The relationship between ITCZ location and cross 1 equatorial atmospheric heat transport ; from the 2 seasonal cycle to the Last Glacial Maximum

6 We quantify the relationship between the location of the intertropical convergence zone 7 (ITCZ) and the atmospheric heat transport across the equator (AHTEQ) in climate models 8 and in observations. The observed zonal mean ITCZ loaction varies from 5.3◦S in the Boreal 9 winter to 7.2◦N in the Boreal summer with an annual mean position of 1.65◦N while the 10 AHTEQ varies from 2.1 PW northward in the Boreal winter to 2.3 PW southward in the 11 Boreal summer with an annual mean of 0.1 PW southward. Seasonal variations in the 12 ITCZ location and AHTEQ are highly anti-correlated in the observations and in a suite of 13 state of the art coupled climate models with regression coefficients of -2.7 and -2.4 ◦ PW−1 14 respectively. We also find that seasonal variations in ITCZ location and AHTEQ are well 15 correlated in a suite of slab ocean aquaplanet simulations with varying ocean mixed layer 16 depths. However, the regression coefficient between ITCZ location and AHTEQ decreases 17 with decreasing mixed layer depth as a consequence of the asymmetry that develops between 18 the winter and summer Hadley cells as the ITCZ moves farther off the equator. 19 We go on to analyze the annual mean change in ITCZ location and AHTEQ in an ensem20 ble of climate perturbation experiments including the response to CO2 doubling, simulations 21 of the Last Glacial Maximum, and simulations of the mid-Holocene. The shift in the an22 nual average ITCZ location is also strongly anti-correlated with the change in annual mean 23 AHTEQ with a regression coefficient of -3.2 ◦ PW−1, similar to that found over the seasonal 24 cycle. The ensemble average annual mean ITCZ shift in response to both CO2 doubling 25 and the Last Glacial Maximum forcing is not significantly different from zero and is rather 26 variable between ensemble members. However, the change in AHTEQ is a robust indicator 27 of the ITCZ shift in these experiments. 28