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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.
Western boundary currents (WBCs) are swift, narrow oceanic currents found in all major oceanic gyres. Within the Pacific Ocean, the subtropical gyre WBCs are the Kuroshio Current in the Northern Hemisphere, and the East Australian Current (EAC) in the Southern Hemisphere (Fig. 1a). The Pacific low-latitude, tropical belt WBCs include the Mindanao Current in the Northern Hemisphere and the New Guinea Coastal Undercurrent (NGCUC) south of the Equator, both of which are directly connected to the equatorial Pacific circulation system. Much of modern wind-driven ocean circulation theory was derived from a quest to understand these Pacific Ocean currents. Knowledge of the effect of the Earth's rotation on WBCs, and of the Ekman transport (see Box 1), led to ground-breaking advances: that wind stress (Fig. 1b) is a driving agent of ocean currents, but it is the horizontal gradient rather than the absolute strength that is important; that latitudinal gradients in the effect of Earth's rotation on the horizontal motion cause a flow intensification towards the west of the ocean basins; and that the Pacific wind-forced ocean currents include the equatorial current system, the low-latitude and subtropical WBCs.
Impacts of the Pacific WBCs on the global ocean circulation and climate variability are manifold. First, in winter, the subtropical WBCs are associated with the largest supply of heat and moisture into the atmosphere in the Pacific basin, and are coupled to North Pacific storm tracks1. As cold and dry air comes into contact with the warm water carried poleward by the subtropical WBCs, heat and moisture are extracted from the surface. Second, the Indonesian Throughflow (ITF), the only low-latitude inter-ocean current, flows from the Pacific to the...