Enhanced Warming over the Global Subtropical Western Boundary Currents

The subtropical western boundary currents carry warm tropical water to the midlatitudes, vent large amount of heat and moisture to the atmosphere along their path, and thus profoundly affecting jet streams, midlatitude storms, as well as carbon uptake. The potential changes of these highly energetic and nonlinear currents under a greenhouse gas forcing have drawn significant concerns, yet detecting such changes remain largely hindered due to very limited observations. Here, based on different reconstructed sea surface temperature datasets and newly developed century-long ocean and atmosphere reanalysis products, we estimate the linear trends and 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 may be attributed to a synchronous poleward shift and/or intensification of global subtropical western boundary currents associated with a systematic change of winds over both hemispheres. Such an enhanced warming may reduce thermodynamic drive for the ocean to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a timely need of a long-term monitoring network of the global western boundary currents and their extensions.