Enhanced warming over the global subtropical western boundary currents

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake1–4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns5–7, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions. The increase of carbon dioxide and other greenhouse gases in the atmosphere has been the major driver of surface warming of the Earth over the twentieth century, a warming that is projected to continue in the foreseeable future8. The oceans take up both heat and carbon dioxide, buffering this surface warming. The recent uptake of heat by the ocean9 has led to a significant increase in sea surface temperature (SST) in both the tropics and high latitudes, with strong warming in the tropical Indian Ocean, and weak warming over the deep-water-formation region in the high latitudes10,11. Studies so far have focused on tropical warming patterns8,12–14 owing to their profound influence on atmospheric deep convection and global atmospheric teleconnections. However, an important region with some of largest air–sea fluxes of heat, moisture and carbon dioxide in the world’s oceans is the convergence zone of extratropical western boundary currents. The poleward flowing warm/salty subtropical western boundary currents meet