It doesn't have to be a very significant break, not even a full degree. But a sharp break will form an invisible wall that they'll stack up against; it's an awfully big ocean if they aren't concentrated somehow. Especially good is to find a peninsula of warm water extending into cooler water, which often shows up after a big blow. Very big breaks, like 2 degrees are good too, but often they will be clogged with detritus on the cold side, so if you drift over you will be constantly pulling your gear. Also, on more moderate breaks its not uncommon to find the big fish (25# and up) on the cold side. Don't underestimate the importance of bottom contour either. Just because its 2000' deep doesn't make it inconsequential. There's lots more tricks too, but they only come out if I'm at the party.
Never gone on them recreationally, but many years on the commercial harvest has made me pretty confident in my tuna skills.