Right, I had this question on my ATPL Met eggs and ham the other day. I chose 'climb' but it did appear in my KDRs.
From what I understand Jetstreams are formed by the fact that the Tropopause above the equator is colder and higher (-70deg 18klms high). At the poles the Tropopause occurs a lot lower and is a lot warmer (-30deg 8klms high).
Above these two points is the isothermal layer. The reletively warm polar air naturally begins to flow toward the equator and the colder air above the equator begins to move south toward the pole. This causes a sort of figure of 8 flow of the winds forming the jetstreams.
So if your flying south and the TAT (TAT=OAT +
some of the temp rise due to aircraft froward movement) is rising I would sayyour headed for the jet core. Because CAT occurs on the lower Ploar side of the jet core I would climb??? But like I said I still got the question incorrect who knows what thier logic is
Rocket