'Persistent' ENSO sequences: how unusual was the 1990-1995 El Nino?

A pronounced climatic pattern, synonymous with protracted El Nino activity, persisted during much of the first half of the 1990s. The impact of this a nomaly was primarily a consequence of its duration, which was much longer t han the life cycles that have marked a number of the well-documented major El Nino events over the last 30-odd years. Depending on which oceanic or at mospheric parameters or which regions of the Indo-Pacific basin are examine d, this recent pattern has been described as either a 'sequence of' El Nino events or a 'persistent' El Nino episode. Such an occurrence has been attr ibuted to a variety of causes, ranging from an enhanced greenhouse effect t o volcanic dust to a major change in the earth's climate system. Much of th e above conjecture has occurred because the recent El Nino sequence/climati c anomaly has been considered with regard to only contemporary data and eve nts. This study first expands this perspective by examining evidence for bo th protracted El Nino and La Nina phases of the El Nino Southern Oscillatio n (ENSO) in historical instrumental data. However, since the presence of su ch signals in records of relatively short length is of limited statistical significance, recourse to:reconstructions based on longer proxy records is necessary. A reconstruction of the Southern Oscillation Index (SOI) derived using a multiple regression model incorporates tree-ring records from ENSO -sensitive regions of the Pacific, including the southwestern USA, Mexico a nd Indonesia. This reconstruction shows a number of 'persistent' El Nino an d La Nina event sequences through time. Due to their generally lower and di ffering temporal and spatial resolution, the length and amplitude of palaeo -events cannot always be compared directly among different proxies or with events in the various instrumentally based records. Nevertheless, the recon struction demonstrates that features indicative of 'persistent' event seque nces have occurred prior to the period of instrumentally based indices. Thi s finding is supported by documentary evidence from other ENSO-sensitive re gions across the Indo-Pacific basin.