Poleward energy transports

The implication of Figures 3.1 and 3.2 is that there must be poleward transports of energy by the atmosphere and oceans that warm the polar regions and cool the equatorial regions. The uneven solar heating results in a mean poleward decline in tro-pospheric temperatures and hence the height of pressure surfaces, inducing a pressure gradient. The atmosphere works to reduce these gradients. In lower latitudes, where the Coriolis parameter is fairly small, the poleward atmospheric energy transport is associated with the thermally direct Hadley circulation cells. In the middle and higher latitudes, the time-mean tropospheric flow is primarily westerly (west to east), representing an approximate balance between the pressure gradient and Coriolis force (geostrophic equilibrium). The poleward energy transport is primarily accomplished via baroclinic eddies (associated with surface cyclones and anticyclones) and long waves that represent disturbances in the westerly flow. The differential solar heating also results in density gradients in the ocean, resulting in a net poleward transport of oceanic sensible heat, which plays a major role in low latitudes.

Estimates of the poleward transports by the atmosphere and ocean required to account for the TOA radiation imbalance shown in Figure 3.1 have been provided in numerous studies. The general approach has been to calculate the required transport from satellite-derived TOA radiation fluxes, the atmospheric transport from global analyses and then estimate the ocean transport as a residual. At present, direct estimates of ocean transports from ocean velocity and temperature are considered unreliable. The best available estimates of the annual mean atmospheric and ocean transports are from the study of Trenberth and Caron (2001). They make use of the ERBE radiation data and atmospheric transports calculated from the NCEP/NCAR and ECMWF ERA-15

Latitudinal Variation Energy Budget

Figure 3.3 Zonal averages from the ERBE period of the mean annual energy transport required by the net radiation budget at the top of the atmosphere (RT), the total atmospheric transport (AT) and the ocean transport (OT). Units are petawatts (PW) (from Trenberth and Caron, 2001, by permission of AMS).

Figure 3.3 Zonal averages from the ERBE period of the mean annual energy transport required by the net radiation budget at the top of the atmosphere (RT), the total atmospheric transport (AT) and the ocean transport (OT). Units are petawatts (PW) (from Trenberth and Caron, 2001, by permission of AMS).

atmospheric reanalyses (see Chapter 9). Instead of obtaining the ocean transports as a residual, they used surface heat fluxes inferred from the atmospheric energy budget. Their estimates also contain adjustments for mass and energy balance. Details are provided in that paper and a companion study by Trenberth et al. (2001).

The resulting estimates of the required annual transport and the atmospheric and oceanic contributions are given in Figure 3.3. The poleward ocean transport dominates only between 0° and 17° N. At 35° N, close to where the total peak transport occurs in both hemispheres, the atmospheric transport accounts for about 78% of the total in the Northern Hemisphere and 92% in the Southern Hemisphere. In general, a greater proportion of the required total transport is contributed by the atmosphere than the ocean as compared with older estimates.

Thermodynamically, the atmosphere can hence be viewed as the principal engine that pumps heat from equatorial sources to sinks in the Arctic and Antarctic. If there were no meridional exchange, the polar regions would be much colder, and the equatorial regions much warmer, than observed. Put differently, the polar regions, while certainly cold, are not nearly as cold as would be expected on the basis of their annual incoming solar radiation. Because the Earth's rotational axis is inclined 23.5° with respect to its orbital plane, we experience seasonality in the latitudinal distribution of incoming (and net) solar radiation, expressed most strongly in the polar regions (see Figure 2.1 for effects on day length). During the winter of each hemisphere, the solar declination is negative, and incoming solar radiation in the polar regions is small or zero, depending on the latitude. The temperature gradient between the equator and poles is therefore strongest, as is the poleward atmospheric transport. During summer,

Figure 3.4 The zonalmean annual cycle of poleward atmospheric energy transport, averaged for 1979-2001 (PW). Negative values in the Southern Hemisphere mean transport to the south (towards the South Pole) (from Trenberth and Stepaniak, 2003, by permission of AMS).

the solar declination is positive, and there is a more even latitudinal distribution of solar heating. This weakens the atmospheric temperature gradient, and the poleward atmospheric transport is correspondingly smaller (Figure 3.4).

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