Emergent climate projection discrepancies stem from spurious rainband

As climate change unfolds, global climate models enable us to understand how anthropogenic influences are shaping the record-breaking climate anomalies occurring year after year. The distinctive pattern of anomalies in Pacific temperatures, with cooling in the east and southeast, and accelerated warming in the west, has received particular attention due to its widespread impacts on global and regional climate and because it departs from the more uniform warming anticipated by climate models. The apparent inability of climate models to simulate this warming pattern limits our ability to understand why it is occurring. Here we show that climate models’ Pacific warming pattern discrepancy stems from their simulation of a spurious Inter-Tropical Convergence Zone (ITCZ) rainband south of the equator—commonly referred to as the “double-ITCZ bias”—in a region that is dry year-round in observations. Models without a double-ITCZ bias produce anthropogenically forced warming patterns that more closely resemble observations as well as enhanced unforced decadal variability throughout the Indo-Pacific. We show that these features arise from the amplification of patterned climate anomalies when the equatorward propagation of temperature anomalies from the Southeast Pacific is unobstructed by a double ITCZ. Our findings suggest that observed tropical Pacific trends may contain a stronger forced component than previously recognized and that reducing the double-ITCZ bias is critical for improving the reliability of future climate projections.