If I contribute to my ROTH IRA, but later in the year find I earn too much, what happens?

It's a pain in the rump to accounting, and further, MAGI over $124,000 but less than $139,000 has a reduced amount of contribution. Roth IRA contributions are using after tax money and you can apply an early year contribution to the prior year. At this income level the up to one year earnings tax savings on the growth of the contribution is not worth the correction and any chance you'll go over the $124K means you should wait until calculation of taxes.  

https://www.irs.gov/retirement-plans/plan-particip...

When you make an illegal contribution you have to fully back it out and any gain is also withdrawn as taxable income, or loss as a capital loss. You need to fix it before filing the tax return of that tax year. You work with proportions of the total account to figure it all out from the time you made the contribution.

https://www.irs.gov/publications/p590a#en_US_2019_...

You must make the fix before filing taxes or it messes it up further.

It was a mistake to contribute through the year.

Best bet is getting on the phone with the account administrator because they have to characterize the fix correctly. You must withdraw the contribution and also any gains on the contribution. Let the administrator determine it if possible especially if a loss is also involved.

Bottom line - fix it before filing, or you complicate it further by 1040X.

Anyone who's smart enough to be earning $160,000 a year should be smart enough to hire a financial advisor rather than seeking free advice on YA. That being said, your contributions do not have to be evenly divided throughout the year. You can contribute more in December.