the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Precession driven low-latitude hydrological cycle paced by shifting perihelion
Abstract. Paleoclimate proxies reveal a significant precessional impact on the low-latitude hydrological cycle. Classical theory suggests that precession modulates the inter-hemisphere summer insolation difference, and hence controls the meridional displacement of the Inter-Tropical Convergence Zone. Accordingly, low-latitude precipitation variations are expected to be in-phase (for the Northern Hemisphere) or anti-phase (for the Southern Hemisphere) with the Northern Hemisphere summer insolation. However, increasing number of proxies, particularly those absolutely dated ones, reveal that variations in terrestrial precipitation at different low-latitudes follow distinct precession rhythms that are very often out-of-phase with hemispheric summer insolation. The mechanism underlying such spatial complexity remains elusive. In this study, we argued that the precession driven low-latitude hydrological cycle is paced by shifting perihelion, rather than the hemispheric summer insolation. More specifically, 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 asynchronous terrestrial precipitation maxima at different latitudes. Our hypothesis, supported by both model simulations and geologic records, suggests that the insolation in individual seasons is equally important in shaping the orbital scale climate changes at low-latitude. This offers an alternative dynamical interpretation for the complex evolution of low-latitude hydrological cycle under precessional forcing.
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CC1: 'Comment on egusphere-2024-2778 - Be precise, use the proper wording', Marie-France Loutre, 14 Oct 2024
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This is a very technical comment on the paper but not at all a review on the scientific findings, and their validity.
The authors wrote “The latitude of the perihelion is introduced as the latitude of Sun’s zenith point when perihelion occurs. This latitude also represents the latitude of maximum incoming solar radiation at the top of atmosphere.”
As far as I know the zenith is defined for an observer on the Earth but not for the Sun. What the authors mean is probably something else. Is it the angular distance of the Sun (at the perihelion) from the zenith, i.e. co-latitude?
« the latitude of maximum incoming solar radiation » Do the authors mean on the day that the Sun reached the perihelion ? In that case when perihelion occurs at summer/winter solstice the maximum incoming solar radiation is at the pole (north/south), not between the tropics.
“Currently, perihelion happens in boreal winter … About 11 kiloyears ago, perihelion occurred in boreal summer … » My understanding is that it means 180deg in 11kyr, which does not correspond to « around 20.4 minutes per year ».
The authors should clearly be more careful in their explanation. For example, they wrote ‘when perihelion occurs’. As soon as the eccentricity is not zero, there is a perihelion. Therefore, it always ‘occurs’. The authors probably meant something else but it is unclear what.
Citation: https://doi.org/10.5194/egusphere-2024-2778-CC1
Data sets
AWI-ESM precessional cycle simulation Hu Yang https://zenodo.org/doi/10.5281/zenodo.13681175
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