The earth is sometimes described as a heat engine, as the atmosphere and ocean convey the surplus of heat from radiation in the tropics to the poles, which receive a deficit of radiation. On the whole, the earth receives as much energy as it emits on average on average in time and space, when the planet is in equilibrium. This transport of heat drives atmospheric and oceanic circulation.

A particular latitude is not in radiative balance (surplus in tropics/deficit at poles). Also the planet is not in radiative balance for a while when the planet experiences a change in climate forcing, like from enhanced greenhouse or from a change in incoming solar radiation. I mentioned that the radiative imbalance from the increase in greenhouse gases in the last century is about 1-2 W/m². This includes the so-called "radiative forcing" from increasing greenhouse gas (the change in outgoing longwave radiation from ONLY the increased greenhouse gases) plus the nearly immediate radiative changes due to the climate response, which includes an increase in water vapor in the atmosphere and an increase in surface temperature and a loss of sea ice and snow, etc.  It would take many centuries to thousands of years for the net radiation to return to zero even after if we could stop increasing greenhouse gases.

The layout of the planet is such that the tropics (30S to 30N) comrise 50% of the planet and the extratropics (30 to 90 deg in both hemispheres) make up the rest. Note this is a tropical centric view. It is also common to break the extratropics into midlatitudes (30 to 60 deg) and polar regions (60 to 90 deg) which cover 37% and 13% of the surface, resp. About 70% of Earth is covered by ocean and 30% is land.

The tropics receive about 2.5 time more incoming solar radiation (insolation) than the polar regions. However for the temperature profile on the planet today, the outgoing longwave radiaion is more uniform (see Fig 4.2). Consequently there is an energy imbalance at a given latitude. The difference between the net solar radiation minus the emitted outgoing longwave radiation  is called net radiation. I used both Ein-Eout and Fin-Fout.

If it weren't for circulation, the planet would adjust to this energy balance and the tropics would become much warmer and the poles would be much colder.

Tropical circulations move heat from source to sink regions and have enourmous consequences for season and regional weather. The three main circulations are the Hadley, monsoon and Walker circulations. We will return to the third one when we discuss El Nino.

The Hadley circulation can be understood by beginning with the recognition that on an annual mean basis the greatest surface heating from sunlight occurs along the equator. This causes the air to be buoyant there and it rises (1st panel). Convective plumes reach up to the stratosphere, which is very stable and resists upward motion (plumes of air are no longer buoyant once they reach the stratosphere). This creates divergence of air near the tropopause along the equator (2nd panel) and reduces the mass of air in the column. The loss of mass reduces the pressure at the surface and this drives convergence at the surface as the surrounding air is drawn towards the low pressure due to the pressure gradient force (3rd panel). Finally the sinking branch away from the equator must be invoked to close the circuit and conserve mass (4th panel).