The relationship between ITCZ location and atmospheric heat transport across the equator

The distribution of tropical precipitation and the atmospheric heat transport in the deep tropics are both primarily controlled by the strength and location of the Hadley cell with the atmosphere transporting heat away from the precipitation maximum. We quantify the relationship between the location of the inter-tropical convergence zone (ITCZ) and the atmospheric heat transport across the equator (AHT_EQ) in climate models and in Nature. Seasonal variations in the ITCZ location and AHT_EQ are highly negatively correlated in Nature (R² = 0.99) and in a suite of state of the art coupled climate models. Similarly, the inter-annual variability of ITCZ location and AHT_EQ are negatively correlated in the both Nature and coupled climate models. Lastly, the change in ITCZ location is negatively correlated with the change in AHT_EQ in an ensemble of climate perturbation experiments including the response to CO₂ doubling, simulations of the last glacial maximum, and simulations of the mid Holocene. The regression coefficient between ITCZ location and AHT_EQ is approximately 3 degrees of latitude per PW and is statistically indistinguishable between the seasonal, inter-annual, and climate change time scales and between models and Nature. These results suggest that a 1 degree latitude shift of the ITCZ demands approximately 0.3 PW of heat transport between the two hemispheres in the atmosphere. The latter must be balanced by a hemispheric asymmetry in the energy flux to the atmosphere, either by surface fluxes (associated with ocean heat transport and storage) or radiation at the top of the atmosphere (TOA). We go on to explore the source of the hemispheric asymmetry in energy fluxes to the atmosphere and conclude that, in Nature, the Northward heat transport in the ocean is responsible for the annual mean position of the ITCZ in the Northern Hemisphere. In contrast, radiative anomalies at the TOA are responsible for ITCZ shifts at the seasonal, inter-annual, and climate change time scales.