Final answer:
Upwelling and thermal changes are the two things that can cause deep-ocean and surface waters to mix.
Step-by-step explanation:
The two things that can cause deep-ocean and surface waters to mix are upwelling and thermal changes.
Upwelling refers to the rising of deep ocean waters that occurs when prevailing winds blow along surface waters near a coastline. This brings the cold, nutrient-rich deep waters to the surface, resulting in mixing with the surface waters.
Thermal changes can also cause mixing. When the temperature of the ocean surface increases, it creates a temperature gradient, and the denser, colder deep-ocean water can move upwards to mix with the warmer surface waters.