Global markets are riding a "high" on central bank money printing, but a change at the top of the Federal Reserve posed the biggest threat to the rally, Bob Janjuah, Nomura's uber-bearish strategist, told CNBC on Friday. Janjuah said Janet Yellen, Fed vice-chairman since 2010 and a likely successor to Bernanke when he steps down in January 2014, had the potential to scare the bond markets.