Response of tropical climate and circulations to precession of the perihelion

We present results of idealized atmospheric general circulation model simulations designed to understand how tropical atmospheric circulations and precipitation respond to changes in the time of year at which perihelion occurs. In particular, we analyze changes in the seasonality of precipitation and changes in the intensity and location of annual-mean precipitation, which arise through a nonlinear rectification of the seasonal cycle because the precession of perihelion does not affect the annual-mean insolation. An idealized continent that has a simple treatment of land surface hydrology allows us to assess how precipitation changes differ between land and ocean, and how the presence of a continental surface affects the tropical climate. Tropical circulations and their changes are examined to determine the extent to which the changes in water vapor convergence and precipitation can be viewed as "thermodynamic" (i.e., dominated by changes in water vapor concentration with fixed circulation) or "dynamic" (i.e., dominated by changes in circulation with fixed water vapor concentration). In aquaplanet configurations, the changes in net precipitation are largely thermodynamic and circulation changes are small; however, the presence of a land surface provides additional nonlinearities that can alter the subtropical hydrological cycle's response to the precession-forced insolation changes.