In early February, Janet Yellen is set to leave the Federal Reserve and be replaced by the new Fed chair nominee, Jerome Powell. Now, to be clear, the Fed chair along with the bank governors do not set central bank policy. Policy for most central banks around the world is dictated in Switzerland by the Bank for International Settlements. Fed chairmen like Janet Yellen are mere mascots implementing policy initiatives as ordered. This is why we are now seeing supposedly separate central banking institutions around the world acting in unison, first with stimulus, then with fiscal tightening.
However, it is important to note that each new Fed chair does tend to signal a new shift in action for the central bank. For example, Alan Greenspan oversaw the low interest rate easy money phase of the Fed, which created the conditions for the derivatives and credit bubble and subsequent crash in 2008.