The further toward the poles you go, the more mental healthcare is needed, so it makes sense they would have the experience and infrastructure! (Completely serious.)
This isn't necessarily the case with my situation. There was only one hospital for where I lived in Marquette, Michigan. My mental healthcare was pertaining to gender dysphoria and not depression which I think you're associating with longer winters. Anyway place I grew up was filled with religious nuts to make things short. They forcibly prescribed me with antipsychotics while ignoring my right & modern medical approach of gender dysphoria. Montreal is a large city compared to that place and is progressive like California. So basically where you're located when it comes to social structure is huge.
I think I might have heard this at some point in the past.
Anyway, I found a reference to support your claim:
Also consistent with the conclusions of Torrey1 and Saha et al,2 our analyses shown in table 3 found a strong tendency for prevalence to increase with latitude.[1]
I'm not aware from the article of how the illness "schizophrenia" foreshadows all of mental health. My illness being Gender Dysphoria.
Vitamin D deficiency from what I've read is common everywhere. It does make sense that longer winters would impact a person such as staying inside more. Yet I doubt any evidence can confirm lack of Vitamin D is associated with the illness schizophrenia.