Astronomical driven low-latitude hydrological cycle not paced by summer insolation

For a century, the hemispheric summer insolation is proposed as a key pacemaker of astronomical climate change. In high latitudes, these climate changes are characterized by cyclical expansion and retreat of ice sheets. While the low-latitude climate changes are featured by strong variations in the hydrological cycle, with dominant precessional variations. Existing studies argued that precession determines the inter-hemispheric summer insolation difference, thus regulating the North-South seesaw of the ITCZ. However, an increasing number of geologic records, especially those absolutely dated ones, reveal that terrestrial precipitation shows asynchronous precessional evolutions that are very often out of phase with the summer insolation. The underlying mechanism, despite being highly debated, however, remains unclear. In this study, we proposed that the astronomically driven low-latitude hydrological cycle is paced by shifting perihelion, rather than the Northern (or Southern) Hemisphere summer insolation. Precession of the Earth’s rotation axis alters the occurrence season and latitude of perihelion. When perihelion occurs, increasing insolation raises the moist static energy over land faster than over ocean due to differing thermal inertia. This thermodynamically moves the tropical convergence precipitation from the ocean to the land, contributing to enhancing the terrestrial precipitation over the latitudinal rain belt. As perihelion shifts towards different latitudes and seasons at different precessional phases, this leads to asynchronous terrestrial precipitation maxima at different latitudes. We present both model simulations and geological records to support our hypothesis. Our results suggest that the insolation in individual seasons is equally important in shaping the orbital scale climate changes at low latitudes. This offers new insight into the Milankovitch theory.