I wouldn’t draw any conclusions about ‘appears to be a 60 year cycle’ from a graph that doesn’t even cover 120 years. It’s possible there are long term ocean current cycles causing such a phenomenon, but I don’t think we know that to be the case.
I expect that the difference in temp between northern and southern hemispheres has continued to climb over the last ~10 years not shown in the graph, as the global temperature has continued to climb.
captainccs is correct in relating this difference to there being more land up north than in the south. The north, where there is more land, warms more quickly because there is less thermal mass closely linked to the surface. If we assume both north and south are exposed to the same amount of incoming energy, more of that energy stays at/near the surface over land, where it can only penetrate a couple feet under ground. In the ocean, the sun’s rays can penetrate deep into the ocean before being converted to heat, and while most of it is converted to heat in the top 10 meters or so, the ocean water mixes, so some of that heat can be carried even lower.
Now, with global warming as more of the sun’s incoming energy is trapped near the Earth’s surface by increasing concentration of heat-trapping gasses like CO2 and CH4, in areas over the ocean, more of that energy is transported down below the surface area than over land, so it warms less at the ocean surface than the land’s surface.