Two seasonal cycles of tropical sea surface temperature under orbital forcing

Earth’s current climate seasonality is assumed to be driven by Earth’s axial tilt. However, a recent study by the first author and collaborators argued that orbital eccentricity plays a significant role in the annual cycle of the Pacific cold tongue, suggesting the same could be true more broadly for Earth's seasonal climate contrary to conventional wisdom. Following one of Mark's favorite baseball dictums - "hit 'em where they ain't" - we explore the relative roles of Earth’s axial tilt (‘tilt effect’) and orbital eccentricity (‘distance effect’) on the seasonal cycle of sea surface temperature (SST) in the Tropics, decomposing the two contributions from a set of simulations spanning the space of eccentricity and longitude of perihelion. A simple model of tropical SST seasonality is derived that separates the contribution from the two effects and allows for the SST seasonality to be reconstructed for any given longitude of perihelion and eccentricity (the latter to within realistic range).

Most of the variance in tropical SST seasonality is explained by three terms: the annual and semiannual contributions from the tilt effect, and the annual contribution from the distance effect. Variation in the net tropical SST seasonality under precession can be readily explained by changes in the amplitude (which increases linearly with eccentricity) and phase (which changes linearly with the longitude of perihelion) of the distance effect annual contribution, with the tilt effect contribution being unchanged. Finally, the annual tropical SST response to the distance effect reveals a thermodynamic contribution with a tropics-wide warming peaking ~1.5 months after perihelion, and a dynamic contribution characterized by a cooling of the Pacific cold tongue peaking ~5.5 months after perihelion. Several other tropical ocean regions with an outsized contribution by the distance effect to their SST seasonal cycle are identified and discussed, including the equatorial Indian Ocean and southwestern equatorial Pacific.