JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101,NO. C5, PAGES 12,209-12,216, MAY 15, 1996 Pacific low-latitude western boundary currents and the Indonesian throughflow RogerLukas Department of Oceanography, University of Hawaii, Honolulu ToshioYamagata Department of Earth and Planetary Physics, University of Tokyo,Tokyo,Japan Julian P. McCreary Oceanographic Center, Nova Southeastern University, Dania, Florida Abstract.In thisintroductory paper, we reviewobservations and theories of the low- latitude western boundary currents in the Pacific Ocean andthe Pacific-to-Indian Ocean throughflow, including the newresults presented in thisspecial section. In addition, we summarize issues and questions thatremain to be addressed. In particular, we point to the need for completing the zero-order description of the mean andseasonal variation of the circulations of the region. Introduction The Pacific low-latitude western boundary currents (LLWBCs) have been thesubject of increasing attention during the past decade for a number of reasons. Forone thing, little was known about themrelative to theircounterpart flowsin theAt- lantic and IndianOceans. For another, theyarebelieved to influ- ence theheat budget of thewarmpoolof thewestem equatorial Pacific, whichapparently plays a key role in theEl Nifio-South- em Oscillation (ENSO) [cf. Webster andLukas, 1992].Finally, theyprovide the source waters for the Indonesian throughflow (ITF), whichlikely play an important role in the global ocean thermohaline circulation [Gordon, 1986]. (Following the view of Wyrtki [ 1961 ] we treat the circulation in the Indonesian seas aspartof the low-latitude westem boundary circulation of the Pacific Ocean.) In the last decade, several nations initiated oceano- graphic programs (e.g., Western Equatorial Pacific Ocean Circu- lation Study(WEPOCS), United States-People's Republic of China Cooperative Studies of Air-Sea Interaction in theTropical Westem Pacific (US-PRC), Chinese Academy of Sciences (CAS), Japanese PacificClimateStudy (JAPACS)) within the frame- work of theTropical Oceans-Global Atmosphere (TOGA) pro- gram andWorld Ocean Circulation Experiment (WOCE) in or- derto followup onthepioneering observational efforts of Japa- nese and French investigators [Yosida etal., 1959; Tsuchiya, 1961; Masuzawa, 1968,1969; Masuzawa andNagasaka, 1975; Nitani, 1970, 1972; Hisard etal., 1969; Colin et al., 1973a, b;Rougerie and Donguy,1975; Guan, 1986] in the western equatorial Pa- cific Ocean. These efforts led to a zero-order description of the Pa- cific LLWBCs, a description that, however, remains incomplete. LLWBCscan beoperationally defined as those westem bound- ary flowsthat aredirectly connected to the strong wind-driven zonal flows of thetropics. Theyareanimportant component of theequatorial heat and mass budgets, contributing a large frac- Copyright 1996 by theAmerican Geophysical Union. Paper number 96JC01204. 0148-0227/96/96JC-01204509.00 tion of the cross-equatorial transport of mass, heat, and salt; thus they are animportant element of theglobal climate system. The LLWBCs offerthepotential for substantial water mass modifi- cation through enhanced diapycnal mixing withinthese strong boundary currents and epipycnal mixing intheir confluence zones. The LLWBCs also play arole in the therm0haline circulation of the World Ocean; the relationship between wind-driven and buoyancy- forced flows in these important areas is intriguing but Unclear. All three oceans have LLWBCs, even though the oceans dif- fer considerably fromoneanother in thecharacter of thewestern boundary and thenature of thewind forcing. The tropical west- emboundary in theAtlantic isdominated by a NW-SE tilt, while the Indian Ocean western boundary has a NE-SW slope. The Pacifictropical western boundary is quite irregular, including many offshore island groups and a pathway through Indonesia to the Indian Ocean. Thelocal windforcing oftheAtlantic LLWBCs isrelatively steady, whileseasonally reversing monsoon circula- tions dominate thelocal forcing in theIndian and western Pacific Oceans. The western PacificLLWBCs are subject to the most wind stress variability: In addition to the strong monsoon, there ispronounced interannual variability associated withtheENSO. There are significant differences between the LLWBCs in the Atlantic,Pacific,andIndian Oceans, no doubt attributable to the different boundary configurations andforcing in each basin. In theAtlantic,theNorth Brazil Current flows northward across the equator, retroflects intotheNorthEquatorial Countercurrent (NECC) and the Equatorial Undercurrent (EUC), and sheds anti- cyclones tothe north [Richardson etal., 1994]. $chott etal. [1995] provide a review of the upper ocean circulation along thetropi- cal westem boundary of the Atlantic. •! n the Indian Ocean, the Somali Current also crosses the equator, but it reverses direction twiceeach yearwith the monsoons. The comprehensive review of low- latitude ocean circulation by Knox andAnderson [1985] provides a useful summary for the IndianOcean including the LLWBCs. More recently, Visbeck andSchott [1992] address the Indian Ocean westem boundary circulation through comparison of observations with a general circulation model. In thePacific, the New Guinea Coastal Undercurrent and the Mindanao Cur- rent (MC) flow equatorward, feeding boththe NECC andthe flow from the Pacific to the Indian Ocean (Indonesian 12,209