1. Making water move
How it is mixed & transported
Vertical transport & mixing achieved by differences in density of different seawater masses
What effects the density of seawater?
Temperature
Salinity
Density affected more by
temperature than salinity,
because the former is more
variable in the ocean

2. Vertical mixing in the ocean
Density profile with depth at tropical and polar latitudes
Remember the thermal profiles with depth at tropic, temperate and polar latitudes
Polar regions essentially isothermal to the bottom, with coldest temperatures near the surface in winter
In temperate & tropical regions a warmer surface layer lies above a deep, cold layer
Warm water less dense than cold
In a density-stratified water column, warm surface water floats on colder water below
Temperate & tropical regions stable, polar regions-density unstable

3. Dense surface waters at the poles sink
At the poles in winter, cold surface water is more dense than underlying warmer water
Moreover, formation of sea ice at high latitudes concentrates salinity further contributing to increased density
The greater density of cold, saline polar waters causes them to sink and form the water at the bottom of all the world’s oceans at all latitudes by moving towards the equator
It takes hundreds of years for a water mass sinking at the poles to complete its journey

4. Deep Ocean Circulation
All circulation below 1000 m is accomplished principally by density differences of polar water masses and their transport both down in the water column and to lower latitudes
The density of a particular water mass determines what layer, or depth beneath the surface it resides at

5. Surface ocean circulation
What moves water horizontally across the ocean? ?

6. Surface ocean circulation
What moves water horizontally across the ocean?
Wind
Coriolis effect (due to the earth being a sphere that rotates from west to east)
The Coriolis force is that of the earth’s rotation on the movement of particles

7. Coriolis force • Particles moving toward the equator
move from low to high eastward
velocity; this lag deflects them to
the west
A particle at the equator travels east at 1613 km/h to complete a full rotation in one day
Particles nearer to the poles travel eastward much more slowly because the circumference is less
So, particles traveling from the equator towards the poles move from areas of high to low eastward velocity
They will have a relative deflection further east

8. Coriolis force
Whether moving towards or away from, the equator, the effect is the same:
In the northern hemisphere, parcels of water are defelcted to the RIGHT
In the southern hemisphere, parcels of water are deflected to the LEFT

9. Combining the effects of planetary winds and Coriolis in the movement of oceanic surface waters
Planetary wind system driven by differential heating of earth’s surface
Solar insolation is greatest near the equator and least near the poles
A given amount of sunlight spread over a larger area if it strikes at an angle: therefore, lower density of energy
Solar radiation must pass through more atmosphere at poles than at equator
Because of earth’s tilt, this pattern varies seasonally

10. Building a model of oceanic circulation
Planetary wind system driven by differential heating of earth’s surface
•Solar insolation is greatest near the equator and least near the poles
-A given amount of sunlight spread over a larger area if it strikes at an angle: therefore, lower density of energy
-Solar radiation must pass through more atmosphere at poles than at equator
•Because of earth’s tilt, this pattern varies seasonally

11. Building a model of oceanic circulation
•Warm air rises, cold air sinks
•Air at equator heated, so rises 30 60 N S

12. Building a model of oceanic circulation30 60 N S
•As surface air rises, it is replaced by
surface air flowing from north & south
•Risen equatorial air expands as pressure
is less high in the atmosphere
•Expanded air cools and starts to descend
but is pushed north and south by air
rising behind it.
•As air high in the atmosphere is deflected
north or south, coriolis acts on it pushing it
eastward; consequently it sinks to the surface
at ~30° latitude
•This air warms as it descends (takes up moisture
which is why the world’s deserts predominate at
these latitudes

13. Building a model of oceanic circulation30 60 N S
Building a model of oceanic circulation
•The first cell, or tropical cell,
drives the other 2 cells which
are more or less passive
•Blue arrows represent the winds
blowing across the earth’s surface
(on globe showing effect of coriolis
force on wind with red arrows)
•As wind blows over the surface
water, the water (because of Coriolis
force) does not travel in the same
direction as the wind
•Water is deflected at an angle of 45°
relative to the wind

14. Ekman Spiral
Friction of the surface water layer acting on layers beneath it, causes underlying layers to be dragged in motion.
They too are deflected by the coriolis force
As the influence of wind is greatest at the air-sea interface, the effect decays exponentially with depth and eventually ends
wind
45°
Surface current

15. Ekman Transport
The sum of these vectors over all depths results in a direction of net transport of water of 90° to that of the wind
90° to the right of the wind in the northern hemisphere
90° to the left of the wind in the southern hemisphere
wind
Surface current
45°
Ekman transport

16. Building a model of oceanic circulation
•Blue arrows represent Ekman transport of water
•D=Divergence (planetary upwelling of water)
•C=Convergence (water masses converging) N D 60 C 30
Oceanic cross-section D
At depth
surface 30 C L 30 60 60 D H S L H
H=High oceanic pressure
L=Low oceanic pressure L
Upwelled waters

17. Building a model of oceanic circulation
•Subsurface flow of water from high
to low areas of oceanic pressure
•Water masses deflected, as always,
by Coriolis force
•Results in mid-latitude, high
pressure oceanic gyres in both
northern & southern hemispheres
due to the presence of land
masses. N L H L H L S

18. Comparisons between ocean basinsH H H H
• Both oceans have equatorial currents that flow west
• An equatorial countercurrent that runs east along the equator
• Oceanic high pressure systems at 30° N&S that generate gyres in the ocean
• Polar current systems

19. Differences between N & S hemispheres
More land in northern hemisphere results in smaller polar current systems in both Pacific & Atlantic compared to southern hemisphere
In southern hemisphere, the west wind drift is circumpolar because virtually no land in its path at 60°S latitude; not only surface moving east, but sub-surface geostrophic flow as well.
Western intensification- weaker in south Pacific because of a leaky boundary associated with Indonesia
Eastside boundaries - at 60°S latitude, swiftly, moving open ocean to east because there is no land, except area between Palmer Peninsula of Antarctica and South America;
a string of islands that that act as a weir blocking free flow of water
This pile up of water is deflected north and becomes the exceptionally strong Humboldt (or Peru-Chile) current

20. Polar wind speed and wave height differences