Stock selloff reflects global growth uncertainties

Why are stocks down today? There are several reasons:

1) This is a quadruple witching expiration, the quarterly expiration of stock index futures and options, and individual stock futures and options, with large volume and volatility at the open and close;

2) Oil is down almost 4 percent, a proxy for global growth;

3) The Federal Reserve has sent a message to investors: it is more concerned about global growth than it had let on.

Read MorePisani: Did the Fed just make a third mandate?

Let's focus on the Fed for a moment. The central bank clearly stated it was concerned about the state of the global economy and its impact on the U.S. Bank of America/Merrill Lynch summed it up Friday morning: "The Fed acknowledged our concerns for global weakness, and this acknowledgement is a bearish signal for risk assets."