Diaz, H. F., and R. S. Pulwarty, 1992: A comparison of Southern Oscillation and El Niño signals in the tropics. In El Niño: Historical and Paleoclimatic Aspects of the Southern Oscillation, H. F. Diaz and V. Markgraf (Eds.), Cambridge University Press, 175-192.
A contingency table and spectral analysis of the El Niño and Nile River flood event records compiled by W. Quinn and colleagues is performed. The purpose of this study is to assess and compare the long-term variance characteristics of these two measures of the El Niño/Southern Oscillation (ENSO) phenomenon. Several other modern indices of ENSO are also examined, such as sea surface temperature in the upwelling regions of the eastern equatorial Pacific, the Tahiti-Darwin sea-level pressure index, and rainfall in the normally dry areas of the central equatorial Pacific. Consistent with previous studies of long-term variations in El Niño development in relation to the occurrence of SOI-negative episodes, it was found that the relative timing of these two manifestations of the ENSO system have varied in relation to one another over the past century. Another important feature of changes in the El Niño system is the fact that the spectral signature of El Niño event data shows a concentration of variance within relatively narrow bands at both short and long time scales. Relatively rapid transitions have occurred in the frequency of occurrence of El Niño events since the mid-16th century. A comparison of the El Niño and Nile River flood event record shows strong similarities, but also some differences. Over the common period 1821 to 1941, one difference noted is that return intervals for Nile flood deficit (all categories) are longer than those of moderate and stronger El Niño events. For the two strongest event categories, the first half of the common record has very nearly the same return period, whereas for the second half, the return interval for strong and very strong El Niños has been about twice as long as that for severe (categories 3 and 4) Nile flood deficits. Nevertheless, as a measure of the long-term variability of the greater ENSO system, the Nile flood intensity record compiled by Quinn (this volume) represents a very useful contribution to ENSO studies.