Case studies in decadal climate predictability

It is well established that, based on knowledge of the initial conditions, important aspects of climate are predictable up to a year ahead. This predictability is primarily associated with the El Niño Southern Oscillation (ENSO). But is climate predictable further ahead? To what extent does knowledge of initial conditions constrain longer-term climate forecasts? In this study we investigate whether there is evidence that climate variables are potentially predictable beyond the generally accepted limit of ENSO predictability. Recognising that predictability is likely to be dependent on initial conditions we consider model-based case studies rather than measures of average predictability. The atmosphere-ocean coupled Hadley Centre HadCM3 model was used in this work. Starting from a pre-indstrial state, a single run was integrated with observed changes in greenhouse gases, volcanic aerosols and the solar cycle for the period 1860-1950. Still using observed forcings, an ensemble of five members was started from the 1950 state and integrated for another 50 years. Each case study consists of two further ensembles started with contemporaneous intial conditions from two different members of the five member ensemble. Four case studies have been completed using ensemble members with opposing large, persistent, regional differences in sea surface temperatures and ocean heat content. The results show clear evidence that knowledge of initial conditions can constrain predictions of climate variables beyond ENSO predictability. In the particular case studies considered, the aspects of climate that are predictable include North Atlantic sea surface temperatures, western European and southeastern US 1.5m temperatures, tropical and subtropical precipitation inthe Pacific and tropical precipitation in the Atlantic. However, predictability of climate variables is strongly dependent on the initial conditions. What are the mechanisms that give rise to these predictable signals? In most cases it seems to be simple persistence of ocean heat content anomalies, but there is also evidence of other mechanisms including ocean dynamics and ocean-atmosphere interaction. We are carrying out further work to interpret and clarify these mechanisms.