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samenvatting paleoceanografie (PT. 1)- UU
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Samenvatting vallis ch 1 t/m 6 + globaal de nog niet besproken delen van ruddiman part I. Lesmateriaal voor eerste deel van de cursus.
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Paleoceanografie (PT. I)CHAPTER 1, BASICS OF CLIMATE
Ellipticity or eccentricity, not in fact constant and varies on timescales of about 100,000 years because of the
influence of other planets on Earth’s orbit; these variations may play a role in the ebb and flow of ice ages)
- Farthest distance from the sun = aphelion
- Closest distance from the sun = perihelion
Obliquity → responsible for the seasons → angle of earth’s rotation (41.000 yrs)
Average depth of the oceans ≈ 4kms
½ mass of the atmosphere in lowest 5k and 95% in lowest 20k → mass of atmosphere is tiny compared to
oceans. Weight atmosphere → atmospheric pressure ≈ 1*105 Pa. Pressure at bottom of oceans ≈ 4*107 Pa.
The atmosphere is composed of nitrogen, oxygen, carbon
dioxide, water vapor, and a number of other minor
constituents (see table). Water vapor content is not
constant through the atmosphere → humidity / rainfall /
drought.
Earth’s temperature is maintained by a balance between
incoming radiation from the sun (So =S/4 = 342 W/m2)
and radiation emitted by earth itself.
Atmosphere has significant effect on balance → greenhouse gasses (H2O, CO2, and other constituents) →
absorb infrared (longwave) radiation emitted by earth itself → causes temperatures to be much higher (makes
earth habitable). A given amount of radiation is spread over a larger area at higher latitudes → colder
temperatures.
Axis of rotation is fixed in space, but rotates
around the sun →orientation relative to sun
varies:
July → more rad. on NH (summer), less on SH
(winter)
Equinoxes → equal radiation NH/SH
Coldest/warmest days of year occur few weeks
after predicted → due to thermal inertia of the
oceans → decay in onset of warm/cold days.
Eccentricity, minor factor in seasonality
Solar radiation → surface warms → emits radiation.
Black body: emits (and absorbs) electromagnetic radiation w/ perfect efficiency.
- All radiation is absorbed.
- Body appears black unless it emits its own visible radiation → unless T = 0K it will emit its own rad.
- Amount of rad. emitted increases with T → F (flux of emitted radiation / unit area (Wm2) = σT4.
- Emits radiation over range of wavelengths → peak at wavelength inversely proportional to it T
, • Wien’s law = λpeak = b/T with b = 2.898*10-3mK
- Sun emits visible light, earth emits infrared light → molecules in our atmosphere absorb infrared
quite efficiently but are fairly transparent to solar radiation → greenhouse effect
Simple energy-balance model (EBM)
- Fraction α = albedo = solar radiation reflected back ≈ 0.3
- Solving for T → 255K ≈ -18C → too cold because didn’t account for greenhouse-effect.
- Net in = Net out
Greenhouse effect:
- When solar radiation reaches the surface, a fraction is reflected, and the rest is absorbed → warming
of the surface. The fraction reflected = planetary albedo ≈0.3. cloud albedo is higher (.5-.9) but surface
is lower (.1), but higher for ice and snow.
- Surface is warmed by absorbed radiation → emits radiation upward due to surface T
(longwave/infrared). Atmosphere ≠ transparent to infrared/longwave due to greenhouse gasses →
absorb and re-emit infrared radiation down back to earth → earth absorbs it back up.
- What are greenhouse gasses ? (See table)
• H2O and CO2 have dominant effect, H2O(clouds and shit) varies around the globe and CO2
remains constant through atmosphere (changes by natural causes, vast changes due to
burning of fossil fuels).
• Higher T → more water vapor H2O → higher T (positive feedback).
• If atmosphere is dry → adding CO2 makes big difference to GH effect. If a lot of water vapor
is present → long wave radiation CO2 would absorb already has been absorbed by water
vapor
• Amount of water vapor adjusts to the level of other GH-gases → thus feedback and not
primary forcing.
Mathematical model of greenhouse effect:
- Assumptions:
• The surface and the atmosphere are each characterized by a single temperature, Ts and Ta
• The atmosphere is completely transparent to solar radiation.
• Earth’s surface is a blackbody.
• The atmosphere is completely opaque to infrared radiation, and it acts like a blackbody.
The parameter Є , called the emissivity or the
absorptivity = 1 for blackbody (the
atmosphere is a blackbody and absorbs all the
surface infrared radiation)
At surface:
At atmosphere:
, or Earth is not a perfect blackbody → T would be too high with these
calculations.
“leaking” of longwave radiation → Є ≠ 1 but lower (about .75). Leaking can be compensated with:
Є increases due to greenhouse gasses.
New balance:
at surface
at atmosphere
, CHAPTER 2, THE OCEANS: A DESCRIPTIVE OVERVIEW
Average depth of oceans ≈ 3.7km, deep trenches up to ≈10km. Volume ≈ 1.3*1018m3, mass ≈1.4*1021kg. Only
about 2% of water on earth is frozen, mainly ice sheets of antarctica and Greenland. Ice on land and ocean has
high albedo. Origin of salt = weathering and erosion + outgassing of chloride from earth’s interior and leaching
of sodium from ocean floor.
Salinity varies spatially:
- High of 37‰ in surface waters of subtropics → evap removes fresh water.
- Low of 32 ‰ at high northern latitudes where rain brings fresh water and evap is small.
Variation in salinity & temperature → density variation in oceans
ρ0 = 1.027 × 103 kg m−3 , T0 = 10°C, S0 = 35 g kg−1, βT and βS are the coefficient of thermal expansion p.27.
Salinity plays greater role in density when temperatures are low.
Ocean has a moderating effect on the climate:
- Slows down progression of global warming.
- Generates winds that bring milder winter and cooler summers near coasts.
Temperature distribution at surface (SST) most affects our climates.
Oversimplification of ocean circulation components:
- Quasi horizontal circulation driven by wind and gives rise to the great ocean gyres.
- Meridional overturning circulation (MOC), in which cold dense water sinks at high latitudes before
moving equatorward and rising in low latitudes.
In mid/high latitudes large sale circulation is characterized by the gyres, between 15° and 45° circulation is
dominated by subtropical gyres. This circulation is driven by the winds, westerlies (wind from west) in mid
latitudes and the generally easterly trade winds in low latitudes. Circulation occurs mainly in upper few
hundred meters.
The meridional component is much stronger in the west of the oceans → western boundary currents. The
equatorward return flow in all subtropical gyres is spread over a much greater longitudinal extent and so is
much weaker per unit area.
Northern hemisphere → subpolar gyres in the poleward regions. Consequence of strong midlatitude westerly
winds and weak easterly winds on high latitudes. Intense western boundary currents flowing equatorward.
Deepwater formation at high latitudes due to high density of water.
The Antarctic Circumpolar Current (ACC)
- High latitudes, southern hemisphere → no other continents connect to Antarctica → water is free to
circulate around the globe.
- Flow predominantly east-west, resembles that of the atmosphere.
- Average glow rate of 120Sv up to 150Sv → gulf stream (comparison) = 30Sv
Meridional overturning circulation (MOC)
- Circulation in meridional
- Horizontal variations can be important.
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