Hi John,
Climate is definitely a factor with Chinese elm. During one particularly warm winter up here in Northern California, one of mine retained its leaves all winter. They fell when the new growth pushed them off in the spring.
In San Diego, you may find this tree behaves in a semi-deciduous way. It may keep some leaves all winter, "hedging" its chances.
Be sure to keep checking and watering as needed so long as there are green leaves on the tree. If it should go fully dormant its water needs will decrease significantly.
Unless you get a some nighttime frost or extended periods in which the daytime temperature hangs in the 50s (F) or below, your tree may look like this all winter. You could certainly defoliate it, but the tree may have other ideas about how to spend the San Diego "winter"
By the way, it looks like you could air layer a nice [i:0b240bc299]shohin[/i:0b240bc299] (miniature size bonsai) using the top portion of the tree.
Regards,
Matt