"We teach our students that the Earth’s axial tilt is responsible for the seasons. But for the eastern equatorial Pacific - which is best known as the hotspot for El Nino - it turns out that its seasonal cycle has a sizable contribution coming from the annual variation in earth-sun distance as well. Awkwardly, the latter is what we tell our students how not to think about the seasonal cycle." - Professor John Chiang
During his sabbatical, Professor John Chiang conducted research that re-examines the annual cycle of the Pacific cold tongue. By reproducing the cold-tongue seasonal behavior in various Earth-system models, he revealed that the annual temperature cycle in the Pacific cold-tongue region is influenced by the shape of Earth’s orbit, as well as by the planet’s axial tilt. The annual cycle from the distance effect is slightly longer than that for tilt — around 25 minutes, currently — so over a span of about 11,000 years, the two annual cycles go from being in phase to out of phase, and the net seasonality undergoes a remarkable change, as a result.
According to Honghai Zhang of University of Houston, Chiang's work "calls for rethinking of what causes seasonal cycles not only in the tropical Pacific, but also beyond — especially given that seasonal cycles are often taken for granted and rarely (if ever) questioned.”
Nature Senior Editor, Michael White, also commented: "It is rare for a paper to successfully address the “wait a minute, why doesn’t this make sense?” question in climate science, in which many topics are deeply entrenched. Chiang et al. do exactly that, elegantly staging a series of model experiments to unravel some of the complexities governing the variability of the tropical Pacific. The work should break off a fresh bout of research into the orbital controls of climate variability, more generally."
Interested in learning more about this project? Read the blurb by the Berkeley News site, check out Chiang's published findings and research briefing in Nature, and visit Professor John Chiang's faculty page for more information about his research.