Jet stream poleward migration leads to marine primary production decrease

• The poleward migration of the Northern Hemisphere subtropical jet stream impacts marine primary production on the long-term. • 40% decline in chlorophyll concentrations in the Northwestern Mediterranean Sea over the 2000–2023 period associated to the northward shift of the jet stream. • The long-term impact of jet stream position on marine primary production, extends beyond seasonal components. • Cascading effects of reduced marine primary production may significantly impact marine ecosystems, fisheries, and the carbon cycle. Jet Streams (JS) are powerful upper-tropospheric winds that significantly influence weather and climate. As anthropogenic climate change alters temperature gradients, subtropical JS are expected to shift poleward, which can have unforeseen consequences on midlatitude Earth systems. Here, we demonstrate, for the first time, the impact of the steady poleward migration of the Northern Hemisphere subtropical JS on Marine Primary Production (MPP). Using over two decades of data (2000–2023), we establish a direct relationship between the JS latitudinal position and MPP variability in the Northwestern Mediterranean Sea. The observed northward migration of approximately 75 km over the study period aligns with a consistent decline in chlorophyll concentrations, representing a 40 % reduction, with rates reaching up to −5% per year. This is attributed to the steady northward seasonal shift of the JS position, which drives changes in northern wind-stress and Ekman pumping, subsequently reducing upwelling occurrence and intensity. While the primary influence of JS position on MPP is seasonal, we demonstrate that its impact extends to non-seasonal components as well. Unlike other studies linking JS shifts to short-term wind stress variations and isolated upwelling events, our findings highlight a long-term impact on MPP. Our findings suggest that JS dynamics is the dominant driver of MPP variability in the Northwestern Mediterranean Sea and point to equivalent situations in other marine regions worldwide. The cascading effects of reduced MPP have the potential to significantly impact marine ecosystems and resources, with broader implications for fisheries and the carbon cycle.