The wild U.S. stock market of late is in the throes of forming a top, the chief global strategist of Morgan Stanley Investment Management told CNBC on Wednesday.

The volatility of the past four sessions is "the first crack," Ruchir Sharma said in a "Squawk Box" interview. "Bull market tops tend to be a process not an event." He said nobody can predict the day-to-day movements in the market, but he believes it will be "directionally lower" over the next 12 to 18 months.

"The reason ... is that the trifecta, which has been driving the global stocks over the past 12 to 18 months as a big sort of tail wind, is now going to turn into a headwind," Sharma said. The "trifecta" of stronger economic growth, still-low inflation and world central bank stimulus isn't going to last forever, he said.

"It's unlikely that global growth from here, including the U.S. growth, is going to accelerate much from here," said Sharma, who is also head of emerging markets at Morgan Stanley Investment Management. "In the second half, we're going to likely see global growth disappoint."

Wall Street opened under heavy pressure again Wednesday, quickly shot strongly higher and then continued the recent crazy swings. Tuesday, also marked by huge losses and gains throughout the session, closed in the plus column, with the Dow Jones industrial average rocketing 567 points higher.

However, the Dow on Tuesday did briefly dip into correction territory, which is defined as a decline over time of at least 10 percent from the most recent 52-week high. The index logged its last record on Jan. 26.

The Tuesday advance broke a two-session losing streak that saw the Dow sink more than 7 percent and basically wipe out all of the gains for 2018. But even with all the volatility, as of Tuesday's close, the Dow was still up nearly 36 percent since Donald Trump won the presidency in the November 2016 election.