Frauen, Claudia (2010) ENSO mechanisms and interactions in a hybrid coupled recharge oscillator model. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 135 pp.

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Abstract

The El Niño Southern Oscillation (ENSO) mode is the most important source of interannual climate variability. It has its origin in the interactions of the atmosphere and the tropical Pacific Ocean but the teleconnections of ENSO reach far beyond the tropical Pacific. Although the understanding of ENSO has improved greatly, there are still aspects of ENSO that are not yet well understood. Some of these aspects are the seasonality of ENSO, i.e., the nature of ENSO events to peak in boreal winter, and the asymmetry of ENSO, i.e., the fact that El Niño events are, in general, stronger than La Niña events. Also, the possible effects of a changing climate on ENSO, and the influences of the tropical Indian and the tropical Atlantic Ocean on ENSO are areas of ongoing studies. For this work, the hybrid coupled model RECHOZ was developed consisting of the ECHAM5 atmospheric general circulation model, the simple recharge oscillator ocean model in the tropical Pacific and a simple mixed layer ocean model outside the tropical Pacific. Despite the simplistic and, by construction, linear representation of ocean dynamics in the RECHOZ model, it is able to simulate the main statistical features of El Niño, including variance, period, seasonality, skewness, and kurtosis.

This model was used to study the seasonality of ENSO and the nonlinearities in the ENSO cycle. Analyses of the model show that atmospheric properties are responsible for the seasonality and nonlinearity of ENSO. A nonlinear relationship between the zonal wind stress and the sea surface temperature (SST) is causing the El Niño-La Niña asymmetry.

With the aid of sensitivity experiments, the effects of changes in the mean state of the tropical Pacific on ENSO due to atmospheric feedbacks were studied and the influences of the tropical Indian and the tropical Atlantic Ocean on ENSO were analysed. Analyses of the sensitivity experiments show that changes in the mean state of the tropical Pacific have a strong influence on the amplitude and frequency of ENSO. However, the results strongly depend on the pattern of the changes. An El Ni~no-like warming pattern leads to a strong increase in ENSO variability and shifts the period of ENSO towards longer timescales. For the tropical Atlantic Ocean no clear influence on ENSO can be detected. In contrast, the tropical Indian Ocean has a strong damping effect on the SST variability in the tropical Pacific and reduces the period of the ENSO cycle.

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