A new study published in the journal Geophysical Research Letters reveals that inaccuracies in how climate models simulate sea surface temperatures (SSTs) can influence the position of the North Pacific westerly jet stream. This jet stream is a powerful band of winds that flows from west to east in the upper atmosphere at midlatitudes and plays a significant role in shaping weather patterns across East Asia, including Japan. The research, led by Yusuke Ushijima and others, highlights how these inaccuracies, or "biases," in SST simulations can cause the jet stream to shift further south than it would under real-world conditions. The study compared results from the Coupled Model Intercomparison Project Phase 6 (CMIP6), an international effort to evaluate climate models, with other simulations that used observed SST data. In 34 out of 35 models, the westerly jet was found to be positioned farther south when the models used their own simulated SSTs, as opposed to real-world SST data. This suggests that the way climate models represent ocean temperatures has a direct impact on the behavior of the jet stream. To confirm this relationship, researchers conducted additional experiments using atmospheric models. When they added the SST biases found in climate models to observed SSTs, the simulations reproduced the southward shift of the jet stream. Further experiments showed that biases in the northwestern Pacific region are especially influential in this shift. This region is known to have a significant impact on atmospheric circulation patterns in the broader North Pacific. The findings underscore the importance of accurately representing sea surface temperatures in climate models. Improving these representations could lead to more reliable simulations of atmospheric circulation patterns that affect weather and climate in East Asia. This, in turn, could enhance the accuracy of future climate projections, helping scientists and policymakers better understand and prepare for regional climate changes.