Two-dimensional planar nitrogen jets in supercritical thermodynamic conditions are simulated using a high-resolution numerical method (which consists of a sixth-order compact difference scheme and a localized artificial diffusivity method) with the aim at exploring its unique characteristics. Effects of injection temperature, chamber pressure, and the equation of state on supercritical jet behaviors are investigated. Throughout the investigations, two major unique characteristics are found under the transcritical conditions. One unique characteristic are found in the mean temperature profile, as the slower increase of jet temperature, in the case of transcritical injections. The cause for the unique feature is simply explained by the corresponding temperature-density (T-ρ) diagrams. Another unique feature of supercritical jet flows appears in the generation of different flow scales. The power spectrums and the flow fields quantitatively and qualitatively show the different features of flow scales due to the injection conditions. In the transcritical injection, the production of smaller flow scales is considerably enhanced relative to the other injection cases, due to not only its higher density ratio, but also its abrupt variations (leap) of δρ/δT in the T-ρ diagrams. This study indicates, for the present conditions used, that the unique characteristics of supercritical jet flows appear in the mean temperature distributions and the generation of different flow scales, which are simply yet effectively explained by the T-ρ diagrams.