Transcript Document

Lecture 7: The Oceans (1)
EarthsClimate_Web_Chapter.pdf, p. 22-24
General features of oceans
Area: covers ~70% of Earth’s surface
Volume: ~97% of all the water on Earth
Depth: ~4 kilometers
Albedo: 5-10%, lowest on Earth’s surface
Heat capacity: high; thermal inertia: high
Temperature: less variable than in the atmosphere
Freezing point: –1.9°C, not
Salinity:
0°Cwater and dissolved salts; most common salt: table salt (NaCl).
Density: 1034-1035 kg/m3 (greater than pure water 1000kg/m3)
Average salinity = 35 parts per thousand (ppt) or 3.5% by weight
Density depends on temperature and salinity:
Cold water  high density
Formation of sea ice  high density
Evaporation  high salinity  high density
Precipitation and river discharge  low salinity  low density
Two main forms of circulation
Surface currents: wind-driven, horizontal, surface waters, fast
Deep-ocean circulation: thermohaline, vertical, deep waters, slow
Surface is not level due to currents, waves, atmosphere pressure, and variation
in gravity.
Vertical Profiles of Temperature and Salinity
A. Unlike the atmosphere, which is
heated from below, oceans are
heated from above, primarily by
Sun, largely at the Equator.
B. Two overall layers
1. Thin, warm, less dense surface
layer well mixed by turbulence
generated by wind
2. Thick, cold, denser
deep layer that is
calm and marked by
slow currents
3. Thermocline is the
boundary between the
layers
Surface currents
A result of three primary forces
Wind-induced drag force
Coriolis force
Pressure gradient force
Hemispherical Gyres
Warm currents (moving out of
the tropics): e.g., Gulf Stream
Cold currents (moving away from
the poles): e.g., California Current
Effects of surface winds
on the oceans
1. Currents converge
toward Equator following
Trade Winds and ITCZ
2. Westward flow along
Equator (i.e., North and
South Equatorial Currents)
3. Equatorial Currents turn
poleward where they
encounter land barriers (e.g.,
Gulf Stream)
4. Eastward flow of currents is
enhanced by the Westerlies
5. Currents turn toward the
Equator where they
encounter land barriers,
completing the gyres
Effects of surface currents on heat transfer
The equator-to-pole energy transport by the ocean is important in
reducing the pole-to-equator temperature differences.
Currents moving out of the tropics carry heat poleward
Currents moving away from the poles carry cold water equatorward
Heat Transfer in the North Atlantic Ocean
Image by NOAA’s
AVHRR Satellite in
June of 1984.
The warm Gulf
Stream current
(27°C, 80°F)
redistribute heat by
swirling through the
cooler water to the
north and east.
Ekman Spiral
a. The Coriolis effects
cause surface current
to move 20–45° from
the wind direction
(45° in theory)
b. Deflection continues
with depth, forming a
spiral
c. To depth of 100 m
d. Net transport of
water is 90° from
the wind direction
More on subtropical gyres
1. Trade winds blowing SW push
shallow waters toward the
NW.
2. Mid-latitude westerlies
blowing NE push surface
water to the SE.
3. A lens of warm water piles in
the center (2 meters higher
than the surrounding ocean).
4. Spinning clockwise in the
northern hemisphere – a
balancing act of pressure
gradient force and Coriolis
force.
Upwelling and Downwelling
a. Deflection of water away from
continent
b. Upwelling of deeper water to
replace surface water
c. Commonly nutrient rich
a. Deflection of water towards
continent
b. Downwelling of surface
water to push deep water
Cold Water Upwelling
Maps of west coast
sea surface
temperature
indicate regions of
significantly cooler
water that has up
welled from below.
Zones of upwelling in global oceans
Summary:
• What percentage of the earth does ocean cover and how deep
the ocean is on average?
• Why is ocean so important in determining Earth’s climate?
• What is the ranges of sea water density, freeze point, salinity
and albedo?
• What determine the ocean currents?
• What role does ocean play in heat transport of the climate
system?
• What cause upwelling and downwelling of ocean water?
Summary:
• What percentage of the earth does ocean cover and how deep
the ocean is on average?
– Covers 72% of the earth and about 4 km deep.
• Why is ocean so important in determining Earth’s climate?
– greatest storage for water and heat for the climate system.
• What is the ranges of sea water density, freeze point, salinity
and albedo?
– 1.034-1.035 X1000 kg/m3, -1.9C, 3.4~3.5%, 5-10%
• What determine the ocean currents?
– Wind stress, Coriolis forcing, sea level gradient
• What role does ocean play in heat transport of the climate
system?
– Subtropical gyros transport warmer water poleward and cooler
water equatorward.
• What cause upwelling and downwelling of ocean water?
– Wind stress and Ekman effect