Impact of vegetation changes on the dynamics of the atmosphere at the Last Glacial Maximum |
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Authors: | Michel Crucifix Christopher D Hewitt |
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Institution: | (1) Met Office, Hadley Centre for Climate Prediction and Research, FitzRoy Road, Exeter, Devon, EX1 3PB, UK |
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Abstract: | Much work is under way to identify and quantify the feedbacks between vegetation and climate. Palaeoclimate modelling may
provide a mean to address this problem by comparing simulations with proxy data. We have performed a series of four simulations
of the Last Glacial Maximum (LGM, 21,000 years ago) using the climate model HadSM3, to test the sensitivity of climate to
various changes in vegetation: a global change (according to a previously discussed simulation of the LGM with HadSM3 coupled
to the dynamical vegetation model TRIFFID); a change only north of 35°N; a change only south of 35°N; and a variation in stomatal
opening induced by the reduction in atmospheric CO2 concentration. We focus mainly on the response of temperature, precipitation, and atmosphere dynamics. The response of continental
temperature and precipitation mainly results from regional interactions with vegetation. In Eurasia, particularly Siberia
and Tibet, the response of the biosphere substantially enhances the glacial cooling through a positive feedback loop between
vegetation, temperature, and snow-cover. In central Africa, the decrease in tree fraction reduces the amount of precipitation.
Stomatal opening is not seen to play a quantifiable role. The atmosphere dynamics, and more specifically the Asian summer
monsoon system, are significantly altered by remote changes in vegetation: the cooling in Siberia and Tibet act in concert
to shift the summer subtropical front southwards, weaken the easterly tropical jet and the momentum transport associated with
it. By virtue of momentum conservation, these changes in the mid-troposphere circulation are associated with a slowing of
the Asian summer monsoon surface flow. The pattern of moisture convergence is slightly altered, with moist convection weakening
in the western tropical Pacific and strengthening north of Australia. |
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