The Nikkei stock index briefly turned positive and the Topix climbed over 1 percent Wednesday morning after opening sharply lower, boosted by banks and other sectors oversold on Monday’s global rout. At 9:50 a.m., the 225-issue Nikkei Stock Average had risen 10.01 points, or 0.03 percent, from Tuesday to 34,687.47. The broader Topix index was up 34.54 points, or 1.42 percent, at 2,468.75.