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Abstract Previous studies have noted the asymmetry in the annual cycle of zonal mean surface air temperature, defined as the difference in the lengths of warming and cooling periods. Pronounced north‐south hemispheric differences in this asymmetry, by up to 40 days, were attributed to the eccentricity of Earth's orbit. However, we propose that the dominant factor comes from the difference in the land‐sea fraction between hemispheres, because the asymmetry is strongly influenced by the annually varying heat capacity and land‐sea interactions. The oceanic temperature annual cycle generally features a longer cooling period than warming due to the seasonal variation in ocean mixed layer depth, and exhibits the opposite situation when there is seasonal sea ice. Land‐sea interactions impact the zonal mean temperature annual cycle by resulting in an earlier winter trough of the downstream oceanic temperature and delaying the summer peak in west coasts. 

Abstract 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 thetilt effect), and the variation in the Earth–Sun distance due to the Earth’s elliptical orbit around the Sun (hereafter thedistance effect). The seasonal insolation change between aphelion and perihelion is only ~ 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 ~ 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. 

We explore the relative roles of Earth’s axial tilt (‘tilt effect’) and orbital eccentricity (‘distance effect’) on the seasonal cycle of tropical sea surface temperature (SST), decomposing the two contributions using simulations of an Earth System model varying eccentricity and longitude of perihelion.  This dataset archives model output produced in this investigation using the Community Earth System Model version 2, and MATLAB code for analyzing the data. 

We explore the relative roles of Earth’s axial tilt (‘tilt effect’) and orbital eccentricity (‘distance effect’) in generating the seasonal cycle of tropical sea surface temperature (SST), decomposing the two contributions using simulations of an Earth System model varying eccentricity and longitude of perihelion. Tropical SST seasonality is largely explained by the annual contribution from tilt, but with significant contributions from the semiannual contribution from tilt and annual contribution from distance, especially in regions where the tilt annual contribution is relatively small. Precessional changes to tropical SST seasonality are readily explained by the distance annual component whose amplitude increases linearly with eccentricity and whose phase changes linearly with the longitude of perihelion, while the tilt contributions remain essentially unchanged. As such, the annual cycle contribution from distance can become significant at high eccentricity (e > 0.05) and dominate the SST annual cycle in some regions of the Tropics. The annual cycle tropical SST response to the distance effect consists of a tropics-wide warming peaking ∼2 months after perihelion consistent with a direct thermodynamic effect, and a dynamic contribution characterized by a cooling of the Pacific cold tongue peaking 5-6 months after perihelion. For current orbital conditions, the thermodynamic contribution acts to dampen the tropical SST seasonal cycle of the northern hemisphere from the tilt influence and amplify it in the southern hemisphere. The dynamic contribution acts to shift the Pacific cold tongue seasonal cycle arising from tilt to earlier in the season, by ∼1 month.