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

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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. Plain Language Summary The Intertropical Convergence Zone (ITCZ) is a wide region of increased precipitation around the Equator. The position of the ITCZ changes over time. It moves to the north during summer in the Northern Hemisphere and to the south during boreal winter. In addition to these seasonal shifts, the ITCZ also changes over longer timescales. We have used a climate model to analyze ITCZ shifts over the Last Millennium. We have found that changes in the positioning of the ITCZ over the Atlantic Ocean are mainly linked to variations in solar luminosity and volcanic events, while for the Pacific and Indian Ocean, internally driven fluctuations in atmosphere and ocean states are dominant. The understanding of the mechanisms behind ITCZ shifts allows for a better assessment of tropical hydroclimatic events, such as persistent droughts and floods that are associated with alterations in the intensity and duration of tropical monsoons, as well as for better understanding its responses under intensified climate change conditions.
CRUE-CSIC (Acuerdos Transformativos 2022). © 2022 The Authors. We gratefully thank the IlModels (CGL2014-59644-R) and GreatModelS (RTI2018-102305-B-C21) projects. JES was supported in part by the US National Science Foundation grants OISE-1743738 and AGS-2101214.