Decadal Modulation of ENSO: Is it Real?

Both observations and millennia-long climate model simulations show decadal variations in the amplitude, dominant timescale, and spatial pattern of El Niño Southern Oscillation (ENSO) in the tropical Pacific ocean. Such decadal variations may obscure long-term trends in ENSO characteristics, and their nature needs to be understood. However, in spite of many investigations, it is still unclear whether decadal ENSO variations are driven by deterministic changes in dynamics, by shifts in the pattern and amplitude of the stochastic forcing, or whether they occur purely by chance. This presentation examines the nature of decadal ENSO changes focusing on the 1976/77 “climate regime shift”. Using a multi-variate linear inverse modeling technique, it is shown that the changes in ENSO characteristics during this climate shift did not happen by chance, but were related to changes in the system dynamics. Changes in dynamics can be expected to be associated with low-frequency variations in the background state of the tropical Pacific, which would alter the dominant ENSO feedbacks. Possible mechanisms underlying these dynamical changes, including variations in the strength of the Pacific Subtropical-Tropical Cells, as well as the possibility of nonlinear rectification processes will be discussed.