A Tale of two Annual Cycles: The Remarkable Change in Pacific Cold Tongue Seasonality Unver Orbital Precession
Event Type: Webinar (Zoom Access)
Speaker: John Chiang, University of California – Berkeley
I will argue that the surface temperature of the Pacific cold tongue possesses not one, but two distinct annual cycles. One originates from the Earth’s axial tilt (“tilt effect”), and in accord with current understanding of how the cold tongue annual cycle arises. The other annual cycle is driven by the variation in Earth-Sun distance (‘distance effect’) from orbital eccentricity. As the two annual cycles possess slightly different periodicities, over a precession cycle the two annual cycles interfere resulting in a complex evolution of net cold tongue seasonality. The annual cycle amplitude from the distance effect increases linearly with eccentricity, and is comparable to the amplitude from the tilt effect at the largest eccentricity values over the past half million years. Mechanistically, the distance effect on the cold tongue arises through a seasonal longitudinal shift in the Walker circulation and subsequent wind forcing on the tropical Pacific ocean-atmosphere system. Implications of this finding for paleoclimate, and for our understanding of seasonality, will be discussed.