Climate sensitivity to land surface variability from minutes to years

The first four modes of variability of land surface fluxes and state variables are the diurnal cycle, synoptic variability (perturbations on the diurnal cycle from minutes to weeks), the annual cycle, and interannual variability (perturbations on the annual cycle from months to decades). The mean diurnal and annual cycles are forced by the geometry of the earth-sun system, but synoptic and interannual variability have dominant components driven from within the ocean-land-atmosphere system. Ocean variability exists mainly on longer time-scales, due to its high viscosity and heat capacity, but variability of the land surface is prevalent on all time scales. We are exploring the relative contributions of land surface variability on each of these time scales to overall climate variability using a global atmospheric general circulation model coupled to a state-of-the-art land surface scheme. By systematically removing one or more modes of land surface variability from the coupled system, we can control the time scales over which feedbacks are allowed to operate. This allows for assessment of the impact of each mode on climate variability across seasonal and interannual time scales. Land surface variability is controlled in one of two ways, either by specifying state variables (a la prescribed SST experiments) or by limiting evaporative fluxes in a governed manner. There are spatial and seasonal variations in the sensitivity of climate to land surface variations, and local sensitivities can be as large or larger than those to global variations in SST. The diurnal cycle plays a significant role in modulating precipitation, even on variations at the interannual time scale.