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| Titel |
Sensitivity of Pliocene Arctic climate to orbital forcing, atmospheric CO2 and sea ice albedo parameterisation |
| VerfasserIn |
Fergus Howell, Alan Haywood, Steven Pickering |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250127325
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| Publikation (Nr.) |
 EGU/EGU2016-7188.pdf |
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| Zusammenfassung |
| General circulation model (GCM) simulations of the mid-Pliocene Warm Period
(mPWP, 3.264 to 3.025 Myr ago) do not reproduce the magnitude of Northern
Hemisphere high latitude surface air and sea surface temperature (SAT and SST)
warming that proxy data indicates. There is also large uncertainty regarding the state
of sea ice cover in the mPWP. Evidence for both perennial and seasonal mPWP
Arctic sea ice is found in analyses of marine sediments, whilst in a multi-model
ensemble of mPWP climate simulations, half of the ensemble simulated ice-free
summer Arctic conditions. Given the strong influence that sea ice exerts on high
latitude temperatures, a better understanding of the nature of mPWP Arctic sea
ice would be highly beneficial in understanding proxy derived estimates of high
latitude surface temperature change, and the ability of climate models to reproduce
this.
In GCM simulations, the mPWP is typically represented with fixed orbital forcing,
usually identical to modern, and atmospheric CO2 concentrations of ∼ 400 ppm. However,
orbital forcing varied over the ∼ 240,000 years of the mPWP, and it is likely that atmospheric
CO2 varied as well. A previous study has suggested that the parameterisation of sea
ice albedo in the HadCM3 GCM may not reflect the sea ice albedo for a warmer
climate, where seasonal sea ice constitutes a greater proportion of the Arctic sea ice
cover. These three factors, in isolation and combined, can greatly influence the
simulation of Arctic sea ice cover and the degree of high latitude surface temperature
warming.
This paper explores the impact of various combinations of potential mPWP orbital
forcing, atmospheric CO2 concentrations and minimum sea ice albedo on sea ice extent and
high latitude warming. The focus is on the Northern Hemisphere, due to availability of proxy
data, and the large data-model discrepancies in this region. Changes in orbital forcings
are demonstrated to be sufficient to alter the Arctic sea ice simulated by HadCM3
from perennial to seasonal, although only when atmospheric CO2 concentrations
exceed 300 ppm. Reduction of the minimum sea ice albedo from 0.5 to 0.2 is also
sufficient to simulate seasonal sea ice, with any of the combinations of atmospheric
CO2 and orbital forcing. Compared to a mPWP control simulation, monthly mean
increases north of 60∘N of up to 4.2∘C (SST) and 9.8∘C (SAT) are simulated. However,
data-model comparisons show the model temperatures still fail to match the proxy data
temperatures. It is suggested that further high latitude warming may be achieved
through adjustments to cloud parameterisation, although the gap between model
and data temperature in simulations, even with significantly reduced sea ice cover
compared to the control, suggests that agreement may still be difficult to achieve. |
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