The phrase "warm subsurface" refers to an area below the surface of something (like the ground or water) that is relatively hot or has a higher temperature compared to the surrounding or surface area. Full definition
Intruding water maintains a thick layer of warmer subsurface water several hundred meters thick. (landscapesandcycles.net)
Lead Scientist Jennifer MacKinnon operates controls of equipment that's been deployed from the Sikuliaq to monitor the turbulent mixing of warmer subsurface water with cooler surface water. (fm.kuac.org)
Studying how that turbulence mixes relatively warm subsurface water with colder water at the surface. (fm.kuac.org)