The Seasonal Cycles of Tropical Sea Surface Temperature from Earth’s Axial Tilt and Orbital Eccentricity

Abstract 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 the 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. Significance Statement It is commonly assumed that climate seasonality is caused by Earth’s axial tilt. However, this is not necessarily the case for the tropics, where the annual variation of insolation from changes to the Earth–sun distance due to orbital eccentricity is significant. Here, we decompose the contributions from tilt and eccentricity on the seasonality of tropical sea surface temperature as simulated by an Earth system model. Orbital eccentricity drives an appreciable portion of tropical sea surface temperature seasonality even in today’s low-eccentricity orbit, modifying the seasonality driven by Earth’s axial tilt. At high orbital eccentricity ( e > 0.05), the response to Earth–sun distance changes can dominate the annual cycle of sea surface temperature in some regions of the tropics.