ENSO Feedbacks and Associated Time Scales of Variability in a Multimodel Ensemble

Abstract

The background state of the equatorial Pacific determines the prevalence of a ''slow'' recharge oscillatortype ENSO over a ''fast'' quasi-biennial surface-driven ENSO. The first is controlled to a large extent by the thermocline feedback, whereas the latter is related to enhanced zonal advective feedback. In this study, dynamical diagnostics are used to investigate the relative importance of these two feedbacks in the Coupled Model Intercomparison Project and its relation with the differences in ENSO-like variability among the models. The focus is on the role of the mean oceanic surface circulation in controlling the relative weight of the two feedbacks.By the means of an intermediate-type ocean model of the tropical Pacific ''tuned'' from the coupled general circulation model (CGCM) outputs, the contribution of the advection terms (vertical versus zonal) to the rate of SST change is estimated. A new finding is that biases in the advection terms are to a large extent related to the biases in the mean surface circulation. The latter are used to infer the dominant ENSO feedback for each CGCM. This allows for the classification of the CGCMs into three groups that account for the dominant feedback process of the ENSO cycle: horizontal advection (mainly in the western Pacific), vertical advection (mainly in the eastern Pacific), and the combination of both mechanisms.Based on such classification, the analysis also reveals that the models exhibit distinctive behavior with respect to the characteristics of ENSO: for most models, an enhanced (diminished) contribution of the zonal advective feedback is associated with faster (slower) ENSO and a tendency toward a cooler (warmer) mean state in the western-to-central Pacific Ocean. The results support the interpretation that biases in the mean state are sustained/maintained by the privileged mode of variability associated with the dominant feedback mechanism in the models. In particular, the models having a dominant zonal advective feedback exhibit significant cold SST asymmetry (or negative skewness) in the western equatorial Pacific.

Influence of Recent Stratification Changes on ENSO Stability in a Conceptual Model of the Equatorial Pacific

“…For the CGCMs, preindustrial simulations are used with the concentration of greenhouse gases fixed to 1850 estimates, which makes it appropriate for studying the physical mechanisms of natural climate variability. They correspond to the models selected in Belmadani et al (2010), except for BCCR-BCM2.0. They correspond to the models selected in Belmadani et al (2010), except for BCCR-BCM2.0.…”

“…One established understanding of ENSO is that it is a near-neutral mode of oscillation that arises from a destabilization of the equatorial Pacific-mean circulation rather than from external forcing alone (Neelin et al 1998;Philander and Fedorov 2003). Those models indeed simulate their own mean state with a large and complex set of compensating ENSO feedbacks, among which a critical challenge is to identify which feedbacks are most influential on the simulated ENSO modulation (Collins et al 2010;Kim and Jin 2011;Belmadani et al 2010). This relation is also of particular interest in a context of global warming, for which climate projections by coupled general circulation models (CGCMs) show an overall agreement on a modification toward a warmer mean state of the equatorial Pacific, but an overall disagreement on an increase or decrease of ENSO activity (Guilyardi et al 2009).…”

ENSO's non-stationary and non-Gaussian character: the role of climate shifts

“…8). The latter is composed of the most 'realistic' models according to recent works (Van Oldenborgh et al, 2005;Guilyardi, 2006;Capotondi et al, 2006;Belmadani et al, 2009). The models that were excluded from this group (BCCR-BCM2-0, CCCMA-CGCM3.1, GISS-AOM, GISS-E-H) were shown to simulate a biased ENSO variability (see aforementioned studies for more details).…”

Sensitivity of ENSO to Stratification in a Recharge–Discharge Conceptual Model

“…In particular, the diversity of the mean state characteristics in these models goes along with a wide range of ENSO dynamics Guilyardi 2006;Belmadani et al 2010). For instance, in these models, a cooler SST mean state tends to be associated with a larger (reduced) contribution of the zonal advective (thermocline) feedback strength (Belmadani et al 2010). Changes in ENSO variability in recent decades Lee and McPhaden 2010) also suggest that a warmer state may influence the ENSO dynamics through the change in the balance between the two feedbacks.…”

“…This conceptual view has been particularly fruitful for interpreting more complex systems (Mechoso et al 2003), which include the coupled general circulation models (CGCMs) of the Coupled Model Intercomparison Project phase 3 (CMIP3) (Meehl et al 2007), and their sensitivity to change in mean state. In particular, the diversity of the mean state characteristics in these models goes along with a wide range of ENSO dynamics Guilyardi 2006;Belmadani et al 2010). For instance, in these models, a cooler SST mean state tends to be associated with a larger (reduced) contribution of the zonal advective (thermocline) feedback strength (Belmadani et al 2010).…”