Dai, NiThe El Niño-Southern Oscillation (ENSO), originating in the tropical Pacific, is the most significant mode of interannual variability of the ocean-atmosphere system. ENSO can modulate global climate through teleconnections with significant socioeconomic consequences, especially in the Tropics and along the western coastline of the Americas. It is thus important for the general circulation models (GCMs) of the oceans and atmosphere to simulate ENSO and its regional hydroclimate impacts with some fidelity. Although our understanding of the ENSO structure and dynamics has improved in the past few decades, its modeling remains challenging. Analysis of climate simulations produced by the Coupled Model Intercomparison Project Phase 5 (CMIP5) GCMs and long-term global precipitation datasets as well as recent high-resolution atmospheric reanalyses provides insights on improving the ENSO simulation as well as the recent and projected ENSO-related changes under global warming. A classification of CMIP5 models into two groups is developed on the basis of pattern correlation of the precipitation climatology and the ENSO-related precipitation anomalies with their counterparts in the 20th Century Reanalysis (20CR) and a statistically reconstructed precipitation dataset (REC). ENSO-related diabatic heating, atmospheric circulations, and air-sea interaction in the two model groups are then assessed using the state-of-the-art high-resolution atmospheric reanalysis, ERA-Interim, whose representation of tropical diabatic heating is considered optimal. The better performing model group simulates the ENSO-related features well, while the underperforming group exhibits severe biases, including deficient equatorial precipitation in both climatology and ENSO precipitation anomalies. This group also simulates a more westward-located and less robust ENSO precipitation/diabatic heating anomaly center together with weaker associated Walker and Hadley circulations and air-sea interaction compared to the better performing group. Regarding multidecadal and centennial change in ENSO variability during the 20th and 21st centuries, ENSO-related SST anomalies strengthened in the later part of the last century, while the changes in ENSO-related precipitation were diverse and included both zonal shift and intensification. The underperforming group of models exhibits a robust increase and zonal shift of ENSO-related precipitation, SST and diabatic heating in the 21st century. The other group shows an increase in ENSO precipitation in the central-eastern equatorial Pacific, with related intensification of diabatic heating anomalies in the mid-to-upper troposphere.enDissertationClimate changeAtmospheric sciencesClimate ChangeCMIP5 ModelsEl Niño-Southern OscillationModel BiasesPrecipitationReanalyses