Download Blue Planet Lecture 2006

What we wish to learn Today:
1.
What is the composition and physical
structure (layering) of the ocean?
2.
How and why do ocean waters
circulate?
3.
What role does the ocean play in
climate change?
Causes of Climate change
A. Tectonic
B. Orbital
C.
??
D.
??
1. Composition of the Oceans
Density
– weight of the water. Density increases with more
salt and decreases with warmer temperatures
Fresh water has ~0.02% dissolved salt (density = 1 g/cm3)
Sea water has ~3.5% dissolved salt (density = 1.024-1.028 g/cm3)
Ocean Salt composition
Salinity
Number of grams of dissolved
salt in a kilogram of sea water.
Ranges from 33–38 g/kg in
open oceans
Ocean Structure - The ocean
is “stratified” into layers
• Stratification is based
on water density – light
water floats on top of
denser, heavier water and
creates a stable structure
• The surface layer is
separated from the bottom
layer by a transition zone
The transition is called a
pycnocline (for density)
or a
thermocline (temperature)
or a
halocline (for salt)
Atlantic Ocean Temperature
Temperature oC
The tropics have higher heat input from the sun, and
temperatures there are warmer than at the poles
www.ewoce.org
Atlantic Ocean Salinity
Salinity (ppt)
(pss 78)
The tropics have more heat to drive evaporation, concentrating the
ocean salts. The Gulf Stream moves this salt northward. The poles
also have greater freshwater input from ice caps and glaciers.
www.ewoce.org
2. Ocean Circulation
• Ultimately driven by solar energy
– Distribution of solar energy drives global winds
• Surface currents
– Affect surface water above the pycnocline (10-15%
of ocean water)
– Driven by major wind belts of the world
• Deep currents
– Affect deep water below the pycnocline (up to 90%
of ocean water)
– Driven by density differences (not by winds)
– Larger and slower than surface currents
The Gulf Stream is a major surface current
Red is warmer water
Ocean Surface Currents
Surface currents are driven by wind and follow
the circulation pattern of the atmosphere
Coriolis “Force”
Not a true “force”, but an apparent
movement or deflection to account for being
on a rotating body instead of fixed in space
The Coriolis effect deflects
moving fluids (atmosphere
and ocean) to the right in
the Northern Hemisphere,
and to the left in the
Southern Hemisphere
The Eckman “Spiral”
Surface currents are
deflected due to the
Coriolis effect, and
flow at ~45° to the
surface wind direction
Currents “spiral” to the right
at depth, so the net “Eckman
Transport” of water will be
90o to the right of the wind
(N. Hemisphere)
Along coastal areas,
Ekman transport can
cause upwelling and
downwelling of water
Coastal Upwelling
Northern Hemisphere
Water movement offshore (due to
Coriolis effect and Eckman
transport) results in upwelling as
cold water rises to replace
surface water
Ekman transport either moves surface water away from
shore, producing upwelling, or moves water toward
the shore, producing downwelling
West Coast
Southern Hemisphere
West Coast
Southern Hemisphere
Upwelling
Downwelling
Illustration of
Coastal Upwelling
off California.
Blue and green
are cooler
waters upwelling
close to shore,
while red
represents
warmer waters
further offshore
July 1992 AVHRR Ch 4
El Niño-Southern
Oscillation (ENSO)
• El Niño = warm surface current in
equatorial eastern Pacific that occurs
every 3-8 yrs around Christmas time
• Southern Oscillation = change in
atmospheric pressure over Pacific Ocean
accompanying El Niño
• La Niña = opposite of El Niño, cool phase
with enhanced upwelling
El-Nino Southern
Oscillation (ENSO)
Upwelling of cool, nutrient-rich
water supports productive fishery
Pacific Ocean
NOAA/PMEL/TAO
Warm surface water blocks
upwelling, fishery collapses
Illustration of the very strong 1997-1998 El Niño
See also: http://www.vets.ucar.edu/vg/
http://www.pmel.noaa.gov/tao/jsdisplay
El Niño impacts and recurrence
Impacts on global
climate can be large $8-10 Billion for the
1982-83 event
Temperature and Precipitation
Typical interval for El
Niños is 3-8 years,
which is too short to
explain longer-term
climate fluctuations
Deep Water Formation and Currents
• Warm saline water
moves north by the
Gulf Stream
• Water cools in Arctic
• Sea ice forms and
increases salinity
• Water becomes dense
and sinks
Sinking water produces a deep current that travels
worldwide. Deep-water also formed near Antarctica.
This movement is called “Thermohaline Circulation”
(heat and salt)
Global “Conveyer Belt” circulation
Flow in conveyor is ~ 30 times larger than all river flow
3. Ocean Circulation and Climate
• Heat re-distribution (keeps Europe warm)
• Monsoons, Cyclones, Hurricanes, El Nino
• Upwelling can create coastal deserts on land
• Great thermal buffer (high heat capacity)
• Oceans absorb ~1/3 of anthropogenic CO2
Can ocean circulation explain abrupt climate shifts?
Ice core data
Melting of
the Ice Cap
may have
stopped or
reduced the
Conveyor Belt
(o/oo)
18O

Sea-surface Temp C
Younger
Dryas
Time (thousands of years B.P.)
Modes of Atlantic Thermohaline Circulation
Abrupt climate change due to switches between 3 modes
Rapid
Cooling
The large Earth in the center
shows the cold climate state
prevailing during most of the
Normal Ice Age
The bottom shows an advance
of the Conveyor Belt into the
Nordic Seas, resulting in a
warm anomaly and
Rapid Warming
Normal
Ice age
Rapid
Warming
The upper globe shows climate
when the Conveyor Belt
collapses, resulting in
Rapid Cooling
Modeled changes in surface air temperature
caused by a shutdown of the North
Atlantic Deep Water formation are large!
Water Freshening in the North
Atlantic Ocean is increasing due
to climate warming
Melting of Greenland Ice-sheet
Salinity is decreasing (ocean is freshening)
If the water gets too fresh, deep water
can’t form (the surface freshwater can’t
sink, no matter how cold it gets)
Model projections of changes in ocean circulation cause
disruptions of the carbon cycle in a warmer world
• 50%
slowdown
in N.
Atlantic
circulation
• 20-30%
reduction
in oceanic
CO2 uptake
Sarmiento, 1998
Take Home Message:
Remember, Rome wasn’t built in a day,
It just burned down
in one…
Summary:
• Solar heat inputs create surface winds, which in turn
drive the surface ocean currents.
• Differences in water density allow deep-water to sink
near the poles, which establishes a 3-dimensional
thermohaline current that encircles the globe (the
Conveyor Belt).
• Ocean circulation strongly controls climate on Earth
through heat transport, upwelling, El Niño, etc.
• Weakening of the deep-water formation and Conveyor
Belt circulation probably affected paleoclimate over the
last 100,000 years.
• Abrupt, rapid climate changes can be strong and can be
caused by a breakdown of the atmosphere-ocean
interactions that control climate.