ENSO evolution
The 1976/77 regime shift was characterized not only by an increase in the background SST, but also by a change in the ENSO evolution pattern. Before the shift El Niño events tended to develop first along the coast of South America and then spread westward. This is known as the “canonical” ENSO scenario described by Rasmusson and Carpenter (1982). After the 1976/77 shift, ENSO events developed first in the central Pacific and then spread eastward (e.g., Wang 1995). Trenberth and Stepaniak (2001) introduced the Trans-Niño Index (TNI) and used it in conjunction with the more traditional Nino 3.4 index (N3.4) to trace the evolution of ENSO events. They found that the TNI lead N3.4 by 3 to 12 months prior to the climate shift in 1976/77 and also followed N3.4 but with opposite sign 3 to 12 months later. However, after the 1976/77 shift, the signs of the TNI leads and lags were reversed.
It appears that the evolution of ENSO events is now returning to its canonical scenario. The recent weak to moderate El Niño events of 2002/03, 2004/05, and 2006/07 were all preceded by peaks in TNI (8, 9, and 11 months earlier, respectively). Similarly, the strong La Niña event of 2007/08 was preceded by a minimum in the TNI 8 months earlier (Fig. 3)