Tropical climatic phase lags and Earth's precession cycle

Abstract

The Earth's precession cycle causes changes in the seasonal cycle of incident solar radiation, affecting high-latitude and tropical climate in the period range ∼19–24 kyr (refs 1–3). Atmospheric general circulation models (GCMs) have shown that thermally forced monsoon circulations are enhanced by the strengthened seasonal cycle of solar heating that results when summer solstice occurs near perihelion^4,5. However, observations indicate that the tropical climatic response reaches a maximum thousands of years after solstice–perihelion alignment^2,6. Because the GCM experiments included slowly evolving boundary conditions (such as ice sheet extent and surface albedo), whose influences on tropical climate are not well understood, reasons for the observed lag are unclear. We have addressed this uncertainty by examining the response to the precession cycle. Our model suggests that some features of the monsoon circulation may reach maximum intensity ∼3,000 yr after solstice–perihelion alignment as the result of a direct thermal response to the astronomical forcing.