MERIDIONAL ATMOSPHERE AND OCEAN HEAT TRANSPORTS Kevin E. Trenberth and Julie M. Caron National Center for Atmospheric Research * , P.O. Box 3000, Boulder, CO 80307, USA trenbert@ucar.edu 1. INTRODUCTION AND METHODS Radiative processes continually act to cool the high latitudes and warm the low latitudes of planet 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; Savijärvi, 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 ground water 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), 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 Savijärvi (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 70°S in the Carissimo et al. and Savijärvi 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. We have computed new results of meridional ocean and atmosphere heat transports based upon energy balance computations of the atmosphere, adjusted to fit physical constraints, using the reanalyses from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The approach used is to estimate the atmospheric energy transports directly from analyses of measurements within the atmosphere. The analysis focuses on the period from February 1985 to April 1989 when there are reliable top-of-the-atmosphere (TOA) radiation data from Earth Radiation Budget Experiment (ERBE). We compute monthly means of all quantities and combine the average monthly means over the ERBE period to produce an annualized mean. Then we compute the net surface heat fluxes as a residual and use those to infer the ocean heat transports. We make several adjustments to the fluxes so that physical constraints are satisfied in order 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. The details along with the surface fluxes are given in Trenberth et al. (2001). Implied ocean transports are compared with those from direct ocean measurements and from coupled models as well as derived results from ECMWF in Trenberth and Caron (2001). Results of the zonal means of the quantities from this study are available online at http://www.cgd.ucar.edu/cas/catalog/ohts/. * The National Center for Atmospheric Research is sponsored by the National Science Foundation.