## 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

 (a) 4oo CM F soo 2oo X 100 n> > o ra a -100 ir -200 (b) 25o CM 2oo h 150 X 100 p 5o n> o c -50 (c) 10.0 7 5 o E 5.o fc X 2.5 o.o (D 5 -2.5 -5.0

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 ¿t Precipitat on A Evp - ' —^ oration __— r —^ . / Net Latitude so 6o 9o North Figure 4 Zonally and annually averaged components of the energy and water balance at the top of the atmosphere and the surface from the time period 1980 to 1990. (a) The graph shows the fluxes of net solar radiation (incoming minus reflected, red line, 'solar'), terrestrial radiation (outgoing long-wave radiation, blue line, 'terrestrial'), and the difference between both (black line labeled 'net') at the top of the atmosphere. The positive values of net radiation in the Tropics (i.e., more solar radiation is absorbed than terrestrial radiation emitted to space) indicates that heat is transported by the atmosphere and ocean systems toward the polar regions, where net radiation is negative. (b) Surface energy balance components of absorbed solar radiation (red line, 'solar'), net emission of terrestrial radiation (blue solid line, 'terrestrial'), latent heat flux associated with evaporation (blue dashed line, 'latent heat'), sensible heat flux (blue dotted line, 'sensible heat'), and the residual (black line, 'net'). The residual consists of the effects of ocean heat transport and heat fluxes due to freeze/thaw of sea ice. (c) The atmospheric water budget, reflected by annual mean precipitation (red line), evaporation (blue line), and the difference ('net', black line). Regions where evaporation exceeds precipitation ('net' is negative) are regions where the atmosphere gains moisture, which is transported by the atmospheric circulation to regions where precipitation exceeds evaporation ('net' is positive). The plots were created using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data sets. Data sets have been obtained from the ECMWF data server. o transport and is further explained in Figure 5. The emergent magnitude of poleward heat transport can be understood by the hypothesis of maximum entropy production (MEP). Different amounts of heat transport are associated with different top-of-atmosphere imbalances, surface temperature gradients, and magnitudes of entropy production associated with the atmospheric circulation. A maximum in entropy production results from the tradeoff between flux and force: a greater poleward heat flux results in a lower temperature gradient and smaller force that is needed to maintain the circulation. This maximum in entropy production is associated with maximum dissipation of kinetic energy and corresponds to an atmospheric circulation at maximum strength. The hypothesis that the atmospheric circulation is maintained at a state of MEP has been confirmed with more detailed calculations with atmospheric circulation models and may have wider-ranging applications beyond turbulent processes within the atmosphere.

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