Orbital eccentricity and Earth’s seasonal cycle – an opinion, and the ‘eccentriseason’ idea

Earth’s eccentricity is known to vary over long period cycles (see figure 3.4.1. here), but is currently near the lower end of its scale. In a recent (July 2024) short opinion article, Chiang and Broccoli argue for ‘the important role that orbital eccentricity can play in seasonality’. They also highlight the concept of ‘eccentriseasons’, which Beaufort and Sarr define as “seasons occurring at low latitude in response to the cycles of the Earth-Sun distance”. Quoting from the opinion piece:

We argue that Earth’s orbital eccentricity should be given due consideration as an annual cycle forcing in its own right in studies of Earth’s seasonal cycle.

There are two sources of seasonality arising from Earth’s orbit around the Sun.

Earth’s axial tilt (hereafter the tilt effect) produces a seasonal cycle of insolation at a given latitude because of the angle that the surface makes to the sun’s incoming rays.

Earth’s orbital eccentricity (distance effect) provides an annual variation in the solar flux because of the varying distance between the Earth and Sun.

In practice, it is assumed that the tilt effect dominates the Earth’s seasons.

Earth Science textbooks note that the distance effect is negligible since Earth’s orbital eccentricity is relatively small (e ~ 0.0167, meaning that the Earth-Sun distance at aphelion is ~1.67% longer than the mean) and the solar flux changes only by ~7% between aphelion and perihelion.

This assumption extends to the research literature on the seasonal cycle, where the relative roles of tilt versus distance is rarely addressed except in a handful of studies [1–3].

As a result, there is a curious gap in our understanding of how Earth’s seasonal climate responds to orbital eccentricity.

However, orbital eccentricity produces seasonal radiative changes that are comparable in magnitude to transient climate forcings commonly considered in climate studies.

The decrease in insolation absorbed by the Earth at aphelion (relative to the annual mean) is ~8 W/m2. This can be compared to the peak radiative forcing resulting from shorter-lived volcanic eruptions like Pinatubo (-3.2W/m2) [4] resulting from increased reflection by aerosols.

Moreover, while the annual cycle of insolation is dominated by tilt at most latitudes (Fig 1A and 1B), near the equator the annual cycle of insolation is dominated by the distance effect (though the tilt effect does produce a large semiannual cycle) (Fig 1C).

For atmospheric circulation and related climate quantities, their seasonal cycle can depend on nonlocal insolation; if we were to use the globally-averaged insolation as a measure, its annual cycle comes entirely from the distance effect (Fig 1D).
. . .
Our argument has profound implications for the concept of seasonality. Seasonality refers to periodic and generally predictable behavior over the course of a calendar year.

However, the superposition of the tilt and distance effects (assuming the two amplitudes are comparable) can lead to a wholesale change in the seasonality of a region over precessional timescales, since the year defined by the distance effect (the Anomalistic year, from perihelion to perihelion) is slightly longer (by ~25 minutes currently) than the year defined by the tilt effect (the Tropical year, from solstice to solstice) [2].

Beaufort and Sarr [link below] found a gradual and consistent transition in the seasonality of tropical ocean surface temperature with the timing of perihelion in simulations with high orbital eccentricity (e~ 0.054), evidencing the important role that orbital eccentricity can play in seasonality (Beaufort and Sarr goes on to propose the concept of ‘eccentriseasons’ which they define as “seasons occurring at low latitude in response to the cycles of the Earth-Sun distance”).

These effects are not just limited to the deep tropics: Chiang and Broccoli [8] showed that the distance effect can account for an appreciable fraction of the annual cycle for features as poleward as the southern hemisphere westerlies.

Our hypothesis also has implications for paleoclimate.

Full article here.
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Related – Eccentricity forcing on tropical ocean seasonality – Luc Beaufort and Anta-Clarisse Sarr (June 2024):
‘We introduce the concept of “eccentriseasons”, referring to distinct annual thermal differences observed in tropical oceans under high-eccentricity conditions, which shift gradually throughout the calendar year. These findings have implications for understanding low-latitude climate phenomena such as the El Niño–Southern Oscillation (ENSO) and monsoons in the past.’

Source: https://tallbloke.wordpress.com/2024/09/05/orbital-eccentricity-and-earths-seasonal-cycle-an-opinion-and-the-eccentriseason-idea/