A new climate model supporting multi-resolution meshes in the ocean component has been established at the Alfred Wegener Institute (AWI) in Bremerhaven. The atmospheric component is ECHAM6 with T63L47 setting, while the ocean is simulated by the AWI multi-resolution model FESOM, supporting triangular unstructured meshes. Two multi-century simulations with ECHAM6-FESOM, REF and TRO, document the beneficial role of an increased tropical ocean resolution for ENSO simulations. REF features a tropical ocean resolution of about 1°, TRO employs more than 0.25° in a narrow equatorial band, with resolution gradually decreasing to 1° as in REF. Outside the tropical belt (15°N to 15°S), both meshes are identical. REF and TRO simulate a mean climate comparable to some of the best CMIP5 models. In TRO, however, both the cold tongue SST bias and the western Pacific SST standard deviation bias appear to improve along with the Nino-3 index statistics. Also, advanced ENSO diagnostics including the Nino-3.4 seasonal variance, the annual cycle representation, and its interaction with ENSO tend to improve. The robustness of these improvements is analyzed and their physical explanations are explored.