Mid-holocene changes in the global ITCZ: meridional structure and land–sea rainfall differences

Abstract Changes in the global and annual Intertropical Convergence Zone (ITCZ) during the mid-Holocene are quantified in this study, using simulations from the Paleoclimate Modelling Intercomparison Project Phase 4. We find an ensemble mean northward migration of ITCZ position by 0.2 $$^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> , 0.3 $$^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> , and 0.8 $$^\circ $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> using three different metrics, which is consistent with proxy evidence of a slight (within one degree) northward shift during the mid-Holocene. The width of the ITCZ exhibits no changes during this period, while the strength has a minor weakening by 0.9% and 0.6% based on precipitation and streamfunction metrics, respectively. Moreover, there is an ensemble mean increase in northern-to-southern hemisphere precipitation (4.6%) and $$\omega $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ω</mml:mi> </mml:math> ratio indices (12%) during the mid-Holocene, suggesting a more uneven and right-skewed meridional structure of the ITCZ with a greater asymmetry. Additionally, the moisture budget analysis indicates the eddy flux term and dynamic term primarily enhance the inter-hemispheric asymmetry of precipitation and poleward migration of both the northern and the southern rain belts within the ITCZ. Further investigation reveals that the northward migration of annual rainfall within the ITCZ predominantly occurs over terrestrial regions, which therefore dominate the change towards more asymmetric structure of the zonally averaged ITCZ. Moisture budgets for land and ocean separately indicate that the land-sea difference in the annual rainfall changes is primarily due to different changes in evaporation.