Decadal potential predictability of twenty-first century climate

Decadal prediction of the coupled climate system is potentially possible given enough information and knowledge. Predictability will reside in both externally forced and in long timescale internally generated variability. The potential predictability investigated here is characterized by the fraction of the total variability accounted for by these two components in the presence of short-timescale unpredictable noise variability. Potential predictability is not a classical measure of predictability nor a measure of forecast skill but it does identify regions where long timescale variability is an appreciable fraction of the total and hence where prediction on these scale may be possible. A multi-model estimate of the potential predictability variance fraction (ppvf) as it evolves through the first part of the twenty-first century is obtained using simulation data from the CMIP3 archive. Two estimates of potential predictability are used which depend on the treatment of the forced component. The multi-decadal estimate considers the magnitude of the forced component as the change from the beginning of the century and so becomes largely a measure of climate change as the century progresses. The next-decade estimate considers the change in the forced component from the past decade and so is more pertinent to an actual forecast for the next decade. Long timescale internally generated variability provides additional potential predictability beyond that of the forced component. The ppvf may be expressed in terms of a signal-to-noise ratio and takes on values between 0 and 1. The largest values of the ppvf for temperature are found over tropical and mid-latitude oceans, with the exception of the equatorial Pacific, and some but not all tropical land areas. Overall the potential predictability for temperature generally declines with latitude and is relatively low over mid- to high-latitude land. Potential predictability for precipitation is generally low and due almost entirely to the forced component and then mainly at high latitudes. To the extent that the multi-model ppvf reflects both the behaviour of the actual climate system and the possibility of decadal prediction, the results give some indication as to where and to what extent decadal forecasts might be possible.