1. Wind driven, Coriolis modified Ekman and geostrophic flow
Surface circulation
Wind driven, Coriolis modified
Ekman and geostrophic flow

2. Surface currents - patterns
Similar in all basins
At low latitudes, have large, “closed” gyres
Gyres elongated in the E-W direction
Gyres centered on the subtropics (~30oN or S)
West-directed flow at N and S equatorial currents
East-directed flow ~ 45oN and S
N-S directed flow at eastern and western boundary currents

3. Surface currents - patterns
West-directed flow driven by tradewinds (coming from N or S-east
East-moving equatorial countercurrents
Western boundary currents are distinct, narrow (< 100km), swift (>100 km/day) and deep (2 km)
Eastern boundary currents are broad (>1000 km), weak (~10s km/day) and shallow (~500 m)
Have smaller, less developed polar gyres in N
Have circumpolar “gyre” in the S

4. Transverse currents
E-W currents driven by the trade winds (easterlies) and mid-latitude westerlies
Link the boundary currents
Equatorial currents
Moderately shallow and broad
Pile up water on west side of basin (W Atl is 12 cm [8”] higher than Pac; W Pac is 1 m higher than E Pac)
Eastward flowing currents at mid-latitudes are weaker (wider and slower) than equatorial currents
Differences in land mass distribution in N and S hemispheres affects flow

5. Why is this important? Processes in surface, wind-driven layers
Change in
productivity rates
affects atm CO2?
Assume these
fluxes are balanced
Change in
burial rates
affects atm CO2?
Why is this important? Processes in surface, wind-driven layers
are different but connected to processes in deep waters.
Fig. 8-9

6. (inorganic/tectonic)
Short-term
Long term (organic)
Long term
(inorganic/tectonic)

7. Link with short term C cycle
In surface oceans
“adds” back CO2
Onset of modern plate tectonics “turns this on”

8. Thermohaline Circulation

9. Thermohaline circulation
Vertical water movement
Driven by density differences (can be very small)
Remember temperature and salinity diagrams and the properties of water
Temperature and salinity profiles (with depth)
Salty water is denser than fresh water
Cold water is denser than warm water
Density gradients with latitude (due to temperature differences of surface waters)
Polar water has the most uniform density (weakest pycnocline) so is least stable

10. Fig. 5-6

11. Polar water column is
least stable; most
uniform density
weakest pycnocline
Surface is cold
Deep water is cold
everywhere
Easiest to rearrange
Formation of ice
excludes salt
Seawater freezes at
–2oC

12. Warming Cycle:
1. March
2. May
3. June
4. August
Cooling Cycle:
1. August
2. September
3. October
4. November
5. January

13. Temp. sea water

16. Wind

17. Most of the ocean is cold
Temperature degree C

20. Thermohaline circulation
As for the atmosphere, there are convergence and divergence zones where water masses collide or diverge
Important for global heat balance
Deep circulation and basin exchange of water, material, and heat

21. Thermohaline circulation
Deep circulation is driven by density differences
Horizontal movement along density surfaces
Movement is very slow (0.1 m/s)
Three layer ocean
surface mixed layer
Pycnocline
Deep water
Deep water formed at 2 places – N Atlantic and Weddell Sea (Antarctica)
Connection between surface and deep water
Diffusion (slow and along density gradients)
Mixing (e.g., storms)
Upwelling (polar, equatorial and coastal)

22. Deep circulation is
like a conveyer belt that
moves heat and water

23. Water masses Possess identifiable properties
Don’t mix easily – flow above or beneath each other
Surface water – to about 200 m
Central water – to bottom of main thermocline
Intermediate water – to about 1500 m
Deep water – to about 4000 m (not the bottom)
Bottom water – in contact with seafloor
Retain characteristics when the mass was formed at the surface (heating/cooling, evaporation/dilution)

24. Formation of deep water
Antarctic bottom water – densest water in the ocean – S (34.65%o), T (-0.5oC), dens (1.0279)
Weddell Sea in winter – when ice freezes and get brines
Sinks and creeps North
Pacific and Atlantic – 100’s to 1000’s of years to get to northern basin
North Atlantic deep water
Formed in the Arctic but escapes only through channels
Warm, salty water chills (heat is transferred to air)
Atlantic and Pacific deep water is less dense than Antarctic bottom water

26. Mediterranean deep water
Salty water (38 %o)
Underlies central water mass in Atlantic
Warmer than other deep water so not as dense

28. Isopycnal is a line of constant density
Different combinations of temp and salinity can make water of the same density
Isopycnal is a line of constant density
Mixing of water masses can increase the density
Increasing
density

31. Age of a water mass Oxygen and C isotopes
Water picks up oxygen and CO2 only at surface by exchange with atmosphere
Loses oxygen as it ages (respiration, reaction with rocks); 14C decays as it ages
Water masses mix slowly
Antarctic bottom water in the Pacific retains its character for up to 1600 years

32. Fig. 8-11

33. -70‰
-110‰
-150‰
-170‰
-230‰
-210‰
∆14C
This diagram represents the flow at a depth of 4000 m; the strange-looking continent/ocean configuration is what
we would obtain if the oceans were drained to this depth. (After W.S. Broecker and T.-S. Peng, Tracers in the Sea,
New York: Eldigio Press, 1982, Figure 1-12.)

34. Water residence time Bottom currents are slow
Antarctic bottom water ~1600 years
Other bottom water – years (time it takes to rise to the surface)
Fast bottom currents on bottom around objects
Surface currents are faster
Surface water – on the order of years
N Atlantic gyre may take about 1 year to complete a circuit

35. Deep water circulation
Flow may be slow but still modified by Coriolis

36. Convergence zones Two water masses meet
Usually one will go under the other (density)
If the same density, may mix to create a new and denser water mass (remember T-S diagram) – this is caballing
Formation of N Atlantic intermediate water, Antarctic intermediate water and Antarctic bottom water produced by mixing

37. S-curve tracks density
with depth
Points a and b on an
Isopycnal so are the
same density, despite
different temperatures
and salinities
If the two water masses
mix, will result in
denser water!

38. Convergence zones Areas of downwelling
Antarctic convergence zone at 50-60oS
South of this is the Antarctic Circumpolar Current
Subtropical convergence zone (40-50oS)
N boundary of Subantarctic Surface Water
Equatorward of subtropical convergence zone is Central Water which is the warmest and saltiest water
Subtropical convergence (45-60oN)
Arctic convergence – ill-defined because of land mass in the N

40. Divergence zones Areas of upwelling Tropical divergence
Antarctic divergence

41. Upwelling Sinking water must be offset by upwelling of water
Water sinks in a localized area, relatively rapidly
Water rises gradually over larger areas
When water moves to surface, must be transported to a pole to sink again
Diffuse upwelling maintains a permanent thermocline (1 cm/d)

42. Maintained by diffuse
upwelling

44. Main thermocline

45. Thermohaline flow
Sinking of surface water most pronounced in the North Atlantic
Water moves at great depths toward Southern hemisphere and wells up into the surface in the Indian and Pacific Oceans (~1000 years)
Slow circulation that crosses hemispheres superimposed on rapid flow of surface water in gyres
Some heat travels from S Pacific, across Indian Ocean and into the Atlantic
Flow distributes gases, solids, nutrients and organisms among ocean basins

46. Fig. 5-12

49. Fig. 8-9

50. The Redfield Equation 106 CO2 + 16 NO3- + HPO42- + 122 H2O 
(CH2O)106(NH3)16(H3PO4) O2
Can also develop similar ratios for trace elements required for growth (e.g., Fe, Mn, Zn, etc.)

51. Box Fig. 8-1

52. Shallow upper mixed layer
and main thermocline
Uptake by phytoplankton
Short residence times
Regeneration during particle sinking
Ocean circulation and/or vertical mixing
Long residence times
Thermohaline circulation

53. O2 decreases, nutrients increase as deep water is transported/ages
Norwegian
Sea
POM
POM
POM
POM
Atlantic
Indian
Pacific
Nutrient regeneration and O2 consumption
O2 decreases, nutrients increase as deep water
is transported/ages

54. supersat’d.
undersat’d.

55. plus nutrients fixed into the organic matter
and pCO2

56. Coastal Upwelling
Major upwelling
zones

57. Wind-induced upwelling
Note that transport goes to the right in the N Hemisphere

58. Seasonal upwelling due to seasonally reversing monsoonal winds

59. Seasonal upwelling Summer
Wind direction resulting from differential pressure over sea and land superimposed on a SeaWiFS chlorophyll image for the southwest monsoon. High chlorophyll concentrations in the western Arabian Sea are due to coastal upwelling
Winter
Schematic showing snow cover extent and wind direction superimposed on a SeaWiFS chlorophyll image for the northwest monsoon season. High chlorophyll concentrations are due to nutrients inputs from of winter convective mixing
(see animation at

60. Equatorial upwelling
Equator
SE trades

62. Net burial of CH2O in sediments
CO2 from the atmosphere
O2 to the
atmosphere
Brings nutrients
back for production
(100 %)
(> 99%)
Net burial of CH2O in sediments
(< 1%)
Fig. 8-9

63. For example phosphorus
The Oceanic P Cycle (all fluxes are 1010 mole P/yr)
River input
Surface Ocean
[PO4]avg = 0.15 µM
For example phosphorus
in the whole ocean 35
187.5
Downwelling
Particle flux
Ocean
volume
Ocean
P concentration
7.5
215
Upwelling
Whole Ocean
[PO4]avg = 2.05 µM
Input = output
Deep Ocean
[PO4]avg = 2.2 µM
Burial in
sediments 35

64. The Oceanic P Cycle (all fluxes are 1010 mole P/yr)
River input
Very different in its
component parts
Surface Ocean
[PO4]avg = 0.15 µM 35
187.5
Downwelling
Particle flux
7.5
215
Upwelling
Deep Ocean
[PO4]avg = 2.2 µM
Burial in
sediments
Whole Ocean
[PO4]avg = 2.05 µM 35

65. Meanders and eddies
Currents are water so, lack well-defined edges so have friction with adjacent boundaries
Meanders as it flows poleward
Sometimes get rings (warm and cold core) pinched off
Warm-core eddies rotate CW (in the N hemisphere)
Cold-core eddies rotate CCW (in the N hemisphere)
Can be 1000 km in diameter and persist for years, effects may be felt to the bottom and may be important for mixing up deep water (& nutrients)

66. Colder productive water
Warm, clear water

67. Eddies Meanders pinch off Trap warm or cold water at their center
Cold-core eddies in the Gulf Stream
Warm-core eddies N of the Gulf Stream
Cold-core eddies very nutrient rich and productive

72. Warm-core ring
Cold-core ring

73. Kuroshio eddies in
natural color!
Green is phytoplankton

74. Retain momentum of current
Formation of Rings
Retain momentum of current
Warm-core rings spin CW (N hemisphere)
Cold-core rings spin CCW (N hemisphere

76. Rings can penetrate to great depth To the seafloor?
Cold ring has low pressure “upwelling” in the center (Coriolis deflection to the right in N hemisphere)
while warm ring has high pressure downwelling in the center (same reason).

77. El Niño/La Niña
Normally get surface winds across the tropical Pacific moving East to West (easterlies or trade winds)
High pressure over eastern Pacific and Low pressure over western Pacific maintains this pattern
But, these pressure systems reverse or collapse with some periodicity (every 3-8 years)
Winds reverse and trade winds weaken or collapse
Change in atmospheric pressure is Southern Oscillation
Serious effects on productivity

78. El Niño/La Niña Wind-driven equatorial currents weaken or collapse
Warm water “sloshes” or flows eastward
Get east-flowing warm water around Christmas-time and this is El Niño
Combine terms and get El Niño Southern Oscillation (ENSO)

79. El Niño
El Niños occurred in the E Pac during , , 1993, 1994 and ,
Vary in strength

80. Normal
air pressure

81. Normal
SST with
Upwelling

82. Storms over land increase (CA)
Deeper thermocline
Failed upwelling or downwelling

83. El Niño
SST with warm
current and little
or no upwelling

86. Normal
Warm pool

91. Effects on productivity

92. ENSO events Thermocline deepens in the Eastern Pacific (EP)
Productivity decreases in EP
Sea level rises in EP
Increases rain and storms on adjacent land
Milder temps on west coast?
This year’s CA storms?

93. La Niña Not always right after an El Niño even
Colder-than-normal event
Thermocline decreases from normal (more intense upwelling)
Resumption and strengthening of trade winds
Winter temps warmer in SE and cooler in NW
Effects opposite of El Niño

94. North Atlantic Oscillation
Seesaw of atmospheric mass between subtropical high and polar low
Dominant mode of winter climate variability in the North Atlantic
Positive NAO index is stronger than usual subtropical high and deeper than normal Icelandic Low
– stronger than average westerlies at mid-latitudes
Increased pressure difference results in
Stronger winter storms crossing Atlantic
More northerly track to storms
Warm and wet European winter and cold and dry N Canada and Greenland winter
Mild and wet winter in Eastern USA