Croll Revisited: Why Is the Northern Hemisphere Warmer Than the Southern Hemisphere?
The question of why, in the annual-mean, the Northern Hemisphere is warmer than the Southern Hemisphere is addressed, revisiting an 1870 paper by James Croll. We first show that ocean is warmer than land in general which, acting alone, would make the Southern Hemisphere with greater ocean fraction warmer. Croll thought it was caused by greater specific humidity and greenhouse trapping over ocean than over land. However, for any given temperature, greenhouse trapping is actually greater over land. Instead, oceans are warmer than land because of smaller surface albedo. However, inter-hemispheric differences in total albedo are negligible because the impact of differences in land-sea fraction are o set by Southern Hemisphere ocean and land reflecting more than their Northern Hemisphere counterparts. In agreement with Croll, it is shown that northward cross-equatorial ocean heat transport is critical for the warmer Northern Hemisphere. This is examined in a simple box model based on the energy budget of each hemisphere. The inter-hemispheric difference forced by ocean heat transport is enhanced by the positive water vapor-greenhouse feedback, and is partly compensated by the southward atmospheric energy transport. To fully explain the temperature difference in this way, requires a northward ocean heat transport at the extreme of observational estimates. A better fit to data is found when a larger basic state greenhouse trapping in the Northern Hemisphere, conceived as imposed by continental geometry, is imposed. Therefore, despite some modifications to his theory, analysis of modern data confirms Croll's 140 year-old theory that the warmer Northern Hemisphere is partly because of northward cross-equatorial ocean heat transport.
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