I have not done any barrel work. All of my ideas come from reading. That being said, it is my understanding that:
The temperature fluctuations encourage the beer to move in and out of the wood of the barrel. Of course this increases the exposure to wood and wood flavor in the beer. There is more concern with extreme daily changes than with annual changes.
You should read the Wild Brews book, if you haven't already.
鯰 a.k.a. なまず a.k.a. Catfish