Get ready for the 'return of volatility'

With Labor Day behind us and summer unofficially over, markets should get busy again.

"I think September and October could live up to their reputation for the return of volatility in the markets," Jeff Kleintop, chief global investment strategist at Charles Schwab, told Business Insider on Friday.

"I wouldn't be surprised to see a correction take place again. It's been a while ... we're probably due for a little bit of a pullback."

Volatility could fling stocks either upwards or downwards. However, Kleintop expects that stocks would repeat the historical trend of pulling back during these months.

September is historically the worst month of the year for stocks. Bespoke Investment Group noted in its most recent "September seasonality" report that the Dow has averaged a drop of 0.96% in September, and has ended with positive returns only 42% of the time.

The quietness of the summer in markets is one reason why some investors anticipate that things will get busy, with everyone back at work.

Aside from this calendar trend, Kleintop said one of the main reasons why volatility could return is that analysts are too bullish on earnings.

He said earnings estimates for the MSCI All Country World Index out to 12 months are relatively high compared to where they've been in the previous 12. Analyst outlooks are the most optimistic they've been since 2009, Kleintop said, and this "doesn't seem to be warranted."

This sentiment is also true for US stocks, he said, where profits for companies that belong to the benchmark S&P 500 are set to decline on aggregate for a fourth straight quarter in Q3, extending an earnings recession.

Kleintop added that the several upcoming central bank meetings and elections in the US and Spain are other events that could unnerve investors.

"I think all that spells the return of volatility in September and October, though probably not a bear market," he said

The start of trading on Tuesday was quiet again, with futures on the three main indexes up by about 0.1%. The S&P 500 has not gained or dropped by more than 1% for nearly two months.