Decadal and longer term changes in El Niño–southern oscillation characteristics

Abstract

AbstractThe time variations of El Niño–southern oscillation (ENSO) amplitude A, period P, and their relationship were studied using observed Niño‐3 sea‐surface temperature (SST) (GISST 2.2, GISST 2.3) and the southern oscillation index (SOI). Different tendencies in the A–P relationship were found in the decadal and longer term variations.On the decadal time scale, the SOI shows a statistically significant positive correlation between ENSO period and amplitude. This result was confirmed by analysing the observed Niño‐3 SST data as well.A negative relationship between longer variations in A and P was then found for certain subintervals of the observed SST time series available, e.g. for 1921–80 and 1903–94, and the dataset representing the latter period is the GISST 2.2 dataset. These longer variations can be interdecadal, and/or secular trends in the A (positive) and P (negative). In contrast, the SOI showed no trends in P and A; consequently, there were no significant P–A correlations found in the SOI data on these time scales.The ECHAM4/OPYC3 model was then tested in order to determine whether it could reproduce the tendencies found in the observed data. The ECHAM4/OPYC3 SST showed a negative A–P relationship in the simulation with increasing greenhouse‐gas concentration. This tendency did not manifest itself in the control run. The model SOI agrees with observation results, showing a positive correlation between ENSO period and amplitude on decadal time scales. Copyright © 2004 Royal Meteorological Society