Emergence of Subsurface Warming in the Southern Ocean Gateway Between New Zealand and Antarctica

We analyze temperature variability in the surface and intermediate layers of the Southern Ocean from 1994 to 2025 using a 30-year summertime record of Expendable Bathythermograph (XBT) measurements collected along the PX36 line across the New Zealand–Antarctica chokepoint of the Antarctic Circumpolar Current (ACC). Significant subsurface warming emerges along the northern flank of the current, while waters south of the Polar Front exhibit no long-term trend. Within the main water masses of the region, Subantarctic Mode Water and Antarctic Intermediate Water warm by ∼0.22 and ∼0.16°C decade−1 respectively, driven largely by strong temperature increase during the last decade. These results reveal this chokepoint as a hotspot of Antarctic Intermediate Water variability and suggest warming in its upstream source regions, underscoring the need for sustained, high-resolution monitoring of this key sector of the Southern Ocean.