The basic explanation for this is Vitamin D. There's a strong role for Vitamin D in the immune response and IBD patients (including myself) often have low Vitamin D levels. So stop postponing that vacation to Hawaii already!
Here's an excerpt:
"A leading explanation for this north-south gradient in the risk of ulcerative colitis and Crohn's disease may be differences in exposure to sunlight, or UVB radiation, which is generally greater in southern latitudes," wrote Dr. Hamed Khalili, of Massachusetts General Hospital in Boston, and colleagues.
"UV radiation is the greatest environmental determinant of plasma vitamin D, and there is substantial experimental data supporting a role for vitamin D in the innate immunity and regulation of inflammatory response," they noted.