A NOTE ON ORBITAL CONTROL OF EQUATOR-POLE HEAT FLUXES

We note that orbital (Milankovitch) variations, in particular the prec ession of the equinoxes, can lead to profound variations in the flux o f heat from the tropics to higher latitudes. The mechanism involves ch anging the intensity of the Hadley circulation by varying the maximum displacement from the equator of the zonally averaged surface temperat ure maximum in summer. The precession of the equinoxes causes this qua ntity to vary by more than a factor of 2. The intensity of the Hadley circulation has a major influence on the heat fluxes in the winter hem isphere. Summer heat fluxes are generally small. Although the precessi on cycle is characterized by periods in the neighborhood of 20000 year s, the variations are modulated by the eccentricity whose variation is dominated by periods in the neighborhood of 100000 years and 400000 y ears. We show how the fact that both small and large heat fluxes lead to low snowfall (and, hence, small glacial accumulation) causes the de modulation of the heat flux leading to dominant eccentricity periods i n the resulting glaciation.