FY 1987

The Equatorial Undercurrent: 100 years of discovery

McPhaden, M.J.

Eos Trans. AGU, 67(40), 762–765, doi: 10.1029/EO067i040p00762 (1986)

The Equatorial Undercurrent is a narrow ribbon of eastward flow centered on the equator in the upper thermocline. It is a permanent feature of the general circulation in the Atlantic and Pacific oceans and is present in the Indian Ocean in northern winter and spring during the northeast monsoon. It reaches speeds of 50–100 cm s⁻¹ below the westward flow of the South Equatorial Current, and in the Pacific transports as much mass on average (40 × 10⁶ m³ s⁻¹) as the Florida Current, which feeds the Gulf Stream. The first observations of the Equatorial Undercurrent were made 100 years ago in 1886 by the Scotsman John Young Buchanan in the Gulf of Guinea. These observations were soon forgotten, however, and nearly 70 years were to pass before observations of the Pacific Undercurrent by Townsend Cromwell and Raymond Montgomery inspired more comprehensive ocean surveys and dynamical theories of equatorial circulation. This article reviews the chronology of historical events surrounding the multiple discoveries of the Equatorial Undercurrent and summarizes our present understanding of its dynamics.