The southern hemisphere eddy-driven jet has experienced a poleward movement over the recent few decades, associated with a positive SAM trend. Explanations of this mainly stressed the response to the external forcings, while part of them also relate the SAM variability to the ENSO influence at the inter-annual timescales. However, little research has isolated the main factors to its variations at decadal timescales. The external radiative forcing and internal variability are well separated here by exploiting a hierarchy of Community Earth System Model (CESM) model outputs, including Preindustrial Control, Large Ensemble and Pacemaker runs. Model simulations suggest that, during the hiatus period, the external forcing and 3 tropical basin SSTs all contribute to the positive SAM trend and mid-latitude jet poleward migration, where the eastern Pacific SST anomaly, in the form of negative IPO, is identified as the dominant driver. Longer time IPO composite and PI control results further support the role of the IPO as a key internal contributor to jet movement and enhanced SAM variability. Physical mechanisms such as Rossby wave teleconnections and eddy-mean flow interactions are discussed but need further model experiment verification.