Tropical circulation and hydrological cycle response to orbital forcing

摘要

The intensity of the two major atmospheric tropical circulations, the Hadley and Walker circulation, has been analyzed in simulations with the Kiel Climate Model (KCM) of the early Eemian and the early Holocene, both warmer climate epochs compared to the late Holocene, or pre-industrial era. The KCM was forced by changes in orbital parameters corresponding to the early and late Holocene (9.5kyr BP and pre-industrial) and the early Eemian (126kyr BP). An intensification of the Southern Hemisphere (SH) winter Hadley cell and a northward extension of its rising branch, the Intertropical Convergence Zone, relative to pre-industrial are simulated for both warm periods. The Walker circulation's rising branch is shifted westward towards the Indian Ocean due to an increased zonal tropical sea surface temperature (SST) gradient across the Indo-Pacific Ocean, which drives enhanced easterlies over this region. The simulated vertically-integrated water vapor transport across the Equator shows the strongest response for the SH winter (boreal summer) Hadley cell over the Pacific Ocean due to an enhanced cross-equatorial SST gradient in the tropical Pacific during the early Holocene and the early Eemian. The orbitally-induced increase of the cross-equatorial insolation gradient in the tropical Pacific leads to a strengthening (weakening) of the wind speed and enhanced (reduced) evaporative cooling over the southern (northern) tropical Pacific, which reinforces the initial radiatively-forced meridional SST gradient change. The increased cross-equatorial insolation gradient in combination with the strong wind-evaporation-SST feedback and changing humidity are important mechanisms to enhance the SH winter Hadley circulation response to orbital forcing.