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. We found that the cold tongue possessed not one but two seasonal cycles: a ‘tilt season’ one driven by tilt (and which corresponded to our prevailing understanding), and a ‘distance season’ driven by orbital eccentricity. The year associated with the distance effect is about ~25 minutes longer than the annual period arising from the tilt effect. It meant that, over time, the phase of the distance seasons shift relative to the tilt seasons. 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 cold tongue annual cycle is not small: 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).

We are currently exploring this hypothesis in my lab group, funded by an NSF P4Climate grant. Our working hypothesis is that the distance effect mechanistically works its influence on the Earth’s seasonal climate through a thermal contrast between the ‘marine hemisphere’ centered over the Pacific ocean, and the ‘continental hemisphere’ centered over Africa. The seasonal variation in this thermal contrast leads to an east-west shift of the Walker circulation, and which in turn influences global climate through climate teleconnctions.

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