Catharctic is an expression of extreme emotion, like, say, the random chopping up of objects and people when angry. I think a catharsis involves heat and energy. I don't think you can be sad and catharctic at the same time, but I'm not sure exactly. My definition involved some Greek tragedy where a woman killed her kids to get back at her husband. I'll have to check on that.
EDIT: Nevermind, checked the real definition.
ca·thar·sis Audio pronunciation of "catharsis" ( P ) Pronunciation Key (k-thärss)
n. pl. ca·thar·ses (-sz)
1. Medicine. Purgation, especially for the digestive system.
2. A purifying or figurative cleansing of the emotions, especially pity and fear, described by Aristotle as an effect of tragic drama on its audience.
3. A release of emotional tension, as after an overwhelming experience, that restores or refreshes the spirit.
4. Psychology.
1. A technique used to relieve tension and anxiety by bringing repressed feelings and fears to consciousness.
2. The therapeutic result of this process; abreaction.
I guess, in some way, The Sunset Tree can be considered cathartic. I'm not an expert. I'd say it's more...moody, mabe?