1. National Institute for Environmental Studies, Tsukuba, Ibaraki, 305-0053, Japan
2. Communications Research Laboratory, Tokyo, 184-8795, Japan
3. Earth Observation Research Center/NASDA, Tokyo, 106-0032, Japan

Correspondence to: Yukari N. Takayabu¹ Correspondence and requests for materials should be addressed to Y.N.T.

Abstract

The role of the Madden–Julian oscillation—a global atmospheric wave in the tropics that is associated with convective activity and propagates eastwards with a period of about 30–60 days (refs 1, 2)—in triggering El Niño events has been discussed before^{3, 4, 5, 6, 7, 8}. But its possible connection with a termination of El Niño has yet to be investigated, despite the difficulty in explaining the timing of El Niño terminations by the basic wind-induced oceanic-wave processes^{9, 10}. For the extreme 1997–98 event, the mechanism of both onset and termination have been investigated³, but the reason for the abruptness of the termination has yet to be resolved. Here we present global data of precipitation, sea surface temperatures and wind speeds that show a precipitation system associated with an exceptionally strong Madden–Julian oscillation travelling around the Equator in May 1998. The propagation of this atmospheric system was associated with an abrupt intensification of the easterly trade winds over the eastern equatorial Pacific Ocean. Combined with the already shallow equatorial thermocline in the central and eastern Pacific Ocean at that time³, these strong winds provided the triggering mechanism for the observed accelerated ending of the 1997–98 El Niño event.