Download Atmospheric and Oceanic Circulation

Atmospheric and Oceanic Circulation
• Equatorial region receives more solar
insolation than polar regions
• Primary mechanisms for redistributing heat on
the Earth
– Atmospheric circulation
– Oceanic circulation
– Latent heat transfers
Poleward Heat Transport
Graedel and Crutzen- Atmospheric Change
Atmosphere
Troposphere8 km (poles) to
18 km (tropics)
The atmosphere is
heated from below
Similar to Ruddiman Box 2-1- online
Atmospheric Circulation
•
Based on simple principles
1.
2.
3.
4.
5.
•
Hot air rises
Rising air creates low pressure at ground level
Cold air sinks
Sinking air creates high pressure at ground level
Air flows from high pressure to low pressure
regions
As air rises and cools, water vapor condensesrain
Atmospheric Circulation
LP
No rotation
No land
Hot air rises at the equator
Cold air sinks at the poles
HP
HP
LP
Garrison, Essentials of Oceanography
Atmospheric Circulation
With rotation (coriolis)
3 cells/hemisphere
Series of gears
HP
LP
HP
Hadley Cell
Major winds:
Trades (NE and SE)
Westerlies
Polar easterlies
Intertropical Convergence Zone (ITCZ)
- caloric equator
- doldrums
High and low pressure systems
LP
Similar to Ruddiman 2-16
online
Atmospheric Circulation
Hadley cells
Subtropical High
Precipitation vs.
Evaporation
Latent heat transport
Polar frontal zone
(jetstream)
Ruddiman 2-16
ICTZ
Atmospheric
Pressure
Systems
ITCZ
Monsoonal
Circulation
See Kump, Kasting
and Crane 4-21
Surface Winds
Surface Ocean Temperature
Temperature- 3000m
Ocean Circulation
Structure of the ocean:
Mixed Layer – stirred by wind
Thermocline- rapid cooling
Deep zone- constant, cold
Garrison
Ocean Circulation
Depth to the thermocline
varies with latitude and
season (intensity of the
wind)
Stratification
vs.
overturning
Ruddiman 2-25 online
Ocean Circulation
• Surface Circulation
(mixed layer ~ top 100m)
– Wind driven circulation
– Broken up into gyres by physical boundaries
• Gyres
– Circular flow around the periphery of a basin
Surface Ocean Circulation
Garrison- note cross equatorial circ in Atlantic is missing in Ruddiman’s figure 2-21
Surface Ocean Circulation
Ruddiman 2-21- online
Surface Winds
Ekman Spiral
KKC 5-3a
Energy rather than water spiraling downward
Ekman Transport
Similar to Ruddiman 2-22- online
Surface Ocean Circulation
Due to coriolis
(Ekman Transport)
Water tends to build up
In the center of the
Gyres.
Sea level ~2m higher
At the center of the gyre
Ruddiman 2-23 online
Surface Ocean Circulation
Surface Ocean Circulation
• Western Boundary currents
– Center of “hill” offset to western margin
– Water circulates around this high
– Channeled, fast, strong on western boundary
• Transport warm water poleward (100’s km/day)
• Gulf Stream, Kuroshio Current, Brazil
Current…
– More diffuse, slower on eastern boundary
• Transport cool water equatorward (10’s km/day)
• California Current, Humbolt or Peru Current…
Gulf
Stream
Upwelling Regions
Garrison
Surface Ocean Circulation
• Upwelling
• Mechanism for bringing cool, nutrientrich waters to the surface
• Wind induced vertical movement of
water leads to divergence
– Coastal upwelling
– Equatorial upwelling
Coastal Upwelling
Ekman Transport
Ruddiman 2-27 (online)
Equatorial Upwelling
Ekman Transport
Ruddiman 2-27 online
Convergences and Divergences
Garrison
Equatorial Upwelling
Ekman Transport
N. Eq. Current
NE
Tr
es
d
a
Divergence
Eq. Countercurrent
ITCZ
Convergence
SE
T
S. Eq. Current
Equatorial Divergence
rad
es
Equator
Vertical Motion
Garrison
Deep Ocean Circulation
Thermohaline Circulation
• Ocean is density stratified
–
–
–
–
Temperature
Salinity
Cold and salty = dense
Both properties are produced at the surface
and transported into the deep ocean by
downwelling at convergence zones
Deep Ocean Circulation
Deep Ocean Circulation
• Layers with distinct densities are called
water masses (identifiable by their temp
and salinity)
–
–
–
–
–
Surface Waters
Thermocline Waters
Intermediate Waters
Deep Waters
Bottom Waters
Deep Ocean Circulation
• Today Deep/Bottom Waters mostly form
(seasonally) in the:
– North Atlantic- Norwegian-Greenland Sea
• North Atlantic Deep Water (NADW)= 4°, 34.9
ppt
– South Atlantic- Weddell Sea
• Antarctic Bottom Water (AABW)= -0.4°, 34.66
ppt
Water is cold (high latitude) and salty (salt
rejection in SH, Med outflow in NH))
NADW
North Atlantic Deep Water
Greenland
Iceland
US
NADW
Broecker and Peng, Tracers in the Sea
Norway
NADW
60°N
Norwegian
Greenland Sea
70°N
Export heat to Europe
Cooled in winter
Gulf Stream
8°C; 35.3 ppt
NADW
4°C; 34.9 ppt
Sinks
Mixes
(MOW)
AABW
Sea ice
AABW
Sea ice
-0.5°C
34.6 ppt
Intermediate Ocean
Circulation
• Intermediate waters form at
convergences
– water not cold enough or salty enough to
sink to seafloor
• Antarctic Intermediate Water (AAIW) 2-4°, 34.2
ppt
Whole Ocean Circulation
Ruddiman 2-26 (online)
Whole Ocean Circulation
Garrison
Deep Ocean Circulation
• Circumpolar Deep Water (CPW or CDW)
– Below Westwind Drift wind mixes the water from
the surface to seafloor
– Combines AABW, NADW (AAIW, Pacific water)
– ACC- “great mixmaster of the world” (Broecker)
– Feeds out into deep Indian and Pacific Oceans
Circumpolar Water
Open University, Ocean Circulation
Ocean Conveyor Belt
Links:
Surface and
deep
circulation
Pinet
Ocean Conveyor Belt
IPCC 2001
Ocean Circulation
Garrison
Ocean Circulation
• Intermediate and Deep Waters sink into the ocean in
just a few places (downwelling)
• Flow and maintain integrity throughout the oceans
because of density contrasts (thermohaline circulation)
• Mix upward gradually along isopycnal surfaces
• Also mix upward from shallow depths by upwelling
(<1000m)
Ocean Circulation
Garrison
Global Conveyor
Note upward mixing all along the path!
Thus, return flow = deep water formation
Particulate flux- alters chemistry along the flow path
Broecker and Peng, 1-13, Also OC Fig. 218b
Global Conveyor
• How long does it take?
~ 1500 yrs
• Records of pathway (tracers)
– Oxygen
– Nutrients (nitrate or phosphate)
– Tritium
– CFC’s
– Nd isotopes
Global Conveyor
• Age of water
– Defined as time since it was in contact with the
surface (ventilation)
• Longer it’s away from the surface = more
decay of organics
– Decrease is O2
– Increase in nutrients (Cd/Ca for geologic
record)
– Increase in silicate (hard part regeneration)
– Heavier δ13C
O2 as a Tracer of Circulation
Modified from Broecker and Peng, Fig. 1-11
Nitrate as a Tracer
Modified from Broecker and Peng, Fig. 1-10
Ocean Conveyor Belt
Pinet