Data are examined from two cruises of the R/V Natsushima in the region 5¿N to 5¿S, 160¿E to 160¿W during January--February 1989 and January--February 1990. These cruises were conducted in the climatically sensitive region of the central equatorial Pacific cold tongue and the western equatorial Pacific warm pool during extremes of the Southern Oscillation index (SOI). Associated with high SOI values in January--February 1989, anomalously strong trade winds prevailed along the equator between 160¿E and 160¿W, the cold tongue penetrated west of the date line, surface dynamic height (relative to 1000 dbar) sloped up to the west along the equator by 17 dyn cm between 160¿W and 160¿E, and the depth of the equatorial thermocline increased from 100 m at 160¿W to 150 m at 160¿E. Flow in the surface layer was to the west at speeds up to 60--80 cm s-1 in the South Equatorial Current, and volume transport in the upper 100 m (300 m) was 52 Sv (98 Sv) to the west across the date line between 5¿N and 5¿S. Flow and hydrographic conditions were markedly different 1 year later when the SOI was at a negtive extreme.

Winds became westerly west of the dateline at speeds up to 10 m s-1, the zonal slope of the upper thermocline and of surface dynamic height reversed, the surface layer warmed by 2¿--3 ¿C between 160¿W and 160¿E, and the cold tongue disappeared from the western Pacific. Zonal currents reversed and flowed eastward at speeds of 20--60 cm s-1 in the upper 75 m along the equator between 155¿E and 175¿E. Conversely, east of 180¿ easterly surface winds and westward surface flow prevailed, so that near the date line both surface zonal winds and currents were convergent. Associated with this convergence, surface dynamic height rose by 12 dyn cm, and a relatively sharp zonal gradient in water mass properties developed in the surface layer. Net westward volume transport across the dateline between 5¿N and 5¿S in the upper 100 m (300 m) was reduced to 20 Sv (47 Sv) in early 1990, or about 50% of the transports observed the previously year. Diagnosis of the depth integrated zonal momentum balance for both cruises indicated that the changes in upper ocean transport could not be interpreted as a linear equilibrium response to local wind forcing. ¿ American Geophysical Union 1993