The dominant periodicities are the same as those from astronomical calculations of changes in the Earth’s orbital parameters.
Our Planet’s Music has Many Beats:
(a) 1 DAY: Day- Night – Day Spins around its axis
(b) 1 Year: Winter – Summer – Winter Orbits the Sun
(c) 23,000 Years: Stronger Summer, Stronger Winter Precession
(d) 40,000 Years: Tilted axis rocks back and forth OBLIQUITY
(e) 100,000 Years: Eccentricity of the orbit varies
MILANKOVITCH CYCLES
SUN
Seasonal Changes in Sunlight as a function of latitude:
ANNUAL Average
PRECESSION redistributes
sunlight in time so that the
the average over a year is
zero
OBLIQUITY redistributes
sunlight latitudinally so that
the average over the globe
is zero.
WARM
COOLING TREND + AMPLIFYING 40K CYCLES
NONLINEAR
40 .. 80 .. 120K
THRESHOLD THRESHOLD ?
40K WORLD
Vt+1 - Vt = a + bt + c(Vt – V*) Obliquity(t)
0 1 2 3 4 5 6
x 106
1
1.2
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3x 10
7
time
tem
perature
temperature versus time
Ice Ages:
19th Century: Evidence for Ice Ages in the northern Hemisphere
Milankovitch: ICE ALBEDO
Sunlight at 65N in July is the controlling factor
Why is precession unimportant? Huybers
What causes the 110K cycle?
There is no 100K cycle! 40K…80K….120K..
Huybers and Wunsch
What caused the huge cycles of the past million years?
What feedbacks other than ice-albedo increased climate sensitivity?
Measurements in low latitudes provide an unexpected answer!!
OBSERVATIONS: Obliquity variations affect mostly high latitudes….glaciers
Precession affects mostly low latitudes …. monsoons
Surprising Recent Observations:
The tropical Pacific responds mainly to Obliquity!
The records for SST are very similar to Ice Volume records
~ 3 Ma Cold Surface Waters Start Appearing in Low Latitudes
March 1998 - El Niño Apparently conditions similar to these persisted up to 3 million years ago
October 1988 - La Niña
Tropical Sea Surface Temperature patterns strongly influence Rainfall patterns and hence the global climate.
Sea Surface Temperatures (Degrees Centigrade)
W E W E
Dep
th
Latitude
Temperature along a section in the mid-Pacific (152W)
El Nino La NinaAdiabatic:
_ _ _ _ _ _ _ __________ El Padre La MadreDiabatic:
_ _ _ _ _ _ _ ______________
1000m
4000m
A shallow thermocline permits two different processes for varying
sea surface temperatures
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
Diabatic:
_ _ _ _ _ _ _ ______________
To remain cold, the ocean
must have a circulation
An Easterly Wind blowing over a Rotating, Spherical Shell of Fluid maintains a Meridional Overturning Circulation
winds
How the Flux of Heat across the Ocean Surface affects its Subsurface thermal structure.
The Response, the Heat Gain, increases in amplitude as the Period of the forcing increases. Note that there is also a change in the phase of the response.
Response to Imposed Changes in Heat Loss in high latitudes. In the Four Different experiments the amplitude of the forcing remains constant but the period changes from 2 to 5 to 10 to 20 years.
2 years 5 Years
20 Years10 Years
Heat Flux across the Ocean Surface
IN A WARM WORLD: A small oceanic heat loss implies a small oceanic heat gain which requires a deep tropical thermocline.
IN A COLD WORLD ……. A large oceanic heat loss implies a large oceanic heat gain which requires a shallow tropical thermocline.
The Heat Budget determines the Depth of the Oceanic Thermocline
The ocean acts as a low pass filter:
The seasonal cycle in high latitudes can not induce
a seasonal cycle at the equator
UP to 3 Million Years Ago: Drifting continents cause Global Cooling
Shoaling Thermocline
Around 3 Million Years Ago: Glaciers Appear in high northern latitudes
Ice albedo feedback
Cold Surface Water appear in low latitudes
air-sea interactions and stratus cloud feedback
3 Million Years ago onward: Trend in global ice volume
shoaling of the thermocline*
expansion of stratus clouds
oceanic salinity
Amplification of 40K Oscillations
THE ROLE OF SALINITY TEMPERATURE SALINITY
• Salinity introduces an indirect mechanism for interfering with the oceanic heat budget
The MELTING of ICE, and variations in EVAPORATION and RAINFALL can profoundly affect the oceanic circulation, hence
Sea Surface Temperatures and the Global Climate.
1. Salinity and temperature have comparable effects on density.
2. The effects of temperature and salinity are in opposition because of heavy rainfall in cold high latitudes. (This introduces the possibility of singularities.)
3. Boundary conditions for temperature and salinity are different Heat Flux = a(T – T*) …. negative feedback! Salt Flux = b(Evaporation – Precipitation) No negative feedback.
Surface Salinities
The Slow, Deep Thermohaline Circulation
The Shallow, Swift Wind-driven Circulation
Antarctic Circumpolar Current
D = k1/3
D2 = D*2 + /T..
.
Temperature
Along the Equator Looking Down
Numerical Experiments in which the oceanic
surface waters are freshened in high latitudes.
Fedorov JPO
Hea
t T
rans
port
Freshening m/year
1.022
1.024
1.026
1.028
Sur
face
Den
sity
gm
/cm
3
41 K WORLD Nonlinear WorldWarm World
Threshold
Warm World (El Nino?)
Trend + Cycles
Since 3 Ma, feedbacks (glaciers, tropical stratus clouds) caused
Cooling Trends
that led to
Thresholds
followed by
Rapid Warming
followed by
Cooling Trend
Milankovitch forcing (Obliquity) is a pace maker
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