The authors quantify the relationship between the location of the intertropical convergence zone (ITCZ) and the atmospheric heat transport across the equator (AHTEQ) in climate models and in observations. The observed zonal mean ITCZ location varies from 5.3°S in the boreal winter to 7.2°N in the boreal summer with an annual mean position of 1.65°N while the AHTEQ varies from 2.1 PW northward in the boreal winter to 2.3 PW southward in the boreal summer with an annual mean of 0.1 PW southward. Seasonal variations in the ITCZ location and AHTEQ are highly anticorrelated in the observations and in a suite of state-of-the-art coupled climate models with regression coefficients of −2.7° and −2.4° PW−1 respectively. It is also found that seasonal variations in ITCZ location and AHTEQ are well correlated in a suite of slab ocean aquaplanet simulations with varying ocean mixed layer depths. However, the regression coefficient between ITCZ location and AHTEQ decreases with decreasing mixed layer depth as a consequence of the asymmetry that develops between the winter and summer Hadley cells as the ITCZ moves farther off the equator. The authors go on to analyze the annual mean change in ITCZ location and AHTEQ 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 shift in the annual average ITCZ location is also strongly anticorrelated with the change in annual mean AHTEQ with a regression coefficient of −3.2° PW−1, similar to that found over the seasonal cycle.

The Southern Ocean has shown little warming south of the Antarctic Circumpolar Current (ACC) over recent decades, and Antarctic sea-ice cover has been modestly expanding 1 . Along the northern flank of the ACC, however, the upper ocean has been warming rapidly 2, 3 . Using observations and general circulation model simulations, we show that these patterns -of delayed warming south of the ACC and enhanced warming to the north -are fundamentally shaped by the Southern Ocean's meridional overturning circulation: wind-driven upwelling of unmodified water from depth damps warming around Antarctica; greenhouse gas induced heat uptake is largely balanced by anomalous northward heat transport; and heat is preferentially stored along the northern flank of the ACC, where surface waters are subducted.Further, we find that these processes are primarily due to passive advection of the anomalous warming signal by climatological ocean currents; changes in atmospheric and oceanic circulations play a secondary role. These findings suggest that the Southern Ocean responds to greenhouse gas forcing on the timescale over which the deep ocean waters that are upwelled 1

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