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]
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.
And there's probably at least quite a few things that aren't mental illness that benefit from mental healthcare.