The U.S. trade deficit jumped 15 percent in May, reaching its highest level since October 2008 and prompting some economists to question whether international trade will continue to provide a boost to the economy in the second half of the year. The shortfall, which reflects the difference between U.S. imports and exports, had been gradually narrowing over the first few months of the year, providing a welcome sign that American businesses were selling more abroad while fewer dollars were going toward foreign goods and services.