge of the Southern
Ocean (Fig. 6).
 Surface waters that form AAIW get dense (cool) enough to sink to 800-1000m
 AAIW spreads northward penetrating to almost ~ 20°N in all ocean basins (Pacific,
Atlantic, Indian oceans) (Fig. 7)
3. The outflow from the Mediterranean Sea (often called Med Water) is another Intermediate
Water (Fig. 8)
 Med Water (MW) sinks to about 1000m and spreads throughout the N. Atlantic only (not
nearly as wide spread as AAIW)
 MW has very high salinity (S~36.5), which increases its density, yet it is warm (
~12ºC).
 Thus the location of the tongue of high salinity at 1000m caused by MW input illustrates
the clockwise circulation path of water parcels at this depth in the N. Atlantic. (Fig. 8)

MW is produced (formed) at a relatively slow rate (~2 Sv) and therefore has a much
smaller distribution than AAIW
6. In the N. Pacific, an intermediate water is formed (called North Pacific Intermediate Water or
Pacific Subarctic Intermediate Water) in the northwest corner of the N. Pacific near the
Kamchatka Peninsula and in the Sea of Okhotsk and sinks to a depth of about 500-700m.
(Fig. 7)
E. Seawater Properties of Deep and Intermediate Waters
1. The various Deep and Intermediate Waters produced in the ocean have distinct potential
temperature and salinity characteristics because their surface cooling and salinity histories are
different.
-thus we can use potential temp and salinity to identify the presence of these various
water masses in water parcels located in the Deep Sea.
2. In the Atlantic Ocean, two deep (AABW, NADW) and two intermediate water masses: (AAIW
and MW) are formed and are potentially present in water parcels in the deep Atlantic.
 the core (maximum presence) of each of these water mass sources are found at different
depths: AABW at the bottom 4000-5000m, NADW at ~2000-3000m, AAIW at ~8001000m and MW at ~1000-1200m
 the depth at which these deep and intermediate waters are primarily found depends on the
density at which these water masses reach when they sink
– the denser the surface water when it sinks, the deeper the water penetrates into the
Deep Sea
3. The average , S and  properties of these Deep and Intermediate Waters are:

 NADW has  = 3°C, S=34.95,  =27.84

 AABW has  = -1°C, S = 34.65,  =27.87

 AAIW has  = 3.5°C, S =34.2,  =27.2

 MW has  = 12°C, S =36.5,  =27.7

4. The presence of water which originally comprised AABW, NADW and AAIW is found in
water parcels throughout all three deep ocean basins (Atlantic, Indian and Pacific)
-water that originally sank in the regions of AABW, NADW and AAIW formation is
transported throughout the Deep Sea via deep currents and mixing
-in contrast, the influence of Med Water is found only in the Atlantic Ocean because its
formation rate is small and essentially diluted (by mixing) beyond recognition in the S. Atlantic
and, likewise, the presence of N. Pacific Intermediate Water is found only in the N. Pacific
because its formation rate is small
5. If AABW, NADW and AAIW were the only sources of heat and salt input to the Deep Sea,
then the  and S properties of any water parcel in the Deep Sea, wherever we found them, would
be a mixture of the  and S properties of AABW, NADW and AAIW.
 This situation would occur if there are no significant in situ sources or sinks of heat and
salt in the Deep Sea and if there were no mixing with other (e.g., thermocline) waters.
 A simple analogy is a bathtub with two faucets, one of which discharges blue water and
the other yellow water so the water in the tub will be either yellow (close to the one
faucet), blue (close to the other faucet) or green (a mixture of the two source waters)
Question: What process that we haven’t mentioned might affect the temperature of some water
parcels in the deep water? (Hint: occurs along some mid-ocean ridges).
6. A depth profile of salinity (and Pot Temp) in the South Atlantic (at ~40ºS 30ºW) is affected by
the inputs of AAIW, NADW and AABW (Fig. 9)
-there is a S minimum at ~800m which results from AAIW input
-there is a S maximum at ~2700m which results from NADW input
-there is a S minimum at the bottom which results from AABW input
7. The effect of AAIW, NADW and AABW input on the depth profile of Potential Temperature
is more subtle than for salinity but still noticeable. (Fig. 9)
8. The depth profile of salinity in the S. Atlantic (Fig. 9) illustrates the influence of horizontal
mixing and circulation on the vertical (depth) distribution of seawater properties in the ocean.
9. The influence of different sources of deep water on the  and S distributions can be more
clearly seen if  is plotted versus S, in what we call a T-S plot (Fig. 9)
• to construct a T-S plot, the Pot Temp and Salinity values measured at each depth
in the depth profile are plotted in T-S space and then a line is drawn through these points
 find where the  and S properties for the measurements at surface, 2000m and 4000m lie
on T - S plot
 the “S” shape in a T - S plot is caused by the inputs of deep and intermediate waters from
AAIW, NADW and AABW
 the input of water parcels with the distinct  and S properties of AAIW, NADW and
AABW show up as bends (inflections) in the T-S plot
10. On a T-S plot for the Atlantic Ocean, the location of the AAIW, NADW, AABW and MW
waters sources are distinct (separated from each other). (Fig. 10)
 Recall that the mean  and S values for NADW is  =3°C and S=34.95, for AABW is  =
-1°C and S=34.65 and for AAIW is  =3.5°C and S=34.2
- In terms of density, the sequence from lower to higher density is:
AAIW<MW<NADW<AABW
- Identify this density trend in Fig 10.
11.Let’s look at where the T-S values measured on a depth profile in the Atlantic Ocean lie
relative to the T-S properties of AAIW, NADW and AABW. (Fig. 11)
– the numbers on the T-S curve represent depth where 6=600m, 40=4000m, etc.
-Below 2000m, the measured T-S values fall along on a straight line connecting the T-S
properties of NADW and AABW
-this linear (straight line) portion of the T - S curve indicates that over this depth region
the water is essentially a mixture of only two sources, that is, AABW and NADW (as will be
discussed below)
-between 800m and 2000m, the T-S curve does not follow a straight line connecting the
T-S properties of AAIW and NADW
-this means that there is another source of T and/or S in this depth interval, which
is most likely input from Med Water
-one result of Med Water input is that water parcels with the T and S characteristics of
pure AAIW (=3.5ºC and S=34.2) are not found at the site of this measured depth profile even
though the measured T and S depth profiles are clearly influenced by input of AAIW water.
F. Determining the contribution that each water source (e.g., AAIW, NADW and AABW)
makes toward the T and S measured on a water parcel in the Deep Sea
1.Let’s assume steady-state conditions for  and S in the Deep Sea
-that is, the measured  and S of a water parcel isn’t changing with time and that T and S
are conservative (neither produced or consumed in-situ).
-the steady-state assumption is probably a very good one for the Deep Sea where the
residence times are very long and changes are very slow.
2. We’ll start with the simplest situation, that is, when there are only two water sources
contributing to the T-S properties of water parcels (Fig. 12).
-In this case, the T and S properties of the all the water parcels must fall on a straight line
(in T-S space) that connects the T and S properties of the two water sources.
-an example of this situation is seen in Fig. 9 in the depth region between ~2700m and
the bottom in the S. Atlantic (40ºS) where there is a linear section of the T vs S plot with the two
water sources being NADW and AABW
Class Problem: Plot (on Fig. 12) the T and S properties represented by a 50-50 and 25-75 mix of
the two source waters (I and II), where source water I has  = 10ºC and S = 36 and water II has 
= 4ºC and S =34. Demonstrate that any mixture of these two water sources has a pot temp and
salinity that falls along a straight line connecting the two water sources.
3. In practice, oceanographers measure the  and S of a water parcel in the Deep Sea and then
calculate the proportion or fraction that a deep or intermediate water source (like NADW,
AABW, AAIW, MW) contribute towards the measured  and S values.
-to calculate this proportion, one must know (assign) the  and S properties of the water
sources in addition to measuring the  and S properties of the water parcel.
4. Determining the proportion of a deep water source present in a water parcel is very useful in
testing deep ocean circulation theories and circulation models
- for example, does the mixture of AABW, NADW and AAIW predicted by a circulation
model or theory for a water parcel at a specific location in the Deep Sea agree with the
mixture of these water sources calculated from measured  and S
- if not, maybe the theory or model estimates of formation rates of AABW, NADW and
AAIW or mixing rates in the Deep Sea are incorrect
5. We can determine the fraction of each end-member water mass in any given sample by using
either a graphical or mathematical approach. This works because the temperature and salinity
most deep water parcels are not affected by in-situ processes, i.e. that is,  and S are conservative
properties of seawater and are only affected by mixing
-this situation is the same as mixing being the only process affecting the color of water in
a bath tub which has inputs of yellow and blue source waters, as described above
-a possible (but not too common) exception to the conservative characteristic of Potential
Temperature can occur near mid-ocean ridges where input of geothermal heat and hot
fluids can warm deep waters.
6. Finding the fraction of two water sources (e.g., NADW and AABW) present in a water parcel
using a graphical method. (Fig. 12)
● on a T-S figure, plot the measured T and S of the two water sources (I and II, in this
example) and the measured T and S of the water parcel (R, in this example)
●



on a T- S plot, the T and S properties of the water parcel must fall on a straight line
connecting the T-S points of the two water sources (assuming there are only two water
mass sources and that T and S are conservative)
on a T-S plot, measure (using a ruler) the length of the line between the observed T, S
point measured on the water parcel [R] and the T, S point of the water source which is
closest (source II in this case)
the ratio of this line length to the entire line length between water source I and II is the
fractional contribution of I (F1), where F1 = b/(b+a), to the measured T and S of water
parcel R
for example, if the measured  and S are 6.0°C and 34.7, respectively, then the water
parcel contains 33% water source I and 67% water source II.
7. Finding the fraction of three water sources (e.g., AAIW, NADW and AABW) present in a
water parcel using a graphical method. (Fig. 13)
 When there are three water sources contributing to the T and S properties of water
parcels, then the T-S of all water parcels must fall within the triangle defined by the T-S
points for the three water sources.
(This situation is analogous to the T and S properties having to fall on a straight line
for mixing of two water sources.)
 Find the fraction of each source water contained by a water parcel with measured T and S
- First, draw the triangle with corners determined by the T and S point for each
of the three water sources
- Second, draw lines from the measured T-S point of each of the three water
sources (I, II and III in the figure) through the measured T-S point of the
water parcel [R] until the side of the triangle is reached.
- Third, measure the ratio of the length of the line between the measured T-S
point and the point where the line intersects the opposite side of the triangle
to the entire line length between the T-S point of the water source and the
intersection with the triangle side. This ratio equals the fraction of the
measured T-S property of the water parcel that was derived from that water
source.
8. For example, in Fig. 13, the fraction of water source III present in water parcel “R” is the
length of line segment “f ” divided by the length of line segment “f+e”
- e.g., similarly, the fraction of I = b/(b+a) and fraction of II = d/(c+d)
-the fractions of I, II and III are about 0.38, 0.44 and 0.18, respectively applying the
graphical approach to Fig. 13
-NOTE: the sum of the three fractions should equal 1.0 (if not, you’ve incorrectly
calculated the fractions)
9. For a real example, NADW is actually made up of three water sources formed separately in the
Labrador, Norwegian and Greenland Seas. (Fig. 14)
10. NADW can be separated into a shallow (~2000m) and deep (~3500m) component with the
-the deeper component, called Lower NADW (seen as the circle inside the triangle in Fig.
14), is composed of water derived from all three source regions with highest contribution
from Greenland Sea water
Class problem: Graphically estimate the fraction of each of the three water sources (LSW, GSW,
and NSW) that make up Lower NADW, which is represented by the circle in the middle of the
cluster of points in Fig. 14.
- In this figure, LSW represents Labrador Sea Water, GSW represent Greenland Sea
Water and NSW represents Norwegian Sea Water.
- Use the centers of the circles to represent the T-S points of the sources waters and water
parcel.
-You should get fraction estimates of around 0.20, 0.17 and 0.63 for LSW, NSW and
GSW, respectively
11. Our estimate for the overall average  and S for NADW of 3.0ºC and 34.95 implies that
NADW is composed mostly of Labrador Sea water and less from Greenland and Norwegian Sea
waters
12. For other examples of using T-S plots to determine water mass contributions look at
Questions 6.9 and 6.10 in the Open University textbook.
13. A mathematical approach can be used to calculate the fraction of each water source in which
you set up enough equations to calculate the fraction of each source water (2 equations for 2
sources, 3 equations for 3 sources, etc.). For three sources, the three equations describe the
temperature, salinity and mass balances. By solving them simultaneously, each fractional
contribution (F1, F2 and F3 representing the fraction of each water source) is determined.
equation 1:
equation 2:
equation 3:
F1 + F2 + F3 = 1
(Mass balance)
F1*T1 + F2*T2 + F3*T3 = Tm (Heat balance)
F1*S1 + F2*S2 + F3*S3 = Sm (Salt balance)
-where F is fraction of the water source present in the water parcel. T1 and S1 are the Pot
Temperature and Salinity water source #1 (same for water source #2) and Tm and Sm are the Pot
Temp and salinity measured on the water parcel of interest.
-It is much easier to solve for F1, F2 and F3 using the graphical approach.
-Note: You only need to know the graphical approach, not the mathematical approach, to
determining F1, F2 and F3 from T and S measurements.
F. How does mixing affect the depth profile of T and S?
1. Water parcels mix with each other as a result of turbulence in the ocean.
 mixing occurs most easily (and thus fastest) along surfaces of equal density (isopycnals),
(which are oriented sort of horizontally in most places) because there is no buoyancy
effects to overcome by mixing
 mixing occurs most slowly between or across isopycnals (sort of vertically) because
buoyancy effects have to be overcome by mixing
 mixing occurs at several spatial scales, that is, via large scale organized motions like
currents, via large scale features like eddies, at smaller scales via turbulent (random)
motions of water parcels and at the smallest scales via molecular motion.
 the length scale being considered often determines which process dominates, generally
turbulent mixing dominates on short length scales (<1m to 10km) whereas currents and
eddies dominates over longer length scales (100-1000kms)


turbulence is an effective mixing process in both the Deep Sea and surface ocean
turbulence can mix both vertically and horizontally, however, the vertical mixing rate is
much slower (~106x slower) than the horizontal (along isopycnal) mixing rate because
vertical mixing has to overcome stability (σ/Z) in most regions of the ocean whereas
along isopycnal mixing does not
2. We’ll use a hypothetical example to demonstrate how mixing affects the vertical shape of
depth profiles of T and S and the shape of the T-S plot when three water sources are present (Fig.
15)
a. Start out with three distinct water sources in three separate layers (top row in Fig 15).
Notice the shapes T and S depth profiles and the T-S plot.
b. Look at the effect of turbulent mixing on the T and S depth profile and T-S plot (middle row
of figure). Explain why the T-S plot has a sharp angle.
c. After continued mixing, the T and S values of the middle layer no longer equal the initial T
and S of the middle layer. Explain why this occurs with continued mixing. The effect of
continued mixing is to “round” the sharpness of the angle.
d. Generally, the further away from the formation region of the source water (e.g., NADW)
that one measures a T and S depth profile, the longer the waters parcels have had to mix
and the smoother (less sharp) the T-S curvature. Also the fainter the original T and S
properties of the source water appear.
Question: What would the T-S plot look like in Fig. 15 after an infinite amount of time was
allowed for mixing to occur?
3. T-S plots with rounded curves, rather than sharp angles, are typically seen in the deep ocean
(Fig. 16)
4. The T-S plot also provides information about the stability of the water column if the depths
along the T-S curve are indicated. (Remember, density increases as you decrease temp and
increase salinity and isopycnals (constant  ) appear as curved lines on T-S plot. (Remember

why?)
-stability is maximized where the T-S line crosses isopycnals over the shortest depth
interval (i.e., perpendicular to isopycnals) and yields the highest Δ /Δz

-in contrast, if the T-S plot follows (parallel to) an isopycnal surface, then there is little
stability
Question: Over what depth interval is the water column stability the greatest based on the T-S
plot in Fig. 16? Over what depth interval is the water column stability the least? Consider depths
>100m only.
5. An unexpected situation occurs when water masses with two different temperatures and
salinities, but with the same density, are mixed together. The resulting mixture has a density
which is greater than the starting density of the original water masses.
 This is more easily seen in Fig 10 where the curvature of lines of constant density is more
clear

if you make a mixture of two water masses with different T and S properties but which
lie on the same isopycnal, then the T and S property of the resulting mixture follows the
straight line connecting the two water mass sources. This mixture, however, is denser
than the density of either source, that is, the density of the mixture lies on a denser
isopycnal than the isopycnal of the two sources.
 this occurs because the relationship between temperature and density is not linear,
especially at cold temperatures (<2°C)
 this phenomenon is called cabelling
● For example: make a 50-50 mixture of two water masses with 1= 2° and S1= 35.04 (
=28.00) and 2= 8.5° and S2=36.0 (=28.00). You might predict that the mixture would have a 
 =28.00, however, the mixture (T=5.25° and S=35.52) has a  of 28.04, slightly but
significantly denser than the source waters.
6. NOTE: Although T-S properties can tell us what proportion of each deep water mass is
present in a water parcel, the T-S properties cannot tell us is the rate at which the deep water
masses have formed or circulated. We have to use other methods, which provide ages of water
parcels, to determine how long it takes for water parcels to circulate and mix in the deep sea.
G. The Conveyor Belt circulation pathway in the Deep Sea
1. The T and S properties of the deep water in the Atlantic Ocean indicate that deep water sinking
in the both the far north near Greenland (NADW) and in the far south near Antarctica (AABW)
flow toward the equator (equatorward)
 Henry Stommel predicted this observation based on theory in the early 1950s (Fig. 17)
 Stommel’s theory indicated that the deep equatorward flow should occur in intensified
western boundary currents while the poleward return flow in the Deep Sea should occur
via weaker currents in the middle and eastern portions of the deep ocean basins
 In the late 1950s, Stommel’s postulated southward intensified flow of deep water along
the western edge of the deep N. Atlantic was verified using neutrally buoyant floats
- this deep current was called Deep Western Boundary Current
 Because deep flows generally are very slow (except in the intensified western boundary
currents), it has been difficult to verify whether the Stommel’s theorized flow pattern
really exists
2. The reasons why deep western boundary currents are present are dynamical and related to the
earth’s spin
 Stommel theorized that there must be equatorward flow of deep water formed in the polar
regions to balance the loss of deep water due to upwelling. This upwelling of deep water
is needed to balance the input of deep waters (like NADW and AABW) to the Deep Sea
and offset the downward diffusion of heat resulting from turbulent mixing.
 Stommel hypothesized that the equatorward flow would be concentrated in intensified
western boundary currents
 Note: Our theoretical understanding of abyssal circulation is still evolving. As our
observations of deep circulation pathways improve, our ability to verify the theory of
deep circulation will improve.
3. Stommel’s theorized flow path indicates that deep water that forms in the far N. Atlantic
(NADW) should flow southward all the way to the Antarctic Circumpolar Current (ACC) at ~5060°S (Fig. 17).
-In contrast, most of the northward (equatorward) flow of the AABW gets entrained into
the eastward flow of the ACC, but some passes underneath the ACC and flows northward at the
bottom along the western boundary of the S. Atlantic
4. The NADW and AABW that enter into the eastward flow of the ACC are transported into the
deep basins of the Indian and Pacific Oceans.
5. In the Indian and Pacific oceans, the deep water enters in the southwestern corner of the basin
and spreads northward along the western boundary. Some of this deep water eventually flows
southward out of each basin back to the ACC in the central and eastern portions of the Indian and
Pacific basins (Fig. 17).
a. Deep water circulation simulated by state-of-the-art General Circulation Models
(GCM) verify the essential pathway of deep flow predicted by Stommel’s
theorized circulation
6. How does the deep water produced in the Atlantic and carried to the Indian and Pacific oceans
return to the surface in the N. and S. Atlantic to complete the round trip?
 deep water warms during its trip from the AtlanticIndianPacific (via downward
mixing of heat from the warm upper ocean)
 this warming increases the buoyancy (decreases the density) of the deep water and causes
it to rise. Eventually this density decrease is sufficient to allow the deep water to rise
through the thermocline and, ultimately, become part of the surface ocean circulation.
7. Note: At steady-state, the rate of deep water returning to the surface ocean has to equal the
total deep water formation rates in the N. Atlantic (NADW) and Weddell Sea (AABW).
- that is, there is a mass flow balance between deep water input (NADW+AABW) and
loss (upwelling) in the deep sea at steady-state
-However, we don’t know well the pathway that returns deep water to the surface ocean
-oceanographers think a substantial amount of the upwelling from the Deep Sea occurs in
the Southern Ocean (south of 50ºS) in the region of the Antarctic Circumpolar Current (see Fig. 6)
8. Ultimately the upwelled deep water must return to the site of deep water formation to complete
the circulation circuit
● this general path of deep water flow gave rise to a Conveyor Belt analogy (Fig. 18)
-although the actual flow path is not this simple, the analogy is convenient
 in the Pacific, the major surface return path is through the Indonesian archipelago
 return flow of surface currents to the Atlantic, from the Indian and Pacific, occurs via the
Agulhas Current around the tip of S. Africa
 once in the Atlantic, ~15Sv gets incorporated into the Gulf Stream flow and is ultimately
advected to the far N. Atlantic to cool, sink and start all over again as NADW
9. Some deep water returns to the Weddell Sea without ever contacting the surface.
- this is the deep water that is entrained during the sinking of AABW
10. Our best estimate is that ~25Sv of the upwelled deep water returns to the regions where deep
water is formed in the far north and south Atlantic Ocean via surface circulation pathways
-about 15 Sv of surface water flow into the North Atlantic to form NADW and about 10
Sv of surface flow to Antarctica, where it sinks and entrains another ~10 Sv of deep water to form
AABW.
-thus our best estimate is that there is a total of about 35 Sv of deep water forming in
today’s ocean, ~15Sv via NADW and ~20 SV via AABW
Class Problem: Calculate the upwelling velocity (m/s) of deep water needed to balance 35Sv of
deep water forming in the polar regions if this upwelling occurred uniformly over the entire ocean
(ocean surface area = 360x1012 m2). How easy would it be to measure this rate directly? How
does this upwelling velocity compare to the average current speeds in the Deep Sea?