Water and Salt Balances
• So far, we covered the heat equation and heat balances, which
cause ocean temperatures to change.
• Water (or salt) balances cause ocean salinities to change.
Some facts:
• Oceans contain over 97% of water on earth
– Ice (polar ice caps, icebergs, glaciers) ~2%
– Ground water, soil, lakes, rivers, etc ~ 0.6%
• Total dissolved salts in the ocean ~ 5 x 1019 kg
• Dissolved solids brought by rivers ~ 3 x 1012 kg/year
(relatively small)
• Therefore, we can assume the conservation of salt: the total
amount of dissolved salt in the oceans is constant.
• What changes salinity?
– evaporation leaves salt behind  salinity increases
– precipitation (rain) dilutes water  salinity decreases
– rivers bring fresh water  salinity decreases
– when seawater freezes salt is released  salinity increases
– when ice melts fresh water released  salinity decreases
Note: global climate change may impact all those processes
(mm/yr)
More evaporation
in mid-latitudes
More precipitation
in low latitudes
Using the principles of conservation of mass and conservation
of salt we can solve some interesting problems
(steady state solutions)
input
=
output
mass: ρiVi + ρfR + ρfP = ρoVo + ρfE
Salt: ρiViSi = ρoVoSo
(f=freshwater; i=input; o=output)
[ρVS = (kg/m3)(m3/s)(g/kg)=g/s]
But assuming ρi  ρo  ρf (changes in V, E, P much larger than in ρ)
(1) Vo – Vi = (R+P) – E (net transport in strait balanced by surf.+river)
(2) ViSi = VoSo (in and out salt transports are balanced)
Examples of applications of the conservation
principles (“box models”)
Mediterranean Sea: evaporation > precipitation
 more salty than Atlantic  net flow into Med. Sea.
Black Sea: precipitation+rivers > evaporation
 less salty than Med. Sea  net flow out of Black Sea.
Strait of
Gibraltar
Bosphorus
Salinity of the Med Sea
(1) Vo – Vi = (R+P) – E
(2) ViSi = VoSo
Conservation of mass
Conservation of salt
Mediterranean Sea
Over the sill at the Strait of Gibraltar, know
Si and So well, and have an estimate of
Vi = 1.75 Sv (1 Sv = 106 m3/s).
from (2)  Vo = 1.68 Sv
From (1)  (Vo-Vi) = (R + P) - E = - 0.07Sv
negative means that E > (R + P) in the
Mediterranean.
Black Sea
Vi = 0.006 Sv. from (2)  Vo = 0.013 Sv
From (1)  (Vo-Vi) = (R + P) - E = +0.007Sv
positive means that E < (R + P) in the Black
Sea.
Residence time- the time it would take to replace all
the water (also called “flushing time”)
Med. Sea
input
Vi=
1.75 Sv
(5.5 x 104 km3/year)
Total
Volume=
3.8x106 km3
Residence time
Volume/Vi=
70 years
Black Sea
0.006 Sv
(0.02 x 104 km3/year)
0.6x106 km3
3000 years
Implications: MS is well ventilated with high oxygen content,
while BS has no oxygen below 200m and is stagnant with
little mixing downward.
In reality, the flushing time and water properties (e.g., salinity)
are more complex than the simple box model and involve other
effects such as mixing and wind forcing
small basin mixing
(limited to narrow fronts)
large basin-wide mixing
fronts
Some implications of salt and fresh water
balances for climate change:
• Melting ice in high latitudes will reduce sinking of dense
waters and slow down the thermohaline circulation
• Reduced Arctic sea ice will expose more water and
increase absorption of radiation by earth
• Increased rainfall and river runoffs brings more dissolved
materials to oceans that will change PH of oceans
(biological implications for coral reefs)
Note that there are now global satellite
observations of Sea Surface Temperature
(SST), Sea Surface Height (SSH), Surface
Waves, Surface Wind, Cloud Cover, Sea-Ice
Cover, etc., that are needed for weather/ocean
prediction and climate studies…
but there were no global observations of ocean
salinities… until June 2011
“The Aquarius science goals are to observe and model the processes that
relate salinity variations to climatic changes in the global cycling of water and
to understand how these variations influence the general ocean circulation.
By measuring salinity globally and synoptically every month for 3 years,
Aquarius will provide an unprecedented new view of the ocean's role in
climate.”
What results are expected from Aquarius?
How does it work?
• measuring microwave (1.43 GHz)
emission from the sea surface
• calculate “brightness temperature”
which is related to dielectric
properties which is affected by Sea
Surface Salinity (SSS)
• need to know Sea Surface
Temperature (SST) very accurately,
from other satellites
• need atmospheric corrections, etc.
(from Oceanography, Vol. 21, No 1)
Salinity in the coastal ocean:
Note that in coastal regions there are much larger and faster
variations in salinity than in the open ocean- they result from
the interaction of river flows with topography, coastal currents,
local winds, etc.
Next is an example from the Gulf of Mexico….
Simulations of salinity and river
runoffs in the Gulf of Mexico
Atchafalaya
Mississippi
Mid-term Exam#1, Mon. 9-23-2013
Material Covered: Lectures # 01-07 ; Knauss Chapters 1-4
• Ocean observations
– What we observed
– How we observed: local instruments, remote sensing,
Eularian & Lagrangian methods, etc .
• Properties of Seawater
– Distribution in the world oceans, how T,S change, why?
– Density and the Equation of State (UNESCO & linear)
– Static stability, speed of sound (SOFAR channel)
• Air-sea heat fluxes and balances
– The heat balance equation QT=Qs-Qb-Qe-Qh-Qv
Qt
T 
– How water temperature changes by heat fluxes:
hC p
– How local imbalances cause temperature changes (seasonal, daily, etc.)
• Salt/water fluxes and mass transports
Mid-term Exam#1, Mon. 9-23-2013
• 20% of course grade
• Closed books, no calculators, no cell phones
• 1 hour (up to 15min extra time if needed)
• What type of questions?
–
–
–
–
Multiple choice
Short questions and definitions
Interpretation of data, simple graphs
Explanation of processes, simple equations
• Answer only what you have been asked! (long answers on
irrelevant material will not help…)
Good Luck