lecture 8: orbital variation and insolation change (chapter 7)
TRANSCRIPT
Lecture 8: Orbital Variation and Insolation Change
(Chapter 7)
Earth’s revolution around the sun on the ecliptic
Orbit Today
Tilt of the earth’s axis of rotation
Tilt Effect
Tilting and season
Tilt and Season
Extreme tilt
Extreme Tilts
Earth’s revolution around the sun on the ecliptic
Perihelion/Aphelion Today
Change of tilt
Eccentricity of the earth’s orbit
Change of Eccentricity
Precession of equinoxes (wobble and shift of perihelion)
Perihelion
Aphelion
Precession of angle (between perihelion and equinox axes)
Extreme Solstice (large eccentricity * large tilt)
0ka
11ka
Modulation of Precession Index by Eccentricity
Precession Index Modulated by Eccentricity
Seasonal insolation
Where precession signal is large, why?
Where tilt signal is large, why?
What signal is dominant in the annual mean, why?
What is the phase of each signal, in different hemisphere?
~10%
Seasonal insolation
Insolation time series of different months
Apr, 1
Kepler’s laws: equal area!
May 1|Calender=Apr 1 +30 days
May 1|Celestial=Apr 1 +30o
Jul 1|Celestial=Apr 1 +90o
Jul 1|Calender=Apr 1 +91 days
Calendar months (fixed-day) vs. Celestial months (fixed-degree, or fixed-angular)
Calendar vs. Celestial months
Starting Vernal Equinox
Chen et al., 2010, Clm Dyn
0 ka 126 -0 ka, Calendar month
Celestial month Calendar-Celestial
126 ka (PH June) -- 0 ka (PH Jan)
Caloric months (relative warmth)
Caloric summer is the 182 days of insolation more than the other 182 days (Caloric winter)
Searching for orbital signal in climate records
simple
complicated
Milutin Milankovitch was a Serbian engineer and meteorologist - born in 1879 he attended the Vienna institute of technology graduating in 1904 with a doctorate in technical sciences. He then went on to work in the University of Belgrade where he spent time working on a mathematical theory of climate based on the seasonal and latitudinal variations of solar radiation received by the Earth. Milankovitch proposed that the changes in the intensity of solar radiation received from the Earth were effected by three fundamental factors. The first is called eccentricity, a period of about 100,000 years in which the nearly circular orbit of the Earth changes into a more elliptical orbit. The next factor is called obliquity, a period of about 41,000 years where the Earth's axis tilt varies between 21.5 and 24.5 degrees. The final factor is called precession, a period of approximately 23,000 years where the Earth's axis wobbles like a spinning top.
Milankovitch TheoryOrbital theory of glaciations and climate model
(2) a simple climate model
Milankovitch(1920):(1) accurate calculations of insolation change due to orbital changes
Koeppen and Wegner (1924): give strong support to linking cool summers to initiation of glacials
Sensitivity experiments: Response of temperature to changes in orbital parameters
116 ka 11 ka
Rejection of Orbital Theory
JEK - 2014
Simpson reported LARGE summer temperature chargesAnd LARGE winter temperature changes, but he concluded that these extremes cancelled in the annual average. Therefore Milankovitch’s idea was unimportant.
Simpson (1940):
New observations from marine sediments resurrect Orbital Theory
JEK - 2014
Hays, Imbrie and Shackleton, 1976
Marine observations Spectra with orbital period peaks
but, relative magnitude ?
Power spectral analysis
Proof of orbital forcing!
Spectral analysis
...)2sin(
)sin(
...)2cos()2sin(
)cos()sin(
)]cos()sin([)(
22
110
22
110
0
tc
tcb
tbta
tbtab
ntbntatf nn
22nnn bac
where
Fourier analysis
is the power (amplitude) at frequency nnT /2or period
but, relative magnitude ?
Power spectral analysis
Proof of orbital forcing!
Undersampling!
Homework set 3
• Insolation forcing
• Power spectrum
End of Lecture 8
