Daniel,
Let's compare your pond to an ocean. Compared to the surface of the earth, how much area does your pond cover? Does your pond have currents that constantly flow from warmer water to cold water? Does your pond have cold areas and warm areas or is it actually a uniform temperature?
The only thing that your pond has in common with the ocean is that it is water, it is not an ocean. Is it reasonable for you to expect something that is not an ocean to behave like an ocean?
Let's compare your pond to an ocean. Compared to the surface of the earth, how much area does your pond cover? Does your pond have currents that constantly flow from warmer water to cold water? Does your pond have cold areas and warm areas or is it actually a uniform temperature?
The only thing that your pond has in common with the ocean is that it is water, it is not an ocean. Is it reasonable for you to expect something that is not an ocean to behave like an ocean?