Transcript The Role of Oceans - University of Texas at Austin

Oceanic Circulation
I. How oceans work
II. Surface currents
III. Deep Currents
IV: The Air-Sea Interactions (ENSO)
The Role of
Oceans in Global
Climate Variability
The Role of Oceans in Hydrological Cycle
97% of the Earth’s free water
86% of the global evaporation
78% of global precipitation
I. How oceans work
1.
2.
3.
4.
Area: covers 70% of the Earth’s surface
Volume: 97% of all the water on the Earth
Depth: 4 kilometers
Density: 1034-1035 kg/m3 (Pure water: 1000 kg/m3) over 90% of
the ocean. Depends on temperature and salinity.
cold water  high density
loss of water by evaporation  increase salinity  high density
precipitation and river discharge  decrease salinity  low density
5.
6.
7.
8.
Heat capacity: high
Temperature: less variable than in the atmosphere
Freezing point: – 1.9°C, not at 0°C because of salinity
Surface is not level due to currents, waves, atmospheric pressure
differences, and variations in gravity.
9. Two main forms of circulation:
wind-driven circulation (horizontal, surface waters, fast)
thermohaline circulation (vertical, deep waters, slow)
I. How oceans work (Cont’d)
Relative proportions of dissolved salts in seawater
Annual Mean Ocean Surface Temperature
Annual Zonal Mean Ocean Surface Temperature
Annual Mean Ocean Surface Salinity
Annual Zonal Mean Ocean Salinity
Ocean Meridional Overturning (Global)
Ocean Meridional Overturning in Atlantic
I. How oceans work (Cont’d)
A. Heated primarily by Sun, largely at the Equator, with global heat
transfer by ocean currents  profound effect of oceans on
climate
B. Two overall layers
1. Thin, warm, less dense surface layer well mixed by turbulence
generated by wind
2. Thick, cold, more dense deep layer that is calm and marked by slow
currents
3. Thermocline is the boundary between the layers
Mixed Layer Processes
Vigorous mixing processes lead to uniform conditions within the surface mixed layer.
Mixed Layer Processes
I. How oceans work (Cont’d)
Quiz
1. Which of the following is NOT true about the oceans?
A. Approximately 97% of the total water on Earth is located in the oceans
B. The oceans cover about 70% of Earth’s surface
C. The average depth of the oceans is about 4 km.
D. The sea surface is level for all the oceans.
E. Because it takes far more energy to change the temperature of water than land or air, water warms
up and cools off much more slowly than either.
2. The three largest reservoirs of water at the earth’s surface in decreasing order of volume are:
A. oceans, ice caps/glaciers, and ground water
B. oceans, ice caps/glaciers, and soils
C. oceans, lakes/rivers, and soils
D. oceans, atmosphere, and biosphere
E. oceans, ground water, and soils
3. What is the primary salt in the ocean?
A. methane
B. CO2 C. FeO2
D. CaSO4
E. NaCl
4. Which of the following increase salinity?
A. Evaporation
B. Precipitation.
C. Formation of sea ice.
E. A and C only.
D. River runoff.
II. Surface currents
II. Surface currents
A. Three primary
forces
1. Global wind
patterns
cause surface
ocean
currents
because of
frictional drag
2. Coriolis effect
3. Pressure
gradients
B. Global wind patterns cause surface ocean
currents because of frictional drag
July
1. Creation of
waves
2. Creation of
hemispherescale gyres
January
3. Decrease in
current
speed with
depth
C. Coriolis effect
1. Earth rotation speed is greatest at Equator falling to zero at Poles
2. Conservation of angular momentum
a. Deflection to the right for component of Equator-to-Pole flow in
Northern Hemisphere
b. Deflection to the left for component of Equator-to-Pole flow in
Southern Hemisphere
C. Coriolis effect (Cont’d)
3. Creation of
Ekman Spiral
a. To depth of 100
m
b. Surface current
moves 20-45o
from the wind
direction (45o in
theory)
c. Deflection
increases with
depth, forming
a spiral
d. Net transport of
water is 90o
from the wind
direction
C. Coriolis effect (Cont’d)
3. Creation of Ekman Spiral
a. To depth of 100 m
b. Surface current moves 20-45o from the wind direction (45o in
theory)
c. Deflection increases with depth, forming a spiral
d. Net transport of water is 90o from the wind direction
D. Pressure gradients
1. Differences in
water height (i.e.,
piling of water
against a continent
because of the
wind)
2. Density differences
because of
temperature or
salinity
3. Atmospheric
pressure
differences
E. Geostrophic currents
- balance of Coriolis
force by pressure
gradient
F. Land forms barriers
to global ocean
currents
G. Resultant overall
current pattern
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
6. Significant heat transfer
a. More solar heating and evaporation at Equator, less at Poles  salinity
and temperature Equator/Pole gradient
b. Warm and salty water carried poleward
Effects of surface currents
7. Zones of upwelling
a. Deflection of water
away from continent
b. Upwelling of deeper
water to replace
surface water
c. Commonly nutrient
rich
7. Zones of upwelling
III. Deep Currents
A. Thermohaline-driven currents
1. Greater salinity  greater density
2. Lower temperature  greater density
3. Equilibrium is sought in global-scale deep
currents (conveyor belt)
B. Global Conveyor Belt (Cont’d)
1. Warm and salty
shallow water from
Tropics is carried
northward through
Atlantic
2. Water cools in North
Atlantic and sinks east
of Greenland as cold
and salty dense water
3. North Atlantic Deep
Water flows toward
the Equator and into
the South Atlantic,
where it meets yet
colder and denser
Antarctic Bottom
Water flowing toward
the Equator
2
1
3
B. Global Conveyor Belt (Cont’d)
4. Combined flow eastward
below Africa and into the
Pacific Ocean
5. Deflected by Asia
6. Journey through Pacific
causes warming and slow
rise to surface
7. Warm and less dense
Pacific surface waters
move south and west to
converge toward Equator
as shallow surface water
to continue warming
6
5
7
4
B. Global Conveyor Belt (Cont’d)
8. 1,000 years for a complete
cycle
9. Conveyor Belt aided by
imbalance between loss
of water by evaporation in
Atlantic and gain of water
by precipitation and runoff
10. Bering Strait prevents
free exchange between
Arctic and Pacific Oceans
 inhibits Arctic-Pacific
Conveyor Belt
The Role of Oceans
I. How oceans work
II. Surface currents
III. Deep Currents
IV: The Air-Sea Interactions (ENSO)
The Air-Sea Interactions
A. Inter-Tropical Convergence Zone (ITCZ)
B. El Niño - Southern Oscillation, aka ENSO
El Niño : Spanish for boy child; Christ child; referring to warm water
conditions and associated consequences, frequently occurring
around Christmas
Southern Oscillation: A seesaw pattern of reversing surface air
pressures at opposite ends of the Pacific Ocean
Walker Circulation (zonal convection) refers to the localized zonal
(west-east) convection cell of atmospheric circulation above the
Pacific Ocean. The easterly trade winds are part of the low-level
component of the Walker circulation.
C. La Niña: (girl child) Opposite extreme of El Niño
A. Inter-tropical Convergence Zone (ITCZ)
1.What is ITCZ?
Region near the equator where the wind systems (trade winds) of
the Northern Hemisphere and Southern Hemisphere meet.
2. Position changes with season. North of equator in July,
(generally) south of equator in January.
3. Broad trough of low pressure, typical of cloudiness,
thunderstorm, and heavy precipitation.
4. More intensely developed in the western Pacific because of
warm water pool of surface seawater (T = 31 °C) and Asian
monsoons.
5. Influences the ocean currents and salinity.
El Nino:
How it
Works
Known for over 400 years ...
Occurs during Christmas season ...
• “Christ Child”
• “The Boy”
• “The Little One”
Food Chain
Nutrients
Fish
Sea Birds
People
B. El Niño - Southern Oscillation, a.k.a. ENSO
1.What is ENSO?
ENSO is a periodic climatic phenomenon caused by oceanatmosphere dynamics in the tropical Pacific Ocean.
ENSO Process: Every two to seven years, strong
westward-blowing SE trade winds subside. Warm water
moves back eastward across the Pacific, like water
shifting in a giant bathtub. The warm water and shifting
winds interrupt the upwelling of cool, nutrient-rich water.
El Niño and SOI
El Niño : Noticed at
Christmas time by
Peruvian fisherman;
warm coastal waters
leading to poor
fishing
El Niño / Southern
Oscillation:
an oscillation in the
surface pressure
(atmospheric mass)
between the SE
tropical Pacific and the
Australian-Indonesian
regions.
Reading
the
Index
2. Importance of understanding ENSO
a. Severe rapid
climate
variations
caused by El
Niño are natural
b. El Niño has
important
consequences
for famine and
economics
around the
globe.
3. Basics of ENSO
Western equatorial Pacific has some of the world's warmest ocean
water. To the east, cool water wells up, carrying nutrients (see
Oceans lecture) that support large fish populations
Oscillation of surface water in the Pacific
4. What causes ENSO?
large-scale internal waves
What Causes ENSO?
During the warm phase of
ENSO (El Niño phase) the
West Pacific Warm Pool
and its associated low
pressure atmospheric
center migrate to the east
along the equator. The
water is transported by
these internal waves.
Kelvin waves cross the
Pacific in 2-3 months and
lead to a warming of the
surface waters and rise in
sea level along Peru.
As sea level rises and warm
water accumulates in the
eastern Pacific, Rossby
waves are generated that
move west across the Pacific.
The time it takes for these
waves to cross the Pacific is
strongly dependent on the
latitude at which the wave is
traveling (near the equator,
they take a 9 month journey;
12 degrees from the equator,
they take 4 years).
The Rossby waves reach the
Western Pacific, travel along
the coast as Kelvin waves to
the equator and turn east and
begin another crossing of the
Pacific.
What Causes ENSO?
5. If it’s a tropical phenomenon, why does it
affect us in Texas?
Teleconnection: during ENSO, warmer, moister air than usual
is generated in the eastern part of the ITCZ in the Pacific.
This changes atmospheric circulation patterns and deflects
the jet stream that goes west-east across US at a more
easterly longitude than during non-ENSO year. As a result,
different amounts of moisture and heat than usual are
delivered to different parts of the world
El Nino and Texas
El Nino and Texas
• December - March: TX wetter and cooler
• More wet and cool in S. Texas
• Experience: flooding, high number of storms
originating from Gulf of Mexico and tropical Pacific
because of the strengthened subtropical jet stream
• Protect your property: homeowners insurance
6. What’s
good and
bad about
the effects
of ENSO?
a. This
depends on
where you
live and
what your
lifestyle is
b. 1997-1998 event and the media’s response
7. Measuring El Nino
Measuring El Nino
Weather Stations
•
•
•
Stevenson Screen
Temperature
Precipitation
Weather Station: Colorado
Measuring El Nino
Buoys
Measuring El Nino
Satellites
Satellites: Temperatures
Measuring El Nino
Sea Surface Temperatures
Measuring El Nino:
Coral Bleaching