What Determines Meridional Heat Transport in Climate Models?

The annual mean maximum meridional heat transport (MHTMAX) differs by approximately 20% among coupled climate models. The value of MHTMAX can be expressed as the difference between the equator-topole contrast in absorbed solar radiation (ASR*) and outgoing longwave radiation (OLR*). As an example, in the Northern Hemisphere observations, the extratropics (defined as the region with a net radiative deficit) receive an 8.2-PW deficit of net solar radiation (ASR*) relative to the global average that is balanced by a 2.4-PW deficit of outgoing longwave radiation (OLR*) and 5.8 PW of energy import via the atmospheric and oceanic circulation (MHTMAX). The intermodel spread of MHTMAX in the Coupled Model Intercomparison Project Phase 3 (CMIP3) simulations of the preindustrial climate is primarily (R 5 0.72) due to differences in ASR* while model differences in OLR* are uncorrelated with the MHTMAX spread. The net solar radiation (ASR*) is partitioned into contributions from (i) the equator-to-pole contrast in incident radiation acting on the global average albedo and (ii) the equator-to-pole contrast of planetary albedo, which is further subdivided into components due to atmospheric and surface reflection. In the observations, 62% of ASR* is due to the meridional distribution of incident radiation, 33% is due to atmospheric reflection, and 5% is due to surface reflection. The intermodel spread in ASR* is due to model differences in the equator-to-pole gradient in planetary albedo, which are primarily a consequence of atmospheric reflection differences (92% of the spread), and is uncorrelated with differences in surface reflection. As a consequence, the spread in MHTMAX in climate models is primarily due to the spread in cloud reflection properties.