https://sniperinmahwah.wordpress.com/2016/01/26/hft-in-the-b...
There are apparently shortwave links across the Atlantic:
https://sniperinmahwah.wordpress.com/2018/05/07/shortwave-tr...
https://sniperinmahwah.wordpress.com/2018/07/16/shortwave-tr...
These should have a lower latency than fibre links although the shortwave link will probably have rather a low data rate.