The tropical precipitation response to orbital precession

Orbital precession changes the seasonal distribution of insolation at a given latitude but not the annual mean. Hence, the correlation of paleoclimate proxies of annual-mean precipitation with orbital precession implies a nonlinear rectification in the precipitation response to seasonal solar forcing. It has previously been suggested that the relevant nonlinearity is that of the Clausius–Clapeyron relationship. Here it is argued that a different nonlinearity related tomoisture advection by the atmospheric circulation is more important.When perihelion changes from one hemisphere’s summer solstice to the other’s in an idealized aquaplanet atmo-spheric general circulation model, annual-mean precipitation increases in the hemisphere with the brighter, warmer summer and decreases in the other hemisphere, in qualitative agreement with paleoclimate proxies that indicate such hemispherically antisymmetric climate variations. The rectification mechanism that gives rise to the precipitation changes is identified by decomposing the perturbation water vapor budget into ‘‘thermodynamic’ ’ and ‘‘dynamic’ ’ components. Thermodynamic changes (caused by changes in humidity with unchanged winds) dominate the hemispherically antisymmetric annual-mean precipitation response to precession in the absence of land–sea contrasts. The nonlinearity that enables the thermodynamic changes to affect annual-mean precipitation is a nonlinearity of moisture advection that arises because precession