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| Titel |
Influence of LGM boundary conditions on the global water isotope distribution in an atmospheric general circulation model |
| VerfasserIn |
Thejna Tharammal, Andre Paul, Ute Merkel, David Noone |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250050102
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| Zusammenfassung |
| Because of differences in the saturation vapor pressures between the isotopic forms of the
water molecule (H216O, H218O, HDO), an isotopic fractionation occurs during any phase
transition such as evaporation or condensation. This is a temperature-dependent process,
which causes the ratios of the heavier to the lighter isotopes in different reservoirs of the
hydrological cycle to vary depending on the atmospheric conditions in the source region or
along the way. So the isotopic information may be utilized to make inferences about climate
change.
The Last Glacial Maximum (LGM, around 21,000 years B.P) represents the largest
change in climate during the last 100,000 years. A strong cooling in both hemispheres is
recorded in many proxy records. We conducted a series of experiments using a water isotope
tracers-enabled atmospheric general circulation model (Community Atmosphere Model
version 3.0, CAM3.0-Iso), by changing the individual boundary conditions (greenhouse gases
(GHG), albedo, topography and ice sheets) each at a time to LGM values. In addition, we
carried out a combined simulation with all the boundary conditions being set to the
values at the LGM. A pre-industrial (PI) simulation with boundary conditions taken
according to the PMIP (Paleoclimate Modelling Intercomparison Project) protocol
was performed as the control experiment. The experiments are designed in order
to analyze the temporal and spatial variations of the isotope distribution with the
change of the individual climate factors and to verify the isotope proxy records for
LGM.
In agreement with previous studies, the experiments show that at high latitudes δ18O in
precipitation (δ18Oprecip) is strongly correlated with the local surface temperature, while in
the tropics δ18Oprecip is strongly correlated with the amount of precipitation. Furthermore, a
strong seasonality in the global distribution of δ18Oprecip is observed. The change in
topography (due to the change in land-ice cover) plays a significant role in reducing the
surface temperature and δ18Oprecip over North America. Exposed shelf areas and their
related surface albedo, combined with the LGM vegetation and surface type distribution,
reduce the Northern Hemisphere surface temperature and δ18Oprecip further. A global mean
cooling of 4.1Ë C is observed in the simulation with complete LGM boundary conditions
compared to the control simulation, which is in agreement with previous experiments using
the fully coupled community Climate System Model (CCSM3.0). Large reductions in
δ18Oprecip over the LGM ice sheets are highly correlated with the temperature decrease over
them.
Our results show that the change in ice-sheet topography has the largest effect in lowering
the surface temperature and precipitation hence δ18Oprecip in the northern hemisphere during
the LGM. The reductions in surface temperature and δ18Oprecip values in the GHG
experiment are low in comparison with the localized and prominent reductions in the surface
temperature and δ18Oprecip concentration of topography and albedo experiments,
even though the reduced GHG bring about lowered surface temperature all over the
globe. The lack of an active ocean component and hence positive ocean-atmosphere
feedback mechanisms in the model is expected to have minimized the effect of the
reduction of GHG concentrations to LGM levels on climate in the GHG experiment. |
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