Sea-surface temperatures off southern California from Scripps Pier and from Koko Head, Hawai‘i, were examined to determine what impact regime shifts that occurred in 1976–1977 and 1988–1989 had on environmental conditions at each location. Cumulative sums were employed to enhance the detection process. The cumulative sum time histories revealed major turning points at both locations at the time of the 1976–1977 event. At both locations, increases in temperature were indicated, consistent with the phase change in the Pacific Decadal Oscillation that took place at that time. The cumulative sums also indicated major turning points at both locations during the 1988–1989 event. A new procedure called the method of expanding means was employed to determine the long-term impact of these events. By comparing means before and after a given event it is possible to observe the magnitude of the change and to what extent it is sustained. For the 1976–1977 regime shift, temperatures increased rapidly and remained consistently higher, by ∼1°C for 2–3 yr at Scripps Pier. This increase occurred over a period of approximately 7 months and accounts for more than half of the total warming that has occurred at that location since 1920. At Koko Head, a similar response was observed with a sustained increase of approximately 0.5°C. The oceanic response to the 1988–1989 event was quite different. At Scripps Pier, temperatures before and after this event did not show any tendency to converge to significantly different values out to periods of 2–3 yr. At Koko Head, mean temperatures did converge to slightly different values after 1 yr, with mean values being consistently lower after this event (∼−0.4°C). It was shown that in some cases changes associated with these events could be identified in the original data, but without the help of cumulative sums, it is usually not possible to make a clear distinction between changes of interest and other sources of variability. Finally, decorrelation time scales for the records at both locations were estimated and found to be on the order of a year, implying spatial scales that are at least synoptic (tens to hundreds of kilometers).
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Vol. 63 • No. 1