## Dynamics

Atmospheric and oceanic motions redistribute heat and mass at the hemispheric scale, thereby playing a critical role in the surface energy and water balance. Motion results from differences in heating, but motion also interacts with these forcings, as seen in the radiative imbalance at the top of the atmosphere (Figure 4).

### Atmospheric Motion

Atmospheric dynamics are driven by density differences that are caused by differential heating and cooling. As the surface absorbs solar radiation and the atmosphere aloft cools by emitting terrestrial radiation, a difference in density is generated that drives vertical convection. The heat carried by convection in form of sensible and latent heat cancels out these differences in heating and cooling. The same applies for large-scale horizontal motion that is caused by the radiative heating imbalance due to the zonal variation of solar radiation.

The atmospheric circulation has important consequences as it provides the driving force for the hydrologic cycle (Figure 4), shaping the large-scale patterns of precipitation (see section titled 'Global energy balance and climate').

### Oceanic Motion

Oceanic motion is set into motion by the same principles, except that differences in salinity also result in density differences, with saltier water being more heavy than freshwater at the same temperature. The resulting circulation is therefore known as the thermohaline circulation. This provides an important link to the hydrologic cycle as it sets the freshwater balance of the oceans.

However, compared to the atmospheric heat transport of = 5 x 1015W, recent estimates of oceanic heat transport of 1-1.3 x 1015W make it noticeably smaller. This is evident in Figure 4, which shows that the top of atmosphere net imbalance - showing the combined effect of atmospheric and oceanic heat transport - is significantly larger than the net imbalance at the surface, which reflects mainly the effect of oceanic heat transport.

Hypothesis of Maximum Entropy Production

The radiative imbalance at the top of the atmosphere exemplifies the importance of atmospheric heat

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