Recent trends in the seasonal and temporal behaviour of the El Nino-Southern Oscillation

Trends in the seasonal and temporal behaviour of the El Nino-Southern Oscillation over the period 1958-2007 have been assessed using two indices of the phenomenon, NINO3.4 and a non-standardised Southern Oscillation Index (SOI). There is no evidence of trends in the variability or the persistence of the indices, nor in their seasonal patterns. There is a trend towards what might be considered more "El Nino-like'' behaviour in the SOI (and more weakly in NINO3.4), but only through the period March-September and not in November-February, the season when El Nino and La Nina events typically peak. The trend in the SOI reflects only a trend in Darwin pressures, with no trend in Tahiti pressures. Apart from this trend, the temporal/seasonal nature of the El Nino-Southern Oscillation has been remarkably consistent through a period of strong global warming.