干旱区生态环境知识资源中心

Our understanding of the climates that have existed on Earth through its history has increased tremendously through a combination of geophysical fluid dynamics, geological evidence, and numerical modelling. Evolution of our Earth's orbit redistributes incoming solar radiation latitudinally and temporally and is believed to have been responsible for changing the strength of the south Asian monsoon, expanding and contracting desert regions, and perhaps even initiating the (geologically) recent cycles of glaciation. Evolution of the atmosphere itself, in terms of the amount of atmospheric greenhouse gases in it, affects the amount of water vapour in the atmosphere, global temperatures, and meridional temperature gradients. Topographic forcing by the massive continental ice sheets that have existed in the past is believed to have significantly altered the jet stream circulation.

All of these factors - the distribution of incoming solar radiation, atmospheric greenhouse gases, and topograhic forcing affect the mean baroclinic structure of our atmosphere, the amount of baroclinic wave activity present, and the eddy heat and momentum fluxes associated with these eddies. The equatorward flux of easterly momentum during the barotropic decay phase of these waves, in particular, plays a key role in determining the strength of upper level convergence and subsidence in the subtropics and, hence, the low-level meridional pressure gradient that helps to maintain the tropical trade winds. Through a series of numerical experiments with a coupled atmosphere-ocean general circulation model, it is shown that all of the factors listed above have played a role in determining the amount of baroclinic wave activity in climates of the Earth's past and that changes in tropical circulations are consistent with the notion that the baroclinic eddy field plays an important role in determining the mean state in the tropics.