Abstract:

Modeling obliquity and CO₂ effects on Southern Hemisphere climate during the past 408 ka

The effect of obliquity and CO₂ changes on Southern Hemispheric climate is studied with a series of numerical modeling experiments. Using the Earth System Model of Intermediate Complexity LOVECLIM and a coupled general circulation model MIROC we show in time slice simulations that phases of low obliquity enhance the meridional extratropical temperature gradient, increase the atmospheric baroclinicity and intensify the lower and middle troposphere Southern Hemisphere Westerlies and storm tracks. Furthermore a transient model simulation is conducted with LOVECLIM which covers the greenhouse gas, ice-sheet and orbital forcing history of the past 408 ka. This simulation reproduces reconstructed glacial/interglacial variations in temperature and sea-ice qualitatively well and shows that the meridional heat transport associated with the orbitally paced-modulation of middle troposphere Westerlies and stormtracks partly offsets the effects of the direct shortwave obliquity forcing over Antarctica, thereby reinforcing the high correlation between CO₂ radiative forcing and Antarctic temperature. The overall timing of temperature changes in Antarctica is hence determined by a balance of shortwave obliquity forcing, atmospheric heat transport changes and greenhouse gas forcing. A shorter 130 ka transient model experiment with constant CO₂ concentrations further demonstrates that surface Southern Hemisphere Westerlies are primarily modulated by the obliquity cycle, rather than by the CO₂ radiative forcing.


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