If your 2015 U.S. income is $0, then you won't be able to contribute to either a Roth or Traditional IRA for 2015.
With a Roth IRA, normally excess contributions are subject to a 6% excise tax. However, if you withdraw your contribution before the October 15 extension deadline along with any earnings that your excess contribution earned, it will be treated as if the excess contribution never happened. The earnings that you withdraw will need to be added to your 2015 income.
To do this, contact your IRA custodian and tell them that you need to do an excess contribution withdrawal. They will have a form for you to fill out. They can also tell you how much income from your excess contribution that you'll need to withdraw.
This is discussed in Publication 590-A:
Withdrawal of excess contributions. For purposes of determining excess contributions, any contribution that is withdrawn on or before the due date (including extensions) for filing your tax return for the year is treated as an amount not contributed. This treatment only applies if any earnings on the contributions are also withdrawn. The earnings are considered earned and received in the year the excess contribution was made.