On the estimation of climate normals and anomalies

Summary The quantification of the interannual component of variability in climatological time series is essential for the assessment and prediction of the El Niño–Southern Oscillation phenomenon. This is achieved by estimating the deviation of a climate variable from normal conditions, defined by its baseline level and seasonal patterns. Climate normals are currently estimated by simple arithmetic averages calculated over the most recent 30-year period ending in a year divisible by 10. The suitability of the standard methodology has been questioned in the context of a changing climate, characterized by non-stationary conditions. The paper contributes to the literature by proposing a regularized real-time filter based on local trigonometric regression, optimizing the estimation bias–variance trade-off in the presence of climate change, and by introducing a class of seasonal kernels enhancing the localization of the estimates of climate normals. Application of this filter to sea surface temperature series in the Niño 3.4 region and to zonal and trade wind strength in the equatorial and tropical Pacific regions illustrates the relevance of our proposal.