Which word describes a state of being sullenly melancholy or gloomy?

The term "morose" accurately describes a state of being sullenly melancholy or gloomy. It characterizes an emotional state marked by a pervasive sadness and a tendency to be withdrawn or brooding. This word evokes feelings associated with deep introspection and a lack of engagement with the outside world, aligning with the idea of a gloomy disposition.

In contrast, the other words suggest positive emotional states. "Joyful" and "cheerful" reflect happiness and a light-hearted attitude, while "upbeat" denotes a sense of optimism and positivity. These terms are antithetical to the concept of melancholy, which is rooted in a more somber emotional context. Thus, "morose" stands out as the fitting descriptor for the gloomy state referred to in the question.