1.
Sea Ice and Density-Driven Ocean Circulation
- Ice and Climate
- Density and Water Masses
- Thermohaline Circulation
Reading:
4th Ed., Ch 9 Secs 17-22, Ch 6 Secs 12-14
5th Ed., Ch 9 Secs 17-23, Ch 6 Secs 14-17
Graphic: Gerlache Strait, Rear Adm. H.D.Nygren, photographer, courtesy of NOAA.
2. Ice and Climate
Polar ice is an important part of the Earth's climate system
Ice is much "brighter" than land or water and reflects the sun's energy back to
space very efficiently
A world with less ice would absorb more solar energy
Graphic: View of Earth as seen by the Apollo 17 crew traveling toward the moon,
Dec. 7, 1972. Photo courtesy of NASA.
3. The "Ice-Albedo" Feedback
- As ice melts, patches of open water develop
- These patches absorb energy and heat
- This heats the ocean, further melting the ice
4. Climate and Ice - North and South
- Arctic Ocean (north) - About 2 meters of ice floats on the ocean
- Antarctic Continent (south) - 1000's of meters of ice rests on the land
A small amount of heating in the Arctic can have a larger impact on climate than
a small amount of heating in the Antarctic
Graphic: (left) Sea ice in the Arctic, (right) land ice in the Antarctic
(thicknesses not to scale).
5. Ice in the Arctic
The Arctic has "permanent" (multi-year) ice and ice that persists only until
summer
Graphic: Sea ice cover in the Artic in February (left) and September (right).
Courtesy of National Snow and Ice Data Center.
6. Climate Change and the Arctic
Many climate models predict that the Arctic will warm significantly due to
increases in atmospheric CO2
Graphic: Surface air temperature change at the time of CO2 doubling (colors)
predicted by an ensemble of climate models with CO2 increasing 1% per year. From
the Intergovernmental Panel on Climate Change (IPCC).
7. Land-Based Observations Support Climate Model Predictions
Graphic: The Grinnell Glacier in Glacier National Park, Montana. Left: 1938,
Right: 1981. Courtesy of USGS.
8. Sea Ice Draft: 1950's-1970's vs 1990's
Measurements of sea ice draft (amount of ice below the water line) indicate less
ice in the Arctic recently compared with the 1950's through 1970's
Graphic: Decrease in Arctic sea ice draft. Graph derived from Rothrock et al.,
1999, courtesy of National Snow and Ice Data Center.
9. Density-Driven Circulation
Heating, cooling, precipitation and evaporation change the density of seawater
The density driven circulation is the motion that arises as dense water sinks
below light water under the influence of gravity
Graphic: Ross Ice Shelf, Antarctica. M.Van Woert, photographer, courtesy of NOAA.
10. Density of Fresh Water
The density of fresh water depends on temperature
Fresh water reaches maximum density at about 4oC
Ice is less dense than fresh water
Graphic: (top) Pinet Fig. 5.5b, see Garrison, 4th Ed. Fig. 6.3, pg 148, 5th Ed.
Fig. 6.3 pg 140, (bottom) icebergs near Antarctica, M.VanWoert, photographer,
courtesy of NOAA.
11. Density of Seawater
Seawater density depends on temperature and salinity
Cold salty water is more dense than warm fresh water
Two samples of water at different temperatures and salinities can have the same
density
Graphic: Density as a function of temperature and salinity. The curved lines are
lines of constant density. Garrison 4th Ed., Fig. 6.11, pg 154, 5th Ed., Fig.
6.13, pg 148.
12. What Happens as Sea Water Freezes?
As the ocean cools and freezes, sea ice forms
The "salt" in the sea is not incorporated into the ice, it is left behind in the
water
Net result:
- "fresh" ice
- very cold, very salty water
- heat moves from the ocean to the atmosphere
Graphic: Looking over broken sea ice to open water, photo by M.VanWoert NOAA
NESDIS OAR, courtesy of NOAA.
13. Sea Ice and Bottom Water*
Bottom water:
- Very cold winds blow over the ocean, forming sea ice
- The seawater that is not incorporated into the ice is very cold and salty and
sinks as "bottom water“
Deep water:
- Winds cool water enough for it to sink, but not enough to form sea ice
14. Evaporation and Mediterranean Sea Water
- Not all dense water is produced near the poles...
- Evaporation creates very salty warm water in the Mediterranean Sea
- This spills over the Gibralter sill and sinks in the central North Atlantic
Graphic: Garrison, 4th Ed., Fig. 9.21, pg. 227, 5th Ed., Fig. 9.23, pg 219.
15. Intermediate Water Production at Convergence Zones
(arrows indicate direction of water motion)
Where ocean currents converge, dense water can sink under lighter water leading
to a new class of water masses (Intermediate Water)
Convergence zones occur due to winds
16. Regions Where Dense Water Forms
Deep & Bottom Water
- form due to atmospheric cooling
Med Sea Water
- forms due to evaporation
Intermediate water
- forms due to convergence of ocean currents
Graphic: Garrison, 4th Ed., Fig. 9.20, pg 227, 5th Ed., Fig. 9.22, pg 219.
17. Water Mass Layering (Atlantic)
Antarctic Intermediate Water - flows northward
North Atlantic Deep Water - flows southward at mid-depths
Antarctic Bottom Water -flows northward along the bottom
Others: Mediterranean Sea water, Central water, Surface water
Graphic: Garrison, 4th Ed., Fig. 9.23, pg 228, 5th Ed., Fig. 9.25, pg 220.
18. Themohaline Circulation - The Concept
Thermo = temperature
Haline = salt
Themohaline = Temperature and salinity (i.e., density) driven circulation
Cold, salty (dense) water sinks in the poles
Warm, fresh (less dense) water floats in the tropics
Graphic: Garrison, 4th Ed., Fig. 9.22, pg 228, 5th Ed., Fig. 9.24, pg 220.
19. Themohaline Circulation - Linking the Global Ocean
1. Sinking at poles in Atlantic
2. Deep flow of cold, salty water into Indian and Pacific
3. Upwelling in Pacific
4. Return flow at surface via Indonesean Passages
Graphic: Pinet, Fig. 6.16b, see Garrison, 4th Ed., Fig. 9.25, pg 230, 5th Ed.,
Fig. 9.27, pg 222.
20. Preview of Next Lecture
Climate Change and Life in the Greenhouse
Reading:
4th Ed., Ch 18 Sec 21
5th Ed., Ch 18 Sec 22
Graphic: Sunrise over the Beaufort Sea in the Arctic. The potential impacts of
climate change may be felt early and very severely here. Rear Adm. H.D. Nygren,
photographer. Courtesy of NOAA.