"If U.S. rates fully normalize over a three-year period (fed funds near 4 percent by early 2018) and Bank of Japan rates still are at zero, you could perhaps see dollar-yen at 140."
Kathy Lien, longtime forex strategist who started BK Asset Management, predicts the pair will hit 110 by the end of the year and 113 to 115 next year.
"We'll be moving into a period [next year] where central banks are tightening—including Bank of England and the Fed—and the global economy will be more stable. That's the perfect backdrop for carry trades to return," Lien said.
In a carry trade, investors buy higher-yielding currencies with higher interest rates and fund them by selling lower-yielding currencies, typically the yen. Usually they thrive during environments of economic and financial calm and stability, better known as risk-on.
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David Woo's long-held forecast is 108 by the end of 2014, and 112 by the end of 2015, though "the risk is to the upside of our forecasts," said Woo, head of global rates and currencies research at Bank of America Merrill Lynch.
Bears say the decline in the yen will be fueled by the very medicine trying to help the ailing Japanese economy, with the BOJ forced to fight the weakness with more monetary stimulus and quantitative easing, a prescription for a weaker currency.
"They have the worst demographics in the world. They are importing energy, unless they go back to nuclear which there have been some fears of understandably. They have a negative birth rate. They have huge government debt. Do you have any ideas? I think all you pretty much have is debasing, " according to Gundlach.
He says weakening the yen via QE is the only option, "the mother of all debasement."
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Japan's economy shrank 7.1 percent in the most recent quarter at an annualized rate, thanks in large part to frozen consumer spending after the government increased the sales tax by 3 percent.
There are indications Bank of Japan Gov. Haruhiko Kuroda is open to doing more to stimulate the economy.
On Thursday, Kuroda met with Prime Minister Shinzo Abe and assured him that he would do more if needed, especially if the BOJ is still failing to meet its 2 percent inflation target.
"Should conditions emerge where the target becomes difficult to meet, we are ready to make without hesitation adjustments to policy, additional easing or whatever," Kuroda told reporters after the meeting.