Unraveling the drivers of climate variability during the Mid-Pleistocene Transition (MPT) remains a central challenge in paleoclimate research. This interval marked a shift from 41-kyr to 100-kyr glacial cycles associated with larger ice sheets. While previous studies emphasize interactions between climate and Northern Hemisphere ice sheets, Antarctica’s role remains unclear. We use the Parallel Ice Sheet Model to simulate Antarctic Ice Sheet evolution over the last 3 million years, applying a climate index approach. Our simulations show that between 1.9 and 0.8 Ma, several Antarctic drainage basins underwent structural re-organization at different times, including the formation of a stable marine-based West Antarctic Ice Sheet (WAIS). We analyze the drivers of these thresholds and their associated state transitions. Our findings reveal tri-stability in the Thwaites basin and suggest that WAIS thresholds and their complex interactions amplified ~100-kyr climate variability before and during the MPT, providing new insights into long-term climate dynamics.