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

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  • 1 Departamento de Física, Universidad de Alcalá, Acalá de Henares, Spain
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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.

Corresponding author address: Dr. Francisco Alvarez-Garcia, Departamento de Física, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain. Email: franciscoj.alvarez@uah.es

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.

Corresponding author address: Dr. Francisco Alvarez-Garcia, Departamento de Física, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain. Email: franciscoj.alvarez@uah.es

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