Whenever I update custom fields, etc. in production mode, I need to update them in the sandbox I have as well. Is there a way to sync the Enterprise WSDL between the two?
As far as I'm aware, you can't sync them as such in any kind of real time. But you can deploy changes to fields from Production to a Sandbox.
As a side note, I'd personally recommend making any changes to fields/workflows/anything within a sandbox then deploying those changes to production. You never know what could break!
One option would be to move away from the Enterprise WSDL completely and use the Partner WSDL instead. This can make coordinating deployments between the sandbox and production much easier.
With the Partner WSDL, you no longer need to update the service definition for every field change. Instead, you query the metadata for the sObject in question to determine if the field is accessible and what the type is. Your integrating code can then respond accordingly.