It has been argued that there was a period of prolonged ENSO conditions between 1990–95 so anomalous that it is “highly unlikely” to be due to “natural decadal-timescale variation” [Trenberth and Hoar, 1996]. This conclusion follows from their study of the Darwin sea level pressure anomaly record, which found that the 1990–95 period would occur randomly about once every 1100–3000 yrs. Taking into account the uncertainty in number of degrees of freedom in the Darwin time series, we find that conditions like those of 1990–95 may be expected as often as every 150–200 yrs at the 95% confidence level. Student's-t, ARMA, and Bootstrap/Monte Carlo tests of the time series all yield similar results. We therefore suggest that the 1990–95 period may plausibly be an aspect of the natural variability of the tropical Pacific.
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