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 uptake 1-4 . The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns 5-7 , 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.

Along the Philippine coast in the western Pacific, the North Equatorial Current (NEC) bifurcates into the northward flowing Kuroshio and the southward flowing Mindanao Current. Using both the linear, time-dependent Sverdrup theory and a high-resolution, nonlinear reduced-gravity model, this study investigated the changes in the NEC-Mindanao Current-Kuroshio (NMK) system induced by large-scale surface wind forcings. Using the Florida State University monthly wind stress data from 1961 through 1992, we show that the seasonal bifurcation of the NEC occurs at the northernmost position in October and the southernmost position in February. While the meridional migration of the basin-wid e trade wind has a relatively small effect in shifting the bifurcation latitude (by about 100 km), the monsoonal wind along the low-latitude western Pacific is effective in inducing a large northward excursion of the NEC's bifurcation in the fall season. On the interannual timescale, the positive wind stress curl of the trade wind tends to intensify and shifts the zero wind stress curl line northward prior to E1 Nifio-Southern Oscillation (ENSO) events. With a lag of about i year this shift induces the bifurcation of the NEC to occur at a higher latitude. During the La Nifia years the NEC generally bifurcates at a lower latitude. No significant seasonal fiuctuation•s are found in the transport of the NEC near the Philippine coast. Seasonal changes in the Mindanao Current and the Kuroshio are, however, significant, and their transports tend to fluctuate 180 ø out of phase, due to the different speeds of the baroclinic Rossby waves at their respective latitudes. The Kuroshio (the Mindanao Current) has a seasonal minimum (maximum) transport in fall when the NEC bifurcates at the seasonally northernmost latitude. The interannual changes in the inflow NEC are largely controlled by the basin-wide, wind stress curl anomalies. While the quasi-biennial changes are confined only to the southern limb of the NMK system, signals with ENSO timescales are •found to influence the midlatitude, subtropical circulation via the Kuroshio. Introduction After encountering the western boundary along the Philippine coast, the North Equatorial CUrrent (NEC) in the Pacific Ocean bifurcates into the northward flowing Kuroshio and the southward flowing Mindanao Current (Figure 1) [ Nitani, 1972 ]. If the surface wind forcing is steady, it is easy to show from the Sverdrup dynamics that the NEC's bifurcatiOn should occur where the zonally averaged transport stream function is zero. In reality, the surface wind field changes both in time and in space and these changes can exert a particularly large impact upon the NEC-Mindanao Current-

Pacific Ocean western boundary currents and the interlinked equatorial Pacific circulation system were among the first currents of these types to be explored by pioneering oceanographers. The widely accepted but poorly quantified importance of these currents-in processes such as the El Niño/Southern Oscillation, the Pacific Decadal Oscillation and the Indonesian Throughflow-has triggered renewed interest. Ongoing efforts are seeking to understand the heat and mass balances of the equatorial Pacific, and possible changes associated with greenhouse-gas-induced climate change. Only a concerted international effort will close the observational, theoretical and technical gaps currently limiting a robust answer to these elusive questions.