Welcome back, Wall Street. Traders and investors returned to work today greeted by tumbling oil prices and renewed worries about the health of the global economy. The Dow dropped 331 points, or 1.9 percent, posting its biggest loss since October. Crude oil sank below $50 a barrel for the first time since April 2009. Meanwhile, Greece’s political future looked uncertain, and the euro dropped to a nine-year low. Expect more volatility as oil prices head to $40 a barrel and change.