On Changing El Nino: A View from Time-Varying Annual Cycle, Interannual Variability, and Mean State

This study investigates changes in the frequency of ENSO, especially the prolonged 1990-95 El Nino event, in the context of secular changes in the annual cycle, ENSO interannual variability, and background amen state of the tropical eastern Pacific sea surface temperature (SST). The ensemble empirical mode-composition (EEMD) method is applied to isolate those components from the Nino-3 SST index for the period 1880-2008. It is shown that the annual cycle [referred to as a refined modulated annual cycle (MAC)] has strong interannual modulation and secular change in both amplitude and phase: a clear transition from increasing to decreasing amplitude around 1947/48, with both linear trends before and after this turning point statistically significant and the amplitude decreasing by 14% since then, and a significant phase delay trend for the period 1881-1938, but hardly any thereafter. A clear transition from significant deceasing to increasing by about 30% in the amplitude of the ENSO interannual variability around 1937 is also found. When El Nino events are represented as the collective interannual variability, their frequency is found to be almost equivalent to that of La Nina events after 1976. A method for conducting synthetic experiments based on time series analysis further reveals that the apparent prolonged 1990-95 El Nino event was not caused solely by ENSO interannual variability. Rather, the 1991/92 warm period is attributable to an interannual variation superimposed by change in the background mean state; the 1993 warm period is attributable to change in the mean state; and the 1994/95 warm period is attributable to a residual annual cycle, which cannot be fully excluded by a 30-yr mean annual cycle approach. The impact that changing base periods has on the classification of ENSO events and possible solutions is also discussed.