Earth's climate is divided into zones, with the tropics (hot, humid regions near the equator) and midlatitudes (temperate regions) being particularly important. The boundary between these zones affects weather patterns, rainfall, and ecosystems worldwide. Scientists have struggled to predict where this boundary should be located using basic physical laws.

We developed a theory that explains this boundary using the balance between heat and mechanical forces in the atmosphere. We created a 'thermal-to-mechanical ratio' that measures how 'tropical' a region is. When heat dominates, the region behaves tropically. When mechanical forces from temperature differences become important, the region behaves more like midlatitudes.

Our theory resolves an apparent contradiction: while global warming causes the tropics to expand, El Niño events cause the tropics to shrink temporarily. This seems backwards since El Niño also warms the planet. In fact, global warming heats the entire planet gradually, weakening atmospheric circulation and allowing tropical expansion. El Niño creates intense local heating in the eastern Pacific that strengthens circulation to redistribute heat, causing tropical contraction. This theory provides a unified framework for understanding how climate boundaries respond to both natural variations like El Niño and human-caused climate change, helping predict future climate impacts.