The Disappearing Subsurface Western Boundary Current in the Northern Hemisphere Under a Warming Climate

Oceanic meridional heat transport (OHT) reaches its maximum near 20°–30°N, with values around 2 petawatts (PW; 1 PW = 10¹⁵ W), accounting for approximately 30% of the total heat transport in the Earth system. Within this latitude band, strong poleward-flowing subtropical western boundary currents (WBCs) play a dominant role in OHT due to their high potential temperature and swift flow velocities. This study uses an ensemble of 10 high resolution climate models and focuses on the projected changes in the two major Northern Hemisphere WBCs—the Gulf Stream and the Kuroshio Current—and their impacts on heat transport under global warming.

For the Gulf Stream, high-resolution models successfully capture the eastern branch of the current near the Bahamas, known as the Antilles Current. As a subsurface current centered around 500 m depth, the Antilles Current exhibits relatively weak mean volume transport (5 Sverdrups; 1 Sv = 10⁶m³/s) and heat transport (0.3 PW), an order of magnitude lower than the Florida Current, the primary branch of the Gulf Stream. However, under global warming, the projected reduction in the Antilles Current (3.8 Sv) is comparable to that of the Florida Current, resulting in a 0.17 PW decline in heat transport. This accounts for the majority of the total decrease in meridional heat transport across 26.5°N in the North Atlantic.

Similarly, we examine changes in the Kuroshio Current and its subsurface branch, the Ryukyu Current. Ensemble-mean results from four eddy-resolving climate models indicate that between 1950 and 2050, the Kuroshio Current in the East China Sea strengthens by 1.2 ± 0.6 Sv, while the Ryukyu Current weakens rapidly by 6.2 ± 2.5 Sv. This leads to a net reduction of 5.0 ± 2.6 Sv in total transport for the Kuroshio system, accompanied by a 0.3 PW decrease in heat transport. This trend is consistent with observational estimates over the period 1958–2022. The underlying mechanisms include a weakening of the subtropical wind field, which reduces total transport in both the Kuroshio and Ryukyu Currents. In addition, enhanced ocean stratification under global warming causes the current system to shoal and weakens flow topography interactions, contributing to the observed strengthening of the Kuroshio Current and the concurrent rapid weakening of the Ryukyu Current.