Recent equatorward shift of the summer North Atlantic jet dominated by internal climate variability.

The North Atlantic jet (NAJ), a fast-flowing westerly wind in the upper troposphere, influences the movement of extreme weather systems and affects the safety of commercial flights. In recent decades, the summer NAJ exhibited a substantial equatorward shift over the eastern Atlantic, contrasting with the poleward shift of zonal-mean jet. However, whether this equatorward shift is driven by external forcing or internal variability remains unclear. Here, we show that the recent equatorward shift of the summer NAJ was dominated by internal climate variability. Using simulations from state-of-the-art numerical models and reanalyses, we identify the recent decadal component of the North Atlantic warming hole in summer as the key factor. Through turbulent heat release, this distinctive pattern altered the local atmospheric thermal structure, causing the shift of the summer NAJ via thermal wind response and eddy feedback. However, this internal variability-dominated situation is not expected to persist. Our results indicate that as early as the 2050s, the latitudinal shift of the summer NAJ is projected to emerge beyond the range of internal climate variability.