Interdecadal Amplitude Modulation of El Niño–Southern Oscillation and Its Impact on Tropical Pacific Decadal Variability*

Abstract

The amplitude of El Niño–Southern Oscillation (ENSO) displays pronounced interdecadal modulations in observations. The mechanisms for the amplitude modulation are investigated using a 2000-yr preindustrial control integration from the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1). ENSO amplitude modulation is highly correlated with the second empirical orthogonal function (EOF) mode of tropical Pacific decadal variability (TPDV), which features equatorial zonal dipoles in sea surface temperature (SST) and subsurface temperature along the thermocline. Experiments with an ocean general circulation model indicate that both interannual and decadal-scale wind variability are required to generate decadal-scale tropical Pacific temperature anomalies at the sea surface and along the thermocline. Even a purely interannual and sinusoidal wind forcing can produce substantial decadal-scale effects in the equatorial Pacific, with SST cooling in the west, subsurface warming along the thermocline, and enhanced upper-ocean stratification in the east. A mechanism is proposed by which residual effects of ENSO could serve to alter subsequent ENSO stability, possibly contributing to long-lasting epochs of extreme ENSO behavior via a coupled feedback with TPDV.

Understanding ENSO Diversity

“…In unforced climate simulations, epochs with greater incidence of the weaker, CP-like El Niños appear to steepen the equatorial time-mean thermocline slope and zonal SST gradient (Rodgers et al 2004;Ogata et al 2013). Indeed, an analysis of SST observations over the past three decades (Lee and McPhaden 2010) suggests that the increasing amplitude of El Niño in the central Pacific contributed to the well-observed multidecadal warming in this region, thus enhancing the zonal SST gradient.…”

“…WWE prevalence, in turn, depends on the zonal extent of the west Pacific warm pool, which is a function of both the background climatological state and ENSO (Eisenman et al 2005;Vecchi et al 2006). This could result in a coupled, nonlinear interplay among the WWEs, ENSO, and the decadal background state (Gebbie et al 2007;Zavala-Garay et al 2008;Sun and Yu 2009;Harrison and Chiodi 2009;Ogata et al 2013;Choi et al 2013).…”

“…Model simulations suggest that epochs with an abundance of strong, EP-type El Niños tend to be associated with decadally warm conditions in the east Pacific, and decadally cooler conditions in the west (Choi et al 2011(Choi et al , 2012, which may in large part be due to the rectification of the ENSO signal (Rodgers et al 2004;Schopf and Burgman 2006;Ogata et al 2013). The extent to which decadal variations (either intrinsic to the climate system or ENSO driven) affect ENSO behavior is not yet clear.…”

North American Climate in CMIP5 Experiments. Part II: Evaluation of Historical Simulations of Intraseasonal to Decadal Variability

“…Sun et al 2013, manuscript submitted to J. Climate ;Ogata et al 2013). On other hand, both the bias in the mean state and the bias in the asymmetry may be a consequence of a more fundamental reason: a weak thermal forcing relative to the dissipation (Sun 2000;Liang et al 2012).…”

Inferred changes in El Niño–Southern Oscillation variance over the past six centuries

“…Full model details can be found in Delworth et al (2006). ENSO variability in CM2.1 exhibits multi-decadal fluctuations in amplitude (Wittenberg, 2009;Ogata et al, 2013), and the irregular ENSO mode is Kug et al (2010), and Karamperidou et al (2013).…”