New Approaches for Analysing Projected Changes in the Water Cycle

Thursday, 18 December 2014
Michael L Roderick1, Fubao Sun1, Wee Ho Lim2 and Graham D Farquhar1, (1)Australian National University, Canberra, Australia, (2)Tokyo Institute of Technology, Department of Civil Engineering, Tokyo, Japan
Previous analysis of climate model output using latitudinal averages for (combined) land-ocean has found a simple relation often summarised by the “wet get wetter dry get drier” statement. In this presentation we revisit this relation and ask whether it holds separately over land and ocean and also whether it holds at local (grid-box) scales. We find that it only applies to latitudinal averages over the ocean. It does not hold for latitudinal averages over land and does not hold over either ocean or land at grid-box scales. With that finding established we investigate a new simple emergent relation based on the long standing Budyko framework of catchment hydrology. We find that climate model output over land conforms to the Budyko framework and use that to decompose the projected changes in runoff at individual grid-boxes into a component that is due to change in rainfall and a second component due to a change in net irradiance. With that we are able to show that over land, the projected change in runoff is dominated by the change in rainfall. We present a new formulation of this result including calculation of the sensitivity of runoff to changes in rainfall.