Decadal ENSO amplitude modulations: a nonlinear paradigm

This paper investigates the mechanism which generates decadal modulations in the amplitude of the El Nino-Southern Oscillation phenomenon (ENSO). Our analysis is based on a multicentury present-day climate simulation performed with an ENSO-resolving Coupled General Circulation Model (CGCM). In consistency with observations, it is found that ENSO variance undergoes changes with a time scale of about 10-20 years. This decadal beat is closely linked to the second dominant pattern of tropical (sub)surface temperature variability. The dipole-like characteristic of this mode is generated mainly by the interplay of horizontal, vertical advection and mixing. We suggest a nonlinear mechanism, which is capable of generating decadal tropical climate anomalies as well as decadal ENSO amplitude modulations (DEAMs) without invoking extratropical dynamics. This mechanism is based on the idea of homoclinic orbits. This new paradigm is validated using a low-dimensional ENSO model that is derived empirically from the CGCM simulation. (C) 2003 Elsevier Science B.V. All rights reserved.