It depends on the time of too, imo. What I mean is the difference between water temperature and the ambient temperature.
At this time of the year the water temperature has peaked. The ambient temperature is falling so the upper levels of the water will be cooling first, leaving the deeper levels warmer.
In 6 months time the opposite will be the case.
As the ambient temperature rises, the upper levels of the water will rise before the depper parts, therefore the opposite will be the case: ie the colder water will be the deeper areas.
if that makes sense, sorry rushing as I'm on 1/2 hour lunch break.