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 K THRESHOLD THRESHOLD ? 40K WORLD V t+1 - V t = a + bt + c(V t – V*) Obliquity(t)

Ice Ages: 19 th 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 El Niño Apparently conditions similar to these persisted up to 3 million years ago October 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 Depth Latitude Temperature along a section in the mid-Pacific (152W ) El Nino La Nina Adiabatic: _ _ _ _ _ _ _ __________ El Padre La Madre Diabatic: _ _ _ _ _ _ _ ______________ 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 AntarcticCircumpolar Current D = k 1/3 D 2 = D* 2 +  /  T..

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Temperature Along the EquatorLooking Down Numerical Experiments in which the oceanic surface waters are freshened in high latitudes. Fedorov JPO Heat Transport Freshening m/year

Surface Density 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