Does the ocean have to be in 'thermal equilibrium' ... with what? If you're talking about thermal equilibrium as a function of depth, then no, since there is a thermal gradient. The reason for that gradient is because the surface is being heated while the depths are not. Is there thermal equilibrium in a poker if you hold one end in a fire and the other end in your hand?
Will the oceans ever reach thermal equilibrium, where all depths are the same temperature? Not as long as the planet continues to rotate and undergoes repeated heating and cooling cycles.
Quick answer – yes. To explain why…
I’m assuming that by ‘thermal equilibrium’ you’re meaning that the entirety of the ocean has reached a steady temperature state, as opposed to the definition in physics that relates to the theory of temperature and thermodynamics; it’s the former which this answer is based on.
For simplicity, and taking an overall view of the planet as a whole, we can consider the ocean as consisting of three layers, the upper layer in which the waters are warm and turbulent, the lower layer in which the waters are cold and calm and between the two is the thermocline, the layer across which temperature falls and density increases.
Whenever there’s a difference in temperature there is a flow of heat from the warm to the cool medium – not just in the oceans but in everything. When the temperature of the atmosphere exceeds the temperature of the ocean, heat energy flows from the air to the water. As the air cools (typically at night), that flow is reversed and heat radiates from the water into the atmosphere.
The heat that is absorbed, even in intense sunlight, will only be absorbed by the sea surface layer – the uppermost 50mm to 100mm. The turbulence caused by waves and currents near the surface mixes the warmed water with the cooler water beneath. This mixing occurs in the upper layer of the ocean and the absorbed heat is evenly distributed in the uppermost 100m or so. In a pond or a lake it’s very different, there isn’t the same mixing and the temperature at the surface can be several degrees warmer than just a few cm below the surface.
Below the evenly mixed warm layer the waters are much calmer, and the movement of heat energy comes less from mixing and more from conduction and convection. We’re now into the thermocline. The uppermost part of the thermocline is as warm as the turbulent waters above it, but the thermocline itself is calm.
The upper parts of the thermocline absorb heat from the warmer waters above, the deeper you go the less heat energy there is to be conducted. This creates a sharp decline in the temperature of the water as you descend from about 100m to about 200m (the actual depths vary due to a number of factors). The base of the thermocline is that part where the ocean no longer cools as you descend, at this point there is little variation in temperature even if you were to go much deeper.
So that’s the three layers – a warm top layer, a middle layer across which the temperature declines and a cool lower layer. Now on to the expansion part.
Water has a unique property in that it is densest at 4°C, as the temperature tends toward this value it becomes denser and contracts. This is rather a strange property as it means that introducing heat energy into cold ocean water doesn’t cause it to expand as you would think, but to contract. For practical purposes this is something that happens in the Polar regions where the water is around 0°C (and there’s no thermocline, just a column of water at more or less the same temperature from the surface down to the benthic zone), most of the sea-surface on Earth is warmer than this with an overall average temperature of 17°C.
Thermal expansion will occur regardless as to whether or not thermal equilibrium has been reached. This is a consequence of the atoms and molecules vibrating faster as they become energised by the added heat energy and occupying a greater space.
Because most of the heat energy of the oceans is in the upper layer, this is where most of the expansion will occur when the ocean warms up. Over time some of this heat will dissipate through the thermocline and this too will expand, albeit on a decreasing scale.
At the bottom of the thermocline and deeper, the temperature is typically below 4°C and in time the heat energy that makes it’s way this deep into the ocean will cause contraction. However, very little heat gets this far down and the expansion on the surface is far greater than the contraction at depth.
To give you an idea of scale, if there were no factors affecting the oceans (no Sun, no weather, no currents etc) then it would take about 6,000 years for a state of temperature equilibrium to be reached and the overall temperature would end up at about 3°C, the oceans would contract and sea-levels would fall by about 200mm.
I understand your problem, how does warmth from the surface go deep, I don't think it can as we have a thermocline a sharp layer of two different temperature waters, I do not think heat can transfer down through that, it if it could there wouldn't such a thing as a thermocline in the first place.
The oceans are mysterious anyway, deep ocean water is uniformally cold 3 or 4degrees C, whether that is at the tropics or at the poles (why?) even though there must be some heat escaping from the earths crust,
The way I see it is higher surface air temps prevent the oceans giving off heat to the atmosphere, slowing down the transfer of heat upwards, and increasing ocean temperature and expansion.
However this destroys the myth that the present pause in temperature rise is by heat going into the oceans.
We don't know enough about the ocean to even guess at this time. In theory, you could come up with an answer, but that would not be close to the truth. Take for example, you would have to have a static ocean to consider this. Last year, I read an article that said that scientists were discovering over a thousand new underwater volcanoes each year. Forget categorizing them, but consider just the ocean currents that they produce. That alone would rule out Thermal Equilibrium. There are just too many variables, many of which we probably don't know even exists, to fully comprehend such an idea.
I know that is not much of an answer, but consider this, over the centuries the earth has changed. There are artist's conceptions of people ice skating on the Thames going way back to the 1500s. The Thames, as you well know is ocean level related. That level hasn't change in all those centuries with all the changes of the earth's atmosphere. That should tell you something.
If you heat a metal bar with a blowtorch the metal expands. It doesn't wait until the entire bar has come to a final temperature.
Equilibrium will eventually happen, it just takes millennia. We are continually warming, expanding, and equalizing since the last ice age. It will continue as the atmosphere continues to warm.
Thermal expansion is essentially instantaneous. The density of each layer of water is determined by its temperature. Each layer has to heat to expand.
I really am having a hard time comprehending whether or not oceanic thermal expansion can occur in layers or whether total equilibrium must first be achieved? Please help me get my mind around this issue.
