Phases

Early research of El Niño suggested that there was only one kind of El Niño. It would start along the east coast of South America when SST increased, and would propagate westward toward the central Pacific. This was derived by averaging all of the past El Niño events. However, averaging takes away any indication of how and in which way each event was unique.

For example, American meteorologists E. Rasmusson and T. Carpenter defined the canonical (i.e., typical) El Niño. However, when the 1982-1983 event is added to the original graph, it differs in several respects from earlier ones.

Now scientists realize that El Niño events can form in two different ways: (1) whether SST first heat up either in the eastern or (2) equatorial Pacific.

E. Rasmusson and T. Carpenter also identified several stages of El Niño’s development: antecedent, onset, peak, transition, and maturity. Again, this was based on events before 1980. Thus, scientists later realized after 1982-1983 that El Niño events can begin at different times of the year.

Though there are different types of El Niño events, they still evolve in the same way. Australian meteorologist Neville Nicholls characterized the El Niño process using slightly different terms: a precursor phase, an onset phase, growth and maturity, and decay. [Nicholls, 1987]. In detail:

The Phases of El Nino events

Precursor phase - This phase starts with an intensification of the prevailing weather patterns. The high atmospheric pressure in the eastern Pacific rises, with a corresponding fall in pressure in the west. The easterly trade winds blow harder, pushing more surface water towards Indonesia, where the sea level rises; and there is an corresponding drop in sea level off South America. With a weakening of the surface westward-flowing winds, equatorial and coastal upwelling begins to reduce, and SST in the central and eastern equatorial Pacific begin to warm up. This is the transition phase, moving out of a cold event and toward a warm event of unknown magnitude.

Onset Phase - This occurs around December most of the time. There is a seasonal slackening of the winds of the coast of Peru and Ecuador. At that time, the upwelling of cold water along the coast slows down, and surface water heats up, lasting until March or so. If that seasonal warming were to continue into April or Mat, it is likely that the onset of a warm even of some magnitude would be under way. However, the onsets of some El Niño events [e.g. 1982-83] have occurred later in the year [after August] , when the SST had already returned to normal in April following the seasonal warming.

Growth and maturity phase As the months proceed, the SST in the central and eastern equatorial Pacific become increasingly warmer, and upwelling ceases to bring nutrient-rich cold ware into the sunlit zone of the ocean surface. The sea level pressure in the South Pacific [near Tahiti] drops, and the pressure at Darwin increases. With the weakening of the westward-flowing winds and the strengthening of the eastward-flowing winds, the area covered by the warmed surface water expands in the central and eastern Pacific. SST can increase from 1oC to 4oC or more (as happened in 1982). The sea level in the western Pacific drops a few tens of centimeters, while sea level in the eastern equatorial pacific increases.

Decay phase This phase begins once maximum SST have been reached in the central and eastern equatorial Pacific, and surface temperature begin to respond to changes in wind speed and direction across the basin. The thermocline begins to move in the opposite direction (once again becoming deeper in the west and shallower in the east), and the warm water pool begins to thicken in the western part of the basin. Westward flowing winds again begin to strengthen and eastward flowing winds weaken. Coastal and equatorial upwelling begins to strengthen, bringing more cold, deep water to the ocean’s surface. And the cycle (i.e. oscillation) begins once again toward the onset of a cold phase.
A year and half after the onset, the Pacific weather has returned to normal.