Mid-Holocene Intertropical Convergence Zone migration: connection with Hadley cell dynamics and impacts on terrestrial hydroclimate

Abstract. This study investigates the multiple changes of the Hadley cell (HC) in response to the northward migration of the Intertropical Convergence Zone (ITCZ) and their combined influence on the terrestrial hydrological cycle during the mid-Holocene, using simulations from the PMIP4–CMIP6 archive. Our results show that the annual global ITCZ position shifts northward by 0.2 and 0.3° as a multi-model mean using two different precipitation metrics, which is consistent with proxy evidence of a slight northward shift during the mid-Holocene. As the ITCZ is co-located with the rising branch of the Hadley cell, the northward migration of the ITCZ is accompanied by a northward movement of the inner HC edge. This further results in a contracted and weakened northern HC, while the southern HC expands and intensifies in the mid-Holocene. Specifically, the northern HC width contracted by 1.1 and 0.5°, with strength reductions of 3.7 % and 4.1 %, while the southern HC expanded by 1.2 and 0.6° and strengthened by 2.9 % and 1.8 %, according to the two streamfunction metrics. Meanwhile, changes in stationary and transient eddies significantly influence the cross-equatorial energy transport during the mid-Holocene, complicating the link between energy transport and the ITCZ position. Moisture budget analysis shows that enhanced moisture eddy fluxes mainly contribute to increased terrestrial precipitation in the Northern Hemisphere, particularly in monsoonal regions, while Southern Hemisphere precipitation decreased due to evaporation and dynamic terms. Seasonal analysis indicates that the terrestrial hydrological cycle changes are primarily due to summer dynamics with an amplified inter-hemispheric contrast and asymmetry, while there are minor changes during winter seasons for both hemispheres. Although orbital forcing during the mid-Holocene was symmetric around the Equator in the annual mean, it indirectly drove hemispherically asymmetric changes in annual atmospheric radiation balance. In the Northern Hemisphere, reduced surface shortwave radiation alongside increased atmospheric shortwave absorption indicates that enhanced cloudiness and water vapor play key roles. Moist static energy (MSE) budget analysis reveals that stronger rising motion significantly promotes vertical MSE advection over land in the Northern Hemisphere, enhancing moist convection and precipitation, while reduced rising motion weakens vertical MSE advection in the Southern Hemisphere, suppressing moist convection and precipitation over land. Regionally, ITCZ migration and associated HC changes alter climate patterns, with reduced Northern Hemisphere terrestrial aridity and dryland contraction but enhanced aridity and dryland expansion in the Southern Hemisphere. Multiple proxies support these findings, indicating wetter Northern Hemisphere conditions and a drier Southern Hemisphere, although inconsistencies remain in Australia's aridity pattern. Our results highlight the complex interactions among ITCZ migration, Hadley cell dynamics, the global hydrological cycle, and terrestrial aridity during the mid-Holocene.