Mac loses wi-fi connection after sleep

Since I upgraded to Mountain Lion, every time I wake my Macbook from sleep, I have to manually reconnect with my wi-fi router, and sometimes it takes upwards of a minute before Airport even senses the various routers within range, includng mine. Before upgrading, when I woke it up, the connection was just there, instantly, for all intents and purposes.

What's up? Is there a setting that I need to adjust?