Don't sweat it, the traffic is organic. You're apparently ranking high for a KW on Yahoo and that's bringing in the majority of your traffic. Hopefully, it's not a seasonal keyword or something that people are just searching for because they heard it in the news. If you find that the traffic sustains itself, try your best to maintain your ranking for the KW (by building backlinks). Given your circumstances, a 30% CTR is fine.
It depends a lot on the niche dude. Look below: My music site has a near 30% CTR, and it was much higher at first, but steadily lowered. Certain niches have high CTR, thats all.
Letting Adsense know is the right approach. Just let them know why there's more traffic and the source. If there's click fraud they'll make a financial adjustment.