Ströh, Achim (2007) The impact of global warming on El Niño-Southern Oscillation. (Diploma thesis), Carl von Ossietzky Universitat Oldenburg, Oldenburg, 69 pp.

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Abstract

The newly developed Kiel Climate Model (KCM) is used to study the influence of global warming on the El Nino-Southern Oscillation (ENSO). The global warming scenario used in this study involves increasing the C02 concentration by 1 % per annum until the concentration has doubled and thereafter stabilised. An ensemble of eight global warming simulations were performed. They were compared against a 330 year stable simulation for current climate conditions. In the first step, the simulation of the tropical climate, mean state, ENSO and its dynamics in the control run are assessed by comparing them against observations. In the second step, the changes induced by the global warming scenario in the mean state of tropical Pacific, the simulation of ENSO, and three major ENSO feedbacks are examined. KCM simulates global climate relatively well in terms of mean state, annual cycle, variability, and ENSO. However, like many models, KCM exhibits a 1 °C cold bias in the cold tongue of the equatorial Pacific sea surface temperature (SST), overestimates the variability, underestimates the non-linear effects and is also lacking the typical phase locking to the annual cycle. The simulated global warming scenario is well inside the range of other coupled general circulation models and shows a linear increase in the surface air temperature during the increase of the C02 concentration. The major findings are near uniform warming of the sea surface temperature combined with a deepening and sharpening of the thermocline. These changes in the mean state result in a doubling of the amplitude and an increase in the extreme El Nino events. These changes in the mean state were shown to strengthen feedbacks between thermocline depth and SST and between SST and zonal wind stress, both of these contribute to the increased ENSO variability.

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