Waves along the Equator in the Atlantic

Abstract

Spectra of two hundred days of data from five inverted echo sounders deployed along the equator in 1983?84 and a thousand days of 30 crossings of the equator by the TOPEX/POSEIDON altimeter in 1993?95 are both found to have enhanced variance of sea surface displacement in two frequency bands centered nominally about 50?1 and 25?1 cycles/day. Complex empirical orthogonal function analysis of the two bands finds much of the variance being accounted for by eastward and westward propagating waves, respectively, that are coherent across the basin. The eastward propagating wave is identifiable as a first-mode Kelvin wave with a period of 54 (49) days in the sounder (altimeter) dataset, a basinwide wavelength and a speed of 2.1 (1.8) m s?1. It is suggested that the westward propagating wave is a surface expression of the meridional oscillation of the Equatorial Undercurrent about the equator, presumably induced by a tropical instability wave, with a period of 24 (28) days, a wavelength of 600 (580) km, and a speed of 0.30 (0.23) m s?1. The amplitude variation with time and longitude of both waves is described and, for the Kelvin wave, compared to the zonal wind stress in the west during 1993?95.