Question

What do oceanographers think might make the deep ocean currents mix with the surface currents?

Answer 1

The primary mechanism is the density difference created by salt content and temperatures.

Explanation:

Nice graphics and description here:
http://www.waterencyclopedia.com/Mi-Oc/Ocean-Mixing.html
See also:
The greater the density differences between different layers in the water column, the greater the mixing and circulation. Density differences in ocean water contribute to a global-scale circulation system, also called the global conveyor belt.
https://www.nationalgeographic.org/media/ocean-currents-and-climate/