A relationship between timing of El Nino onset and subsequent evolution

A relationship between the timing of El Nino onset and the subsequent evolution is examined, using 130-year long time series of Nino-3.4 index from 1871 to 2000. It is found that El Nino events can be classified into two major types: one is the onset of which is from April to June (spring type) and the other is from July to October (summer-fall type). Here, the duration of El Nino is defined as the period when the 5-month running mean anomaly of Nino-3.4 index is exceeding 0.5degreesC. As a result, 24 El Nino events are identified, and classified into 10 spring type events and 14 summer-fall type events. In general, spring type events grow greater in magnitude, and take the mature phase around a boreal winter and the evolution is relatively regular. On the contrary, summer-fall type events are relatively weaker in magnitude, and have rather irregular aspects.