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