The onshore and offshore exchange rates for the yuan converged on Tuesday as state-owned banks sold US dollars and bought the Chinese currency in the offshore market to help strengthen the yuan there by as much as 1.5%, The Wall Street Journal reported. The buy-up drove a shortage of liquidity that pushed up borrowing costs, making it prohibitively expensive to make bets against the yuan and effectively putting a floor under the exchange rate. The offshore rate, which normally more closely reflects international investor attitudes, had weakened against the dollar and helped widen a gap that traders said the central bank intervened to narrow.