Weakening of the Walker Circulation and apparent dominance of El Niño both reach record levels, but has ENSO really changed?

Changes in El Niño-Southern Oscillation (ENSO) and the Walker Circulation can be routinely monitored using the Southern Oscillation Index (SOI). Here we show that the lowest 30-year average value of the June-December SOI just occurred (i.e. in 1977-2006), and that this coincided with the highest recorded value in mean sea-level pressure at Darwin, the weakest equatorial surface wind-stresses and the highest tropical sea-surface temperatures on record. We also document what appears to be a concurrent period of unprecedented El Niño dominance. However, our results, together with results from climate models forced with increasing greenhouse gas levels, suggest that the recent apparent dominance might instead reflect a shift to a lower mean SOI value. It seems that global warming now needs to be taken into account in both the formulation of ENSO indices and in the evaluation and exploitation of statistical links between ENSO and climate variability over the globe. This could very well lead to the development of more accurate seasonal-to-interannual climate forecasts.