Hard for me to say - NAO is largely a meteorological phenomenon, which is not my strongest suit. It isn't even sure, I believe, whether the NAO has a coupling effect on oceanic currents. And experts still seem to quibble how to exactly measure it, or define it. So I'm not even sure if it can fully cover "descriptive", as the NAO may include traits that haven't been properly associated to it...

However, if I were to wager, I'd say that the science on NAO is somewhere halfway between describing it and mapping out the dynamics involved.

Does that help?