Earth System Science II – EES 717
Spring 2012
1. The Earth Interior – Mantle Convection &
Plate Tectonics
2. The Atmosphere - Climate Models,
Climate Change and Feedback Processes
3. The Oceans – Circulation; climate and the
oceans
Key Points to Remember
1. The global wind system acts to redistribute heat
between lows and high latitudes.
2. Coriolis force influences the direction of winds as
they move from regions of high pressure to regions of
low pressure.
3. Differential heating between land and water greatly
influences global wind patterns.
4. Mid-latitude weather is systems are cyclones and
anticyclones. Low latitude circulation is characterized
by spiraling Hadley cells.
5. Heat is transported to higher latitudes through
convective motions and latent heat transfer.
6. Ocean-Atmosphere interactions are most intense
(strong coupling) in the tropics.
The Tropics
Pressure and
Winds
Circulation around Highs
and Lows & Redistribution of Heat
Extratropical Circulation – Planetary
Waves
Extropical
Dynamics –
Frontogenesis
Ocean-Surface Conditions Depend on Latitude,
Temperature, and Salinity
North
Temperature
Equator
Latitude
Tropic of Cancer
Tropic of
Cancer
Equator
Tropic of
Capricorn
South
Tropic of Capricorn
Salinity
Temperature
Salinity
Mid Ocean Average Surface Salinity
Table 6-3, p. 166
Sea-surface
temperatures
during
Northern
Hemisphere
summer
Sea-surface
average
salinities in
parts per
thousand
(‰).
Fig. 6-16, p. 168
The Ocean Is Stratified by Density
two samples of
water can have
the same density
at different
combinations of
temperature and
salinity!
Fig. 6-17, p. 169
The Ocean Is
Stratified into Three
Density Zones by
Temperature and
Salinity
a.The surface zone or
surface layer or mixed
layer
b.The pycnocline, or
thermocline or halocline
c.The deep ocean (~
80% of the ocean is
below the surface zone
Temperature (°C)
5
10 15
20
25
Polar
Tropical
Temperate
4,000
6,000
2,000
8,000
3,000
10,000
40
50
60
70
Temperature (°F)
Depth (ft)
Depth (m)
1,000
2,000
Typical
temperature
profiles at
polar, tropical,
and middle
(temperate)
latitudes.
Note that
polar waters
lack a
thermocline.
Water Transmits Blue Light More Efficiently
Than Red
most of the ocean lies in complete blackness
• Dissolved salts
 Major constituents and trace elements
 Conservative/nonconservative constituents
• Major Constituents = [] > 1 part per million






Na+
ClSO4Mg2+
Ca2+
K+
Sodium
Chloride
Sulfate
Magnesium
Calcium
Potassium
86 %
• Trace Elements = [] < 1 part per million
99 %
Gases
• Distribution with depth
Photosynthesis removes CO2 and produces O2 at the
surface
Respiration produces CO2 and removes O2 at all depths
Compensation depth (Photosynthesis = Respiration)
photosynthesis
CO2
O2
respiration
Oxygen and CO2 profiles
O2 Concentrations
Photosynthesis
Bottom water enrichment
oxygen
minimum
CO2 Concentrations
Direct solution of gas from the
atmosphere
Respiration of marine organisms
Oxidation (decomposition) of
organic matter
Surface Currents Are Driven by the
Winds
A combination of four forces – surface
winds, the sun’s heat, the Coriolis
effect, and gravity – circulates the
ocean surface clockwise in the Northern
Hemisphere and counterclockwise in the
Southern Hemisphere, forming gyres.
The North Atlantic gyre, a
series of four
interconnecting currents
with different flow
characteristics and
temperatures.
Surface Currents Flow around the Periphery of Ocean Basins
The Ekman spiral and the mechanism by which it operates.
Surface Currents Flow around the Periphery of Ocean Basins
Consider the North Atlantic.
The surface is raised through wind
motion and Ekman transport to form a
low hill. The westward-moving water at
B ‘feels’ a balanced pull from two
forces: the one due to Coriolis effect
(which would turn the water to the
right) and the one due to the pressure
gradient, driven by gravity (which would
turn it to the left).
The hill is formed by Ekman
transport. Water turns clockwise
(inward) to form the dome, then
descends, depressing the
thermocline.
Hydrostatic Pressure:
p = -rgz
z = depth
g = grav. Acc.
r = density of seawater
PGF per Unit Mass:
1/r x dp/dx = g x tan(q)
In oceanography, dynamic topography refers to the topography of the sea surface
related to the dynamics of its own flow. In hydrostatic equilibrium, the surface of the
ocean would have no topography, but due the ocean currents, its maximum dynamic
topography is on the order of two meters
Dynamics: The forces and motions that characterize a system
Seawater Flows in Six Great Surface Circuits
Geostrophic gyres are gyres in balance between the pressure gradient
and the Coriolis effect. Of the six great currents in the world’s ocean,
five are geostrophic gyres. Note the western boundary currents in
this map.
Boundary Currents Have Different Characteristics
Western boundary currents – These are narrow, deep,
warm, fast currents found at the western boundaries of
ocean basins.
The Gulf Stream
The Japan Current
The Brazil Current
The Agulhas Current
The Eastern Australian Current
Eastern boundary currents – These currents are cold,
shallow and broad, and their boundaries are not well
defined.
The Canary Current
The Benguela Current
The California Current
The West Australian Current
The Peru Current
Boundary Currents Have Different Characteristics
The general surface circulation of the North Atlantic.
Unit for measuring
flow rates (or
volume transported
by ocean currents):
sverdrups
1 sv = 1 million cubic
meters of water per
second
Boundary Currents Have Different
Characteristics
Eddy formation
The western boundary of the Gulf Stream is usually
distinct, marked by abrupt changes in water
temperature, speed, and direction.
(a) Meanders (eddies) form at this boundary as the
Gulf Stream leaves the U.S. coast at Cape
Hatteras. The meanders can pinch off (b) and
eventually become isolated cells of warm water
between the Gulf Stream and the coast (c).
Likewise, cold cells can pinch off and become
entrained in the Gulf Stream itself (d). (C = cold
water, W = warm water; blue = cold, red = warm.)
Vertical Movement of Water
Wind induced vertical circulation is vertical movement induced by
wind-driven horizontal movement of water.
Upwelling is the upward motion of water. This motion brings cold,
nutrient rich water towards the surface.
Downwelling is downward motion of water. It supplies the deeper
ocean with dissolved gases.
Consider: West and East Winds
29
Nutrient-Rich Water Rises near the
Equator
Equatorial upwelling.
The South Equatorial Current,
especially in the Pacific,
straddles the geographical
equator. Water north of the
equator veers to the right
(northward), and water to the
south veers to the left
(southward). Surface water
therefore diverges, causing
upwelling. Most of the upwelled
water comes from the area
above the equatorial
undercurrent, at depths of 100
meters or less.
Thermohaline Flow and Surface Flow:
The Global Heat Connection
The global pattern of deep circulation resembles a vast “conveyor belt” that carries
surface water to the depths and back again. Begin with the formation of North Atlantic
Deep Water north of Iceland, which flows south through the Atlantic and then flows over
(and mixes with) deep water formed near Antarctica. The combined mass circumnavigates
Antarctica and then moves north into the Indian and Pacific ocean basins. Diffuse upwelling
in all of the ocean returns some of this water to the surface. Water in the conveyor
gradually warms and mixes upward to be returned to the North Atlantic by surface
circulation.
Water Masses May Converge, Fall, Travel
across the Seabed, and Slowly Rise
A model of thermocline
circulation caused by heating in
lower latitudes and cooling in
higher latitudes. The thermocline
at middle and low latitudes is
“held up” by the slow upward
movement of cold water.
The water layers and deep
circulation of the Atlantic
Ocean. Arrows indicate the
direction of water
movement. Convergence
zones are areas where water
masses approach one
another.
The Arctic Sea:
Relatively enclosed basin (connection to
the Pacific through the Bering Strait and to
the Atlantic through the Greenland and
Norwegian Seas)
Enclosed nature influences ice cover
Circulation: was originally deduced from ice
flows and drifting ships, supplemented with
direct current measurements and geostrophic
calculations
Other Major Current Systems
Keep In Mind:
Coriolis Force on the Equator is zero
Coriolis Force by 0.5o influences flow of water
Currents To Know:
North Equatorial Current (NEC)
South Equatorial Current (SEC)
Equatorial Counter Current (ECC)
Equatorial Under Current (EUC)
Equatorial Undercurrent (EUC) & Eastward Directed Pressure Gradient
Wind driven water from the surface mixed layer piled on the western side of the basin
Wind stress balances the pressure gradient (Coriolis Force ~= 0 at equator)
Adjustment (depression) of the thermocline on the western end
Baroclinic conditions at depth drive a jet-like current eastward eventually balance by
friction (eddy viscosity)
Waves: the ocean can respond to the winds
in distant places by means of large-scale
disturbances that travel as waves.
Barotropic: surface waves
Baroclinic: density surface (thermocline)
Rossby (Planetary Waves)
Kelvin
Examples of barotropic and baroclinic waves
propagating through the ocean
Most tides are barotropic ‘Kelvin’ waves
Think about what would happen if the wind
stress was dramatically reduced or changed
directions in the case of the Asian Monsoon
Kelvin Waves
Travel eastward along the equator as a
double wave ‘equatorial wave guide’
Travel along coasts (coast on right in the NH
and on the left in the SH)
Balance between pressure gradient force and
coriolis force.
Kelvin Waves
Surface equatorial kelvin waves travel ~200 m/s
Rossby radius of deformation L = c/f
•High latitudes smaller eddies closely trapped
to the coast (increase in planetary vorticity)
•Low latitudes larger radius
Kelvin waves in the thermocline can have dramatic effects,
particularly in low latitudes where the mixed surface layer is thin.
Northward migration of ITCZ in western Atlantic generates
disturbance that propagates eastward
Splits into two coastal Kelvin waves when hits the eastern
boundary
The region of the disturbance where the thermocline bulges
upward cold nutrient rich sub-thermocline water can reach the
surface
4-6 week travel time
Rossby Waves:
Propagate from east to west across basin
Travel along lines of latitude
Move slower than Kelvin waves
Conservation of Potential Vorticity
Example: Waves in the jet-stream
ENSO: El Nino – Southern Oscillation
“Interest in the phenomenon of El Nino goes back to the mid-19th century but it
was the El Nino event of 1972-73 that stimulated large-scale research into
climatic fluctuations, which began to be seen as a result of the interaction
between atmosphere and ocean.”
El Nino events are perturbations of the ocean-atmosphere system
Disturbance – a depression in the thermocline
accompanied by a slight rise in sea-level propagates
eastwards along the Equator as a pulse or series of
pulses (Kelvin Waves)