When I first started playing with the Climate Change puzzle I noticed a few assumption I thought were pretty questionable. You always have to make some assumptions, but you should always keep track of the assumptions and find ways to avoid using the same assumptions over and over and over. One of the funniest assumptions is "equilibrium". You can use Statics to construct an equilibrium or stable condition to work from but it isn't very likely to be a real equilibrium. Since I was just playing with the energy relationships between the SST and Lower Troposphere temperatures (RSS in absolute) I have put together a few charts.
The hemispheres are basically out of balance. At the top of the atmosphere you have a required Ein~=Eout for any short time period and the ultimate Ein exactly equal to Eout over the long haul if the climate it to be stable forever and ever amen.
I an not particularly sure what time frames need to be considered. A planet is a pretty big thermo problem. It looks like there is at least about 10 Wm-2 of "uncertainty" though. I have referenced a few papers in the past on meridional and zonal sst imbalances where Brierley and Toggweiler indicate that the closure of Panama and opening of the Drake Passage reduced overall ocean temperatures while warming the NH with respect to the SH. That would be pretty much what the hemispheric imbalance indicates and ~10 Wm-2 would be roughly 2 degrees depending on what you consider to be the right "global" mean temperature.
Some confuse the ocean heat reservoir with a plain vanilla heat sink. The lowest temperatures are at the poles and the lowest temperatures would be the heat sinks. The Southern pole is a better heat sink than the northern pole. The northern pole though is much more variable than the relatively fixed, thermally isolated, southern pole. If you have a NH or event a "global" cooling event, more energy from the oceans would be transferred north than south relative to the "average". Ocean heat transport is slower than atmospheric heat transport so you would have lags. Since the oceans are stratified, thermocline related to density gradients, you have conditions where the deeper ocean may or may not get involved in the heat transfer game. That would be extremely hard to model heat reservoir characteristics totally different than plain vanilla heat sink characteristics. You can make the oceans are the heat sink assumptions, but once things start diverging from your assumed reality you need to reconsider initial assumptions.
If you are in the "there is no way in hell ocean heat transport can cause 2 degree changes in "global" temperature", you are one that needs to do some serious reconsideration of initial assumptions. The big things about the oceans are the heat sink assumption is that it simplifies the problem to up/down, you don;t have to think as hard about "wall" or pole ward energy advection. Bad move. In the long run the "Wall" energy transfer will be equal to the vertical energy transfer if you have a stable system.
So the southern oceans have to be cooler than the northern oceans in order to somewhat equalize pole ward energy transfer. The northern heat sink just ain't as good as the southern, plain and simple. You still don't know the time scale required for "stable". Right now the southern hemisphere ocean receive more solar energy than the northern and while clouds respond quickly to regulate SST, there is no "law" that says you can't have some energy imbalance or warming of the deepest oceans during a period when the ocean have more available energy. There has to be some limit, Earth doe exist, but you have somewhere around 20,000 year in the precssional cycle. Who can say that a 0.5 Wm-2 positive imbalance for 5,000 years or so with a 0.5 Wm-2 negative imbalance isn't normal for a planet like Earth? Assuming the imbalance has to exactly be zero is another questionable assumption.
In any case, there are a lot better ways to determine pole ward energy flows, but this post shows that you can do a bit more than you might think with data available on Climate Explorer.