caa a22 4500 605473943 CHVBK 20210128100345.0 cr unu---uuuuu 210128e20150301xx s 000 0 eng 10.1007/s00382-014-2147-z doi (NATIONALLICENCE)springer-10.1007/s00382-014-2147-z Croll revisited: Why is the northern hemisphere warmer than the southern hemisphere? [Elektronische Daten] [Sarah Kang, Richard Seager, Dargan Frierson, Xiaojuan Liu] The question of why, in the annual-mean, the northern hemisphere (NH) is warmer than the southern hemisphere (SH) 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 SH, with greater ocean fraction, warmer. Croll was aware of this and thought it was caused by greater specific humidity and greenhouse trapping over ocean than over land. However, for any given temperature, it is shown that greenhouse trapping is actually greater over land. Instead, oceans are warmer than land because of the smaller surface albedo. However, hemispheric differences in planetary albedo are negligible because the impact of differences in land-sea fraction are offset by the SH ocean and land reflecting more than their NH counterparts. In the absence of a role for albedo differences it is shown that, in agreement with Croll, northward cross-equatorial ocean heat transport (X-OHT) is critical for the warmer NH. This is examined in a simple box model based on the energy budget of each hemisphere. The hemispheric difference forced by X-OHT is enhanced by the positive water vapor-greenhouse feedback, and is partly compensated by the southward atmospheric energy transport. Due to uncertainties in the ocean data, a range of X-OHT is considered. A X-OHT of larger than 0.5PW is needed to explain the warmer NH solely by X-OHT. For smaller X-OHT, a larger basic state greenhouse trapping in the NH, conceived as imposed by continental geometry, needs to be imposed. Numerical experiments with a GCM coupled to a slab ocean provide evidence that X-OHT is fundamentally important in determining the hemispheric differences in temperature. Therefore, despite some modifications to his theory, analysis of modern data confirms Croll's 140-year-old theory that the warmer NH is partly because of northward X-OHT. Springer-Verlag Berlin Heidelberg, 2014 Hemispheric temperature difference nationallicence Energy budget nationallicence Cross-equatorial ocean transport nationallicence Greenhouse trapping nationallicence Kang Sarah School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, EB2 Rm801-2, UNIST-gil50, Eonyang-eup, Ulju-gun, 698-798, Ulsan, Republic of Korea aut Seager Richard Lamont Doherty Earth Observatory, Columbia University, Palisades, NY, USA aut Frierson Dargan Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA aut Liu Xiaojuan Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA aut Climate Dynamics Springer Berlin Heidelberg 44/5-6(2015-03-01), 1457-1472 0930-7575 44:5-6<1457 2015 44 382 https://doi.org/10.1007/s00382-014-2147-z text/html Onlinezugriff via DOI BK010053 XK010053 XK010000 Metadata rights reserved Springer special CC-BY-NC licence nationallicence 1 research-article jats NATIONALLICENCE NATIONALLICENCE NL-springer NATIONALLICENCE 856 40 https://doi.org/10.1007/s00382-014-2147-z text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Kang Sarah School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, EB2 Rm801-2, UNIST-gil50, Eonyang-eup, Ulju-gun, 698-798, Ulsan, Republic of Korea aut NATIONALLICENCE 700 1- Seager Richard Lamont Doherty Earth Observatory, Columbia University, Palisades, NY, USA aut NATIONALLICENCE 700 1- Frierson Dargan Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA aut NATIONALLICENCE 700 1- Liu Xiaojuan Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA aut NATIONALLICENCE 773 0- Climate Dynamics Springer Berlin Heidelberg 44/5-6(2015-03-01), 1457-1472 0930-7575 44:5-6<1457 2015 44 382