|Title||Impact of hindcast length on estimates of seasonal climate predictability|
|Year of Publication||2015|
|Authors||Shi, W, Schaller, N, MacLeod, D, Palmer, TN, Weisheimer, A|
|Secondary Title||Technical Memorandum|
It has recently been argued that single-model seasonal forecast ensembles are overdispersive, implying that the real world is more predictable than indicated by estimates of so-called perfect-model predictability, particularly over the North Atlantic. However, such estimates are based on relatively short forecast datasets comprising just 20 years of seasonal predictions. Here we study longer 40-year seasonal forecast datasets from multi-model seasonal forecast ensemble projects and show that sampling uncertainty due to the length of the hindcast periods is large. The skill of forecasting the North Atlantic Oscillation during winter varies within the 40-year datasets with high levels of skill found for some sub-periods. It is demonstrated that whilst 20-year estimates of seasonal reliability can show evidence of overdispersive behaviour, the 40-year estimates are more stable and show no evidence of overdispersion. Instead, the predominant feature on these longer timescales is underdispersion, particularly in the tropics.