How significant was the 1877–78 El Niño?

Literature revealed that the 1877‒78 El Niño corresponded to great famine events around the world. However, the strength and statistical significance of this El Niño event have not been fully addressed in the past, largely due to lack of data. These are studied here using ensemble analysis of the Extended Reconstructed Sea Surface Temperature (SST) version 5.

The ERSSTv5 standard run indicates a strong El Niño event with a peak Niño3 index of 3.5°C during 1877–78, stronger than those during 1982‒83, 1997‒98 and 2015‒16. However, analysis of ERSSTv5 ensemble runs indicates that the strength and significance (uncertainty estimates) depend on the constructions of the ensembles. A 1,000-member ensemble analysis shows that the ensemble mean Niño3 index has a much weaker peak of 1.8°C, and its associated uncertainty is very large during 1877‒78 (2.8°C) but much smaller during 1982‒83 (0.3°C), 1997‒98 (0.2°C) and 2015‒16 (0.1°C). Further investigation indicates that the large 1877‒78 uncertainty is associated with selections of a short (1-month) period of raw-data filter and a large (20%) acceptance criterion of Empirical Orthogonal Teleconnection modes in the ERSSTv5 reconstruction, when observations are sparse. Adjusting the parameters to more realistic values decreases the 1877‒78 uncertainty to 0.5°C while increases the ensemble mean peak Niño3 index to 2.8°C, confirming a strong and statistically significant 1877‒78 El Niño event. Furthermore, the estimated uncertainty during 1877‒78 (0.5°C), 1982‒83 (0.3°C), 1997‒98 (0.2°C), and 2015‒16 (0.1°C) indicate that that these four major El Niño events are statistically comparable in strength and are all significant.