On the meridional heat transport and its partition between the atmosphere and oceans

Abstract

In this thesis I study the meridional heat transport of the climate system and its partition between the atmosphere and oceans using models and data. I focus on three primary questions: (1) What is the total heat transport and its partition between the two fluids? (2) What sets the magnitude of the total heat transport and to what extent is it sensitive to the details of atmospheric and oceanic circulation? (3) How robust is the partition of heat transport between the two fluids and how sensitive is it to bathymetric constraints on ocean circulation? For these studies I employ a series of aqua-planet calculations using a coupled atmosphere-ocean-ice model in which idealized ocean basins impose various geometrical constraints on ocean circulation. A wide range of ocean heat transports and heat transport partitions are found, but with more modest variations in the total heat transport. Differences in the total heat transport are associated with the presence or absence of polar ice which is found to be sensitive to the ability of the ocean to carry heat to high latitudes. These model results, as well as data, are analyzed in the context of earlier work suggesting that the total meridional heat transport should be insensitive to the details of the atmospheric and oceanic circulation. The heat transport partitions in these aqua-planet calculations are also analyzed. The calculations all feature the same gross partition as the present climate with the ocean dominating near the equator and the atmosphere dominating at middle and high latitudes. While this suggests that this overall partition may be a robust feature of the climate system, there are important differences associated with the presence or absence of a meridional barrier to zonal flow in the ocean.

(cont.) These results are diagnosed in the context of simple models and scalings which compare the strength of the atmospheric and oceanic circulations and the energy contrasts across the flows. Parallels are drawn with present and paleo climate. Finally, I produce a new estimate of the total meridional heat transport employing the method of minimum variance estimation, data from the Clouds and the Earth's Radiant Energy System instruments, and a prior estimate. This new estimate yields a peak poleward heat transport of 5.6 ± 0.8 PW at 35°N and 35°S with a northward transport of 0.1 ± 0.9 PW at the equator. This represents a 27% reduction in the standard error relative to the prior estimate. An estimate of the partition is made using direct ocean heat transport estimates with the atmospheric component computed as a residual.