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| Titel |
Transition to a climate state qualitatively different from modern in NCAR CCSM simulations of the Eocene |
| VerfasserIn |
R. Caballero, M. Huber, J. Hanley |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062655
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| Zusammenfassung |
| Recent paleoclimate proxy reconstructions indicate that global-mean surface temperatures
in past warm climates may have been some 10-15°C higher than today. Little is
known about atmospheric dynamics at temperatures this warm. In particular, is
it possible that the general circulation may transition to a qualitatively different
state at warm enough temperatures? We study this question in a set of simulations
using NCAR’s Community Climate System Model in both full- and slab-ocean
configurations with a range of atmospheric CO2 concentrations extending from
preindustrial values (280 ppm) up to 8960 ppm (5 doublings). We find that the
simulation at 4480 ppm CO2 gives a remarkably good fit to recent surface temperature
reconstructions of the early Eocene, and does not suffer from the classical ‘low
gradient’ problem. We also find that the atmospheric general circulation in this and
warmer simulations differs qualitatively from the modern regime in at least two major
ways:
1) When equatorial surface temperatures exceed ~33°C, the model undergoes a transition
to equatorial superrotation, a state with strong annual- and zonal-mean westerlies
on the equator. The transition is driven by zonal momentum convergence due to
large-amplitude transient eddies on the equator. These eddies have a structure similar to
the observed Madden-Julian Oscillation (MJO). The model’s MJO variability is
weaker than observed when simulating the modern climate but increases sharply with
temperature, coming to dominate the tropical variability and mean state of the warmest
climates.
2) Poleward atmospheric moisture transport across midlatitudes decreases with increasing
temperature, the opposite behaviour to that at temperatures close to modern. We account for
this non-monotonic behaviour in terms of a simple diffusive model of the oceanic storm
tracks which suggests that it is due mostly to the drop in mean eddy amplitudes as
temperature increases. |
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