Variations of the zonal pressure gradient (ZPG) and the Equatorial Undercurrent (EUC) are described in the equatorial Atlantic Ocean (between 34¿30'W and 6¿30'E) as observed during eight pairs of Programme Fran¿ais Oc¿an et Climate dans l'Atlantique Equatorial cruises between October 1982 and August 1984. In 1982 and 1983 the annual cycle of weak trades early in the year, followed by comparatively strong and sustained trades, were relatively normal. In the boreal winter of 1984, the extent of the collapse of the trade winds was exceptional, and consequences were observed in the upper layers of the ocean along the equator. The 20 ¿C isotherm depth, representative of the thermocline depth, was unusually deep along the equator in the Gulf of Guinea. A near-zero value of the ZPG was observed from the Greenwich meridian westward. An eastward surface current, separated from the EUC by a layer of reduced velocity, was observed over much the same fetch. West of 10 ¿W the maximum velocity of the EUC showed very little seasonal variation, usually between 70 and 80 cm s-1, except in the boreal fall of 1983 when it exceeded 100 cm s-1.

During this period (in both 1983 and 1984) the EUC transport at 23 ¿W reached 25¿106 m3 s-1, or twice its annual average, which derived not only from the increase in speed but also for a deepening of the current to below 100 m. It was also the season when the ZPG was strongest and has a similar amplitude at both the surface and 50 dbar relative to 500 dbar. The seasonal variations of the EUC in the Gulf of Guinea were even more pronounced. During the boreal summer and fall, the current weakened, while the local ZPG reversed direction compared to the west. The stronger penetration of the EUC into the gulf occurred in the alternate seasons when there was no local reversal in the eastward acting ZPG force. The large changes in the oceanic and atmospheric circulation during the 1983--1984 period are hypothesized to be related to the suppression of the coastal period are hypothesized to be related to the suppression of the coastal Benguela upwelling along the Namibia coast.

This Atlantic equivalent to El Ni¿o occurred 1 year after the exceptionally strong event in the Pacific. ¿American Geophysical Union 1987