Regional and global dynamics of Intertropical Convergence Zones in present and past climates

Rain in the tropics is concentrated in narrow Inter Tropical Convergence Zones (ITCZs), with strong convergence of moist static energy (MSE) below and strong divergence of dry static energy aloft. Recent studies have provided energetic constraints for the position of the ITCZ by assuming that the vertically integrated meridional MSE flux vanishes near it, which makes it possible to relate the ITCZ position to derivatives of the MSE flux near the equator. We examine the relation of the vertically integrated MSE flux and the position of the ITCZ in past and present climates in reanalyses and in simulations of the fifth phase of the Climate Model Intercomparison Project (CMIP5) and the third phase of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the climatological mean positions of the ITCZ and of shallow convergence zones generally agree with the zeros of the meridional MSE flux. The energetic constraints capture the seasonal migration both of the zonally averaged ITCZ and of regional ITCZs over Africa, Asia and the east Pacific. They also capture the transition from single to double ITCZ regimes. During the mid-Holocene, northern hemisphere summer insolation was 7% stronger than today, which increased the southward cross-equatorial MSE flux and lead to a northward migration of the ITCZ. This northward migration is seen in PMIP3 simulations. However, it is insufficiently strong to be consistent with reconstructed precipitation changes, for example, over northern Africa. Idealized GCM simulations with a slab ocean and continents suggest that a transition from a single to a double ITCZ regime may have occurred during this period over Africa. The simulations suggest that small changes in the distribution of ocean heat intake, albedo or zonal MSE fluxes in the tropics can lead to an ITCZ bifurcation, particularly over north Africa. This may account for both the rapidity of changes in and the distribution of reconstructed precipitation over northern Africa during the mid Holocene.