The Role of Internal Variability in ITCZ Changes Over the Last Millennium

Abstract

Tropical hydroclimate is modulated by the position and intensity of the Intertropical Convergence Zone (ITCZ). Reconstructions and simulations of the Last Millennium (LM) suggest latitudinal variations of the ITCZ that impact the hydroclimate over large regions of the world. These ITCZ shifts have been generally linked to external radiative forcing, but analyses of the Community Earth System Model – Last Millennium Ensemble (CESM-LME) demonstrate a significant contribution of internal variability over multidecadal and centennial timescales. In contrast to changes driven by external forcing, which are associated with an asymmetric warming between the Northern and Southern Hemispheres, the contribution of internal variability in the CESM-LME is associated with cooling and warming of the eastern Pacific. While external forcing remains the main driver of ITCZ changes in the Atlantic basin, the contribution of internal variability in the CESM-LME exceeds that of the forcing for the Pacific and Indian Ocean basins.