The phrase "thermal equator" refers to an imaginary line around the Earth where the average temperature is highest. It's like a belt around the middle of the planet where it's consistently warm. Full definition
The positions of those rain belts, in turn, are tied to that of the so - called thermal equator (the ring around the planet's middle where surface temperatures are highest). (scientificamerican.com)
These and other paleoclimate records indicate that rain belts shifted northward along with the thermal equator because of the global heat imbalance. (scientificamerican.com)
Because the Northern Hemisphere has more landmass, it is heating up faster than the Southern Hemisphere, and, as some climate models have suggested, this could push the thermal equator northward, and along with it those key rain belts. (scientificamerican.com)