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 (tilt effect) and that Earth’s elliptical orbit around the Sun (distance effect) plays a negligible role. However, recent research points to a greater role for 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 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 attributes the cold tongue seasonal cycle to the tilt effect, but 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’ driven by tilt (and in accordance with prevailing understanding), and a ‘distance season’ driven by orbital eccentricity. The year arising from the distance effect (perihelion to perihelion) is about ~25 minutes longer than the year arising from the tilt effect (solstice to solstice). 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 and this opinion piece where we outline our argument that the distance effect should be treated as a seasonal cycle in its own right (Chiang and Broccoli 2023, 2024).

My lab group is currently exploring this idea, funded by a 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.

In Chiang and Kong 2025 we decompose the seasonal cycle of tropical sea surface temperature into contributions from the tilt and distance effects. The SST annual cycle from the distance effect consists of two main behaviors: a thermodynamic warming of the entire tropical basin peaking two months after perihelion, and a dynamic cooling of the Pacific cold tongue peaking 5-6 months after perihelion. For today’s orbital conditions (with relatively low eccentricity), the former acts to dampen the tropical SST seasonal cycle of the northern hemisphere from the tilt effect and amplify it in the southern hemisphere, and the latter shift the Pacific cold tongue seasonal cycle arising from tilt to earlier in the season, by ∼1 month. At high eccentricity (e > 0.05), the distance effect becomes prominent and dominates the SST annual cycle in some regions of the Tropics.

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

Chiang, J.C.H., and Broccoli, A.J. Orbital eccentricity and Earth’s seasonal cycle. PLOS Climate, 3(7), p.e0000436 (2024).

Chiang, J.C.H. and Kong, L.Y.L. The Seasonal Cycles of Tropical Sea Surface Temperature from Earth’s Axial Tilt and Orbital Eccentricity. In press for Journal of Climate, June 2025.