Is ENSO Really Changing?

ENSO characteristics vary at decadal/inter-decadal timescales, as revealed by observations, long ocean reanalysis, and climate model simulations. These low-frequency variations are often associated with changes in background ocean conditions, as well as differences in ocean-atmosphere feedbacks, and are often viewed as “regime shifts” resulting from fundamental dynamical changes. In this study we use Linear Inverse Modeling (LIM) to examine the influence of climate system noise upon decadal ENSO modulation. As an example, we focus on the 1976/77 climate “shift”, a well-studied variation of the climate system, which was characterized by a relative large increase in ENSO amplitude and a spectral change to longer timescales. LIM describes the evolution of a dynamical system in terms of a linear operator, encapsulating the system dynamics, and noise. We apply the LIM to the SODA 2.0.2 data over the period 1958-1997 to examine whether the changes observed from the first to the second halves of the record may be due to changes in dynamics, changes in the noise statistics, or simply due to sampling variability. Our results indicate that the observed changes in ENSO amplitude and spectral characteristics are well within random system variations. Results from this study can have important implications for decadal prediction, as well as for the detection of anthropogenic change in the Pacific Ocean.