The Standard & Poor’s 500 Index closed Monday at 2,057.64, falling within 0.2 percent of its average price over the last 200 days, or 2,053.51. The gauge’s relative strength index, which measures the speed of swings, slipped below 35.5, the lowest reading since December. American investors were shaken from a two-month slumber by a global selloff that pushed the Chicago Board Options Exchange Volatility Index up 34 percent, the biggest increase since April 2013.