U.S. stocks close out their worst month in three years

August was a brutal month for investors.

Fears about a slowdown in China's economy and concerns about when the Federal Reserve will raise interest rates pushed stocks sharply lower this month. While the market recovered much of the ground it lost, the Standard & Poor's 500 index still finished August down 6.3 percent, its worst showing since May 2012.

The selling started midway through the month after China shocked investors by devaluating its currency. The move, an effort to boost China's economy, seemed to have the opposite effect. Global investors interpreted the decision as a sign that China's economy, the second-largest in the world, was growing more slowly than anticipated. That combined with another plunge in Chinese stocks sent off red flags in Asia, Europe and the Americas.

The selling was fierce and deep. Trading volume, which typically slows in summer, spiked. The S&P 500 index at one point fell into what's known as a "correction," which is when an index falls 10 percent or more from a recent high.

On Monday the declines were relatively modest. The Dow Jones industrial average gave up 114.98 points, or 0.7 percent, to close at 16,528.03. The S&P 500 lost 16.69 points, or 0.8 percent, to 1,972.18, and the NASDAQ composite lost 51.82 points, or 1.1 percent, to 4,776.51.

The losses had been deeper, but oil prices, which were solidly lower earlier in the day, jumped after the U.S. Energy Department cut its estimate for the country's oil production. The news sent energy stocks higher, making energy the only industry in the S&P 500 to close with a gain.

U.S. crude surged $3.98, or nearly 9 percent, to $49.20 a barrel in New York. Brent crude, the international standard, jumped $4.10 to $54.15 a barrel in London.

It wasn't like U.S. markets were in perfect shape before China's spooked them. Investors had recently trudged through a corporate earnings season which delivered only meager profit growth.

"Earnings growth is waning and stock valuations are either fully valued or even a little overvalued right now. I think the investor complacency we had earlier in the summer has made this market primed to overact to basically anything out there," said Scott Clemons, chief investment strategist at Brown Brothers Harriman Private Bank.

Things are not likely to change in September. Even setting aside the historical reputation of September being one of the toughest months for the market, investors will have to contend with the Federal Reserve's interest rate meeting on Sept. 16 and 17 and more economic data from the U.S. and China that could drastically swing the market from one way to another.

"Concerns over slowing growth in China are unlikely to go away. Many investors have long awaited signs that China was on the verge of implosion," said Bob Doll, chief equity strategist at Nuveen Asset Management.

Investors are also keeping a close eye on the Federal Reserve.

Fed Vice Chairman Stanley Fischer said over the weekend that policymakers still had a "pretty strong case" for raising rates in September. That ran counter to recent market sentiment that China's economic slowdown and global market volatility might prompt the nation's central bank to wait.

Speaking at the Fed's annual gathering in Jackson Hole, Wyoming, Fischer emphasized he was not saying what action the Fed might take at its September meeting, but analysts took his comments to mean he saw the economy moving close to satisfying the Fed's conditions for a hike. The Fed has kept rates ultra-low since the 2008 financial crisis.