Croll Revisited : 1 Why is the Northern Hemisphere Warmer than 2 the Southern Hemisphere ?

7 The question of why, in the annual-mean, the Northern Hemisphere (NH) is warmer than 8 the Southern Hemisphere (SH) is addressed, revisiting an 1870 paper by James Croll. We 9 first show that ocean is warmer than land in general which, acting alone, would make the 10 SH, with greater ocean fraction, warmer. Croll was aware of this and thought it was caused 11 by greater specific humidity and greenhouse trapping over ocean than over land. However, 12 for any given temperature, it is shown that greenhouse trapping is actually greater over 13 land. Instead, oceans are warmer than land because of the smaller surface albedo. However, 14 hemispheric differences in planetary albedo are negligible because the impact of differences 15 in land-sea fraction are offset by the SH ocean and land reflecting more than their NH 16 counterparts. In the absence of a role for albedo differences It is shown that, in agreement 17 with Croll, northward cross-equatorial ocean heat transport (X-OHT) is critical for the 18 warmer NH. This is examined in a simple box model based on the energy budget of each 19 hemisphere. The hemispheric difference forced by X-OHT is enhanced by the positive water 20 vapor-greenhouse feedback, and is partly compensated by the southward atmospheric energy 21 transport. Due to large uncertainties in the ocean data, a range of X-OHT is considered. A 22 X-OHT of larger than 0.5PW is needed to explain the warmer NH solely by X-OHT. For 23 smaller X-OHT, a larger basic state greenhouse trapping in the NH, conceived as imposed 24 by continental geometry, needs to be imposed. Numerical experiments with a GCM coupled 25 to a slab ocean provide evidence that X-OHT is fundamentally important in determining the 26 hemispheric differences in temperature. Therefore, despite some modifications to his theory, 27 analysis of modern data confirms Croll’s 140 year-old theory that the warmer NH is partly 28 because of northward X-OHT. 29