Questions and Answers at end of Topic 2
1. Where in the Deep Sea would you look for the water that has been isolated from the
atmosphere for the longest time?
In the deep water (~3000m) in the far North Pacific.
2. What process causes the salinity to decrease southward in the core of the NADW?
Mixing with the AABW and AAIW, both of which have lower salinity than the NADW.
3. Why is there a salinity maximum at the bottom off New Zealand and an O2 minimum at middepth (~2000m)? (see Fig. 5)
The Salinity maximum is an indication of northward deep water flow from the Antarctic
Circumpolar current into the Pacific basin along the western boundary, in agreement with
Stommel’s theory of deep water circulation. Salinity decreases northward in the deep Pacific as
water parcels mix with lower salinity waters.
The O2 minimum at 2000m in the S. Pacific is evidence for the southward flow of Pacific Deep
water that returns deep water to the Antarctic Circumpolar Current. This deep water is old, since
it is at the end of the circulation pathway in the deep Pacific, and has low O2 because respiration
has consumed oxygen during the time the deep water has spent in the Pacific.
4. Why is there less curvature in the T vs S trends as one goes from the Atlantic to Indian to
Pacific ocean basins (see Fig 7)
The most T-S curvature is found in the Atlantic where there are two distinct deep source waters
(NADW and AABW) and an intermediate source water (AAIW) that cause an “S-shaped” curve
in T-S. (If you connected the T-S properties of these three source waters it would have an “S”
shape). As deep water moves eastward from the S. Atlantic into the deep S. Indian and deep S.
Pacific via the Antarctic Circumpolar Current, mixing tends to blend the T-S properties of these
three source waters. The result is to reduce the T-S “curvature” of the deep waters in the Indian
and Pacific ocean compared to the Atlantic.
5. Why is using  (Pot. Temp.) rather than in-situ temperature important when making a T-S plot?
Because θ corrects for temperature increase resulting from pressure effects and allows us to
compare θ values in the deep sea to θ values at different depths in the water column even at the
surface in deep water formation regions. If we used in-situ temperatures, the temperatures in the
deep sea would change simply as a result of the depth of the water mass. For example, in-situ
temperatures could deviate from a linear mixing line between two source waters because of a
change in depth only (pressure), which would be interpreted on a T-S plot as input from a third
source water. For a Deep Sea heat budget,  provides a pressure-independent indication of the
heat transported by deep water input.
6. If the deep water T and S distributions are unchanging (at steady-state), what process maintains
the shape of the T vs S distribution while mixing continues to reduce the curvature?
Mixing tends to remove the T-S curvature, by mixing water with different T and S properties,
whereas continued advective input of water masses tends to produce T-S curvature. For example,
the input of AAIW at ~1000m contributes water with low salinity. Balancing this input is mixing
with surrounding water, which has higher salinity. At steady-state, the loss of salinity from a
volume of water where AAIW is being input is exactly balanced by the gain of salinity via
mixing. Thus in most regions of the world’s deep sea (except possibly in the deep water
formation regions), temperature and salinity is not changing over time, i.e., T and S in the deepsea is at steady-state, and is maintained by a balance between input and loss of heat and salt.
7. Why can’t the approach of using T and S for determining the fraction of water mass sources be
used for the upper ocean (<1500m)? Hint: Are these shallow water parcels isolated from
atmospheric influences on T and S?
Water parcels in the main thermocline (100-1500m) mix with surface waters and thus their
temperature and salinity properties are influenced by local surface heat input and evaporation and
precipitation. This input of heat and salt make the T-S plot deviate from a simple mixture of
source water input. Look at a cross section of potential temperatures for the Atlantic (Pacific and
Indian) in the upper 1500m. These isotherms give you a good idea of the isopycnal contours,
which is turn, provide a reasonable indication of the preferred pathway of mixing in the ocean.
These shallow isopycnals intersect the surface ocean, where T and S are changed due to solar
heating and evaporation-precipitation. Thus along isopycnal mixing of heat and salinity make the
T-S plot deviate from simple mixing situation encountered in the Deep Sea (where isopycnal
surface reach the surface only in the regions of deep water formation.
8. If you measured a depth profile of NO3 concentration at the equator in the Pacific at what depth
would you expect to find the highest and lowest concentrations below 1500m?
The highest NO3 concentrations would be at ~1500m and the lowest at the bottom (~
4500m). This NO3 distribution occurs because the northward flowing bottom water is younger
and has lower nutrient concentrations than the older and shallower southward flowing deep water.
Since nitrate accumulates as a result of respiration, the older the deep water, the higher its nutrient
concentrations. Another factor is that respiration rates are higher towards the surface than at
depth because there is more organic matter in the water at shallower depths.
9. Explain why the 14C content of water parcels in the Deep Sea does not change over time
despite the constant decay of 14C.
The explanation is similar conceptually to the situation maintaining constant T and S in the Deep
Sea despite mixing constantly trying to dilute the T-S properties of the source waters as they
circulate. For 14C, constant input of deep and intermediate waters (NADW, AABW and AAIW),
with relatively high 14C levels offsets the 14C lost via radioactive decay. For any specific water
parcel, the input of 14C from advection and mixing exactly balances the loss to decay. See if you
can picture how the general circulation pathway in the Deep Sea brings in water with higher 14C
levels into each deep basin (most easily seen for the deep Indian and Pacific oceans).