Mid-Holocene ITCZ migration: impacts on Hadley cell dynamics and terrestrial hydroclimate

Abstract. This study investigates the multiple changes of the Hadley cell (HC) in response to the northward migration of Intertropical Convergence Zone (ITCZ) and their combined influence on terrestrial hydrological cycle during the mid-Holocene, using simulations from the PMIP4-CMIP6 archive. Our results show that orbital forcing increased radiative heating in the Northern Hemisphere, shifting the ITCZ 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. This migration primarily drives the northward movement of the inner HC edge, resulting in a contracted and weakened northern HC, while the southern HC expands and intensifies. 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 stream- function metrics. Enhanced moisture eddy fluxes are a major contributor to increased terrestrial precipitation in the Northern Hemisphere, particularly in monsoonal regions, while Southern Hemisphere precipitation decreased due to evaporation and dynamic terms. 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. Regionally, ITCZ migration and associated HC changes alter climate patterns with reduced Northern Hemisphere terrestrial aridity and drylands contraction, while the Southern Hemisphere has enhanced aridity and drylands expansion. 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, global hydrological cycle, and terrestrial aridity during the mid-Holocene.