A: I haven't found any research on this and so all I can say is that I have found that, like you, a number of Crohnies experience diarrhoea and sometimes nausea on exposure to the cold. Here are my untested hypotheses of what might be happening:
It is well established that stress aggravates Crohn's symptoms and that includes many kinds of stressors. Two stressors that might potentially be involved here are; first, the endocrine response to the cold, acting in a similar way to the "flight or fight" response, creating the spasms and possibly also the nausea. Second is the effect of the shivering response. This latter response could result in spasms either as a direct result of the stimulus to shiver or as a secondary response whereby the shivering is interpreted as stress and the spasm responds to that stress.
The body protects the internal organs from cold in priority to all the limbs and it could well be that evacuation is a defence mechanism, as in, "Why are we wasting all this heat on waste products or partially digested food when we should be maintaining our body heat?". This would operate similarly to the evacuation when fear strikes, as in, - "Why are we processing and carrying around this baggage that will slow us down when we want to run away? initially, this strategy to conserve heat would actually lose heat but would save heat in the longer term.
I haven't studied this in detail as, with the Crohnies who reported this to me as a problem I helped them implement a successful strategy to protect them from these symptoms in the cold without needing further information about the cause. Perhaps I will study it in more detail if the same strategy meets with less success in the future. I hope this helps. - Joy