An Assessment of Differences in ENSO Mechanisms in a Coupled GCM Simulation

Abstract

This study investigates the physical mechanisms involved in the generation and decay of El Niño–Southern Oscillation episodes in a coupled GCM simulation. Warm and cold events found in a 100-yr-long record are separated into groups by means of a clustering technique that objectively discriminates common features in the evolution of the tropical Pacific heat content anomalies leading to the event’s peak. Through an analysis of the composites obtained from this classification, insight is gained as to the processes responsible for the presence of different behaviors. Three classes of warm events were identified. The first is characterized by the westward propagation of warm heat content anomalies north of the equator before the onset of the episode. This propagation characteristic of the delayed oscillator paradigm appears weakened in the decay of the episode. In the second class, local development of heat content anomalies in the northwest tropical Pacific, associated with overlying wind stress curl anomalies, dominates both the generation and the decay of the warm event. In addition, subsurface cold anomalies form in the equatorial western Pacific in association with the poleward flow considered by the recharge–discharge oscillator model. The third class is characterized by a relatively quick development of the warm episode. Attention is focused on the first two classes. The suitability of different conceptual models to explain them is addressed. Previous analyses of the simulation are reviewed throughout this work. Differences between the classes are related to a regime shift that occurs toward the middle of the record.

Tropical Atlantic Variability: Observations and Modeling

“…The short observational period available (~100 years) does not rule out that they are a reflection of internal variability. It is also possible that low-frequency variability changes the frequency of occurrence of different types of variability in the AZM: [107] found in a GCM simulation that, similarly to ENSO [108], different kinds of Atlantic Niño events can be identified and that their teleconnections to ENSO are of a different nature.…”

Impact of global warming on ENSO phase change

“…For this study we use a procedure capable of detecting classes of events (groups that share a distinct evolution), following the technique of AlvarezGarcia et al (2005).…”

“…By means of a clustering technique, we identify the main characteristics of simulated ENSO episodes when the model is forced according to the IS92a IPCC scenario (hereinafter SCR run) and compare the results with a second simulation with present day conditions (hereinafter CTR run). This technique has already been used to analyse ENSO characteristics in a simulation of present day climate variability performed with another coupled GCM, the SINTEX model (AlvarezGarcia et al, 2005). Their study stressed the importance of decadal variability in ENSO generation and impacts.…”

Modeling of El Niño events using a simple model

“…This modified equation system(1),(3) and (5)-(6) was solved numerically (henceforth E2) in a similar way to experiment E1. The value of parameter C T =5.1 was chosen in this experiment, so that the maximum amplitude of variability in T E corresponds to 1.6 o C and so the contribution of both external forcings to this variability would be comparable (that follows from the correlation coefficient between the M(t) and NINO4-index).…”