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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."

Traders work on the floor of the New York Stock Exchange.
Getty Images
Traders work on the floor of the New York Stock Exchange.

This is why we are not getting a joyous stock market rally on a dovish Fed, which we almost certainly would have had a year ago.

Another major concern is the Fed's focus. Instead of traders focusing on the monthly jobs reports, we now have an additional emphasis on "global economic and financial developments." Market participants are clearly having a hard time assessing that phrase. I am as well.

Read MoreFed dovishness may have stopped soothing market

And so are some central bank figures. Reuters reported that Bank of Korea Gov. Lee Ju-yeol told a meeting of commercial bank executives that "A big change to note this time is that the Fed cited global environments and China," and that "This made things complicated for market players and uncertainty remains severe."

What this means is that for some there are less-compelling reasons for owning stocks than there were 24 hours ago. Perhaps continued improvement in the U.S. economy will ultimately convince everyone that the U.S. is the place to be, damn the rest of the world.

Read MoreThis bank just slashed its S&P earnings outlook to zero

Let's hope so. But even then, the message is clear: for the moment, buyers can be patient, there's no need to be aggressive.

  • Bob Pisani

    A CNBC reporter since 1990, Bob Pisani covers Wall Street from the floor of the New York Stock Exchange.

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