‘Persistent’ ENSO sequences: how unusual was the 1990-1995 El Niño?

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Abstract
A pronounced climatic pattern, synonymous with protracted El Niño activity, persisted during much of the first half of the 1990s. The impact of this anomaly was primarily a consequence of its duration, which was much longer than the life cycles that have marked a number of the well-documented major El Niño events over the last 30-odd years. Depending on which oceanic or atmospheric parameters or which regions of the Indo-Pacific basin are examined, this recent pattern has been described as either a 'sequence of El Niño events or a ‘persistent’ El Niño episode. Such an occurrence has been attributed to a variety of causes, ranging from an enhanced greenhouse effect to volcanic dust to a major change in the earth's climate system. Much of the above conjecture has occurred because the recent El Niño sequence/climatic anomaly has been considered with regard to only contemporary data and events. This study first expands this perspective by examining evidence for both protracted El Niñino and La Niñfia phases of the El Niño Southern Oscillation (ENSO) in historical instrumental data. However, since the presence of such 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 and Indonesia. This reconstruction shows a number of ‘persistent’ El Niño and La Niñfha event sequences through time. Due to their generally lower and differing 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 reconstruction demonstrates that features indicative of ‘persistent’ event sequences have occurred prior to the period of instrumentally based indices. This finding is supported by documentary evidence from other ENSO-sensitive regions across the Indo-Pacific basin.