Orbital Eccentricity and Earth's Seasonal Climate

We are taught in school and in college that Earth’s seasons are caused by Earth’s axial tilt (hereafter the tilt effect) and that Earth’s elliptical orbit around the Sun (hereafter the distance effect) plays a negligible role. However, recent research has motivated me to reconsider the role of the distance effect in Earth’s seasonal climate.

In a 2015 paper, Michael Erb and Tony Broccoli found that the Pacific cold tongue seasonal cycle dramatically changed its phase and amplitude as the longitude of perihelion (the angular position of perihelion relative to the autumnal equinox) was altered in the GFDL CM2.1 (note they used a large orbital eccentricity of e = 0.0493). The warm and cold months of the cold tongue, which occurs around March and September with perihelion set to NH winter solstice (similar to today), shifts to June and December when perihelion set to NH summer solstice! This was unexpected as the prevailing understanding was that the cold tongue seasonal cycle was driven by the tilt effect, but that the obliquity was not altered in their simulations.

In Chiang et al. 2022, we solved this puzzle. First, we showed that the cold tongue seasonal behavior occurred in several different Earth system models, indicating that it is a ubiquitous feature. We found that the cold tongue possessed not one but two annual cycles: one driven by tilt (and which corresponded to our prevailing understanding), and another driven by orbital eccentricity. The dynamics of the distance effect annual cycle cold tongue was distinct from that of the tilt effect, arising through a seasonal longitudinal shift of the Walker circulation that then drove a forced annual cycle of the tropical Pacific coupled ocean-atmosphere system.

Even more interesting was the fact that annual period arising from the distance effect (‘Anomalistic year’) is about ~25 minutes longer than the annual period arising from the tilt effect (‘Tropical year’). It meant that, over time, the phase of the distance effect annual cycle shifts relative to the tilt effect annual cycle. Thus, the Pacific cold tongue behavior seen by Erb et al. (2015) is readily explained as the result of the superposition of two annual cycles of comparable amplitudes but with slightly different periods. The amplitude of the distance effect annual cycle is linear with eccentricity and is comparable to the tilt effect for e=0.05, which means that even at today’s relatively low eccentricity (e=0.017), the distance effect amplitude is ~1/3 of the tilt effect amplitude. In other words, it is not negligible!

The speculation is that the distance effect plays a more important role in Earth’s seasonal climate than it is given credit for. Tony Broccoli and I elaborate on this speculation in this position paper where we outline our argument that the distance effect should be treated as an annual cycle in its own right (Chiang and Broccoli 2023). In other words, that one should account for both annual cycles in seasonal cycle studies, separate their respective roles, and give the annual cycle from distance the same due consideration as for tilt. This work is currently ongoing in my lab group.

Chiang, J.C.H., Atwood, A.R., Vimont, D.J. et al. Two annual cycles of the Pacific cold tongue under orbital precession. Nature 611, 295–300 (2022). https://doi.org/10.1038/s41586-022-05240-9. SharedIt Link / Research Briefing / Press Release

Chiang, J.C.H., and Broccoli, A.J. A role for orbital eccentricity in Earth’s seasonal climate. Geosci. Lett. 10, 58 (2023). https://doi.org/10.1186/s40562-023-00313-7