Braconnot et al. (2007b) previously demonstrated the surprising result that, in PMIP2 LGM simulations, the meridional shift of the ITCZ during the boreal summer Japanese dating online differs in sign between the ensemble members. We find here that indeed, and contrary to expectations, two models shift the annual mean ITCZ toward the Northern Hemisphere. We note that, in both cases, these changes are expected given the change in AHTEQ and the relationship between PCent and AHTEQ. We now discuss the cause of the negative (more southward) change in AHTEQ in these two models. In ECHAM5, ?SWWebsites,TOA? increases by 0.24 PW, primarily due to the surface albedo change, and ?OLR? increases by 0.46 PW, primarily due to the Planck feedback and more cooling in the Northern Hemisphere as compared to the Southern Hemisphere. Because the ?OLR? change is larger in magnitude than the ?SWWeb,TOA? change, the ice sheet actually introduces a net source of energy to the atmosphere in the Northern Hemisphere due to radiative overcompensation and AHTEQ moves energy away from the energy source in the Northern Hemisphere, resulting in a northward ITCZ shift. In FGOALS, the total heat transport across the equator is nearly unchanged (?0.05 PW) due to a large radiative compensation with the change in ?SWWeb,TOA? being nearly, but not completely balanced by ?OLR?. Concurrently, ?OHT + S? increases by 0.10 PW either due to more northward ocean heat transport or transient energy storage in the Southern Ocean. As a consequence, AHTEQ decreases by 0.05 PW, largely reflecting compensation between oceanic and atmospheric heat transport, and the ITCZ shifts northward.
3) 6Kyr experiments
The robust annual mean northward ITCZ shift and more southward AHTEQ in the 6Kyr runs is surprising given that there is no interhemispheric contrast of forcing in the annual mean and implies that nonlinearities in the seasonal response to forcing play a vital role in setting the ITCZ location. In the ensemble average, the changes in net radiation at the TOA have a minimal impact on AHTEQ as the 0.06 PW increase in ?SWNet,TOA? (associated with reduced cloudiness in the Northern Hemisphere midlatitudes) is nearly balanced by a 0.05 PW increase in ?OLR? (associated with warming of the northern subtropics). Ultimately, the ensemble average 0.06 PW decrease (more southward) in AHTEQ is a consequence of a 0.07 increase in ?OHT + S?. Our analysis techniques cannot distinguish between a change in cross-equatorial ocean heat transport and a hemispheric asymmetry of transient ocean storage. The annual mean shift in the 6Kyr runs reflects the ITCZ moving farther northward during the boreal summer while the ITCZ migration into the Southern Hemisphere during the austral summer is nearly unchanged relative to that in the PI simulations, as discussed further in our concluding remarks.
4. Summary and you may talk
The seasonal cycle of the location of the ITCZ (PPenny) is highly anticorrelated with the atmospheric heat transport at the equator (AHTEQ) and highly correlated with the interhemispheric contrast of tropical sea surface temperatures (?SST) in both the observations (R 2 = 0.99 and 0.94 respectively) and coupled climate models (R 2 = 0.80 and 0.94 respectively). Cent and AHTEQ is a consequence of their mutual association with the meridional migration of the Hadley cell. The regression coefficient between PPenny and AHTEQ over the seasonal cycle is ?2.7° PW ?1 in the observations and agrees well with the CMIP3 ensemble average of ?2.4° PW ?1 . Similarly, the regression coefficient between PCent and ?SST over the seasonal cycle is +3.3° K ?1 in the observations and agrees well with the CMIP3 ensemble average of +3.7° PW ?1 .