A role for orbital eccentricity in Earth's seasonal climate

The seasonality of Earth's climate is driven by two factors: the tilt of the Earth's rotation axis relative to the plane of its orbit (hereafter the tilt effect), and the variation in the Earth-Sun distance due to the Earth's elliptical orbit around the Sun (hereafter the distance effect). The seasonal insolation change between aphelion and perihelion is only similar to 7% of the annual mean and it is thus assumed that the distance effect is not relevant for the seasons. A recent modeling study by the authors and collaborators demonstrated however that the distance effect is not small for the Pacific cold tongue: it drives an annual cycle there that is dynamically distinct and similar to 1/3 of the amplitude from the known annual cycle arising from the tilt effect. The simulations also suggest that the influence of distance effect is significant and pervasive across several other regional climates, in both the tropics and extratropics. Preliminary work suggests that the distance effect works its influence through the thermal contrast between the mostly ocean hemisphere centered on the Pacific Ocean (the 'Marine hemisphere') and the hemisphere opposite to it centered over Africa (the 'Continental hemisphere'), analogous to how the tilt effect drives a contrast between the northern and southern hemispheres. We argue that the distance effect should be fully considered as an annual cycle forcing in its own right in studies of Earth's modern seasonal cycle. Separately considering the tilt and distance effects on the Earth's seasonal cycle provides new insights into the workings of our climate system, and of direct relevance to paleoclimate where there are outstanding questions for long-term climate changes that are related to eccentricity variations.