Abstract
Assessments of climate forecast skill depend on choices made by the assessor. In this perspective, we use forecasts of the El Niño-Southern-Oscillation to outline the impact of bias-correction on skill. Many assessments of skill from hindcasts (past forecasts) are probably overestimates of attainable forecast skill because the hindcasts are informed by observations over the period assessed that would not be available to real forecasts. Differences between hindcast and forecast skill result from changes in model biases from the period used to form forecast anomalies to the period over which the forecast is made. The relative skill rankings of models can change between hindcast and forecast systems because different models have different changes in bias across periods.
Highlights
Assessments of climate forecast skill depend on choices made by the assessor
While we focus on evaluating El Niño Southern Oscillation (ENSO) forecast skill here, the issues arising in the interaction of forecast processing and skill apply more generally to all climate forecasts
History of ENSO forecast skill Forecasts of ENSO have been issued since the late 1980s8, with more routine forecasts emerging in the 1990’s to include those made through coupled general circulation models (CGCMs)[9]
Summary
Assessments of climate forecast skill depend on choices made by the assessor. In this perspective, we use forecasts of the El Niño-Southern-Oscillation to outline the impact of bias-correction on skill. The model forecast anomalies are generated for the fair and unfair methods by subtracting the reference climatologies in the training (row a) and testing (row c) periods respectively from the forecast values.
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