Estimates Of Meridional Atmosphere And Ocean Heat Transports

[Adam Makin]

AsEarth rotates on its axis and revolves around the Sun, the basic geometry determines an excess of radiation from the Sun is received in the tropics, and there is a deficit in polar regions (Fig. 1.4). Contrasts in temperatures between the two regions would be far greater than observed (Chapter 5) were it not for the dynamic transport of energy polewards by the atmosphere and ocean. The contrast is greatest in winter, when the polar night sets in, with zero incoming radiation in the Arctic or Antarctic, and the contrast in summer is less in the northern versus southern hemisphere owing to the location of large continental land masses in mid- to high latitudes that more readily warm up than the oceans do. How and where the poleward energy transports occur greatly affect local climates.

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The required total heat transport from the TOA radiation RT is compared with the derived estimate of the adjusted ocean heat transport OT (dashed) and implied atmospheric transport AT from NCEP reanalyses (PW)

Radiative processes continually act to cool the high latitudes and warm the low latitudes of the earth, and it is only the poleward energy transport by the atmosphere and the oceans that serves to offset this. Early studies that tried to apportion how much each component contributed first estimated the required poleward heat transport from satellite measurements, then computed the atmospheric transports from observations, and finally computed the ocean transports as residuals. Moreover, this was done using zonal means (Vonder Haar and Oort 1973; Oort and Vonder Haar 1976; Trenberth 1979; Masuda 1988; Carissimo et al. 1985; Savijrvi 1988; Michaud and Derome 1991). This procedure not only assumes that the atmospheric transports are correct, it also assumes they are correct over both land and ocean, yet subsequent analyses (e.g., Trenberth and Solomon 1994) have found that there are implied subterranean transports in land areas, whereas physical constraints ensure that any such transports must be tiny as they can arise only from surface and groundwater flows plus conduction. As estimates of direct global ocean heat transports emerged (Bryden 1993), it became apparent that the atmospheric transports were likely to have been underestimated.

The studies of Vonder Haar and Oort (1973) and Oort and Vonder Haar (1976) for the Northern Hemisphere (NH) and Trenberth (1979) for the Southern Hemisphere (SH), as well as those from Carissimo et al. (1985) and Savijrvi (1988) made use of radiosonde data, but the uncertainties in the atmospheric heat transports are substantial because of lack of observations over the oceans. The uncertainties are apparent at 70S in the Carissimo et al. and Savijrvi results, for instance, where there is no ocean but their residuals imply a large poleward heat transport by the ocean. Moreover, use of global analyses (Masuda 1988) indicated larger estimates of poleward atmospheric transport apparently because radiosondes fail to pick up the substantial heat transports over the oceans. However, there has been a steady trend of increases in the magnitude of the poleward energy transports in both hemispheres as atmospheric analyses have improved, and this has continued with the recent reanalyses. Thus the poleward ocean transports inferred using residual methods have decreased over time.

Hence the analysis focuses on a subperiod from February 1985 to April 1989 when ERBE data are available. The approach is to compute monthly means of all quantities and to combine the average monthly means over the ERBE period to produce an annualized mean. We make several adjustments to the fluxes so that physical constraints are satisfied to provide the best estimates of values in the real world. The constraints are the estimates of long-term changes in heat storage, the transports at the northern and southern limits of our integration, and the requirement that the TOA radiation balance the divergence of atmospheric energy over land.

To evaluate the results for the ocean, we compare the meridional transports of heat with alternative estimates from successful stable coupled climate models that have been run without artificial flux adjustments for several centuries of simulation time (Boville and Gent 1998; Gordon et al. 2000) and from multiple analyses of direct ocean measurements to determine the extent to which independent means of obtaining these quantities have converged. The results show further increases in the poleward atmospheric transports of energy when compared with previous estimates, but results are now at a point at which there is almost no scope for further changes, because the inferred ocean transports would be reduced to values outside the error bars of the direct measurements. Hence there is a convergence of the ocean transport values to be mostly within error bars, which are typically in the range of 0.3 PW (1 petawatt is 1015 W). (Results of the zonal means of the quantities from this study are available online at )

Section 2 outlines the datasets used and the processing and evaluation that have already been carried out and presents results for the atmospheric transports. The implied ocean heat transports are also presented along with the adjustments made to allow for heat storage changes and to satisfy the physical constraints. Section 3 presents the ocean transports from direct ocean measurements and the coupled models and comparisons among the three approaches. It also compares the best estimates of the atmosphere and ocean energy transports with each other. Section 4 presents concluding remarks.

The ECMWF reanalyses (ERA-15) are at T106 resolution with 31 levels in the vertical and a hybrid coordinate that makes a transition to a pressure coordinate above about 100 mb. However, there are continuity problems with the ECMWF reanalyses arising from the positive reinforcement of biases in satellite radiances with those of the assimilating-model first guess (Trenberth et al. 2001b). Two spurious discontinuities are present in tropical temperatures, with jumps to warmer values throughout the Tropics below 500 mb in late 1986 and early 1989, and further spurious interannual variability is also present. These features are also reflected in the specific humidity fields. The temperature discrepancies, which were identified initially using microwave sounder unit data, have a complex vertical structure with height (warming below 500 mb but cooling in the layer above), and these problems affect moist static energy profiles and therefore poleward heat transports. The time series of tropical temperatures from the NCEP reanalyses are more consistent than those from ECMWF, and so only the NCEP results are used to examine the time series of variability.

The surface fluxes can then in turn be integrated meridionally to give the implied ocean northward heat transports (see Trenberth et al. 2001a). Of the products examined in that study (two derived, two NWP model, and COADS, but not including the coupled models dealt with here) only the derived surface fluxes give reasonable implied northward ocean heat transports, because the other three were corrupted by the large systematic biases.

The zonal mean TOA energy budget from the ERBE data (Fig. 1) is used to compute the required poleward heat transport RT, which is presented along with the estimated atmospheric transports AT from both reanalyses for the same period (Fig. 2). Peak values in the NH of about 5.0 PW (see also Fig. 6) at 43N greatly exceed the 3.1 PW of Oort and Vonder Haar (1976) and also those from the Global Weather Experiment ECMWF analyses of 4.0 PW (Masuda 1988). In Fig. 3, we present the mean northward atmospheric energy transports from NCEP as a function of month, because this allows a comparison with those of Oort and Vonder Haar (1976) for the NH. The latter featured peak northward transports of 5.0 PW in December at 63N, values exceeded 4 PW from about mid-November to the end of February, and were less than 2 PW in summer. Figure 3 shows that the maximum poleward transports occur in winter of both hemispheres and exceed 8 PW in the NH, with values much greater throughout the year than those in Oort and Vonder Haar (1976). The peak poleward transport in the SH is not quite as large, but the annual cycle is much smaller.

The implied zonal mean ocean transports, adjusted as discussed below, are computed from the residually derived surface fluxes (Fig. 5) starting at 65N where there is a minimum of ocean available to transport heat northward. Estimates are that the transport through the Bering Strait is 0.2 1013 W, and that in the North Atlantic is 1.4 1014 W (Aagaard and Greisman 1975). Therefore we use 0.14 PW at 65N as the starting point of our integration in the Atlantic. We set the dividing line between the Atlantic and Indian Ocean at 25E, directly south of Africa. The Atlantic and the Pacific are separated at 70W, south of South America. For the Pacific and Indian Oceans, we use 130E from 5S to south of Australia and 100E north of 5S. Although integration of the surface fluxes readily partitions the contributions by basin, the result cannot necessarily be interpreted as a heat transport unless the mass budget is closed. The Indian and Pacific Ocean partition is confounded by the Indonesian Throughflow, so that ocean mass flow in each basin is not closed, and only their sum is meaningful as a heat transport. The computations were done on a T42 grid (2.8) as a compromise between the requirement for high resolution to resolve islands and the ocean basin configurations and the need to smooth the analysis fields to suppress the small-scale noise. The same domains and procedures were used for each product.

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