I don't know the science but i can tell you my experiences.
My Hg gets much better in winter season and so i am able to extend transfusion interval a bit in winter. But then again, winter in Pakistan and specially my city are mild winters. Summer is usually tough at my side and it can be easily notices that if you walk more or stay long in summer heat, you will feel exhausted and that sure is not good specially for thal. In winter i usually don't get tired easily.
But since you have mentioned Canada, i've heard that winters there are too chilly so maybe someone living there can explain how it is like to live in extreme cold conditions.