Source: Geophysical Research Letters
The El Niño Southern Oscillation (ENSO) is an irregular, large-scale variation in ocean temperature and wind over the tropical Pacific Ocean that occurs every few years. The consequences of this event make temporal changes in the predictability of the ENSO important to understand.
A new global coupled retrospective forecast ensemble was developed for the 20th Century using the low-resolution version of the European Centre for Medium-Range Weather Forecast (ECMWF) operational forecasting system SEAS5. The reforecasts have 10 ensembles constructed using 10 realizations of the ocean initial conditions, with starts on November and May of each year and lasting two years. The variability of the ENSO skill is examined using Niño3.4 and the Southern Oscillation Index (SOI) by comparing with reanalysis, as well as using a perfect model set up.
Weisheimer et al.  found two periods of high skill at the beginning and end of the 20th Century and a period with noticeably reduced skill between 1930 and 1960. This period of low predictability cannot be explained by the increase in the quality of observations in recent years and is probably related to changes in the background state of ENSO.
Citation: Weisheimer, A., Balmaseda, M. A., Stockdale, T. N., Mayer, M., Sharmila, S., Hendon, H., & Alves, O. (2022). Variability of ENSO forecast skill in 2-year global reforecasts over the 20th century. Geophysical Research Letters, 49, e2022GL097885. https://doi.org/10.1029/2022GL097885
—Suzana Camargo, Editor, Geophysical Research Letters